BY1-basic biochemistry and cell structure- cell division

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  • Cell Division
    • Chromosomes
      • Made up of DNA, protein and some RNA
        • DNA molecule made up of sections (genes)
      • DNA:  single strand in form of double helix running length of the chromosome
      • DNA molecule made up of sections (genes)
      • Chromosomes only become visible at beginning of cell division
      • Before C.D. DNA molecule makes a copy of its self (chromatids) The identical chromatids lie parallel except the joined, specialised centromere.
      • Chromosome number varies. Humans- 46, fruit fly- 8. Found in matching pairs (homologous pairs)
    • Mitosis
      • Two daughter cells identical to parent (diploid)  Genetic stability.
      • Cell cycle- IPMAT
        • Interphase
          • Longest part of cell cycle
          • Newly formed cell grows and produces organelles lost during previous CD.
          • Quantity of DNA is doubled during this time.
          • Just before next C.D. DNA replicates- forming chromatids joined at centriole.
          • Considerable metabolic activity- energy from ATP required.
          • Chromosomes not visible because chromosome material (chromatin) is dispersed throughout nucleus.
        • Prophase
          • Longest stage of mitosis
          • Chromosomes condense (become shorter and thicker). They become visible as long thin threads- now referred to as a pair of chromatids.
          • Animal cells have centrioles. The centrioles divide and move to opposite poles of the cell.
          • Protein microtubules form from each centriole and the spindle develops- it extends from pole to pole.
          • Towards end, nuclear envelope disintegrates, nucleolus disappears.
          • Pair of chromatids clearly visble lying free in the cytoplasm.
        • Metaphase
          • Chromosomes arrange themselves on equator of the spindle.
          • Then chromosomes become attatched to certain spindle fibres bythe centomere
          • Contraction of the spindle fibres draws the individual chromatids slightly apart.
        • Anaphase
          • Very rapid.
          • The centromere splits, the spindle fibers contract and pull the separated chromatids to the poles (centromere first)
        • Telophase
          • Final stage
          • Chromatids have reached poles of cell and are called chromosomes again. They uncoil and lengthen.
          • The spindle breaks down
          • Centrioles replicate
          • The nucleoli reappear and the nuclear envelope re-forms.
          • In animal cell: Cytokinesis occurs by the constriction of the centre of the parent cell from the outside inwards.
          • In plant cells: a cell plate forms across the equator of the parent cell from the inside outwards and a new cell wall is laid down.
      • Growth and repair. eg, dead surface skin cells replaced by identical ones.
      • Asexual reproduction. No variation between each individual
    • Meiosis
      • Sexual reproduction- 2 gametes fuse to form a zygote
      • To maintain full set of chromosomes (diploid) in each generation, the chromosome number must be halved (haploid) during meiosis.
      • Meiosis involves two divisions: Meiosis I- where chromosome number is halved. Meiosis II- where the 2 haploid cells divide again like in mitosis
      • 4 daughter cells with half the number of chromosomes as the parent cell.
      • During prophase I, homologous chromatids wrap around and then partially repel each other. They remain joined at certain parts called chiasmata. Here, the chromatids may break up and join with an equivalent chromatid. This cross over results in genetic variation.
      • 1st stage of metapase in meiosis- the pairs of homologous chromosomes arrange themselves randomly on the equator of the spindle.So, the combination that goes into each daughter cell in meiosis 1 is also random
      • Provides variation: genotypes mixed when gametes fuse, independent assortment of chromosomes, cross over ( new combinations)


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