Cell division revision notes for EDUQAS A level biology

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each pair, comes from each. parent. > Humans, therefore have 23 pairs of homologous chromosomes, in two sets and are described. as diploid with the symbol 2n (human chromosome number: 2n=46). > Diploid definition: Having two complete sets. chromosomes. > Plady level is the number of complete sets of chromosomes in an organisms. > Organisms with more than 2 complete set. of chromosomes are described as polyploid > Human gametes are haploid as they have. 1 set of chromosomes. Whereas human body cells are diploid as they contain 2 sets of chromosomes. (a): interphase and the main stages of mitosis. > Mitosis produces two daughter cells that are genetically identical to the parent cell and to each other. ) Dividing cells undergo a regular pattern of events know as the cell cycle. This is the sequence of events that take place between one cell division and the next.. > This is a continuous process but includes: ・Interphase, a period of synthesis and growth • Mitosis, a type of cell division that produces. two daughter cells, which have the same number of chromosomes and are genetically. identical with each other and parent cell (Includes prophase, metaphase, anaphase, telophose). • Cytokinesis, the division of the cytoplasm to form two daughter cells. " Interphase > Interphase is the longest phase of the cell cycle, with a lot of metabolic activity. > The newly formed cell grows and its organelles replicate, replacing those lost in the previous division. -Prophose Sphase Metaphase mitosis A-Anaphase T-Telophose cytokinesis > The DNA replicates so its quantity. doubles. The part of interphase in which DWA is replicated is known. as the Sphase (short for synthesis) > Proteins, such as histones and enzymes, are synthesised during interphase, requiring energy from ATP. > Chromosomes not visible because the nuclear material, chromahin, is dispersed throughout the nucleus. Idirection Interfhose Stages in Mitosis 1. Prophase : . • The longest of the four stages of mitosis •The chromosomes condense. They coil, getting shorter and thicker and become visible as long thin threads, They become distinguishable as pairs of chromatids. • Centrioles are present in animal cells. The pairs seperate and move to the opposite ends (poles) of the cell, organising a partner as they move. Protein microtubules form, radiating from each centriale, making the spindle. Spindle fibres extend from pole to pole and from pole to the centromere of each chromosome. • Towards the end of prophase, the nuclear envelope disintegrates and the nucleows disappears. Pairs of chromatids can be seen clearly, lying free. in the cytoplasm. 2. Metaphase • Chromosomes attach to the spindle fibres at their centrometes and line up along the equator of the spindle Shortening of the spindle fibres drows the individual chromatids slightly apart. 3. Anaphase • The centromeres divide into two seperating the sister chromatics in each chromosome The spindle fibres shorten, pulling the two. sister chromatids apart in opposite directions.. Each sister chromatid reaches opposite pales of the cell. 4 Telophase •The chromatids have reached the poles of the cells. and are referred to as chromatids again. •The chromosomes uncoil and lengthen The spindle fibres break down. The nuclear envelope re-forms. The nucleolus reappears. Cytokinesis The division of the nucleus by mitosis is followed. by cytokinesis, the division of the cytoplasm to make two cells. > In animal cells, cytokinesis occurs by the constriction of the parent cell around the equator from the outside inwards. > In plant cells, droplets of cell wall material, a all plate, form across the equater of the parent cell from the centre outwards and they extend and join to form new cell wall. the Annotated Diagram of Mitosis STAGES OF MITOSIS PROPHASE Nucleolus Nuclear envelope META PHASE up! Chromosomes line across the equator of I the spindle attached by their centromeres to the spindle centromere ANAPHASE re- Cenbrioles TELOPHASE nuclear envelope Centrioles nucleolus fre-forming Remains of spindle breaking down < XXX cleavage furrow JE > > centrioles moving to poles of cell ←cell membrane •Pair of chromatids Each centricte reaches a pole and! they organise the production of the spinde microtubules. -spindle Chromatids move to the opposite poles; centromeres first, pulled by the micro- tubules L cell membrane. chromatids (which will reach the poles of the spindle The differences between mitosis in animal cells! and plant cells! ceus Plant Cells Shape Cells become rounded betere mites is No shape change Centrioles Present Absent Cytokinesis Clevage furrow Animal Spindle Occurence Cell plate "develops from outside inwards develops from centre ahundr Degenerates at belophase Remains in new cell wall In epithelia, bone marrow, others in meristems. (b): the significance of mitosis as a process in which daughter cells are provided with identical copies of genes and the process of cytokinesis The Significance of Mitosis > Chromosome number: Mitosis produces two daughter cells that have the same number of chromosomes as the parent all and each other and each chromosome in the daughter cell is an exact replica of those in the parent cell. So mitosis produces cells that are genetically. identical to the parent, giving genetic stability. > Growth: Mitosis allows for replacement of dead cells, repair of tissues, asexual reproduction, growth in animals. and an increase in cell number in plants. > Asexual Reproduction Asexual reproduction produces complete offspring that are genetically identical to the parent. It takes place in unicellular organisms, (such as yeast + bacteria) and in some incests, such as greenfly. (c): the significance of mitosis in terms of damage and disease, repeated cell renewal, damage repair. and healing and unrestriched division leading to cancerous growth The length of the cell cycle is controlled by genes which ensure that mitosis happens where and when it is needed. This allows for the timely replacement and repair of tissues in adults and for correct development. in ensbryos YIf the genes that control the cell cycle are damaged, cells may fail to divide, may divide too frequently or at the wrong time. Radiation, certain chemicals and some viruses can mutate DNA, and DNA sequencing has identified specific gene nuutations, that affect the timing of the cell. cycle > Genes control the cell cycle by acting as a brake, preventing the cell cycle from repeating continually. If such genes are mutated, the brake is damaged and the all may go. immediately from one round of mitosis to the next and alls will replicate too fast. If this happens in solid tissue, a tumour forms. These genes are called tumour suppressor genes because they prevent rapid replication, which would lead to turnour formation. cause cancer, > Therefore, some genes have the potential to if they become mutated or the cell is infected with a vives. Before they are altered, when they don't. cause cancer, they are called proto-oncogens. But once altered and able to cause cancer, they are called gene with potential to cause cancer. (d): the main stages of meiosis and cytokinesis oncogenes YOncogenes A > Meiosis takes place in the reproductive organs of plants, animals and some protochistans, prior to sexual reproduction 7 Meiosis: A two-stage cell division in sexually reprod- ucing organisms that produces four genetically distinct daughter cells, each of which have half the number of chromosomes of the parent cell. > In meiosis, the diploid number of chromosomes is hachred to haploid. When two haploid gametes fuse at fertilisation, the zygote that is formed has two complete sets of chromosomes, one from each gamete, restoring the diploid condition. If the chromosome did not halve during gamete formation, the number of chromosome would double every generation. Meiosis has two divisions 88 Parent cell with one pair homologors Meiosis 1: homologous chromosomes seperate Meiosis 11: chromatids seperate chromosomes. > DNA replication in interphase, is followed by two divisions, called meiosis I and meiosis 11. Each division goes through the same sequence of steps. as mitosis, ie prophase, metaphase, anaphase and telophase. The names of the stages are followed by 1 or 11, to distinguish the two divisions, because the chromosomes. behave differently. > But between the two divisions, there is no more DNA. replication. That only happens once, before meiosis 1. > By the end of meiosis I, the homologous pairs of chromosomes have seperated, with one chromosome of each pair going into either of the two daughter. cells. > Each daughter celly has only one of each homologous pair, so they contain half the number of chromosomes of the parent nucleus. > In meiosis 11, chromatids seperate and the two new. haploid nuclei divide again. > Four haploid nuclei are formed from the parent. nudeus, each containing half the number of chromosomes and every gamere is genetically unique. Meiosis L 1. Prophase Il Paternal and maternal chromosomes came together in homologous pairs. This pairing of chromosomes is called synapsis, and each homologous chromosome is bivalent. Bivalent = the association of the two chromosomes of a homologous pair at prophase 1 of meiosis. The chromosomes coil up, condensing to be come shorter. and thicker, visible as two chromatics. The centriales. seperate and move to the poles of cells. They organise the polymerisation of microtubules, which radiate from them, and the spindle forms. . •Crossing over; Prophase I differs from prophase in mitosis as the homologous chromosomes associate in their pairs, the bivalents. The chromatids wrap around. each other and then partially repel each other but remain joined at points called chiasmara. At a chiasmatay a segment of DNA from one chromated may be exchanged with the equivalent part from a chromatic of the homologous chromosome. This is crossing over and is a source of genetic variation, as it mixes genes. from the two parents in one chromosome. This genetic recombination produces new combination of alleles and a single crossover in meiosis I results in four haplard gameres having a different genetic composition. • By the end of prophase, the nuclear envelope has disintegrated and the nucleows has disappeared. 2. Metaphase I • Pairs of homologous chromosomes arrange at the equator of the spindle. •Independent Assortment - In a homologous pair, one chromosome is from the mother and the other is from the father. They lie at the equator randomly, with either one facing either pole. So a combination of paternal and maternal chromosomes faces each pole. and the combination of chromosomes that goes into the each daughter cell at meiosis I is random with respect to which parent they came from. • Independent Assortment Definition : either of a pais of homologous chromosomes moves to either pole at anaphase I of meiosis, independently of the chromosomes of other hamologous pairs. It producer. new genetic combinations. When independent assortment happens with 2 pairs. of chromosomes, there are 2²=4 possible combinations of Maternal and parental chromosomes in gametes: Mausis OR themselves Meias's I 3. Anaphase I •The chromosomes in each bivalent seperate and, as the spindle fibres shorten, one of each pair is pulled. to one pole, and the other to the opposite pale. Each pole recieves only one of each homologous pair of chromosomes and, because of their random arrangement. at Metaphase, there is a random mixture of maternal. and parental chromosomes. 4. Telophase l In some species, the nuclear envelope reforms around. the haploid group of chromosomes and the chromosome decondence and are no longer visible. •But in many species, the chromosomes stay in their condensed form. 5. Cytokinesis The division of the cytoplasm, making 2 haploid cells. Meiosis 11 1. Prophase ll • The centribles seperate and organise a at right angles to the old spindle. new spindle 2. Metaphase 11 •The chromosomes line up on the equator, with each spindle fibre by its attached to a chromosome centromeve. •Independent assortment happens as the chromatids can face Rither pole. 3. Anaphase 11 •The centromeres divide and the spindle fibres shorter, pulling the chromatids to opposite poles.. 4. Telophase 11 •At the poles, the chromatids lengthen and can no. be distinguished in the microscope. • The spindle disintegrates and the nuclear envelope and nucoli re-form 5. Cytokinesis -Cytokinesis takes place, producing four haploid daughter cells. The significance of meiosis > Meiosis keeps the chromosome number constant from one generation to the next.. > Meiasis produces greater genetic variation in the gametos. due to crossing over in prophase I and independent. assortment in metaphase I and 11 The differences between meiosis I and meiosis || Phase Process Meiasis I Meiasis 11 X Prophase follows DNA replication crossing over ✓ hondlogous pairs, either side of equater chromosomes on equator independent assortment Chomologous chromosome) ✓ (chromatics) Seperation at anaphese chromosomes. 2 chromatid, 4 haploid haploid metaphare aligiument at equaltor Anaphase number of daughter alls Plody of daughter cells! (e): differences between mitosis and Mitosis 1 Generic crossing over Independent assortment. Generic composition 2 Number of divisions Number of doughter cells. Chromosome no in doughter cells Same as parent cell Half of parent all Ploidy of daughter alls of diploid parent. Chiasmata Piploid Absent X melosis None None Meiosis 2 4 Haploid Present In prophase In Mekuphase (and/ Genetically identical with parent cell and each other, Genetically different