Module 5 Syllabus notes.docx

reproduction ============ how does reproduction ensure the continuity of a species? explain the mechanisms of reproduction that ensure the continuity of a species by analyising sexual and asexual methods of reproduction in a variety of organisms ----------------------------------------------------...

reproduction ============ how does reproduction ensure the continuity of a species? explain the mechanisms of reproduction that ensure the continuity of a species by analyising sexual and asexual methods of reproduction in a variety of organisms ----------------------------------------------------------------------------------------------------------------------------------------------------------------- **Reproduction --** process by which organisms replicate themselves. **Asexual** reproduction involves only one parent and gives rise to offspring that are genetically identical to each other and to the original parent. - Does not involve gametes. - The main process of asexual reproduction is mitosis. [Apomixis/Parthenogenesis] - reproduction by special generative tissues without fertilisation. It includes parthenogenesis in animals, in which the new individual develops from the unfertilised (usually female). - E.g., brambles, dandelions. Brynoe's gecko, sharks. [Advantages]: - Reproduction in the absence of pollinators or males. [Disadvantages:] - no genetic diversity. - Can only occur in females. [Regeneration]: parent organism splits and the parts of the organism develop into mature, fully grown individuals. The fragments regenerate into new complete individuals by mitosis and differentiation. - E.g., Planarians Sponges. [Fragmentation]: body of organism breaks into two or more parts, each of which regenerates the missing pieces to form a new, complete individual. E.g., sponges, flat worms, echinoderms. [Advantages]: - These organisms retain some stem cells through their life. - Stem cells can develop into any cell type in the body so they can regenerate body parts (lost through injury) or even reproduce new individuals. **Sexual** reproduction involves two parents who produce offspring that have a mix of the parents' genes and therefore differ from each other and from the parents. The key difference between sexual and asexual reproduction is that one produces individuals with unique combinations of genes (sexual) and the other does not (asexual). [Importance of reproducing for the continuity of a species]: - passing on their genes. - Offspring carry the same/a mix of genetic traits from the parents into the next generation ensuring that the gene pool and the species continues. +-----------------------+-----------------------+-----------------------+ | | Asexual Reproduction | Sexual reproduction | +=======================+=======================+=======================+ | Advantage | - [Quick and | - [Creates more | | | easy] | genetic | | | -- can rapidly | variation]{.under | | | increase | line} | | | population | within a | | | numbers in | population by | | | favourable | randomly | | | conditions. | combining | | | | advantageous | | | - [Only needs one | traits from | | | parent]{.underlin | parents to create | | | e} | a unique | | | -- no time and | individual. | | | energy wasted in | (Essential for | | | finding a mate. | continuity of | | | | species in a | | | - [Large number of | changing | | | offspring in | environment). | | | short amount of | | | | time] | - natural selection | | | -- ideal for | ensures that only | | | populating area | the strongest | | | in a stable | organisms reach | | | environment. | sexual maturity | | | | and can therefore | | | | [pass on | | | | advantageous | | | | traits/adaptation | | | | s. | | | | ] | +-----------------------+-----------------------+-----------------------+ | Disadvantage | - little (random | - [Slower | | | mutations) to [no | process]{.underli | | | genetic | ne} | | | variation]{.under | both in terms of | | | line} - | courtship, | | | could be | copulation and | | | catastrophic if | gestation. | | | environment | | | | changes and wipes | - [high energy | | | out entire | demands]{.underli | | | population. | ne} | | | | -- in finding a | | | | mate, gestation, | | | | and parental | | | | care. | +-----------------------+-----------------------+-----------------------+ | Differences | - one parent | - two parents | | | | | | | - quicker | - slower | | | | | | | - no variation | - more variation | | | | | | | - less energy | - more energy | | | | required. | +-----------------------+-----------------------+-----------------------+ | Similarities | - means of | | | | producing | | | | offspring to | | | | ensure the | | | | continuity of | | | | species. | | +-----------------------+-----------------------+-----------------------+ ### animals: advantages of external and internal fertilisation **Fertilisation** is the union of male and female gametes. [Factors that affect the survival rate and thus impact the continuity of species:] - - Where fertilisation occurs - The environment for zygote development. - Number of offspring produced. - Breeding frequency - Parental care. Characteristics Differences Similarities ----------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------------- **External Fertilisation** **Internal fertilisation** Gametes Large number of male and female gametes produced. Large number of male gametes and fewer female gametes produced. Male and female gametes required -- sperm and eggs. Union Occurs in open water environments Occurs inside the reproductive tract of the female in organisms that live mostly or completely on land. Sperm fertilise the eggs when they unite Conception mechanism Simultaneous release of gametes. Copulation: the male inserts sperm into the female's reproductive tract via penis or cloaca. Sperm will fertilise eggs when in very close proximity to each other; gametes require a watery environment for this to occur. Chance of fertilisation Low, because male gametes are released into a large open area where there is less chance of successfully uniting with female gametes. High, because male gametes are released into a confined space where there is more chance of successfully uniting with female gametes. If male and female gametes are in close proximity to each other, fertilisation will usually occur. Environment for zygote Usually external, in a watery environment that is vulnerable to environmental elements such as temperature, predation, infection and rapid dispersal from the area. Usually internal, in a very protected environment inside the female's body. Temperature is controlled and there is less chance of predation, infection and loss of zygote from the area. Zygote requires a watery environment for development. Number of offspring/zygotes Usually a larger number than in internal fertilisation, but many zygotes perish and so a smaller number of offspring survive. A smaller number of offspring than in external fertilisation, because very few perish (higher success rate). Zygote number is determined by the number of sperm and ova that successfully fuse. Breeding frequency More frequent than in internal fertilisation due to the lower fertilisation success rate. Seasonal and less frequent than in external fertilisation due to higher fertilisation success rate and greater energy costs. Breeding frequency depends on the requirements of the species and the favourability of environmental conditions. Parental investment Usually no parental care. Parental care of eggs and/or developing young is more common. Parental investment is indirectly proportional to the number of gametes produced. **Sexual reproduction in animals** [Fertilisation process:] - Gametes are produced in structure called **gonads**. - The gamete cells are formed by **meiosis** - Cells of an organism normally have chromosomes in pairs -- **diploid**. - The number of pairs is referred to as 'n'. - Humans have 23 pairs of chromosomes, 2n =46 chromosomes. - The process of meiosis halves the number of chromosomes in the cells so that each gamete has only one of each pair -- **haploid**. - When fertilisation occurs, the chromosomes from the egg and sperm come together to form a zygote that is diploid. - Having inherited half its genetic information from each parent. - The offspring formed through sexual reproduction are a unique combination of the parents' genetics. Two haploid cells fuse to form a diploid zygote à zygote divides by mitosis à larger numbers of cells from the embryoà embryonic cells differentiate to form specialised tissues à foetus. Female gametes (egg or ova): - Large, immobile cell. - Contain food stores needed for the development of the embryo. Male gametes (sperm): - Contain limited food supplies. - Usually have a tail (flagellum) for motility).  [Haploid vs Diploid ] Humans have in the body cells (somatic cells) 46 chromosomes. These chromosomes come in pairs, so they have 2 x 23 chromosomes. - Chromosomes are numbered à all body cells have 2x No. 1 (one from mum, one from dad), 2 x No2. (One from mum, one from dad), etc. Scientists say that somatic cells are diploid (2n= 2x 23). Life cycles of sexually reproducing organisms follow a pattern called alternation of generations: - They alternate between haploid (n) and diploid (2n) stages. - Most animals (including humans): - Diploid stage is what we see as everyday body structure and function. - Haploid stage is the unseen internal production of sperm in males and eggs in females. - Time spend in each stage varies between different species. Multicellular organisms are composed of two main types of cells: - Somatic cells = all cells in the body of an organism apart from the sex cells - e.g., skin cells, muscle cells, nerve cells. - Germ cells = cells that give rise to gametes. - Specialised sex cells that combine in sexual reproduction. Advantages Disadvantages --------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------- Fertilisation is less risky, and the young are more likely to survive. Slower reproductive rate -- fewer offspring are produced over a longer time period. Unfavourable (deleterious) genetic variation is eliminated from the population more efficiently. Mates have to be found and accepted as suitable. Finding and competing for a mate can be risky and energetically costly. Generates genetic variation through recombination during meiosis and selects for beneficial genetic variation more efficiently. Recombination during meiosis can break apart beneficial genomic combinations and introduce deleterious variation to populations. Populations are better able to adapt to and survive changing environmental conditions. Potential for spread of sexually transmitted diseases throughout population Improves long-term evolutionary potential of populations Energetically costly; gamete production, mating, gestation and rearing young requires a lot of ongoing energy input form the parent. ### plants: asexual and sexual reproduction **Types of asexual production in plants:** [Vegetation propagation] is when new individuals arise from the portions of the root, stem, leaves or buds of adult individuals and are genetically identical to their parents. - One of the most general techniques of asexual reproduction. - Some adult plants produce vegetative organs -- bulbs, tubers, rhizomes, and suckers, from which new plants can arrive. - The offspring are identical to the parent. +-----------------+-----------------+-----------------+-----------------+ | Definition | Example | Advantages | Disadvantages | +=================+=================+=================+=================+ | Tuber -- | Potatoes. | - Produces | - Competition | | swollen | | rapid | from sister | | underground | | increase in | and parent | | stems with | | number of | plants for | | buds. | | plants | resources. | | | | growing in | | | | | a | - Lack of | | | | favourable | genetic | | | | area. | variation | | | | | which could | | | | - Allows a | protect the | | | | genetically | population | | | | superior | in case of | | | | plant to | changing | | | | produce | environment | | | | unlimited | al | | | | copies of | conditions | | | | itself | or disease. | | | | without | | | | | variation. | | +-----------------+-----------------+-----------------+-----------------+ | Runners/stolons | Strawberries | | | | -- above ground | | | | | modified stems. | | | | +-----------------+-----------------+-----------------+-----------------+ | Rhizomes -- | Ginger | | | | underground, | | | | | horizontal | | | | | modified stems. | | | | +-----------------+-----------------+-----------------+-----------------+ | Bulbs/ corms -- | Onion, tulips | | | | swollen | | | | | underground | | | | | stem which | | | | | produces | | | | | lateral buds. | | | | +-----------------+-----------------+-----------------+-----------------+ **Sexual reproduction in plants:** Structures for sexual reproduction in seed producing plants include: - Seed cones (gymnosperms). - Flowers (angiosperms). Mosses and ferns reproduce sexually forming spores and have a life cycle that alternates generations between haploid and diploid stages. **Flower Structure vs Function**  **Pollination and fertilisation** Diagram of a plant with text and images Description automatically generated with medium confidence Steps for flower sexual reproduction: 1. A haploid pollen grain lands on the stigma 2. The pollen grain produces a tube that penetrates the stigma, down through the style and to the ovary. 3. The pollen grain carries 2 male gamete-nuclei to the ovule. 4. One of the gamete-nuclei fuses with the ovule to form a diploid zygote, the other fuses with a diploid cell to form the endosperm. (The endosperm is triploid and surrounds the growing embryo and provides it nutrients (protein, oil, starch). 5. After fertilisation, the zygote develops into a seed. The seed is protected by a tough outer coat. The ovary grows into a fruit and encases the seed. 6. The seed is then freed from the fruit, either by bursting open to release the seed, by falling to the ground once becoming very ripe, or by being eaten by animals. 7. Once the seed lands in a suitable environment, it germinates and grows into a new plant. [After fertilisation:] - ovule develops into a seed protect by an outer seed coat. - Ovule expands, endosperm forms, zygote undergoes some mitosis to produce a multicellular embryo. (all cells are diploid). - The embryo develops seed leaves (cotyledons), root tip, epidermal and vascular tissues begin to form. - As ovule changes into a seed, the ovary containing the ovule becomes mature fruit. - Fruits are specialised structures that protect the seeds and may enhance seed dispersal. **Pollen and seed dispersal** Seed dispersal: - Beneficial for seeds to be dispersed over a wide distance. - Prevents overcrowding, competition for light, water and soil nutrients. - Fruits may be dry (banksia pods, gum nuts) or fleshy (apples). - Dry fruits have often inbuilt 'explosive' mechanisms for dispersal by air, wind, or water - light seeds floating on air or water. - Fleshy fruits rely on insects, birds or mammals for dispersal. - Animals eat fruit, move along and egest the seeds. [Seeds vs Spores] Seeds: - Multicellular - More facilities for plant survival - Located in the fruit or flower of flowering plants. Spores: - Unicellular - Less facilities for plant survival. - Located underneath the leaves of non-flowering plants. Feature of flower Wind-pollinated flowers Bird-pollinated flowers Insect-pollinated flowers ------------------- ------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------ Petals Small and inconspicuous, usually green or dull in colour Usually large and colourful, red or orange, often a tubular shape, sometimes no petals at all. Usually large and colourful (yellow or blue); may be shaped to encourage specific pollinators. Scent Usually absent Rarely fragrant because birds have little sense of smell. Often present because insects are highly attracted to scents. Nectar none Large amounts of nectar produced in nectary base of flower. Sometimes produced at base of petals, so insect must enter the flower to reach the nectar. Anthers Protrude outside the flower, so pollen is easily blown off by the wind; abundant pollen is produced. Anthers are commonly lower than stigma, colourful, and may not be enclosed by petals. Enclosed within flower, commonly lower than stigma Stigma Stigma protrudes from the flower is often long, feathery and sticky to increase surface area for trapping wind-borne pollen. Higher than anthers, sometimes not enclosed by petals and often colourful Enclosed within flower, sticky and commonly higher than the anthers. Pollen Very small grains, light and powdery, large amounts produced. Sticky or powdery pollen; small amount produced. Relatively large grains and often sticky; small amount produced. **Germination and development.** - Embryos in seeds lie dormant until conditions are appropriate. - Water, oxygen, temperature, and day length are important environmental factors that influence seed germination. - Dormant seeds can wait months, years and even decades to continue propagation of their species. ### fungi: budding, spores **Budding** -- a small bud that separates from the parent to form a new smaller individual. (Unequal division of cytoplasm). - A small bud arises as an outgrowth of the parent body. - The nucleus of the parent yeast is separated into two parents and one of the nuclei shifts into the bud. - The newly created bud divides and grows into a new cell. [Advantages:] - There is no need for another organism to reproduce. - Usually, rapid. - Sponges form internal multiples buds as packets called gemmules during drought. - Makes more individual cells so if parent cell dies, daughter cell increases chance of the species continuing. - Parent cell maintains size reduces risk of predation by larger unicellular organism. [Disadvantages:] - Subject to genetic mutation. - Reduces the amount of diversity. **Spore formation (sporogene****sis) in fungi:** **Spores --** tiny unicellular reproductive cells which are identical to parent. E.g., moulds, and mushrooms. [Advantages:] - There is no need for another organism to reproduce. - Lightweight à can travel relatively large distances. - Large number of spores formed = rapid expansion. - Have thick walls to protect from drying out. [Disadvantages] - Survival rate is low. - Reduces the amount of genetic diversity. ### bacteria: binary fission **Binary fission** is the most common form of reproduction in prokaryotes such as bacteria. It also occurs in some single-celled eukaryotes like the Amoeba and the Paramaecium (protists). (also, anemone -- animal). - The original cell splits into [equal sized cells]. - You can't say which cell is the parent and which is the daughter -- both called daughters. [Advantages:] - There is no need for another organism to reproduce. - Bacteria = rapid and exponential process. - Both cells reduce in size so increased SA:V ratio increases the efficiency of diffusion. [Disadvantages:] - Subject to genetic mutation. - Reduces the amount of diversity. ### protists: binary fission, budding analyse the features of fertilisation, implantation and hormonal control of pregnancy and birth in mammals ---------------------------------------------------------------------------------------------------------- **Reproduction in mammals** [Placental mammals:] - Uterus provides nourishment and protection via placenta and umbilical cord for the developing embryo and foetus until birth. - They are viviparous (give birth to developed, live young). - After birth: babies are nourished with milk and develop a covering of fur. - Humans, horses, dogs, seals, whales, elephants. [Marsupial mammals:] - Underdeveloped joey, protected and nourished in the external pouch after an early birth, allowing another fertilisation to occur internally. - They are viviparous but give birth to underdeveloped live young. - Kangaroos, brushtail possum, wombat, koala. [Monotreme:] - Females lay eggs and each puggle (baby monotreme) develops inside a leathery eggshell, then hatches, and is protected and fed milk by the mother. - They are oviparous (lay eggs in which their young develop). - Platypus, echidna [Common in all mammals:] - Before fertilisation, haploid gametes must be produced by specialised reproductive organs in each parent. - Internal fertilisation must occur. **Male reproductive system** - Paired **testes**, held inside the **scrotum**, which produce and store mature sperm continuously during mating periods, main structures: - **Seminiferous tubules** (sperm cells are formed) - Epididymis (stores sperm cells) - Accessory glands that produce secretions which make up about 95% of the volume of semen. - **Prostate, seminal vesicles, Cowper's gland**s. - Paired system of ducts (**vas deferens**), leading from the testes to the urethra. - **Luteinising hormone (LH)** from the **pituitary gland** (in the brain) to stimulate the secretion of **testosterone** in the testes. - **Penis**, male organ that grows to full size during puberty -- sexual and excretory functions. - **Urethra** tube passes through the penis, delivering urine or semen out of the body (not at the same time). - Becomes erect when ready for copulation à erection is a result from increased blood flow into the spongy tissue. [In testes]: precursor germ cells produce diploid spermatocytes via mitosis à these undergo meiosis to produce four haploid sperm cells. During mating: - Contractions of the vas deferens move sperm towards the urethra. - Secretions of the accessory glands are added, forming the seminal fluid. - Functions: causes the sperm to become motile and provides an alkaline nutritious medium that is rich in proteins, ions, vitamins and fructose sugar. - Flagellum (tail) propels the sperm through the female reproductive tract towards the egg after copulation. - Sperms swim through liquid internal environment in a race to be the first to reach and fertilise the egg. - Head of sperm: contains nucleus with a haploid set of chromosomes, and a cap called the acrosome that contains enzymes used for penetrating the outer layer of the female egg. - Mitochondria in the midpiece produce ATP for energy during the journey through the female reproductive tract. **Hormonal control of the male reproductive cycle** Androgens are commonly referred to as male hormones. - This group of hormones control the development and functioning of male sex organs and secondary sex characteristics. - Testosterone is an androgen produced and released by the Leydig cells in the testes which plays a role in the production of sperm. Gonadotropin-releasing hormone (released by the hypothalamus) regulates the release of Luteinising hormone (LH) and Follicle stimulating hormone (FSH) from the pituitary gland. - LH stimulates the production of testosterone (by the Leydig cells) and FSH stimulates the production inhibin (by the Sertoli cells), which maintains the level of testosterone by inhibiting the release of LH and FSH from the pituitary gland. Sperm are produced by meiosis inside the sperm tubules in the testes and are stored until they are mature. - Each sperm is microscopic -- an enlarged head that contains the haploid nucleus and a long tail that whips from side to side to enable the sperm to move. - During copulation, semen containing sperm is introduced into the vagina of the female and must undertake a race to reach the egg cell. - Although half a million sperm may begin the journey, only a few hundred to a few thousands will reach the ovum. **The female reproductive system** - **A single uterus** where, if an egg is fertilised it implants in the uterine wall, a placenta forms, and the foetus develops until time of birth. - the uterus undergoes changes that are controlled by hormones. - Paired **ovaries** which hold the oocytes (immature egg cells) until puberty when monthly ovulation starts under hormonal control. - Paired **fallopian tubes** (oviducts) connecting each ovary to the uterus. - The open end of each tube has fringe-like structures called **fimbriae** that surround the ovary to catch the eggs when released. - Fertilisation occurs in the fallopian tube. - A **cervix**, a narrow muscular canal 2-3cm length lined with mucous, that connects the uterus and the vagina. - During childbirth the cervix stretches to at least 10 cm width. - During menstruation it is controlled by oestrogen to become softer and more open. - A **vagina**, a muscle-lined canal from the cervix to the genitals, which receives the male penis during sexual intercourse. - Monthly menstrual blood flow from the uterus exits the body through the vagina. - The vagina is also the birth canal for the baby to enter the outside world. - Unlike in males, the urethra (excretion of urine) is separate to the female reproductive tract. Some placental mammals have two uteri (cats, horses, deer, dogs and whale). - Rodents, rabbits have a pair of uteri and cervices joined to a single vagina à allowing reproduction of large litters. Marsupials also have paired reproductive tracts. - Early birth 4-5 weeks of age à marsupials don't have a complex placenta. In monotremes, the uterus only functions to form a leathery eggshell around the embryo. **Hormonal control of the female reproductive cycle** Oestrogens are the main group of female hormones which control the development and functioning of female reproductive organs and secondary sex characteristics. **The menstrual cycle** The cycle of changes in the ovaries and uterus of sexually mature females. **Ovulation** - A female is born with all the immature egg cells (oocytes) already in her ovaries. - After reaching puberty, ovarian cycles commence - Later in life females cease to ovulate àmenopause. - Before a female is born, meiosis has begun in all oocytes (immature egg cells) but is arrested at an early stage. - Once she reaches puberty, pituitary hormones control the continuation and completion of meiosis. - The hypothalamus releases GnRH which stimulates the anterior pituitary gland to release the follicle stimulating hormone (FSH). - Effect on ovary: one or more of the oocytes with resume meiosis and mature within a group of nutritive cells called a follicle. - Only one egg forms from each oocyte during meiosis. - When the oocyte is maturing, it grows much larger by adding nutrients. - Follicles containing a maturing egg release the hormone oestrogen, which causes changes to the lining of the uterus and also acts on the anterior pituitary gland (stimulating it to make LH). - The uterine lining becomes thicker, softer and spongy, and richly supplied with blood vessels in readiness to receive a fertilised egg. - Ovulation is the release of a mature egg and is triggered by a surge of luteinising hormone (LH) released from the anterior pituitary gland in the brain. - The ovum (ripe egg) bursts out of the follicle and is drawn by fluid currents into the fallopian tube. - Eggs can't move; fimbriae move to create a current that sweeps the egg into the fallopian tube. - The egg is moved along the fallopian tube by contractions of the fallopian tube and synchronised movement of cilia on its internal walls. - In the ovary, the burst follicle (now without egg) is called the corpus luteum. - The corpus luteum, stimulated by LH, secretes large amounts of both oestrogen and progesterone. - These hormones cause a further thickening of the lining of the uterus during the latter part of the cycle. - To prepare the uterus to receive and embryo, should fertilisation occur. - Increase in progesterone and oestrogen in blood inhibits FSH and LH secretion of pituitary gland ànegative feedback. - If not fertilised àthe egg passes out of the reproductive tractà corpus luteum slowly disintegrates and stops releasing its hormones à the thickened uterine lining breaks down and menstruation occurs. **[Fertilisation]** - Fusion of two haploid gametes to form a single diploid zygote cell. - The zygote cell contains the genetic material of both the egg and the sperm. - There are equal genetic contributions from the male and the female parents to the zygote. - Occurs in mammals internally, takes place in upper part of oviduct (fallopian tube). - Male inserts his penis into the female's vagina. - Muscular contractions (ejaculation) push semen from his urethra into her vagina. - From vagina sperms swim, using movement of their flagella, through the cervix à into uterus à into fallopian tube. - Timing is important: ovulation must have occurred. - Sperm can survive 3-5 days within the female reproductive tract. Fertilisation occurs in four steps:  **Implantation** - After fertilisation, the zygote starts to divide repeatedly by mitosis to form a ball of undifferentiated cells. - Called a **morula.** - While this is happening, the morula continues to move down the oviduct. - The cells in the morula start to differentiate into an inner cell mass (which will form the embryo and an outer cell mass which will form the placenta. - Called a **blastocyst.** - When the blastocyst reaches the uterus, it is ready for **implantation.** - **Implantation** occurs around day 8 or 9 after fertilisation. - the blastocyst attaches to the uterus wall. - The outer layer of cells sends finger like projections into the uterus wall, and this develops into the placenta. - This is so that the developing embryo can gain nutrition (via diffusion) from the mother's blood supply in the uterus wall. - After implantation, the blastocyst becomes a gastrula over the next five days. - The gastrula has three different layers of cells. - The gastrula becomes an embryo then a foetus.  [Placenta and umbilical cord:] Outer layers of cells in blastocyst initiates the formation of a placenta. - Later an umbilical cord develops by the fifth week of the embryo stage from the remnants of the egg yolks sac. - It replaced the yolk sac as the source of nutrients for the embryo, enabling embryonic blood vessels to reach the placenta. - The umbilical cord stays with the foetus until after birth. The placenta is an exchange organ brining blood vessels of the foetus into close contact with maternal blood. - **Blood of the mother and the foetus do not mix**. - Nutrients and oxygen from the mother diffuse across into the blood of the umbilical vein and move to the foetus. - For removal of waste products and circulation od depleted blood through the umbilical arteries waste diffuses from foetal artery back to the placenta. - After birth the umbilical cord is cut (belly button). - Placenta is also an important source for hormones during pregnancy. **SUMMARY** **[Pregnancy and Birth]** Pregnancy is normally timed from the start of the last period before fertilisation, so pregnancy timings are normally about 2 weeks longer than the time the baby has been developing. - Embryo stage is from implantation (about 10 days after fertilisation to 9 weeks after fertilisation) à 11 weeks of pregnancy. - Foetus stage is from 12 weeks until birth. The embryo and the foetus stages both gain their food and oxygen (and get rid of waste) via the placenta. - Oxygen, glucose, CO2 etc., diffuse between the mothers and embryo/foetal blood supply. - The blood from the embryo/foetus goes to and from the placenta via the umbilical cord. **Development of the embryo** - During embryonic development, major organs of the body are formed from the three primary layers of the gastrula. - This is completed 8 weeks after fertilisation. - After the embryonic stage, the organism has distinct features and is known as a foetus for the remainder of the development. [Embryonic germ layers and cell specialisation] Blastocyst undergoes gastrulation à folding in on itself to form a gastrula with three primary layers of cells: ectoderm, mesoderm, endoderm (germ layers) and two membranes. - Ectoderm (outermost layer of embryo): forms epidermis, hair, peripheral nervous system, brain and spinal cord. - Mesoderm (middle layer of embryo): forms muscle, cartilage, kidney and gonad cells - Endoderm (innermost layer of embryo): forms lungs, bladder, lining od digestive system and respiratory system. **Development of the foetus** - From 9 weeks after fertilisation to 38 weeks of fertilisation the developing organism is known as foetus. - Foetus grows in size and organs continue to develop. - Cells and tissues become specialised to carry out their specialised functions. - Foetus is protected in the amniotic cavity (fluid-filled environment). **Summary of Hormones in pregnancy in mammals.** +-----------------------+-----------------------+-----------------------+ | Hormone | Produced By | Function in Pregnancy | +=======================+=======================+=======================+ | Human chorionic | Developing embryo | - Maintains Corpus | | gonadotropin (hCG). | during first | luteum, which in | | | trimester and then | turn continues to | | | later by the | secrete | | | placenta. | progesterone and | | | | oestrogen. | | | | | | | | - Levels of hCG | | | | decline after 12 | | | | weeks and the | | | | corpus luteum | | | | deteriorates. | | | | | | | | - Its present in | | | | urine of blood | | | | used for | | | | pregnancy tests. | +-----------------------+-----------------------+-----------------------+ | Oestrogen | Corpus Luteum in | - Causes decrease | | | first trimester and | in hormones GNHR, | | | then placenta (by | FSH, LH to | | | foetus adrenal | prevent ovulation | | | glands) | and menstruation. | | | | | | | | - Oestrogen levels | | | | are rising during | | | | pregnancy to help | | | | maintain | | | | pregnancy. | | | | | | | | - Increased levels | | | | of oestrogen in | | | | 3^rd^ trimester | | | | **induces | | | | receptors to form | | | | on the uterus | | | | wall that can | | | | respond to | | | | oxytocin.** | | | | | | | | - High levels | | | | induce labour. | +-----------------------+-----------------------+-----------------------+ | Progesterone | Corpus luteum in | - The hormone of | | | first trimester and | pregnancy because | | | then placenta | it maintains the | | | secretes this after | endometrium. | | | the corpus luteum | | | | deteriorates. | - Causes decrease | | | | in hormones GnRH, | | | | FSH, LH to | | | | prevent ovulation | | | | and menstruation. | | | | | | | | - Stimulates | | | | changes in the | | | | mother's body | | | | (enlargement of | | | | uterus, mucus | | | | plug in cervix, | | | | placental growth | | | | and breast | | | | growth. | | | | | | | | - **Levels continue | | | | to rise during | | | | second trimester | | | | to help maintain | | | | pregnancy and | | | | inhibit uterus | | | | contractions.** | +-----------------------+-----------------------+-----------------------+ | Relaxin | Corpus luteum and | - Relaxes maternal | | | placenta (during | muscle joints and | | | pregnancy), ovaries | ligaments to | | | (during birth). | allow for the | | | | expanding foetus | | | | and relaxes | | | | uterine muscles | | | | and prepares | | | | lining for | | | | implantation. | | | | | | | | - Helps dilate or | | | | open the cervix, | | | | widens pubic bone | | | | and relaxes | | | | pelvic ligaments | | | | to allow passage | | | | of baby. | +-----------------------+-----------------------+-----------------------+ | Prolactin | Pituitary gland | - Levels start | | | | rising in second | | | | trimester of | | | | pregnancy. | | | | | | | | - Stimulations | | | | production of | | | | milk in mammary | | | | glands of | | | | breasts. | +-----------------------+-----------------------+-----------------------+  **Summary of Hormones in birth. (Parturition).** +-----------------------+-----------------------+-----------------------+ | Name of hormone | Produced by | Function in birth | +=======================+=======================+=======================+ | Oestrogen | Placenta | - Contracts | | | | progesterone on | | | | suppressing | | | | contractions. | | | | | | | | - Increases | | | | sensitivity of | | | | uterus for | | | | oxytocin. | +-----------------------+-----------------------+-----------------------+ | Oxytocin | Pituitary gland (of | - Major role in | | | both mother and baby) | birth. | | | | | | | | - Triggers and | | | | maintains labour | | | | causing muscular | | | | contractions of | | | | uterus to | | | | increase as | | | | pressure of | | | | baby's head onto | | | | cervix increases. | | | | | | | | - Causes placenta | | | | to release | | | | prostaglandins. | | | | | | | | - Releases milk | | | | from breast when | | | | baby suckles. | +-----------------------+-----------------------+-----------------------+ | Prostaglandin | Uterus wall | - Helps to initiate | | | | labour and | | | | stimulate uterus | | | | contractions by | | | | reducing | | | | progesterone | +-----------------------+-----------------------+-----------------------+ | Relaxin | Ovaries | - Helps dilate | | | | cervix to allow | | | | passage of | | | | foetus; widens | | | | pubic bones and | | | | relaxes pelvic | | | | ligaments. | +-----------------------+-----------------------+-----------------------+ | Cortisol | Foetus and uterus | - Believed to cause | | | | increase in | | | | oestrogen to | | | | become dominant | | | | hormone, the | | | | stress hormone is | | | | a result of | | | | increased | | | | pressure of | | | | growing foetus | | | | just rise before | | | | birth. | | | | | | | | - Decreases | | | | progesterone and | | | | increases | | | | prostaglandin | | | | production from | | | | placenta. | +-----------------------+-----------------------+-----------------------+ **Feedback is where a stimulus causes and effect which leads to a change on the stimulus.** **[Labour and birth]** A correct balance of hormones is essential to maintain the pregnancy. - hCG is released from the placenta when the embryo has implanted. - It stimulates blood flow to the pelvic area and helps regulate the ovarian hormones. - Progesterone is required at high levels throughout pregnancy with levels steadily rising until the birth of the baby. - Initially the progesterone comes from the corpus luteum, but after six weeks the placenta produces it. - It stimulates early preparation of the uterus for pregnancy and later it prevents lactation and uterine contraction until it is time for birth. - Oestrogen levels rise throughout the pregnancy to work in partnership with progesterone. - It promotes growth of breast tissue in preparation for maternal milk production. Just before human birth: - Balance of oestrogen and progesterone changes. The level of prostaglandins increases à this increases the sensitivity of the cervix and uterus to oxytocin. - Oxytocin causes uterine contractions à labour begins. - Cervix reaches full dilation (10 cm) à oxytocin and adrenaline start final series of muscular contractions. - After baby is delivered: oxytocin causes continuation of uterine contractions until placenta is delivered à uterus shrinks back to normal size (over the next few weeks and months). - Oxytocin works with prolactin to stimulate lactation for feeding the newborn baby. **Flow Chart Pregnancy hormones** evaluate the impact of scientific knowledge on the manipulation of plant and animal reproduction in agriculture --------------------------------------------------------------------------------------------------------------- cell replication ================ How important is it for genetic material to be replicated exactly? The main people responsible for the discovery are Rosalind Franklin, Maurice Wilkins, James Watson, and Francis Crick. Cells contain units of heredity known as **genes** on chromosomes. - Each cell contains two copies of every autosomal gene, on inherited from each parent. - Different variations of the same gene are called **alleles** of that gene. - These versions of the same gene are found in identical positions on a pair of similar chromosomes within cells. - Diploid individuals have two alleles for each gene and haploid cells have only one allele of each gene. **Gene:** - A segment of DNA on a chromosome - Specifies a particular characteristic (e.g., seed colour). - Has two alleles in an individuals and two or more alternative alleles in a population. - **For example** in pea plants, one gene may determine seed colour, while another determines stem length. In humans, genes determine characteristics such as height, eye colour, hair colour and freckles. **Alleles:** - Are alternative forms of the same gene - Occur in pairs in a diploid individual, but two or more alleles for each gene may be present in a population - Segregate during gamete formation (meiosis). - Occur individually in each haploid gamete - Pair during fertilisation, where the diploid condition of an organism is restored during zygote formation. - **For example,** the gene height has two alleles -- tall (T) and short (t). model the processes involved in cell replication ------------------------------------------------ ### mitosis and meiosis The process of mitosis, or cell division, is also known as the **M phase.** - This is where the cell divides its previously-copied DNA and cytoplasm to make two new, identical daughter cells. **Stages of mitosis** Mitosis consists of four basic phases: **Prophase, Metaphase, Anaphase, and telophase.** [Early Prophase:] The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears. [Late Prophase]: the nuclear envelope breaks down. - The chromatin material shortens and thickens by coiling and the DNA separates out into chromosome. - Each chromosome contains two copies of the DNA. - Each copy is called a spindle chromatid, and these are joined by a single centromere. [Metaphase]: [ ] the chromosomes line up across the centre or 'equator' of the cell, each attached to the spindle fibre by a centromere. - Each chromosome consists of two identical sister chromatids. [Anaphase]: begins when proteins in the centromere are cleaved, which allows the sister chromatids to separate. - Each chromatid becomes a chromosome. - The spindle fibres contract and the chromosomes are pulled by their centromeres to opposite ends of the cells. - The spindle fibres contract and, as the sister chromatids begin to separate, they are now known as daughter chromosomes. - They are pulled towards opposite poles of the cell and their movement is assisted by the centromere. [Telophase]: The daughter chromosomes gather at opposite poles of the cell. - The spindle breaks down. - The nuclear membrane and nucleolus reappear. - Nuclear division or mitosis is now complete. - The result is two nuclei with chromosomes identical to each other and to the original nucleus in the parent cell. - The nuclear membrane forms around the two nuclei, now called daughter nuclei. [Cytokinesis:] Division of the cytoplasm occurs, separating the two daughter nuclei so that each is in its own cell. Cytokinesis differs in plant and animal cells. - Animal cells: the cytoplasm constricts in the centre of the cell between the two daughter nuclei and 'pinches off'. Cytokinesis is important because it separates the newly formed daughter nuclei and ensures that each cell only has one nucleus. The outcome at the end of mitosis and cytokinesis is two daughter cells with chromosomes that are identical to each other and to the original parent cell. - The daughter cells then enlarge until they are the same size as the original adult cell. - The nucleus of each cell controls all cell activities. - The ratio between the proportion of nucleus and cytoplasm remains constant. - If the cytoplasm exceeds a certain proportion of the cell, the ability of the nucleus to control its functioning decreases and this may help trigger cell division. **[Meiosis]** Purpose of meiosis: - Create haploid gametes 9with half the number of chromosomes). - Create genetic variation amongst those gametes. Some cells stop dividing (e.g., most brain cells) or pause for a while before dividing again, they are said to be in **G0.** (zero growth). **Homologous chromosomes** Looking at the chromosomes of an individual (their karyotype), we find that the chromosomes come in pairs, called homologous chromosomes. - Each pair consists of two chromosomes which are the same length, (with same position for the centromere) and with the same genes at the same positions (loci) along their lengths. However, one chromosome from each pair comes from one parent, the other comes from the other parent. - while homologous chromosomes have the same genes. They may have different versions of the genes (alleles). - E.g., one chromosome may have a hair colour gene code for "make the protein for red hair" and the other chromosome in the pair codes for "the protein for blonde hair". - More complex (just simplified). **Steps in meiosis** [Meiosis consists of two consecutive divisions:] Meiosis I: Diploid cell becomes two haploid cells with the chromosome number halved. - (homologous chromosomes separate). Meiosis II: The two haploid cells divide again. - Sister chromatids separate. [Stages of Meiosis:] Interphase: - The chromosomes have not yet condensed. - The chromosomes have replicated, and the chromatin begins to condense. Prophase I: - The chromosomes are completely condensed. - The homologous chromosomes pair with one another. Metaphase I: - The nuclear membrane dissolves and the homologous chromosomes attach to the spindle fibres. - They are preparing to go to opposite poles. Anaphase I: - Chromosomes move to opposite ends of the cell. Telophase I and Cytokinesis: - The cell begins to divide into two daughter cells. - Each daughter cell can get any combination of maternal and paternal chromosomes. Prophase II: - The cell has divided into two daughter cells. Metaphase II: - The chromosomes line up on the spindle fibres. Anaphase II: - The two cells each begin to divide. - The chromosomes move to opposite ends of each cell. Telophase II and Cytokinesis: - The formation of four cells à meiosis is over. - Each of these prospective germ cells carries half the number of chromosomes of somatic cells. **Sources of genetic variation during meiosis:** [Crossing over:] - occurs during prophase I. - occurs at random and involves the exchange of part of one chromatid with another. - Produces chromosomes with new combinations of genes. - Produces new variation of gametes. - arms of homologous chromosomes meet and exchange genetic material. - Mixing of maternal and paternal genes causes genetic variation. [Independent assortment:] - Occurs during Metaphase I: - Homologous chromosomes line up at the equator in one of two arrangements: - Maternal copy left/paternal copy right or - Paternal copy left/maternal copy right. - The orientation of each homologous pair is random and is not affected by the orientation of any other homologous pair. - This means an allele of one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome. - Their inheritance is independent of one another. - Alleles of two (or more) different genes are sorted into gametes independently of one another. - i.e., the allele a gamete receives does not influence the allele received for another gene. - Independent assortment will **not** occur if two genes are located on the same chromosome. ### dna replication using the watson and crick dna model, including nucleotide composition, pairing and bodning. DNA or **Deoxyribonucleic acid** is the genetic material in humans and all other organisms. - In eukaryotic cells, DNA is found in the cell nucleus. DNA is composed of a series of subunits called nucleotides, where each nucleotide consists of a sugar (deoxyribose), a phosphate and a nitrogenous base. - DNA has two strands that spiral to form a double helix. Each strand has: - A **sugar-phosphate** backbone -- linked chain of alternating sugars and phosphate molecules. - A **nitrogenous base** -- these bases pair together to form the rungs. The 'rungs' of the ladder (the bases) are [attached to the sugar molecules.] Each rung is made up of two chemicals called **nitrogenous bases**. These bases pair with each other. - Adenine (A) with thymine (T) - Guanine (G) with cytosine (C). The order of the bases creates a unique code for the information stored within DNA and this order then holds the instructions for the production of a specific protein. - The ratio of adenine to guanine and cytosine to thymine could be explained by their **complementary base pairing.** **DNA VS chromosomes vs gene** A chromosome is made of a material called Chromatin. - Chromatin is made of DNA and special structural proteins called **histones.**  Within each long DNA molecule of a chromosome there will be many **genes**. - Each gene is a small segment of DNA with specific base pairs. - A gene will code for a specific polypeptide and create a protein. - it is the combined effect of the proteins coded by your genes that causes your characteristics. Every cell in an organism contains a complete set of the organism's DNA. When cell divide, they must first replicate their DNA. - This ensures the full genetic code can be put into the new cell. Most of the time, DNA is stretched out into long, thin threads. - When a cell gets ready to divide, chromosomes must replicate and then shorten and thicken becoming visible under the light microscope. - The number of chromosomes varies among species. - Humans have 46 chromosomes or 23 pairs. **DNA replication --** the production of two identical double stranded molecules of DNA from one original double helix molecule. - The process of DNA replication in cells is termed **semi-conservative**, as the two strands of the original DNA molecule separate and each gives rise to a new complementary strand. - Ensures that the **genetic material is copied exactly.** - DNA replicates before cell division so that each cell can receive one full and exact copy of the coded instructions that control the basic life functions of the cell. [DNA replication] The process of replication begins at **origin of replication** sites along a chromosome. - These sites are sections that are coded with a specific sequence of nucleotides. - Proteins that start DNA replication detect and attach to these sections causing the two strands to separate to form a replication 'bubble'. - There is a replication fork at the end of each bubble where the unwinding process begins and the bubble opens up further in both directions. - **Helicase** enzymes untwist the double helix at the replication forks and break the bond between the two strands of DNA\> - The separated sections in the bubble act as templates for new complementary strands. The process of DNA replication occurs in three steps: 1. **The DNA double helix unwinds.** Each DNA molecule is a double stranded helix. - An enzyme called **helicase** causes the DNA helix to progressively unwind. 2. **DNA unzips** -- **that is, the two strands separate.** The weak hydrogen bonds break between the complementary bases of the nucleotides on opposite strands and the two DNA strands separate, exposing the nucleotide bases. - Each 'rung' splits down the middle, creating a **replication fork** -- beyond this point, the DNA resembles on half of a 'flat ladder'. - Each strand is made up of a sugar-phosphate backbone and a single row of nucleotide bases. 3. **Nucleotides are added to each single strand:** Each separate strand of the existing DNA molecule acts as a template for the production of a new strand of DNA. - Nucleotides are picked up by the enzyme **DNA polymerase III** and slotted in opposite their complementary base partner on each of the existing strands. - these nucleotides are picked up from a pool of nucleotides in the nuclear sap. - The direction in which nucleotide insertion occurs is antiparallel on the two opposite strands. - On one strand it begins at the replication fork and goes towards the end of the strand. - On the other strand it begins at the end of the single strand and goes towards the replication fork. (Nucleotides can only be added to the 3' end of the growing DNA strand. This means the new strand always grows in a 5'à3' direction).  [Result of DNA replication] Each resulting DNA molecule contains one strand of the existing DNA molecule and a newly synthesised strand. - The replicated DNA molecules rewind into the double helix conformation, like the original molecule. - The end result is that there are two molecules of DNA, each a double-stranded helix, and they are identical to each other and to the original molecule from which they formed. **The significance of DNA replication** DNA has two main functions: 1. **Heredity** -- this relies on **DNA replication.** 2. **Gene expression** -- this relies on **protein synthesiss.** [Heredity and the need for replication:] The genetic material of a cell must be transmitted from: - One cell to another during **mitosis**, allowing for growth and repair and maintenance of an organism. - One generation to another during **meiosis** (e.g., when gametes are formed for sexual reproduction). In each of these situations, it is necessary for the DNA to make an exact copy od itself- that is, DNA replication occurs. - Replication of DNA ensures that the genetic code of a cell is passed on to each new daughter cell that arises from it. - An exact replica or copy of the DNA must be produced so that the new cells have the same, distinctive message that the original cell had. - If DNA replication goes wrong, this has a direct effect on the phenotype of the individual. (mutation). **The complexity of DNA replication** - DNA has to be unwound from its spiral configuration before the strands can be separated. - A large number of physical and chemical reactions take place simultaneously during DNA replication, and so a collection of en enzymes is required to control each reaction. - The two strands of DNA run in opposite direction (antiparallel) and nucleotides can be added in one direction. - DNA replication errors sometimes occur and these need to be correct. **Cell Cycle** Cells in multicellular organisms have a cycle of growth and division. - A grown cell lives for one turn of the cycle and becomes two cells. The cycle repats again and again as a constant source of renewal. - This cell cycle consists mainly of interphase. - Only a small percentage of the cells are in mitosis phase at any given moment; this can be confirmed, in certain cells, by watching a single cell through its entire life cycle. Interphase (the cellular growth phase) is divided into three sub-phases. - **G1 phase** -- cell growth (cellular contents excluding chromosomes are duplicated). - **S Phase** -- DNA replication (each of the 46 chromosomes are duplicated). - **G2 phase --** protein synthesis. Throughout all of interphase, the cell is growing by producing proteins and additional copies of organelles, such as mitochondria. - In the middle stages of interphase, the cell synthesises DNA. - This occurs through the process of DNA replication. - The S phase results in the cell having **x shaped chromosomes**, made of two identical DNA molecules. - When cell divisions (mitosis) occurs the x-shaped chromosomes are separated, to place one copy of the DNA in each daughter cell. - This ensures that the parent cell and the daughter cell have identical chromosomes. Phase of cell cycle Doe cellular growth occur? Does DNA replication occur? Does cell division occur? --------------------- ---------------------------- ----------------------------- --------------------------- Interphase: G~1~ **Yes** Interphase: S **Yes** **Yes** Interphase: G~2~ **Yes** M Phase **Yes** Cytokinesis [Eukaryotic cells usually have multiple chromosomes. ] During many stages of a cell's life cycle, each chromosome contains only one such DNA molecule. - At other times, the DNA molecule doubles; the chromosome then comprises two joined **chromatids**, each made up on one DNA molecule complexed with proteins. - At the particular times when chromosomes are visible in microscopes, the chromatids are joined in a specific small region of the chromosome (**centromere).** - Centromeres direct the movement of chromosomes when a nucleus divides. - A body that has a single centromere, whether it contains one or two DNA molecules, is called a chromosome.  assess the effect of the cell replication processes on the continuity of species -------------------------------------------------------------------------------- **The importance of accuracy during DNA replication.** Since the sequence of bases in DNA makes up the genetic code of an individual, exact copying of this sequence during replication is critical, for two main reasons: - Heredity (inheritance of genes) -- the genetic material transmitted from cell to cell (by mitosis) and from generation to generation (by gametes from meiosis) needs to be accurate. - **Gene expression** (protein synthesis) -- the genetic instructions given to a cell to create its structure and ensure its correct functioning must be accurate. Fidelity of replication: the ability of the genes to accurately replicate. DNA is at constant risk of mutation. - Cells have enzymes that are able to repair incorrect base insertion and other DNA damage that may arise during replication. [Error in mutation] - *Spontaneous mutations* -- natural errors that arise at random in DNA during replication. - *Mutagenic mutations* - errors that arise as result of exposure to cells to environmental factors such as radiation or chemicals. - Environmental factors that change DNA are called **Mutagens.** As the length of time that the cells are exposed to mutagens increases, and the intensity of exposure rises, so the risk of mutation also increases. - There are enzymes in cells to repair both types of mutation, but sometimes DNA errors go undetected, and this results in a permanent mutation. - This incorrected mutation will be replicated in successive divisions and, if occurring in meiosis, passed on to later generations of cells. [DNA repair] The insertion of an incorrect base is common during DNA replication. - When this occurs, a repair enzyme recognises the mismatched base pair, excises the incorrect base and replaces it with the correct base. - This is called **DNA mismatch repair** and is a function of the enzyme DNA polymerase I.  [Gene expression and the need for accurate replication ] How the genetic code works: - Enzymes control the synthesis of cell materials and the biochemistry (metabolism) of everything within each cell and within the whole organism. - The sequence of bases store exact copies of instructions in every cell as well as playing a vital role in the translation of the genetic code into protein. In multicellular organisms, different genes are activated and expressed in each type of cell. - The entire genetic code passed on to each cell must contain a full and accurate set of instructions à when the relevant gene is activated, it functions correctly to make proteins that determine the type of cell it will become. - The activation of genes is regulated by other molecules, such as enzymes (which are proteins), and therefore also needs to be accurately coded for by DNA. - Errors in genes that control the cell cycle may lead to changes in cell division and cell death, leading to cancer. - Mismatch repair of these genes is important. - Replication errors in genes that code for DNA repair enzymes are also linked to cancer. **All living things arise from other living things.** The **continuity of a species** refers to the ongoing survival of species as a result of characteristics being passed down from parents to offspring in a continuous lineage. - This inheritance of characteristics from ancestors to currently living organisms relies on the passing down of consistently **accurate genetic information** and the occasional introduction of variation of some genetic information. - So that species can adapt and survive in a changing environment. Accurate DNA replication à **genetic stability,** whereas **mutation** results in **genetic variation.** - While variation is important for evolution, genetic stability is important for the survival of the individual. - Both genetic stability and variation play an important role in ensuring the *continuity of the species*. [Genetic continuity:] a way of preserving genetic information across generations, dependent on two things: - When a cell divides by mitosis, the resulting two daughter cells must have the same number and type of genes as the original cell. - When two sexually reproducing organisms breed, the resulting offspring must have the same number of genes as the parent organisms and variation in these genes must not be extremely detrimental or lethal. Genetic continuity ensures continuation of a species, because it ensures that new cells or organisms have all the genes they need, in working order, to survive. - A lack of genetic continuity results in disease and sometimes in death and extinction. [Ensuring the continuity of species -- genetic stability] At a *genetic level*, stability arises when chromosomes are replicated accurately to give rise to identical daughter chromosomes. For continuity at the *species level*, successful desirable traits must be passed on, along with some random errors. This allows a species to evolve if an environmental change occurs. - Natural selection acts so that individuals in a population that are best suited to the environment survive and reproduce, passing on their genes to their offspring. - This mixing of parent genes during sexual reproduction, including some that have arisen due to mutation, increase genetic diversity and helps maintain the continuity of species. The mechanisms that have evolved to ensure genetic continuity (passing on of genetic traits) and the surivial and continuity of species include: - Consistent replication prior to cell division. - An orderly distribution of chromosomes when cells dive and when gametes form. - Fertilisation methods that ensure individuals of the same species breed successfully - Methods to ensure embryo survival, such as production of large numbers or protection and nourishment of developing embryos and parental care. - Natural selection so that the fittest survive to reproductive age and pass on their genes. Mechanisms that result in genetic variation in a species include: - Mutation -- changes in DNA due to mutation may be spontaneous or mutagen-induced. - Mixing of parental genes during sexual reproduction (brought about by crossing over and independent assortment during meiosis, and random fertilisation of gametes. [Genetic errors that threaten the continuity of species] The number of DNA repair enzymes present in cells is an indication of how important accurate replication and DNA repair are for survival. - A decrease in the ability of cells to repair DNA during replication is also thought to be responsible for accelerated ageing and may give rise to neurodegeneration. When a mutation is present in a DNA repair gene, the gene may be expressed in an altered form or not expressed at all. Genetic information can only be stored in a stable form and passed on consistently if DNA repair enzymes continuously scan the DNA for errors in replication and replace incorrect or damaged nucleotides. - Natural selection is a mechanism that ensures individuals carrying damaged genes are removed from populations so that the continuity of species is not at risk. dna and polypeptide synthesis ============================= why is polypeptide synthesis important? construct appropriate representations to model and compare the forms in which DNA exists in eukaryotes and prokaryotes ---------------------------------------------------------------------------------------------------------------------- **DNA in Eukaryotes and Prokaryotes** The genetic code is universal -- the same nucleotide base-pairing code is used in all living organisms, both prokaryotes and eukaryotes, to instruct protein synthesis. **Prokaryotic DNA** - Contain a circular strand of DNA. - Has no membrane around it and floats in cytoplasm in the nucleoid region, - DNA codes for proteins that will be made on ribosomes in the surrounding cytoplasm - DNA codes for proteins that will be made in ribosomes in the surrounding cytoplasm. - Many have on are more small rings of non-chromosomal DNA\< called plasmids, floating separately in the cytoplasm. - The genes on these plasmids codes for features that are not essential to the survival of the cell, but often provide the bacteria with a selective advantage, such as resistance to antibiotics. - Plasmids replicate independently of the main circular "chromosome". - Prokaryotic chromosomes are less condensed than their eukaryotic counterpart. - They are naked -- do not have histone proteins. - Genomes are compact. - Contain little repetitive DNA and no introns. **Eukaryotic DNA** - In membrane-bound nucleus - Individual DNA molecules are arranged into a number of separate chromosomes. - Number of chromosomes is not a measure of how complex an organism is. - There is a large proportion of non-coding DNA (DNA that is not directly used to make polypeptides) in sequences called **introns**. The sequences actually coding for polypeptides are called **exons** (only 3% of DNA)\> - Non-coding DNA plays a role in controlling gene expression. - DNA is linear and winds around proteins (histones). - Nucleosomes are formed (DDNA wraps around 8 histones). - Mitochondria and chloroplasts contain their own DNA (=non-nuclear DNA). - This DNA is inherited independently of nuclear (chromosomal) DNA. - Non-nuclear mitochondrial DNA (mtDNA) is found in mitochondria. - mtDNA can be used to trace maternal inheritance. - during fertilisation, only the nuclei fuse, the sperm does not contribute any mitochondria, so all mitochondria are coming from the egg. - mtDNA is a very small circular molecule with only 37 genes. - each mitochondrion contains 5-10 circular DNA molecules, and each cell has between 100-1000 mitochondria. - Mutation rate is higher in mtDNA than in chromosomal DNA. **Summary of the differences** +-----------------------------------+-----------------------------------+ | Prokaryote | Eukaryote | +===================================+===================================+ | Chromosomal DNA is in a region of | Chromosomal DNA is in the | | cytoplasm called the nucleoid, | nucleus, which is separated from | | lacking a membrane. | the cytoplasm by a double-layered | | | membrane. | | There is one chromosome per cell. | | | | There are multiple chromosomes | | | per cell a diploid (2n) or | | | haploid (n) number. | +-----------------------------------+-----------------------------------+ | A circular chromosome without | Linear thread-like chromosomes | | ends (no telomeres). | with ends (telomeres). | +-----------------------------------+-----------------------------------+ | Contains plasmids -- small, | Contains no plasmids but there | | circular DNA. | are other sources of DNA apart | | | from chromosomes -- mitochondrial | | | DNA and chloroplast DNA. | +-----------------------------------+-----------------------------------+ | There is much less DNA than in | There is much more DNA than in | | eukaryotes (thousands to millions | prokaryotes (millions to billions | | of bases, depending on species). | of bases, depending on species). | +-----------------------------------+-----------------------------------+ | There are fewer genes than in | There are more genes than in | | eukaryotes (thousands). | prokaryotes (tens of thousands). | +-----------------------------------+-----------------------------------+ | There is less non-coding DNA | There is more non-coding DNA | | (introns) than in eukaryotes | (introns) than in prokaryotes | | (greater number of genes per | (fewer genes per number of | | number of bases). | bases). | +-----------------------------------+-----------------------------------+ | DNA is in a region called the | DNA is tightly packaged -- coiled | | nucleoid but is not packaged into | around histones forming | | an organelle (less DNA to fit | nucleosomes, which are condensed | | into the cell). | into chromatin and packed as | | | chromosomes into the nucleus. (a | | | lot of DNA to fit into a small | | | space). | +-----------------------------------+-----------------------------------+ | Genes cluster into functional | Genes that code for functionally | | groups, known as operon regions. | similar proteins can be | | (e.g., genes that code for | physically far apart or located | | enzymes in the same biochemical | on different chromosomes. | | pathway are next to each other on | | | the chromosome so all the genes | Eukaryotes have mechanisms to | | for the pathway can be | express these gens at the same | | transcribed and expressed at | time. | | once). | | +-----------------------------------+-----------------------------------+ model the process of polypeptide synthesis ------------------------------------------ **Polypeptide synthesis** [Production of a protein involves the following:] - **DNA**: a gene on the DNA strand provides the information required to make the polypeptide in the form of a designated sequence of bases. - **Messenger RNA (mRNA):** this is a type of ribonucleic acid. - It carries the information from the DNA in the nucleus to ribosomes in the cytoplasm. - Ribonucleic acids are single stranded. - The sugar in the sugar-phosphate backbone is ribose and the base thymine is replaced by uracil. - **Transfer RNA (tRNA):** This type of ribonucleic acid brings amino acids to the ribosome to be linked to build the polypeptide chain. - there are over 20 types of tRNA, a different type of each amino acid. - tRNA has a distinctive clover-leaf shape. - Each type of tRNA contains an anticodon or triplet of bases that recognises, and is complementary to, a codon on the mRNA. - On the opposite end to the anticodon is an amino acid temporarily bound to the tRNA. - **Ribosomes**: A ribosome is made up of two subunits. - It acts as the site for polypeptide syntehsis in the cytoplasm. - It contains theree active binding sutes, which hold the mRNA strand and two tRNA molecules together temproarily during the linkage of amino acids to make the polypeptide chain. - rRNA is the structural part of the ribosomes. - **Enzymes**: As with all chemical rpocesses within the cell, enzymes are involved in caatalysing the reactions at all stages in the process.  ### transcription and translation **Transcription** -- this is the copying of 1 section of the DNA (a gene) to make a messenger (mRNA) strand. It occurs in a similar way to DNA replication except: - Only 1 strand of the DNA is copied (**the template strand)** - The other strand is called **the coding strand.** - Only 1 part of the DNA is copied (**a gene**) - The enzyme that does the copying is called **RNA polymerase.** In eukaryotes, the DNA is in the nucleus, so transcription occurs in the nucleus and the mRNA then travels out (through nuclear pores) to the cytoplasm for translation. - Also there is modification of the mRNA before it leaves the nucleus. **Translation** Requires 3 things: - mRNA (messenger RNA) - tRNA (transfer RNA) - Ribosomes (an organelle made of protein and rRNA (ribosomal RNA)). The sequence of bases in the **mRNA** is "read" in groups of threes: these 3's are called **codons**. - This relies on 3 bases on each **tRNA** which is complimentary to each codon -- these 3's are called **anticodons**. - Each tRNA is also attached to an amino acid. Which amino acid is determined by its specific anticodon. Ribosomes help the correct tRNA pair with the mRNA (based on the complimentary bases pairing between the codon and anticodon) and then join the amino acids together in an extending chain. 1. mRNA moves to the ribosome and attaches to it. 2. tRNA molecules pick up the amino acid that corresponds to their anticodon. 3. Codon and anticodons match up. 4. Peptide bonds form between amino acids. 5. tRNA molecules leave the ribosome and go to pick up more amino acids. 6. The polypeptide chain continues growing until it is complete. 7. mRNA is reused or broken down into free nucleotides. **Reading the genetic code** **Scientists have worked out the exact codon (and anticodon) code which is associated with each amino acid in a polypeptide chain.** - **The start codon: Translation always starts with the codon AUG which matches the anticodon UAC.** - This codes for the amino acid methionine (met)**.** - **The stop codons: These don't code for an amino acid but cause the ribosome to let go of the mRNA and stop translating any further along the mRNA molecule.** **The code for translation normally shown as a table of codons (on the mRNA) and the particular amino acids each group of 3 letters it is translated into.** **Most amino acids are coded for by more than 1 codon -- redundancy.** - **There are 64 possible grouping of 3 letters. But only 20 amino acids that need to be coded for.** ### assessing the importance of mRNA and trna in transcription and translation ### analysing the function and importance of polypeptide expression **Functioning and importance of polypeptide synthesis** [Importance] A polypeptide is a polymer of many amino acids linked by peptide bonds forming a chain and proteins may be made up of more than one polypeptide chain. - Proteins make up the most 'solid' mass in the bodies of many mammals. Polypeptides are synthesised within the structure of the ribosome with some being released into the cytosol and other moving into the lumen of the endoplasmic reticulum where proteins are formed and transported through the tubules to parts that form smooth endoplasmic reticulum near Golgi bodies where the proteins are enclosed in a transport vesicle. In specialised cells, coded instructions for the production of a particular protein are "switched on". - In other types of cells, these genes might be switched off, because these polypeptides are not needed in these particular cells. - this ensures that the cell develops a particular structure, in keeping with the type of tissue to which it belongs. - E.g., in skin tissue, genes for the pigment protein melanin and for the protein keratin will be switched on in each cell, ensuring that the cells become skin cells. - E.g., in muscle cells, the genes for melanin would be present in each nucleus of the muscle cells, but they would be switched off (not expressed into polypeptides), because the protein melanin is not needed in muscle cells. **Variety of polypeptides** The genetic code gives the sequence of amino acids in polypeptides and this sequence determines the biological activity of the polypeptide. - Proteomics is the study of the full protein set encoded in the genome. - The protein set shows the particular polypeptides/proteins involved and interacting when a particular biological activity is occurring. [Variety in polypeptides is due to the following:] - The specific sequence of amino acids. - If there are 20 common amino acids, the number of different polypeptides consisting of 12 amino acids would be 20 to the power of 14 different polypeptides. - The variety of amino acids present gives the polypeptide a high degree of specificity. - The number of amino acids present. - A single protein molecule may have around 50 to 50, 000 or more amino acids in the chain. - The spatial arrangement of the amino acids in the chain causes coiling and folding to give the protein a specific 3D structure. A change in the code (a mutation) during DNA/cell replication can affect the property of the polypeptide coded by that gene. **Mutations** If a mutation causes a change in the DNA code, this will influence the protein produced. - The activities of the cell can be disrupted if a particular protein cannot be produced. - This can even lead to disease. - If a mutation on the beta haemoglobin gene puts thymine instead of guanine at one particular location, the person will suffer from thalassemia major. - Results in abnormalities such as severe anaemia and growth retardation. ### assessing how genes and environment affect phenotypic expression **Genotype vs Phenotype** An organism's final appearance (phenotype) is a result of the interactions between the information held in its genotype and the effect of the environment acting on it during growth and development. - Not all phenotypes are affected by environmental factors. - Blood type, tongue rolling are determined solely by genotype and are not changed due to other factors. - Poor diet is an example of an environmental factor which can affect a person's phenotypic expression. - Someone who has inherited genes to be tall but has a poor diet during childhood may not reach their potential height. - Twin studies can be used to prove if a characteristic is due to environmental or genetic factors. - Since identical twins have exactly the same DNA\< any differences between twins must be due to environmental factors. **Effect of the environment on gene expression** The effect of a gene can be enhanced or masked by variation in the environment. [Hydrangeas] - The acidity of alkalinity of the soil influences the colour of the flowers. - Hydrangeas growing in acidic soil develop blue flowers, whereas those grown in alkaline soil develop pink flowers. **Regulation of the ge