D2.1 cell and Nuclear Division

• Cell division in animal cells: a cleavage furrow forms when actin and myosin proteins form a contractile ring at the equator of the cell just under the plasma membrane. When the proteins contract, they pull the plasma membrane towards the centre, eventually separating the cell into two daughter cells.
• Cell division in plant cells: a cell plate forms at the equator, formed from vesicles carrying carbohydrates, lipids, and proteins fusing together to create the two plasma membranes. Afterwards, other vesicles that carry pectin and cellulose deposit these substances by exocytosis in the gap between the two new membranes leading to the formation of new cell walls, separating the new daughter cells
• The division of the cytoplasm in cytokinesis is usually equal, but there are cases where the division of the cytoplasm is unequal, such as oogenesis in humans and budding in yeast.
• Mitosis results in genetically identical cells, and maintains the chromosome number and genome of cells
• Mitosis is used for growth, repair of damaged tissues, replacement of cells, and asexual reproduction.
• Meiosis results in cells that are genetically different from each other, which can help to maintain genetic diversity, which is essential for evolution to take place.
• Meiosis is the type of cell division used to produce haploid gametes (sex cells) by reducing the number of chromosomes by half.
• DNA replication is a prerequisite for mitosis and meiosis, which occurs during the S phase of interphase in order to create 2 identical DNA molecules (aka sister chromatids) that are held together at the centromere. Together, they form a chromosome.
• During anaphase, one chromatid from each chromosome ends up in one daughter cell, while one chromatid ends up in the other.
• WHen the centromere is split at the start of anaphase, the chromatids are referred to as chromosomes.
• Prior to mitosis, the DNA molecules are loosely coiled to form a complex called chromatin.
• Histone proteins supercoil DNA and form them into structures called nucleosomes (made up of DNA molecules wrapped around 8 histone proteins.)
• In between the nucleosomes, histone H1 and linker DNA are present to facilitate further packaging of nucleosomes to form the chromosomes.
• During prophase, the chromatin gets condensed by supercoiling to form chromosomes.
• Microtubules and microtubule motors are responsible for the movement of chromosomes during cell division.
• Microtubules are tubulin fibers that form part of the cytoskeleton of the cell, which are able to lengthen and shorten in order to enable chromosome movement.
• α-tubulin and β-tubulin form dimers which can be added or removed at the ends of the microtubules to change the length of the tubule. Chromosome movement is facilitated by motor proteins.
• Interphase includes: i) G1: Normal metabolism including protein synthesis, active transport, respiration, etc. ii)S: DNA replication; iii)G2: Normal metabolism + synthesis of organelles/cytoplasm/membrane
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  Prophase: chromatin fibers condense by supercoiling, becoming visible as chromosomes. The chromosomes appear as two identical sister chromatids joined at the centromere. The centrioles move to opposite poles, and the spindle (made of microtubules) beings to form in the cytoplasm. The nuclear membrane breaks.
• Metaphase: the chromosomes move to the equator, and spindle microtubules attach to chromosome centromeres.
• Anaphase: centromeres split as spindle microtubules pull chromatids to opposite poles (post centromere split, the sister chromatids are referred to as sister chromosomes). Sister chromosomes move to opposite poles as microtubules shorten.
• Telophase: Sister chromosomes have arrived at the poles, and the spindle disappears. The nuclear membrane reforms and chromosomes decondense, becoming chromatin.
• Meiosis I: separation of homologous chromosomes (cells go from diploid to haploid)
• Meiosis II: separation of sister chromatids- cells remain haploid.
• Prophase I (Meiosis): the cell has 2n chromosomes (n being the haploid number of chromosomes). There are double chromatids. Crossing over occurs at the chiasmata, which is an exchange of genetic material between non-sister chromatids of homologous chromosomes through breaking and rejoining DNA. This helps to provide new combinations of alleles to increase genetic variety.
• Metaphase I (Meiosis): Spindle microtubules move homologous pairs to the equator of the cell. The orientation of paternal and maternal chromosomes on either side of the equator is random and independent of other homologous pairs. The different position impacts the offspring's genotype. The number of different combinations is 2^n, where n is the haploid number.
• Anaphase I (Meiosis): Homologous pairs are separated. One chromosome of each pair moves to each pole, and the centromere remains intact.
• Telophase I (Meiosis): Chromosomes uncoil. During the interphase that follows, no replication occurs. The reduction of the chromosome number from diploid to haploid is completed. Cytokinesis occurs.
• Prophase II (Meiosis): Chromosomes, which at this point still consist of two chromatids, condense and become visible.
• Sexual reproduction introduces genetic variation firstly through meiosis: there is crossing over, which allows linked genes to be reshuffled to create new combinations called recombinants, and random orientation. The other way is through fertilization and genetic variation: the fusion of gametes is random, which can provide a new and unique combination of alleles.