Mitosis and Meiosis PDF

30/09/23 Mitosis and Meiosis: Learning objectives: de ne the processes of mitosis and meiosis. Learning objectives: understand the key specialisations of sperm and oocytes. Learning objectives: describe how genetic variation arises from parental chromosomes, sister chromatids exchange, chromosomal s...

30/09/23 Mitosis and Meiosis: Learning objectives: de ne the processes of mitosis and meiosis. Learning objectives: understand the key specialisations of sperm and oocytes. Learning objectives: describe how genetic variation arises from parental chromosomes, sister chromatids exchange, chromosomal segregation and chiasma. Mitosis: Parent cell divides to generate t wo identical daughter cells. Daughter cells have same number of chromosomes as the parent. Human somatic cells are diploid. Mitotic cells containing mistakes in DNA can lead to disease if not removed or repaired. Mitotic cell division is highly regulation Some somatic tissues are postmitotic (non dividing) G0 is where is starts and cell is basically thinking do I need to divide. Something may come along and stimulate it. It moves into G1. However, it cannot move into s phase; it needs to pass the G1 checkpoint ( Rb and p53 halt cycle and if alls well inhibition removed by cyclin-dependent kinases ) The next step is DNA replication. The cell needs to ensure the DNA is replicated with high delity. Any errors will be removed or repaired by proteins and there is another checkpoint to ensure this has happened ( intra-S checkpoint ). The cell moves into G2 phase. Cells at this stage are still called diploid cells but each chromosome has been copied. There is another checkpoint ( G2 checkpoint ) to check all is normal. Then the cell enters mitosis, there is another checkpoint ( Met/Ana or spindle checkpoint). If a cell is stopped at a checkpoint it’s hard to resume. The cell becomes quiescent ( reversible ) or senescent which is stable cycle arrest. Another way to recover from damage is apoptosis. Over view of mitosis: Prometaphase is when the chromosomes attach to the spindles. Meiosis: Described as reductive division; number of chromosomes reduced. Meiosis 1: Following DNA replication each chromosome is comprised of t wo sister chromatids. Unique to gametogenesis Pairing (synapsis) of homologous chromosomes Crossing over and exchange of DNA ( recombination ) Reductive division- separation of homologous chromosomes Daughter cells contain 1 set of chromosomes Each chromosome comprised of a pair of sister chromatids Prophase 1: chromosomes that have been replicated condense and pair with homologues to form tetrads. Metaphase 1: Tetrads are held together by chiasmata. Chromosomes arrange themselves on the equator of the spindle. Anaphase 1: homologous chromosomes separate and migrate to opposite poles of the cell. Telophase 1: the chromosomes have formed t wo groups. The cell begins to constrict across the middle. Separates into t wo daughter cells. Meiosis II: Prophase II: the chromosomes of the t wo daughter cells condense again in preparation for a second division. Metaphase II: the chromosomes then migrate to the equator. Anaphase II: the newly separated chromosomes of the t wo daughter cells move to opposite poles of their spindle. Telophase II: the cells constrict across the nuclear membrane. Four haploid nuclei are formed, each one with one member of each pair of chromosomes from the original nucleus. Spermatogenesis: left diagram Oogenesis: right diagram Each chromosome must have at least one Crossing over for them to separate properly. Human oogenesis: partof ovary 1 telophase istayalignedin equator Spermatogenesis: