topic 2

• mitosis - the division of the nucleus
• in mitosis the single diploid cell breaks in half to 2 form 2 identical daughter cells.
• mitosis can be used for asexual reproduction
• the division of the cell is called the cytokenesis
• what are the stages of the 1st stage of mitosis?

  Prophase, , Metaphase, Anaphase, Telophase
• in the 1st stage of mitosis 1-prophase, the chromosomes condense and become visible in the nucleus. each chromosome consists of 2 sister chromatids which join together at the centromere. a pair of centrioles move to the opposite sides of the cell. spindle fibres are formed and attach themselves to each chromosome to the centre of the cell.
• in the 1st stage of mitosis 1-metaphase, the spindle apparatus is formed and the chromosomes line up at the centre of the cell.
• in the 1st stage of mitosis 1- anaphase, the centromeres divide into 2 and spindle fibres shorten. this pulls the sister chromatids apart towards the opposite sides of the cell
• in the 1st stage of mitosis 1-telophase, the chromatids now identify as chromosomes and the spindle apparatus breaks down and the nucleus membrane reform. as the nucleus returns the chromosomes uncoil back to their chromatin state
• what happens in cytokinesis?

  the cell membrane is pulled inward by the cytoskeleton which creates a groove that over time deepens. eventually the groove is so deep that the membrane fuse to form 2 identical cells
• cytokinesis in plant cells have their cell membrane cant furrow so their vesicles from the Golgi apparatus form membrane structures down the centre of the cell. these fuse together to form a central cell membrane which divides into 2
• meiosis usually happens in sexual reproduction and is only used to produce gametes
• how many gametes are formed in meiosis?

  4
• cell division is meiosis starts with a diploid cell and produces 4 haploid cells
• these gametes that are produced are important as these are used for the zygote to form and the zygote will have a diploid number of chromosomes
• in chromosomes there are equal amounts of blood group gene but their alleles are different
• when the chromosomes come together their chromatids wrap around each other which is called a bivalent
• chiasmata - the points where the chromatids joined together
• when the chromatids cross over each other, their DNA is exchanged and the chromosomes are now different. they can also exchange alleles. these are called recombinant chromosomes
• stages of meiosis 1?

  prophase 1, metaphase 1, anaphase 1, telophase 1
• stages in meiosis 2?

  Prophase II, Metaphase II, Anaphase II, Telophase II
• in meiosis stage 1, prophase 1 - the chromosomes condense and become visible. homologous chromosomes link together forming chiasmata which exchanges alleles and crossing linking if performed. the centrioles move to the opposite sides of the cell and the spindle fibres assemble into the spindle apparatus.
• in meiosis stage 1, metaphase 1 - the homologous chromosomes are lined up at the equator of the spindle apparatus
• in meiosis stage 1, anaphase 1 - the spindle fibres shorten and the chiasmata between the chromosomes break
• in meiosis stage 1, telophase 1 - the chromosomes have now reached the opposite ends of the poles of the cell and a nuclear membrane is formed and chromosomes recoil from their chiasmata shape
• after telophase 1 cytokinesis have the cells divided into 2 so there are haploid cells that have homologous chromosomes
• in meiosis stage 2, prophase 2 - chromosomes condense and become visible with spindle fibres starting to develop
• in meiosis stage 2, metaphase 2 - the chromosomes link up at the centre of the equator of the spindle apparatus
• in meiosis stage 2, anaphase 2 - the centromere of each chromosome divides and the spindle fibres shorten. the chromatids are now pulled to the opposite sides of the cell
• in meiosis stage 2, telophase 2 - the chromatids are now identifies as chromosomes and the nuclear membrane starts to reform and the chromosomes recoil
• after telophase 2 cytokinesis occurs and the cell becomes a haploid cells
• independent assortment = the homologous pair chromosomes are randomly organised on each side of the equator
• cells in multicellular organisms have evolved to become more specialised to their function
• before mitosis we have to go through the interphase which consists of 3 stages (g1, s , g3)
• gap phase (g1) - the cell grows and replicates its organelles. this is where protein synthesis happens to build new organelles
• the synthesis phase (S) - the cell replicates its DNA
• the gap phase 2 (G2) - the cell keeps growing until the organelles have to be duplicated
• there a 2 checkpoints in the cell cycle - one before the S phase and one after the S phase as this is the DNA checking for any errors so it reduces the risk of replicating mutated DNA
• specialisation in red blood cells (erythrocytes) - erythrocytes transport oxygen to the cells of the body. these red blood cells are small which gives them a large surface area to volume ratio and that helps carry more oxygen effciently
• specialisation in red blood cells (erythrocytes) - erythrocytes transport oxygen to the cells of the body. these red blood cells are small which gives them a large surface area to volume ratio and that helps carry more oxygen effciently