[BIO] Chapter 14 - Cell Division

© Hee Xin Wei (Copyrighted) Topic 14 : Cell Division

Chapter Analysis FOCUS EXAM WEIGHTAGE •Constitute to around 3% in Paper 2 in the past 5 years •commonly tested in MCQ and structured questions •content heavy to memorise the full cell cycle •identifying microscope images

mitosis meiosis Key Concept

Mitosis importance Mitosis •Mitosis is nuclear division that produces 2 daughter nuclei that is genetically identical to parent nucleus. •Daughter nuclei have the same number of chromosomes and same amount of DNA as parental nucleus Importance of mitosis •Producing genetically identical cells is important so the daughter cells will have all the genes necessary •Mitosis allows growth, repair and asexual reproduction that requires genetically identical cells. 1.Growth - Growth requires increase in number of new genetically identical cells within an organism, which is produced by mitosis for it to increase in size 2.Repair - New cells are produced to replace worn out cells that have been shed or to heal from wounds 3.Asexual reproduction - producing offspring that are genetically identical to the parents as well as to one another.

Mitosis Interphase (not part of mitosis) •Cell spends 95% of its time in this phase •in interphase, the cell prepares for cell division by absorbing nutrients to store energy and building up protoplasm and duplicating organelles •The DNA replicates, producing two identical chromatin threads joined at the centromere. •Total DNA content of the cell doubles •Chromosome number still remains 2n TIPS ✴DNA content is represented by number of sister chromatids ✴Chromosome number is represented by number of centromeres interphase

Mitosis spindle fibers centrioles Prophase •The chromatin condense, coil and shorten to become chromosomes, which are visible under the light microscope. •Each homologous chromosomes consists of two sister chromatids •Nucleolus disappears •The nuclear envelope disintegrates. Metaphase •The chromosomes line up along the metaphase plate or equatorial plate •Centromeres are attached to the spindle fibre Anaphase •Centromere divides •Aister chromatids splits, they are now called a daughter chromosome. Telophase •Nuclear envelope forms around the chromosomes at each pole of the cell. •Nucleolus reforms in each nucleus •The chromosomes in each daughter nucleus uncoil and lengthen to form chromatin threads. Cytokinesis •Cleavage furrows appear in the the cytoplasm between the two nuclei. The furrows deepen and two identical daughter cells are produced. •In plant cells, a cell plate is formed between the two daughter nuclei, dividing the cell into two. Cell plate is formed by the fusion of small fluid-filled vesicles produced by the Golgi apparatus.

Meiosis Meiosis •Meiosis is a reduction nuclear division that produces daughter nuclei containing half the number of chromosomes as the parent nucleus. •A parent nucleus undergoes meiosis to give 4 daughter nuclei (gametes) •Interphase also occurs prior to meiosis Gametes •Gametes are haploid cells which means it contains half the number of chromosomes as compared to normal body cell •For example, a human cell has diploid number of chromosomes, 23 pairs of chromosomes, 46 chromosomes, undergo meiosis to give 4 human gametes that each have 23 chromosomes. phase I parent cell: 2n chromsomes daughter cells: n chromosomes

Homologous Chromosomes •A pair of chromosomes of the same shape, same length, same centromere position and have the same genes with the same corresponding loci. •One chromosome is inherited from the maternal parent, while another one from the pair is inherited from paternal parent •Homologous chromsomes may contain different types of alleles human has 23 pairs of homologous chromosomes

Meiosis phase I Prophase I•The chromatin condense, coil and shorten to become chromosomes, which are visible under the light microscope. •Synapsis occurs where homologous chromosomes pair up •Crossing-over may occur where some DNA is exchanged at chiasma between homologous chromosomes •Nucleolus disappears •The nuclear envelope disintegrates. Metaphase I•The homologous chromosomes line up in pairs along the metaphase plate. •homologous chromsomes independently arrange themselves along the metaphase plate Anaphase I•Homologous chromosomes separate and move to opposite poles of the cell. Telophase I•Nuclear membranes form around the chromosomes at each pole of the cell. •The chromosomes uncoil and lengthen to form chromatin threads. Cytokinesis •Cleavage furrow forms in animal cell or cell plate in plant cell, dividing the cell. •the number of chromosomes is already halved at the end of meiosis 1

Meiosis phase II Prophase II •The chromatin condense, coil and shorten to become chromosomes, which are visible under the light microscope. •The nuclear envelope disintegrates. Metaphase II•The chromosomes line up along the metaphase plate or equatorial plate Anaphase II•Centromere divides •Spindle fibres shorten and separates the sister chromatids and pull to the opposite poles of the cell, forming sister chromosomes Telophase II•Nuclear envelope forms around the chromosomes at each pole of the cell. •Nucleolus reforms in each nucleus •The chromosomes in each daughter nucleus uncoil and lengthen to form chromatin threads. Cytokinesis •Cleavage furrows appear in the the cytoplasm between the two nuclei. The furrows deepen and two identical daughter cells are produced. The end product of meiosis is 4 daughter cells. •The daughter cells have haploid number of chromosomes and x/ 2 amount of DNA.