Reproduction & Inheritance DP Bio Resource Guide PDF

1 Table of Contents D2.1 Cell and nuclear division...................................................

1 Table of Contents D2.1 Cell and nuclear division............................................................................................................................................................. 7 D2.1.1—Generation of new cells in living organisms by cell division.............................................................................7 D2.1.2—Cytokinesis as splitting of cytoplasm in a parent cell between daughter cells..........................................7 D2.1.3—Equal and unequal cytokinesis....................................................................................................................................7 D2.1.4—Roles of mitosis and meiosis in eukaryotes............................................................................................................ 8 D2.1.5—DNA replication as a prerequisite for both mitosis and meiosis......................................................................9 D2.1.6—Condensation and movement of chromosomes as shared features of mitosis and meiosis................10 D2.1.7—Phases of mitosis.......................................................................................................................................................... 11 D2.1.8—Identification of phases of mitosis...........................................................................................................................12 D2.1.9—Meiosis as a reduction division................................................................................................................................. 14 D2.1.10—Down syndrome and non-disjunction................................................................................................................. 15 D2.1.11—Meiosis as a source of variation.............................................................................................................................16 Additional higher level................................................................................................................................................................. 19 D2.1.12—Cell proliferation for growth, cell replacement and tissue repair...............................................................19 D2.1.13—Phases of the cell cycle............................................................................................................................................ 19 D2.1.14—Cell growth during interphase...............................................................................................................................20 D2.1.15—Control of the cell cycle using cyclins................................................................................................................. 20 D2.1.16—Consequences of mutations in genes that control the cell cycle............................................................... 21 D2.1.17—Differences between tumours in rates of cell division and growth and in the capacity for...............21 metastasis and invasion of neighbouring tissue.................................................................................................................. 21 D3.1 Reproduction................................................................................................................................................................................ 23 D3.1.1—Differences between sexual and asexual reproduction................................................................................... 23 D3.1.2—Role of meiosis and fusion of gametes in the sexual life cycle..................................................................... 24 D3.1.3—Differences between male and female sexes in sexual reproduction......................................................... 24 D3.1.4—Anatomy of the human male and female reproductive systems.................................................................. 25 D3.1.5—Changes during the ovarian and uterine cycles and their hormonal regulation...................................... 27 D3.1.6—Fertilization in humans................................................................................................................................................28 D3.1.7—Use of hormones in in vitro fertilization (IVF) treatment................................................................................. 29 D3.1.8—Sexual reproduction in flowering plants............................................................................................................... 29  2 D3.1.9—Features of an insect-pollinated flower................................................................................................................ 30 D3.1.10—Methods of promoting cross-pollination............................................................................................................31 D3.1.11—Self-incompatibility mechanisms to increase genetic variation within a species................................. 31 D3.1.12—Dispersal and germination of seeds.................................................................................................................... 32 Additional higher level................................................................................................................................................................. 33 D3.1.13—Control of the developmental changes of puberty by gonadotropin-releasing hormone and.........33 steroid sex hormones....................................................................................................................................................................33 D3.1.14—Spermatogenesis and oogenesis in humans.....................................................................................................33 D3.1.15—Mechanisms to prevent polyspermy.................................................................................................................... 33 D3.1.16—Development of a blastocyst and implantation in the endometrium........................................................33 D3.1.17—Pregnancy testing by detection of human chorionic gonadotropin secretion........................................ 33 D3.1.18—Role of the placenta in fetal development inside the uterus.......................................................................33 D3.1.19—Hormonal control of pregnancy and childbirth................................................................................................ 33 D3.1.20—Hormone replacement therapy and the risk of coronary heart disease...................................................33 D3.2 Inheritance.....................................................................................................................................................................................34 D3.2.1—Production of haploid gametes in parents and their fusion to form a diploid zygote as the means of inheritance........................................................................................................................................................................................ 35 D3.2.2—Methods for conducting genetic crosses in flowering plants.........................................................................35 D3.2.3—Genotype as the combination of alleles inherited by an organism.............................................................. 35 D3.2.4—Phenotype as the observable traits of an organism resulting from genotype and environmental factors................................................................................................................................................................................................ 35 D3.2.5—Effects of dominant and recessive alleles on phenotype................................................................................ 35 D3.2.6—Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression............................................................................................................. 35 D3.2.7—Phenylketonuria as an example of a human disease due to a recessive allele........................................35 D3.2.8—Single-nucleotide polymorphisms and multiple alleles in gene pools.......................................................35 D3.2.9—ABO blood groups as an example of multiple alleles......................................................................................35 D3.2.10—Incomplete dominance and codominance..........................................................................................................35 D3.2.11—Sex determination in humans and inheritance of genes on sex chromosomes.....................................36 D3.2.12—Haemophilia as an example of a sex-linked genetic disorder.....................................................................36 D3.2.13—Pedigree charts to deduce patterns of inheritance of genetic disorders..................................................36 D3.2.14—Continuous variation due to polygenic inheritance and/or environmental factors................................36 D3.2.15—Box-and-whisker plots to represent data for a continuous variable such as student height........... 36 Additional higher level................................................................................................................................................................. 36 D3.2.16—Segregation and independent assortment of unlinked genes in meiosis................................................36 D3.2.17—Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes.......................................................................................................................................................... 36 D3.2.18—Loci of human genes and their polypeptide products....................................................................................36 D3.2.19—Autosomal gene linkage.......................................................................................................................................... 36 D3.2.20—Recombinants in crosses involving two linked or unlinked genes.............................................................36 D3.2.21—Use of a chi-squared test on data from dihybrid crosses............................................................................. 37  3 D2.1 Cell and nuclear division D2.1.1—Generation of new cells in living organisms by cell division In all living organisms, a parent cell often referred to as a mother cell divides to produce two daughter cells. All living things grow, repair themselves, and reproduce by creating new cells through cell division. A single parent cell splits into two daughter cells, each containing a copy of the parent's genetic material. This process of cell division allows organisms to grow, repair themselves, and reproduce. Traced back through generations, this cell division reveals a continuity of life linking all organisms to the first single-celled life forms on Earth. Overview of Cell Division D2.1.2—Cytokinesis as splitting of cytoplasm in a parent cell between daughter cells Students should appreciate that in an animal cell a ring of contractile actin and myosin proteins pinches a cell membrane together to split the cytoplasm, whereas in a plant cell vesicles assemble sections of membrane and cell wall to achieve splitting. Cytokinesis is the physical separation of a parent cell's cytoplasm into two daughter cells during cell division. It follows nuclear division by mitosis or meiosis. Animal cells use a contractile ring of actin and myosin to form a cleavage furrow, pinching the cell in two. Plant cells build a cell plate from vesicles guided by microtubules, dividing the cytoplasm.  4 D2.1.3—Equal and unequal cytokinesis Include the idea that division of cytoplasm is usually, but not in all cases, equal and that both daughter cells must receive at least one mitochondrion and any other organelle that can only be made by dividing a pre-existing structure. Include oogenesis in humans and budding in yeast as examples of unequal cytokinesis. Cytokinesis often splits the cytoplasm equally, like in growing root tips where cells have similar functions. Unequal division can also occur, with smaller daughter cells still receiving essential organelles like mitochondria to survive. Budding in yeast and egg cell formation in humans are examples of unequal cytokinesis. Budding in yeast: Yeast reproduces asexually by forming a bud, a small outgrowth containing one daughter nucleus and less cytoplasm. The bud matures and separates from the larger mother cell. How Does Yeast Make Bread? Oogenesis in humans: Egg formation (oogenesis) involves unequal cell division. Most cytoplasm goes to one daughter cell (oocyte), which matures, while the other cell (polar body) is much smaller and doesn't develop further. First polar body extrusion oocyte meiosis - In vitro maturation D2.1.4—Roles of mitosis and meiosis in eukaryotes Emphasize that nuclear division is needed before cell division to avoid production of anucleate cells. Mitosis maintains the chromosome number and genome of cells, whereas meiosis halves the chromosome number and generates genetic diversity. Eukaryotic cells reproduce via cell division, which requires nuclear division to ensure each daughter cell gets a copy of the genetic material. There are two main types of nuclear division: mitosis for growth and repair, and meiosis for sexual reproduction.  5 D2.1.5—DNA replication as a prerequisite for both mitosis and meiosis Students should understand that, after replication, each chromosome consists of two elongated DNA molecules (chromatids) held together until anaphase. Before cell division (mitosis or meiosis), a cell copies all its DNA to ensure each daughter cell gets a complete set of genes. What is a Chromosome?  6 DNA exists as single chromosomes before replication, and then replicates into pairs of identical DNA molecules called sister chromatids. Cohesin protein complexes hold these sister chromatids together until cell division is ready to separate them. As DNA condenses during cell division, chromosomes become visible under a microscope, each showing two sister chromatids. Sister chromatids are genetically identical.  7 D2.1.6—Condensation and movement of chromosomes as shared features of mitosis and meiosis Include the role of histones in the condensation of DNA by supercoiling and the use of microtubules and microtubule motors to move chromosomes. Mitosis and meiosis share the need to condense and move chromosomes for proper cell division. Chromosome DNA is extremely long and needs to be compacted to avoid tangles during separation. Condensation involves wrapping DNA around proteins and further shortening through not-yet-fully-understood mechanisms. Microtubules, assembled and disassembled by centrosomes (in animal cells), are used to move chromosomes. Kinetochore proteins on chromosomes attach to microtubules, pulling sister chromatids apart in mitosis or homologous chromosomes apart in meiosis. Mitosis vs Meiosis Mitosis vs. Meiosis: Side by Side Comparison Mitosis vs Meiosis Rap Battle! | SCIENCE SONGS  8 D2.1.7—Phases of mitosis Students should know the names of the phases and how the process as a whole produces two genetically identical daughter cells. M Phase of the Cell Cycle Mitosis: The Amazing Cell Process that Uses Division to Multiply! (Updated) Before entering mitosis, the cell undergoes a lengthy interphase where it grows, replicates its DNA, and prepares for division. Mitosis occurs in four main stages: prophase, metaphase, anaphase, and telophase. People Meet And Talk! - Prophase, Metaphase, Anaphase, Telophase Prophase is the first stage where chromosomes condense. Metaphase follows prophase, and chromosomes move to the center of the cell. During the short anaphase stage, sister chromatids separate and move towards opposite poles. Telophase is the final stage where new nuclei form around the separated chromosomes, and the cell begins to divide.  9 D2.1.8—Identification of phases of mitosis Application of skills: using diagrams as well as with cells viewed with a microscope or in a micrograph. 10  11 Describe the events that occur during mitosis. (6) sequence of stages is prophase → metaphase → anaphase → telophase; chromosomes condense/supercoil/become shorter and fatter in prophase; spindle microtubules grow (from poles to equator) in prophase/metaphase; nuclear membrane breaks down in prophase/metaphase; spindle microtubules attach to the centromeres/chromosomes in metaphase; chromosomes line up at equator in metaphase; centromeres divide / (paired) chromatids separate / chromosomes separate into two chromatids in metaphase/anaphase; (sister) chromatids/chromosomes pulled to opposite poles in anaphase; spindle microtubules disappear in telophase; nuclear membrane reforms around chromosomes/chromatids in telophase; chromosomes/chromatids decondense in telophase;  12 D2.1.9—Meiosis as a reduction division Students should understand the terms “diploid” and “haploid” and how the two divisions of meiosis produce four haploid nuclei from one diploid nucleus. They should also understand the need for meiosis in a sexual life cycle. outline the two rounds of segregation in meiosis. Cells contain chromosomes with genes arranged in a specific sequence. This sequence is mostly stable but can change through mutation, creating new alleles. Homologous chromosomes carry the same gene sequence (but not necessarily the same alleles!) and come in pairs in diploid body cells (2n). Sexual reproduction relies on haploid gametes (n) like sperm and eggs. To prevent a doubling of chromosomes with each generation, meiosis reduces the chromosome number from diploid to haploid during gamete formation. Meiosis, as the reduction division, is crucial in sexual life cycles to maintain the characteristic chromosome number in a species. Meiosis _ Mcgraw Hill.mp4 Meiosis Meiosis (Updated) What is Meiosis? D2.1.10—Down syndrome and non-disjunction Use Down syndrome as an example of an error in meiosis. During meiosis, errors can occur where chromosomes don't separate properly (non-disjunction). This can lead to cells with an extra or missing chromosome, usually lethal. Down syndrome is a rare exception where having an extra chromosome (number 21) isn't fatal, but causes developmental problems. Non-disjunction can also cause sex chromosome abnormalities like Klinefelter's syndrome (XXY) and Turner's syndrome (X). Only up to 3:41! Down syndrome (trisomy 21) - causes, symptoms, diagnosis, & pathology Chromosomes and Karyotypes  13 D2.1.11—Meiosis as a source of variation Students should understand how meiosis generates genetic diversity by random orientation of bivalents and by crossing over. Again! Meiosis _ Mcgraw Hill.mp4 Crossing Over: During meiosis, homologous chromosomes pair up with four chromatids total (two per chromosome). These pairs are called bivalents. Crossing over occurs when non-sister chromatids in a bivalent swap sections of DNA, resulting in an exchange of genes. This exchange creates new combinations of alleles on the chromosomes, increasing genetic diversity in offspring.  14 Random Orientation of Bivalents: In meiosis I, bivalents line up in the center of the cell. Spindle fibers attach to each chromosome, but which pole they attach to is random (50% chance for either). Since chromosomes have different gene versions (alleles), the random orientation determines which alleles end up in each gamete (sperm or egg). The random orientation of multiple bivalents creates a vast number of possible combinations (2^n, where n is the number of bivalents). In humans with 23 chromosome pairs (22 bivalents), this randomness alone generates over 8 million possible gamete combinations. Describe the events that occur during meiosis and how they generate genetic diversity. (10) Sequence of stages is prophase I → metaphase I → anaphase I → telophase I → prophase II → metaphase II → anaphase II → telophase II; Meiosis I Chromosomes condense/supercoil/become shorter and fatter in prophase I; Homologous chromosomes pair up forming tetrads/bivalents in prophase I; Crossing over/recombination occurs between homologous chromosomes in prophase I, forming chiasmata; Crossing over increases genetic diversity by exchanging DNA between non-sister chromatids, leading to new combinations of alleles; Spindle microtubules grow (from poles to equator) in prophase I/metaphase I; Homologous chromosomes line up at the equator in metaphase I; Homologous chromosomes separate and are pulled to opposite poles in anaphase I; Independent/random reassortment of chromosomes during anaphase I increases genetic diversity by producing gametes with different combinations of maternal and paternal chromosomes; Meiosis I is a reduction division, reducing the chromosome number by half, resulting in cells that are haploid at the end of this first division; Nuclear membrane reforms around chromosomes in telophase I OR cells divide in cytokinesis after telophase I; Meiosis II Sister chromatids (now individual chromosomes) separate and are pulled to opposite poles in anaphase II; Nuclear membrane reforms around chromatids/chromosomes in telophase II; chromosomes/chromatids decondense in telophase II; Meiosis involves two successive divisions, resulting in four genetically different gametes; 15 16  17 D2.1.12—Cell proliferation for growth, cell replacement and tissue repair Include proliferation for growth within plant meristems and early-stage animal embryos as examples. Include skin as an example of cell proliferation during routine cell replacement and during wound healing. Not required to know details of the structure of skin. Growth: Cell proliferation allows multicellular organisms to grow. In animals, this is rapid during embryonic and juvenile stages, with examples like bone growth plates. Plants grow through cell division in specialized regions called meristems. Cell Replacement: Worn-out or damaged cells need to be replaced. In the skin's epidermis, new cells are continuously produced in the basal layer, pushing older cells to the surface where they die and flake off. While some debate whether this is true proliferation, cell division does occur. Tissue Repair: Wounds can heal through cell division, replacing lost cells. This relies on the presence of stem cells that can divide and differentiate into specialized cells for repair. Skin is particularly good at repair, with basal cells regenerating the outer layers and deeper stem cells fixing more extensive damage. D2.1.13—Phases of the cell cycle Students should understand that cell proliferation is achieved using the cell cycle. Understand the sequence of events including G1, S and G2 as the stages of interphase, followed by mitosis and then cytokinesis. The Cell Cycle The Cell Cycle (and canc… The cell cycle governs cell growth and division. It alternates between mitosis (nuclear division) and interphase (growth and preparation). During interphase (G1, S, G2), the cell grows, replicates DNA, and prepares for mitosis.  18 D2.1.14—Cell growth during interphase Students should appreciate that interphase is a metabolically active period and that growth involves biosynthesis of cell components including proteins and DNA. Numbers of mitochondria and chloroplasts are increased by growth and division of these organelles. Interphase is the extended cell cycle stage between cell divisions (mitosis or meiosis). During interphase, DNA unwinds and becomes active for transcription (making RNA) and protein synthesis. The cell grows in size by doubling its DNA, increasing cytoplasm volume, and making more organelles like mitochondria and ribosomes. Interphase is a metabolically active time for growth and preparation for cell division. D2.1.15—Control of the cell cycle using cyclins Limit to the concentration of different cyclins increasing and decreasing during the cell cycle and a threshold level of a specific cyclin required to pass each checkpoint in the cycle. Not required to know details of the roles of specific cyclins. Checkpoints: The cell cycle has built-in pauses to ensure everything is ready before moving to the next stage. These checkpoints also stop excessive cell division when a tissue has enough cells. Cyclins and Cyclin-Dependent Kinases: A group of proteins called cyclins work with cyclin-dependent kinases to regulate the cell cycle. Cyclins activate these kinases, which then trigger specific actions needed for each cell cycle phase. Cyclin Levels: Different cyclins accumulate at specific times, and only when they reach a threshold level can the cell cycle progress. This ensures the right actions happen at the right time.  19 D2.1.16—Consequences of mutations in genes that control the cell cycle Include mutations in proto-oncogenes that convert them to oncogenes and mutations in tumour suppressor genes, resulting in uncontrolled cell division. Uncontrolled cell division, often caused by mutations in genes regulating the cell cycle, can lead to tumors. These mutations are inherited by daughter cells during division. Mutagens like certain chemicals and high-energy radiation increase the risk of these mutations. Two main gene types play a role: proto-oncogenes and tumor-suppressor genes. Proto-oncogenes normally control cell proliferation and growth. Mutations can transform them into oncogenes, which actively promote uncontrolled cell division. Often, a single mutation in a dominant proto-oncogene is enough. Tumor-suppressor genes normally act as brakes on cell division or repair DNA damage. Mutations can inactivate them, increasing tumor risk. In these genes, mutations that create truncated, non-functional proteins are common. p53 Tumour Suppressor (2016) by Etsuko Uno wehi.tv Developing a tumor often requires multiple mutations accumulating over time. The vast number of cells in our body and long lifespans make this a possibility, despite the low chance of a single cell acquiring all the necessary mutations. D2.1.17—Differences between tumours in rates of cell division and growth and in the capacity for metastasis and invasion of neighbouring tissue Include the terms “benign”, “malignant”, “primary tumour” and “secondary tumour”, and distinguish between tumours that do and do not cause cancer. How does cancer spread through the body? - Ivan Seah Yu Jun Tumors can be benign or malignant. Benign tumors grow in a mass but don't spread and are unlikely to cause harm. Malignant tumors, also known as cancers, spread through metastasis, where detached cells invade nearby tissues or travel through blood/lymph to form secondary tumors in other organs. The consequences of malignant tumors are more severe due to multiple secondary tumors forming. Certain tissues like breasts, ovaries, testes, and the thyroid gland are more prone to malignant tumors due to hormonal cell division stimulation. While a cancer diagnosis is concerning, effective treatments exist, and tumor growth rates can vary, with some being very slow-growing.  20 Application of skills: Students should observe populations of cells to determine the mitotic index. The micrograph shows mitosis in a cell of an onion (Allium cepa) root tip. (a) Deduce, with a reason, which stage of mitosis is shown. (b) The cells visible in the onion root tip were classified and counted. Calculate the mitotic index.  21 D3.1 Reproduction D3.1.1—Differences between sexual and asexual reproduction Include these relative advantages: asexual reproduction to produce genetically identical offspring by individuals that are adapted to an existing environment, sexual reproduction to produce offspring with new gene combinations and thus variation needed for adaptation to a changed environment. All living things reproduce to create new members of their species. This can be done through sexual reproduction, which combines genetic material from two parents, or asexually, where a single parent replicates itself. Sexual reproduction leads to genetic variation in offspring, allowing them to adapt and evolve over time. Asexual reproduction results in offspring nearly identical to the parent, except for occasional mutations. This difference in genetic variation is a key distinction between sexual and asexual reproduction strategies. Born Pregnant: Aphids Invade with an Onslaught of Clones | Deep Look  22 D3.1.2—Role of meiosis and fusion of gametes in the sexual life cycle Students should appreciate that meiosis breaks up parental combinations of alleles, and fusion of gametes produces new combinations. Fusion of gametes is also known as fertilization. Sexual reproduction in eukaryotes relies on two opposing processes: meiosis and gamete fusion. Meiosis halves the chromosome number in gametes (sperm and egg), preventing a doubling of chromosomes with each generation. Fertilization, the fusion of gametes, creates a new diploid organism with a unique combination of genes from both parents. Meiosis, a key evolutionary step, allows this genetic variation for adaptation and evolution within sexually reproducing eukaryotes. D3.1.3—Differences between male and female sexes in sexual reproduction Include the prime difference that the male gamete travels to the female gamete, so it is smaller, with less food reserves than the egg. From this follow differences in the numbers of gametes and the reproductive strategies of males and females. In sexual reproduction of most eukaryotes (plants and animals), unlike some fungi, two distinct gametes exist: small, motile sperm (male) and larger, non-motile eggs (female). This distinction is known as anisogamy. What Is Sperm?What is sperm made of? medical animation #sperm #fertilization  23 D3.1.4—Anatomy of the human male and female reproductive systems Draw diagrams of the male-typical and female-typical systems and annotate them with names of structures and functions. 24  25 D3.1.5—Changes during the ovarian and uterine cycles and their hormonal regulation Include the roles of oestradiol, progesterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH) and both positive and negative feedback. The ovarian and uterine cycles together constitute the menstrual cycle. How menstruation works - Emma Bryce ovulation and menstrual cycle often called period|medical animationDandelionTeam #ovulation #period - YouTube The menstrual cycle involves two main players: the ovary (egg production) and the uterus (lining preparation). FSH (follicle-stimulating hormone), from the pituitary gland (in the brain), kicks things off by stimulating the ovary to develop follicles, each containing an egg. Follicles produce estradiol, which thickens the lining of the uterus (endometrium) in preparation for a possible pregnancy. Estradiol also initially increases FSH production (positive feedback). As estradiol levels rise, they eventually switch roles and provide negative feedback, inhibiting further FSH production. Meanwhile, rising estradiol levels trigger a surge of LH (luteinizing hormone). This LH surge causes ovulation, the release of a mature egg from the ovary around mid-cycle (14 days).  26 The empty follicle left behind after ovulation transforms into the corpus luteum. This structure produces progesterone, which further stimulates the endometrium to become even more receptive for implantation of a fertilized egg. Progesterone does not directly thicken the endometrium, but rather differentiates it by: Increasing the number of blood vessels (vascularization) to supply nutrients for a potential embryo. Changing the composition of the lining to make it more receptive for implantation. Progesterone also plays a feedback role, inhibiting the production of both FSH and LH. If pregnancy doesn't occur, progesterone levels drop, causing the endometrium to break down and shed during menstruation. This marks the end of the cycle and the beginning of a new one with another rise in FSH. Outline the roles of progesterone and oestradiol in the human menstrual cycle. (6) follicles secrete oestradiol/ FSH stimulates secretion of estrogen; (rapid) increase in oestradiol stimulates FSH/LH production; oestradiol also stimulates repair/thickening of endometrium/uterus lining; LH causes follicle to produce less estrogen/more progesterone; corpus luteum secretes more estrogen/progesterone; progesterone maintains/stimulates thickening of endometrium/uterus lining; estrogen/progesterone inhibit FSH/LH secretion; estrogen/progesterone levels fall after day 21–24 if no embryo/fertilization; lower concentration of estrogen/progesterone allows disintegration of endometrium/uterus lining / menstruation occurs; Award [4 max] if only one hormone is explained. This is not the only question on the menstrual cycle but only one example! Make sure you know it all ! D3.1.6—Fertilization in humans Include the fusion of a sperm’s cell membrane with an egg cell membrane, entry to the egg of the sperm nucleus but destruction of the tail and mitochondria. Also include dissolution of nuclear membranes of sperm and egg nuclei and participation of all the condensed chromosomes in a joint mitosis to produce two diploid nuclei. In fertilization, a sperm, guided by the egg, penetrates the egg's defenses and fuses with its membrane. While the sperm's tail and mitochondria are discarded, the nucleus enters the egg. Before the first cell division, both the sperm and egg nuclei dissolve their membranes, releasing their 23 chromosomes each. Immediately after fertilization, a protective layer around the egg hardens, preventing additional sperm from entering. This ensures that only one sperm fertilizes the egg, preventing the development of an abnormal embryo with too many chromosomes. Remarkably, all 46 chromosomes then participate in a joint cell division, using the same machinery, to form a new cell with two genetically distinct diploid nuclei. This first division of the fertilized egg marks the beginning of a new organism. Fertilization  27 D3.1.7—Use of hormones in in vitro fertilization (IVF) treatment The normal secretion of hormones is suspended, and artificial doses of hormones induce superovulation. In Vitro Fertilization (IVF) is a fertility treatment that bypasses natural fertilization. IVF relies on precise hormonal control. Medications first suppress the woman's natural menstrual cycle by stopping FSH and LH production. High doses of FSH are then introduced to stimulate the ovaries, causing multiple follicles to develop eggs – many more than in a natural cycle. (superovulation) Once follicles reach a certain size, hCG injection mimics the embryo's signal, triggering egg maturation. After egg collection, fertilization with sperm occurs outside the body in a controlled laboratory environment. If fertilization is successful, embryos are transferred back to the uterus, often with additional progesterone to prepare the lining for implantation. How in vitro fertilization (IVF) works - Nassim Assefi and Brian A. Levine IVF People aren’t having babies in Denmark so they made this provocative ad D3.1.8—Sexual reproduction in flowering plants Include production of gametes inside ovules and pollen grains, pollination, pollen development and fertilization to produce an embryo. Understand that reproduction in flowering plants is sexual, even if a plant species is hermaphroditic. Fertilisation and Seed Formation Where Does Fruit Come From? Flowering plants have sex organs within their flowers. Male parts (stamens) produce pollen grains through meiosis, which carry sperm cells. Female parts (carpels) contain ovules where meiosis creates an egg cell/ovule. Pollination, by wind or animals, transfers pollen to the stigma. A pollen tube grows from the pollen grain, carrying sperm to the ovule for fertilization. The fertilized egg develops into an embryo within a seed, even if the plant has both male and female parts. This process, with meiosis and fertilization, qualifies as sexual reproduction.  28 D3.1.9—Features of an insect-pollinated flower Draw diagrams annotated with names of structures and their functions.  29 D3.1.10—Methods of promoting cross-pollination Include different maturation times for pollen and stigma, separate male and female flowers or male and female plants. Also include the role of animals or wind in transferring pollen between plants. Plants often employ strategies to discourage self-pollination, favoring cross-pollination which increases genetic variation. This variation is crucial for adaptation during environmental changes and leads to "hybrid vigor" in offspring – stronger and healthier plants. Mechanisms like different maturation times for pollen and stigma, or separate male and female flowers on the same plant (monoecy) or even separate plants (dioecy), hinder self-pollination. Wind or animals then become essential for transferring pollen between plants, facilitating cross-pollination. Despite some plants being hermaphrodites (having both male and female parts), these mechanisms ensure fertilization between genetically distinct individuals. Pollination Explained D3.1.11—Self-incompatibility mechanisms to increase genetic variation within a species Students should understand that self-pollination leads to inbreeding, which decreases genetic diversity and vigour. Understand that genetic mechanisms in many plant species ensure male and female gametes fusing during fertilization are from different plants. Even with mechanisms to avoid self-pollination, some pollen from a hermaphrodite plant may land on its own stigma. However, many plants have a clever trick called self-incompatibility to prevent fertilization by its own pollen. This system, unlike our immune system that fights foreign invaders, prevents the plant's own pollen from functioning. Self-incompatibility is genetically controlled, with different versions (alleles) of genes determining compatibility. Plants with matching self-incompatibility alleles simply can't fertilize each other, promoting genetic diversity in offspring. This is why some fruit orchards plant multiple varieties – to ensure successful fruit production by encouraging fertilization between genetically distinct individuals.  30 D3.1.12—Dispersal and germination of seeds Distinguish seed dispersal from pollination. Include the growth and development of the embryo and the mobilization of food reserves. Seed dispersal, unlike pollination, happens after fertilization and ensures seeds travel away from the parent plant. This spread reduces competition for resources and helps the species colonize new areas. The dispersal method (wind, animals, hooks) depends on the fruit's structure. Seed Dispersal Sir David Attenborough Gives a Lesson on Seeds | The Green Plane… Seeds and Germination Explained Water Dispersal: Coconuts, with their tough, buoyant outer shells, are adapted for water dispersal. Ocean currents can carry them over vast distances, establishing new coconut palms on distant shores. Seeds like alder and willow have air pockets or lightweight structures, allowing them to float for extended periods on rivers and currents, potentially traveling long distances before germinating in suitable areas. This contrasts with seeds dispersed by wind or animals, which may sink quickly. Explosive Dispersal: Desiccant, a drying agent, might significantly accelerate the "popping" of gorse seed pods by rapidly removing moisture. These pods store tension as they dry and explosively rupture when a threshold is reached, flinging seeds outwards. Wind Dispersal: Samaras, winged seeds like those found in maple trees, are prime examples of wind dispersal adaptations. Their wings create lift and spin, allowing them to travel long distances on wind currents. Animal Dispersal (ingestion or attachment): Many fleshy fruits, like berries and cherries, are brightly colored and attractive to animals. Animals eat the fruit, and the seeds, indigestible by the animal's gut, pass through the digestive system and are deposited in new locations with the animal's droppings, far from the parent plant. Some seeds, like those of burdock plants, have hooks or barbs that cling to animal fur as they brush by. These "hitchhikers" can travel long distances on an animal's fur before snagging and falling off, potentially establishing themselves in new territories.  31 Additional higher level D3.1.13—Control of the developmental changes of puberty by gonadotropin-releasing hormone and steroid sex hormones Limit to the increased release of gonadotropin-releasing hormone (GnRH) by the hypothalamus in childhood triggering the onset of increased luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release. Ultimately the increased sex hormone production leads to the changes associated with puberty. Around the onset of puberty, the hypothalamus starts pulsating GnRH more frequently. This increased GnRH signal triggers the pituitary gland to release more LH and FSH. In turn, these gonadotropins stimulate sex hormone production (testosterone in males, estrogen in females) which drive the physical changes of puberty. D3.1.14—Spermatogenesis and oogenesis in humans Include mitosis, cell growth, two divisions of meiosis and differentiation. Understand how gametogenesis, in typical male and female bodies, results in different numbers of sperm and eggs, and different amounts of cytoplasm. Both sperm and egg production (spermatogenesis and oogenesis) involve mitosis, growth, meiosis (two divisions), and differentiation. However, they differ in results: Males: Continuous mitosis in the testes produces many sperm cells with minimal cytoplasm after meiosis. Females: Mitosis before birth creates follicles containing oocytes. After puberty, meiosis is completed in a mature follicle, resulting in one egg with most cytoplasm and three nonfunctional polar bodies. This explains the vast difference in sperm and egg numbers. Spermatogenesis Meiosis | Oogenesis | Gametogenesis | Human Embryology | Reproductive Physiology  32 D3.1.15—Mechanisms to prevent polyspermy The acrosome reaction allows a sperm to penetrate the zona pellucida and the cortical reaction prevents other sperm from passing through. The acrosome reaction helps a sperm reach the egg. When a sperm binds to the egg's zona pellucida, enzymes from the acrosome break down the zona, allowing the sperm to pass through. The cortical reaction prevents other sperm from entering the egg. Once a sperm fertilizes the egg, cortical granules release enzymes that harden the zona pellucida and change its surface, blocking further sperm binding and penetration. From 2:30 to 4:00 Fertilization D3.1.16—Development of a blastocyst and implantation in the endometrium Not required to know the names of other stages in embryo development. Following fertilization, the zygote divides rapidly, forming a solid ball of cells. Around day 6 or 7, this ball transforms into a hollow structure called a blastocyst with about 250 cells. The blastocyst travels to the uterus and implants in the lining (endometrium) for nourishment. The outer layer of the blastocyst develops projections that connect with the mother's blood supply, while the inner cell mass will eventually become the embryo and then the fetus. Development of blastocyst and implantation D3.1.17—Pregnancy testing by detection of human chorionic gonadotropin secretion Include the production of human chorionic gonadotropin (hCG) in the embryo or developing placenta and the use of monoclonal antibodies that bind to hCG. The developing embryo secretes hCG, a hormone crucial for maintaining pregnancy. Early on, hCG keeps the corpus luteum producing progesterone, which prevents the uterine lining from breaking down. After 8-12 weeks, the placenta takes over progesterone production, but still relies on hCG stimulation. Pregnancy tests detect the presence of hCG, indicating a fertilized egg and potential pregnancy. How Does a Pregnancy Test Work?  33 D3.1.18—Role of the placenta in fetal development inside the uterus Not required to know details of placental structure apart from the large surface area of the placental villi. - Understand which exchange processes occur in the placenta and that it allows the fetus to be retained in the uterus to a later stage of development than in mammals that do not develop a placenta. The placenta, formed by both fetal and maternal tissues, is crucial for human development. Unlike marsupials that give birth early, the placenta allows a human fetus to stay in the uterus for longer due to efficient exchange. Fetal villi with a large surface area maximize contact with maternal blood.  34 Nutrients like glucose and oxygen pass from mother to fetus, while waste products move in the opposite direction. This selective exchange, using various mechanisms, sustains the growing fetus. D3.1.19—Hormonal control of pregnancy and childbirth Emphasize that the continuity of pregnancy is maintained by progesterone secretion initially from the corpus luteum and then from the placenta, whereas the changes during childbirth are triggered by a decrease in progesterone levels, allowing increases in oxytocin secretion due to positive feedback. Progesterone, initially from the corpus luteum and later from the placenta, is key to maintaining pregnancy by preventing uterine contractions. As birth approaches, the fetus signals the placenta to stop progesterone production. This drop in progesterone allows oxytocin secretion to rise, triggering positive feedback that gradually strengthens uterine contractions. The contractions cause the cervix to dilate, amniotic sac to burst, and eventually push the baby out. With the first breath, the baby becomes physiologically independent.  35 D3.1.20—Hormone replacement therapy and the risk of coronary heart disease Menopause is when a woman's ovaries stop releasing eggs and her body naturally produces less estrogen, leading to the end of menstruation and fertility. Hormone replacement therapy (HRT) combats menopausal symptoms like hot flashes and vaginal dryness with estrogen and progesterone. While HRT offers relief and helps prevent osteoporosis, concerns exist about a potential increased risk of coronary heart disease. See B3.2.6 How menopause impacts women's heart health  36 Part 1: How Does New Genetic Information Evolve? Point Mutations - YouTube Genetic Engineering (youtube.com) Genetics Basics | Chromosomes, Genes, DNA and Traits | Don't Memorise - YouTube How this disease changes the shape of your cells - Amber M. Yates - YouTube Huntington disease (Year of the Zebra) What is cystic fibrosis, exactly? - YouTube Cystic Fibrosis: Pathophysiology, Genetics, Symptoms, Diagnosis and Treatments, Animation - YouTube 2-Minute Neuroscience: Huntington's disease - YouTube What It's Like To Be Color Blind - YouTube Understanding Hemophilia - YouTube Are GMOs Good or Bad? Genetic Engineering & Our Food The central park five case: The Central Park Five Official Trailer #1 (2012) - Ken Burns Documentary Movie HD The Central Park Five: A cautionary tale of injustice https://pyxis.nymag.com/v1/imgs/46c/a37/b52d716bf3d6d3f1b80d4e03bcafbce3bf-Jackie-Herbach -wedding-photo-1989-1.2x.w710.jpg Is radiation dangerous? - Matt Anticole  37 D3.2 Inheritance D3.2.1—Production of haploid gametes in parents and their fusion to form a diploid zygote as the means of inheritance Students should understand that this pattern of inheritance is common to all eukaryotes with a sexual life cycle. They should also understand that a diploid cell has two copies of each autosomal gene. Sexual reproduction relies on gametes, which have half the usual number of chromosomes (haploid). Parents create these gametes through meiosis, halving their genetic material. When a sperm and egg (gametes) fuse, they form a zygote with a full set of chromosomes (diploid), containing half from each parent.  38 D3.2.2—Methods for conducting genetic crosses in flowering plants Use the terms “P generation”, “F1 generation”, “F2 generation” and “Punnett grid”.Understand that pollen contains male gametes and that female gametes are located in the ovary, so pollination is needed to carry out a cross. They should also understand that plants such as peas produce both male and female gametes on the same plant, allowing self-pollination and therefore self-fertilization. Mention that genetic crosses are widely used to breed new varieties of crop or ornamental plants. Scientists study inheritance patterns by crossing flower varieties like peas. Pollen from one plant (male parent) is transferred to another plant's stigma (female parent), often using a brush to prevent unwanted pollination. This cross creates F1 generation offspring with seeds containing the next generation (F2). Punnett squares help analyze these crosses. By studying traits like flower color in pea plants, researchers like Gregor Mendel unlocked the basic principles of heredity. This method is still used today to breed new and improved plant varieties. GCSE Biology - Gregor Mendel and the History of Genetics #76 D3.2.3—Genotype as the combination of alleles inherited by an organism Use and understand the terms “homozygous” and “heterozygous”, and appreciate the distinction between genes and alleles. Genes can have different versions, called alleles. These variations can be small or large and arise from mutations. In diploid organisms, we inherit two alleles for most genes, one from each parent. The combination of these alleles is our genotype. Individuals with two identical alleles (BB or bb) are homozygous, while those with different alleles (Bd) are heterozygous. What is an Allele? Quick Definition  39 D3.2.4—Phenotype as the observable traits of an organism resulting from genotype and environmental factors Suggest examples of traits in humans due to genotype only and due to environment only, and also traits due to interaction between genotype and environment. Phenotype refers to an organism's observable characteristics, like eye color or ability to taste bitterness. It results from both an organism's genetic makeup (genotype) and the environment it lives in. For example, eye color is influenced by genes (genotype) but can also be slightly affected by sun exposure (environment). Height is another example where genes determine potential, but nutrition (environment) plays a role in reaching that potential. D3.2.5—Effects of dominant and recessive alleles on phenotype Students should understand the reasons that both a homozygous-dominant genotype and a heterozygous genotype for a particular trait will produce the same phenotype. Gregor Mendel studied pea plants to understand inheritance. He crossed pure-breeding tall plants with dwarf plants and surprisingly, all the offspring (F1 generation) were tall. The F1 generation plants, when allowed to self-pollinate, produced tall and dwarf plants in a 3:1 ratio in the F2 generation. Mendel explained this pattern using alleles. In this case, 'T' for tallness is dominant and 't' for shortness is recessive. An organism with TT or Tt genotype will be tall (dominant phenotype) while tt will result in a dwarf plant (recessive phenotype). This explains why both homozygous tall (TT) and heterozygous tall (Tt) plants look the same. They have the same phenotype (Tall) but different genotypes (TT & Tt). D3.2.6—Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression Phenotypic plasticity is not due to changes in genotype, and the changes in traits may be reversible during the lifetime of an individual. Organisms can adjust to their environment by altering gene expression, which affects their traits. This adaptation, called phenotypic plasticity, is reversible because genes are only turned on or off, not permanently changed.  40 This is especially beneficial in changing environments. For example, increased sun exposure might darken someone's skin through melanin production triggered by gene expression changes. Once sun exposure goes down, gene expression goes back to normal, and skin lightens up again. D3.2.7—Phenylketonuria as an example of a human disease due to a recessive allele Phenylketonuria (PKU) is a recessive genetic condition caused by mutation in an autosomal gene that codes for the enzyme needed to convert phenylalanine to tyrosine. Phenylketonuria (PKU) is a disease caused by a recessive allele on chromosome 12, an autosomal chromosome. This means it affects males and females equally. A mutation in this gene prevents the production of an enzyme needed to convert phenylalanine to tyrosine, leading to a buildup of phenylalanine in the body. Though both parents may be carriers without symptoms (having one recessive allele each), a child with two recessive alleles (from both parents) will have PKU. Early detection and a special diet can prevent intellectual disability and other problems. Up to 1:39 Phenylketonuria - causes, symptoms, diagnosis, treatment, pathology D3.2.8—Single-nucleotide polymorphisms and multiple alleles in gene pools Students should understand that any number of alleles of a gene can exist in the gene pool but an individual only inherits two. The gene pool contains all the genes from a sexually reproducing population. A gene, with its sequence of DNA bases, can have variations called alleles. These variations often involve just a single base difference (SNPs) between alleles. This can lead to multiple alleles existing in a gene pool, even though an individual organism only inherits two copies of that gene. For example, the apple S-gene has over 30 alleles, but an apple tree only has two.  41 D3.2.9—ABO blood groups as an example of multiple alleles Use IA, IB and i to denote the alleles. The ABO blood group system is an example of multiple alleles with three alleles (IA, IB, and i) for one gene. Despite six possible genotypes, there are only four blood types (A, B, AB, and O) because IA and IB are codominant over the recessive i allele. Codominance means both IA and IB alleles are expressed, like in type AB blood (IAIB genotype) where both A and B antigens are present on red blood cells. The i allele doesn't alter the red blood cell glycoprotein, resulting in type O blood (ii genotype). Why do blood types matter? - Natalie S. Hodge D3.2.10—Incomplete dominance and codominance Students should understand the differences between these patterns of inheritance at the phenotypic level. In codominance, heterozygotes have a dual phenotype. Include the AB blood type (IAIB) as an example. In incomplete dominance, heterozygotes have an intermediate phenotype. - Include four o'clock flower or marvel of Peru (Mirabilis jalapa) as an example. Note: When referring to organisms either the common name or the scientific name is acceptable. Codominance and incomplete dominance are two ways genes can be expressed differently. In codominance, both alleles are fully expressed in the heterozygote, like type AB blood (IAIB) where both A and B antigens are present. Incomplete dominance shows an intermediate phenotype in the heterozygote, like pink flowers in four o'clock flowers (Mirabilis jalapa) where a red allele (CR) and a white allele (CW) blend for a pink color. Neither allele is completely dominant over the other in incomplete dominance. Codominance results in a dual phenotype, while incomplete dominance shows a blending of the traits. Genetics incomplete Dominance in…  42 D3.2.11—Sex determination in humans and inheritance of genes on sex chromosomes Students should understand that the sex chromosome in sperm determines whether a zygote develops certain male-typical or female-typical physical characteristics and that far more genes are carried by the X chromosome than the Y chromosome. D3.2.12—Haemophilia as an example of a sex-linked genetic disorder Show alleles carried on X chromosomes as superscript letters on an uppercase X. D3.2.13—Pedigree charts to deduce patterns of inheritance of genetic disorders Students should understand the genetic basis for the prohibition of marriage between close relatives in many societies. D3.2.14—Continuous variation due to polygenic inheritance and/or environmental factors Use skin colour in humans as an example. Application of skills: understand the distinction between continuous variables such as skin colour and discrete variables such as ABO blood group. They should also be able to apply measures of central tendency such as mean, median and mode. D3.2.15—Box-and-whisker plots to represent data for a continuous variable such as student height Application of skills: Students should use a box-and-whisker plot to display six aspects of data: outliers, minimum, first quartile, median, third quartile and maximum. A data point is categorized as an outlier if it is more than 1.5 × IQR (interquartile range) above the third quartile or below the first quartile. Additional higher level D3.2.16—Segregation and independent assortment of unlinked genes in meiosis Students should understand the link between the movements of chromosomes in meiosis and the outcome of dihybrid crosses involving pairs of unlinked genes. D3.2.17—Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes Students should understand how the 9:3:3:1 and 1:1:1:1 ratios are derived. D3.2.18—Loci of human genes and their polypeptide products Application of skills: Explore genes and their polypeptide products in databases. They Should find pairs of genes with loci on different chromosomes and also in close proximity on the same chromosome.  43 D3.2.19—Autosomal gene linkage In crosses involving linkage, the symbols used to denote alleles should be shown alongside vertical lines representing homologous chromosomes. Understand the reason that alleles of linked genes can fail to assort independently. D3.2.20—Recombinants in crosses involving two linked or unlinked genes Students should understand how to determine the outcomes of crosses between an individual heterozygous for both genes and an individual homozygous recessive for both genes. Identify recombinants in gametes, in genotypes of offspring and in phenotypes of offspring. D3.2.21—Use of a chi-squared test on data from dihybrid crosses Students should understand the concept of statistical significance, the p = 0.05 level, null/alternative hypothesis and the idea of observed versus expected results.