IAS Biology Topic 3B: Mitosis, Meiosis & Reproduction 3B.1 PDF

IAS Biology Topic 3B: Mitosis, Meiosis & Reproduction 3B.1 The Cell Cycle What is a Cell Cycle? The cell cycle describes a sequence of reactions that results in the growth of the cell and replication of the genetic material to make...

IAS Biology Topic 3B: Mitosis, Meiosis & Reproduction 3B.1 The Cell Cycle What is a Cell Cycle? The cell cycle describes a sequence of reactions that results in the growth of the cell and replication of the genetic material to make two identical daughter cells through mitosis. The length of the cell cycle is very variable depending on environmental conditions, the cell type and the organism. For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly). Phases of Cell Cycle The cell cycle has three phases: (i) Interphase (ii) Nuclear division (mitosis) (iii) Cell division (cytokinesis) Interphase:  During Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis).  Interphase consists of three phases: o G1 phase o S phase o G2 phase  It is at some point during the G1 phase that a signal is received telling the cell to divide again.  The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids).  This phase of the interphase stage of the cell cycle is called the S phase – S stands for synthesis (of DNA). The S phase is relatively short.  The gap between the previous cell division and the S phase is called the G1 phase – G stands for gap. Cells make the RNA, enzymes and other proteins required for growth during the G1 phase.  Between the S phase and next cell division event the G2 phase occurs. Page 1 of 5  o During the G2 phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired. o Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle). Nuclear Division (Mitosis)  Follows interphase.  Referred to as the M phase – M stands for mitosis.  Cell growth stops during the M phase.  During mitosis, the two identical sister chromatids of each chromosome separates from each other and move to opposite poles of the cell.  This ensures that each new nucleus that forms will contain the exact same genetic information as the original nucleus. Cytokinesis  Cytokinesis is the division of cytoplasm in eukaryotic cells after nuclear division (eg. Mitosis)  Follows M phase.  Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells.  In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed.  In plant cells, the remaining spindle fibres guide Golgi vesicles to the equator of the cell. The vesicles enlarge and fuse together forming a cell plate. The basic structure of the cell wall forms within each vesicle and the vesicle fuse to join the cell wall. Small gaps left between the vesicles form plasmodesmata. Page 2 of 5 Checkpoints in Cell Cycle (Cyclins) The movement from one phase to another is triggered by chemical signals called cyclins. A checkpoint is a stage in the eukaryotic cell cycle at which the cell examines internal and external cues and "decides" whether or not to move forward with division. There are a number of checkpoints, but the three most important ones are:  The G1 checkpoint, at the G1/S transition.  The G2 checkpoint, at the G2/M transition.  The spindle checkpoint, at the transition from metaphase to anaphase. The chemical substances which control the cell cycle are small proteins called cyclins. These build up attach to enzymes called cyclin-dependent kinases (CDKs). The Cyclin-CDK complex that forms phosphorylates other proteins, changing their shape and bringing about the next stage in the cell cycle. Variation in the Amount of DNA and Cell Volume during Cell Cycle Cytokinesis The amount of DNA doubles during S phase of cell cycle due to DNA replication and halves to return to original amount after cytokinesis due to cell division. The cell volume or mass increases to double steadily until cytokinesis. After cytokinesis, the cell divides into two, hence the cell volume returns to original volume. Page 3 of 5 Structure of Chrmosome Chromosomes are responsible for the transmission of the hereditary information for one generation to the next. They contain DNA coiled on positively charged basic protein called histone. The dense clusters formed are called nucleosome. Between divisions of the nucleus each chromosome contains one DNA molecule. Before the nucleus divides a copy of this DNA molecule is made. Therefore, at nuclear division, the chromosome is a double structure, containing two identical DNA structure. The two parts of the chromosome are referred to as chromatids. Each chromatid of a pair contains one of the two identical DNA molecules. The chromatids are held together at a point called the centromere which may occur anywhere at the length of the chromosome. Individual chromosomes cannot be seen very clearly in the period between divisions, known as interphase. This is because the chromosomes become very loosely coiled, long, thin threads spread throughout the nucleus. This material is called chromatin. Chromosomes Chromatin Tightly Packed DNA Unwound DNA Found only during Cell Division Found throughout Interphase DNA is not being used for macromolecule DNA is being used for macromolecule synthesis synthesis Page 4 of 5 Karyotype & Karyogram Karyotype is the number and type of chromosomes present in the nucleus. Karyogram is a photograph or diagram of the karyotype where chromosomes are paired up according to similar shape and size. Page 5 of 5  IAS Biology Topic 3B: Mitosis, Meiosis & Reproduction 3B.2 Mitosis  Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)  Although mitosis is, in reality, one continuous process, it can be divided into four main stages  These stages are: o Prophase o Metaphase o Anaphase o Telophase Prophase  Chromosomes condense and are now visible when stained.  The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere.  The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus). A centrosome is an organelle that consists of two centrioles.  Spindle fibres (protein microtubules) begin to emerge from the centrosomes (which consist of two centrioles in animal cells).  The nuclear envelope (nuclear membrane) breaks down into small vesicles. Page 1 of 4 Metaphase  Centrosomes reach opposite poles.  Spindle fibres (protein microtubules) continue to extend from centrosomes.  Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles.  Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres.  Each sister chromatid is attached to a spindle fibre originating from opposite poles. Anaphase  The sister chromatids separate at the centromere (the centromere divides in two).  Spindle fibres (protein microtubules) begin to shorten.  The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules). Telophase  Chromosomes arrive at opposite poles and begin to decondense.  Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes.  The spindle fibres break down. Page 2 of 4 Importance of Mitosis  Mitosis is how organisms grow and replace old cells/repair tissues (NOT repair cells).  It is also the method which organisms use for asexual reproduction.  It results in genetically identical individuals or clones. Mitotic Index The mitotic index is a measure of the proportion of dividing cells in a cell population. The mitotic index may increase during processes that promote division, such as normal growth or cellular repair. The mitotic index is the ratio between the number of cells in mitosis and the total number of cells. It can be determined by analysing micrographs and counting the relative number of mitotic cells versus non-dividing cells. Applications of Mitotic Index  We can use the mitotic index to identify actively dividing tissues (such as hair cells), including cancerous tissue.  Cancerous tissue is dividing more rapidly than it should.  We can also use the mitotic index to measure the effectiveness of treatments for cancer. If the treatment is working, the mitotic index of the tumour will fall. Page 3 of 4 Observing Mitosis Using Root Tip Squash The flow chart shows how garlic root tips can be treated to observe mitosis in the root cells. Heating the root tip in hydrochloric acid softens the cell walls and hydrolyses pectates holding cells together. This helps to produce a thin layer of cells upon squashing. Acetic Orcein is used to stain chromosomes. Heating with the stain helps to intesify the stain. The root tips were teased using mounted needles to separate the cells. Squashing will produce monolayer/ thin layer of cells You should always first focus a microscope using the low- power objective lens. There are two reasons for this. 1. Firstly, you are less likely to damage a lens by racking it down into the slide – the low-power objective does not reach the slide. 2. Secondly, the field of view using a low-power objective lens is much larger than when using a high- power objective lens. Consequently, using low power, it is easier to find a group of cells that show what you are looking for; in this case cells dividing by mitosis. Page 4 of 4  IAS Biology Topic 3B: Mitosis, Meiosis & Reproduction 3B.3 Sexual Reproduction & Meiosis Difference between Sexual & Asexual Reproduction Asexual Reproduction Sexual Reproduction Involves single organism Involves one or two organism No production of gametes Male and female gametes are produced There is no fusion of gametes It involves fusion of male and female gametes It requires only mitotic divisions It requires meiotic division followed by mitotic division It produces offsprings that are identical to the Offsprings will have somem characters from parent male parent and others from female In this chance of genetic variation is only In this reproduction there is more chance for through random mutation genetic variation due to crossing over, independent assortment and random fertilisation It is not very useful for natural selection in It is highly useful for natural selection in evolution of species evolution of species It occurs by budding, fragmentation, sporulation It occurs due to pollination and fertilisation Gametes Gametes are the “sex” cells and contain only ½ the normal number of chromosomes, called the “Haploid” number (the symbol is n). Ex – Sperm cells and ova are gametes. Meiosis Meio means to reduce. During Meiosis diploid cells are reduced to haploid cells. Meiosis is the process by which “gametes” (sex cells), with half the number of chromosomes, are produced. Meiosis gives rise to cells that are genetically different from each other. During meiosis, the nucleus of the original 'parent' cell undergoes two rounds of division with only one duplication of chromosomes. The two round of divisions are:  Meiosis I (the chromosome number halves (from 2n to n) in the first division of meiosis (meiosis I))  Meiosis II (similar to mitosis) Page 1 of 3 Genetic Variation due to Meiosis In meiosis genetic variation may occur due to:  Crossing over  Independent assortment/ random assortment Crossing Over It occurs in prophase I of meiosis. It is defined as the exchange of chromosome segments between non-sister chromatids in meiosis. It creates new combinations of genes in the gametes that are not found in either parents which can contribute to increased diversity. Page 2 of 3 The pairing of the two chromosomes together creates a tetrad or bivalent. The point where the chromosomes attach is called the chiasmata. The end result is two recombinant chromosomes which have a different combination of alleles than either parent. Independent Assortment Homologous pairs of chromosomes orient randomly at metaphase I of meiosis. In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independent of the other pairs. Importance of Meiosis  Genetic Variation: Meiosis brings about genetic variation through crossing over and random assortment.  Haploid cells: Meiosis results in the formation of haploid gametes, which upon fertilisation restores diploid number of chromosomes in zygote. Page 3 of 3  IAS Biology Topic 3B: Mitosis, Meiosis & Reproduction 3B.4 Gametes: Structure & Function Gametogenesis The production of gametes in the gonads is called Gametogenesis. Spermatogenesis = formation of sperm in testis. Oogenesis = formation of ova in the ovary. The cells of the germinal epithelium in both the testis and the ovary undergo a series of mitotic and meiotic divisions to form haploid gametes. The cells which undergo meiosis are called mother cells. Sperm Mother Cells are known as Spermatocytes. Egg Mother Cells are known as Oocytes. It is important the gametes are haploid so that the diploid number is restored on fertilisation. Spermatogenesis  The production of sperm takes place in the testes in males from puberty onwards.  The testes contain many small tubes, or tubules, known as seminiferous tubules.  Spermatogenesis begins in the germinal epithelium, a layer of cells that makes up the outer layer of the seminiferous tubules.  Cells in the germinal epithelium divide by mitosis, producing diploid cells called spermatogonia (singular spermatogonium).  Of the two daughter spermatogonia cells, one will go on to eventually become a sperm cell, while the other remains in the germinal epithelium where it can continue development.  Spermatogonia differentiate into immature sperm cells called primary spermatocytes.  Primary spermatocytes mature and divide by meiosis.  Meiosis I forms secondary spermatocytes.  Meiosis II forms spermatids. Page 1 of 5   Spermatids formed during meiosis remain associated with the Sertoli cells as they mature into sperm cells, also known as spermatozoa.  Once fully matured, the sperm cells detach from the Sertoli cells and move along the seminiferous tubule lumen towards the sperm duct. Adaptation of Sperm 1. The head contains the haploid nucleus so that diploid chromosome number can be restored upon fertilisation. 2. Have a flagellum (tail) that allows them to swim towards the egg cell 3. Contain many mitochondria that provide energy through aerobic respiration for movement of the flagellum (swimming) 4. An acrosome that contains digestive enzymes to break down the protective glycoprotein layer in the zona pellucida surrounding the egg cell - sperm cells must penetrate this layer in order to fertilise the egg. 5. It has a stream lined body that allows it to move quickly. Oogenesis  The production of ova begins in the ovaries of the female foetus before birth.  The ovaries are surrounded by an outer layer of cells called the germinal epithelium; the cells in this layer divide by mitosis throughout the first 7 months of foetal development to form diploid cells called oogonia (singular oogonium).  This process of oogonia formation stops after 7 months, by which time several million oogonia have been produced. These are all the oogonia that the ovaries will produce in the Page 2 of 5  female’s lifetime. Although the ovaries contain several million primary follicles at birth, many of them degrade throughout a woman’s life and never reach maturity.  During the few months leading up to birth, the oogonia in the foetus’ ovaries grow in size and enter meiosis I, and a layer of cells called follicle cells develop around them. The partially divided oogonia together with their layer of follicle cells are known as primary follicles.  Once the oogonia have developed into primary follicles the oogenesis process pauses until the start of puberty.  When puberty begins, the hormone FSH stimulates the continued development of several primary follicles in the ovary. Only one of the stimulated follicles will reach maturity.  Meiosis I continues and the primary follicle divides to form two new cells.  The division of cytoplasm is not equal and the result of meiosis I is a secondary oocyte along with a very small cell called a polar body. The polar body cell has very little cytoplasm and does not mature further.  The secondary oocyte formed at the end of meiosis I enters meiosis II; at this point, it leaves the ovary, together with its layer of follicle cells, in the process of ovulation.The remains of the follicle that are left behind in the ovary develop later into the corpus luteum.  The secondary oocyte doesn’t finish meiosis II until after a sperm cell enters it, at which point meiosis II finishes just before the nuclei fuse.  The secondary oocyte becomes an ovum very briefly between the end of meiosis II and the fusion of the two nuclei.  A second polar body is produced at the completion of meiosis II. Page 3 of 5 Adaptation of Egg 1. Egg contains the haploid nucleus so that diploid chromosome number can be restored upon fertilisation. 2. Are much larger than sperm cells as most of their internal space contains food to nourish a growing embryo. 3. Have follicle cells that form a protective coating. 4. Have a jelly-like glycoprotein layer, known as the zona pellucida, which forms an impenetrable barrier after fertilisation by a sperm cell has occurred, to prevent other sperm nuclei from entering the egg. Gametogenesis in Plants Most flowering plants are hermaphrodites. They possess both male and female reproductive structures. The female structure is the carpel and it consists of a stigma, style and ovary. The production of female gametes takes place in the ovary. The male structure is the stamen and it consists of an anther and a filament. Pollen grains which contain the male gametes are produced in the anther. Page 4 of 5  Formation of female gametes: o The ovary contains one or more ovules o Within the ovule there is a diploid megaspore mother cell which divides by meiosis to produce four megaspore cells (three of which disintegrate) o One of the megaspores undergoes mitosis three times to produce an embryo sac cell with eight haploid nuclei o One of these nuclei is the female gamete, two are polar nuclei, two are synergid cells and two are antipodal cells  Formation of male gametes: o The diploid microspore mother cells found in the anthers undergo meiosis to produce four haploid microspores o The nuclei of these haploid microspores divide by mitosis once to form pollen grains, but cytokinesis does not occur. These cells mature into pollen grains o Pollen grain cells have two nuclei: a pollen tube nucleus and a generative nucleus o The generative nucleus is the male gamete Page 5 of 5  IAS Biology Topic 3B: Mitosis, Meiosis & Reproduction 3B.5 Fertilisation in Mammals & Plants Transfer of Male Gamete to Female Gamete Pollination (In Plants): The transfer of pollen from anther to stigma is called pollination. Plants rely on the wind or insects to carry their pollen from plant to plant. Mating (In Mammals): Mating is the process by which a male animal transfers sperm from his body directly into the body of the female. Fertilisation It is the fusion of male gamete nucleus with female gamete nucleus. Fertilisation can be of two types: (i) External Fertilisation External fertilisation occurs outside the body, with the female and male gametes discharged directly in to the environment where they meet and fuse. External fertilisation is usually seen in aquatic species, because spermatozoa and ova are very vulnerable to drying and are rapidly destroyed in the air. (ii) Internal Fertilisation Internal fertilisation involves the transfer of the male gametes directly to the female. This does not guarantee fertilisation, but makes it much more likely. Fertilisation in mammals is always internal fertilization. Page 1 of 3 Fertilisation in Plants Plant fertilisation occurs as follows: (i) Pollen grain composed of the pollen tube nuclei and the generative nucleus adheres to the stigma, where it subsequently germinates to produce a pollen tube. (ii) The pollen tube grows down the stigma through the style to reach the ovule. The pollen tube nucleus secretes digestive enzyme which digest the surrounding tissue in style and use it as a source of nutrients. (iii) The pollen tube grows through a gap between the integuments, known as the micropyle, into the embryo sac. (iv) The generative nucleus of the pollen divides to produce two male nuclei which enter the embryo sac. (v) One of the male gametes fuses with the female nucleus to form a diploid zygote. (vi) The other male gamete fuses with two polar nuclei to form a triploid endosperm nucleus which serves as a source of nutrients for the embryo. (vii) The fertilised ovule divides by mitosis to form the embryo consisting of the developing shoot known as the plumule, developing root known as the radicle and one or two cotyledons. The integuments become the seed coat, the ovule becomes the seed and ovary becomes the fruit. Page 2 of 3 Fertilisation in Mammals (i) As sperms move through the female reproductive tract, the acrosome region matures so it can release enzymes and penetrate the ovum. (ii) Many sperm cluster around the ovum and, as soon as the heads of the sperm touch the surface of the ovum, the acrosome reaction is triggered. (iii) Acrosome Reaction: The acrosome in the head of a sperm cell releases enzymes that digest the protective layer around the ovum known as zona pellucida. This allows the sperm to pass through the egg cell membrane. This process is known as the acrosome reaction. (iv) Once the sperm cell nucleus enters the egg cell, the second meiotic division in the ovum takes place providing a haploid ovum nucleus to fuse with the haploid male nucleus to form a zygote. (v) Following fertilisation, ion channels in the cell membrane of the ovum open and close so that the inside of the cell, instead of being electrically negative with respect to the outside, becomes positive. This alteration in charge blocks the entry of any further sperm to prevent polyploidy. It is a temporary measure until cortical reation takes place. (vi) Cortical Reaction: Once the sperm cell nucleus enters the egg cell, the egg cell immediately releases the contents of cortical granules into the zona pellucida by exocytosis. The enzymes released from the cortical granules destroy the sperm-binding sites on the egg cell membrane and cause the zona pellucida to rapidly thicken and harden, forming fertilisation membrane. The fertilisation membrane prevents any more sperm cells from entering, ensuring only one sperm cell can fertilise the egg cell to prevent polyploidy. This process is known as the cortical reaction. What is the advantage of ejaculating many sperms? One sperm alone does not produce enough enzyme to penetrate the protective layers around the ovum. The very large number of sperm released in ejaculation, provides enough acrosome in the oviduct to surround the ovum and digest its zona pellucida. Page 3 of 3