DNA, Mitosis and Meiosis

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  • Created by: Roisind65
  • Created on: 11-02-15 10:11
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  • DNA, Mitosis and Meiosis
    • DNA and Chromosomes
      • Chromosomes
        • a thread like structure made of protein and DNA by which hereditary information is physically [passed from one generation to the next.
        • Chromosomes are only visible as distinct structures when a cell is dividing.
          • for the rest of the time they are widely dispersed throughout the nucleus
        • DNA in chromosomes are held in position by protein molecules.
      • What is an allele?
        • one of a number of alternative forms of a gene.
    • Meiosis and genetic variation
      • 1. in the first division the homologous chromosomes pair up and their chromatids wrap around each other.  By the end of this stage the homologous pairs  have separated with one chromosome from each pair going into the two daughter cells.
        • Portions of there chromatids may be exchanged in a process called crossing over.
          • The chromatids of of each pair become twisted around each other.
            • During this twisting process tensions are created and portions of the chromatids break off.
              • These broken portions then rejoin with the chromatids of it's homologous partner.
                • Usually it is equivalent portions of homologous chromosomes that are exchanged
                  • In this way new genetic combinations are produced.
      • 2. In the second division the chromatids move apart at the end of meiosis, four cells have been formed . In humans, each of these cells contains 23 chromatids.
    • Replication of DNA
      • Nuclear division is the process by which the nucleus divides. There are two types of nuclear division, mitosis and meiosis.
      • Cell division follows nuclear division and is the process by which the whole cell divides.
      • A representative portion of DNA, which is about to undergo replication
        • and enzyme, DNA helicase, causes the two strands of the DNA to separate
          • DNA helicase completes the splitting of the DNA strand. Meanwhile, free nucleotides that have been activated are attracted to their complementary bases.
            • once the activated nucleotides are lined they are joined together by DNA polymerase. The remaining unpaired bases continue to attract their complementary nucleotides.
              • finally, all the nucleotides are joined to form a complete polynucleotide chain using DNA polymerase. In this way, two identical strands of DNA are formed. As each strand retains half of the original DNA material, this method of replication is called the semi-conservative method.
    • Structure of DNA
      • Nucleotide
        • Sugar called deoxyribose
        • Phosphate group
        • and organic base belonging to one of two different groups:
          • (a) single-ring base - cytosine (C) and thymine (T)
          • (b) double-ring base - adenine (A) and guanine (G)
            • adenine always pairs with thymine by means of two Hydrogen bonds.
            • guanine always pairs with cytosine by means of three hydrogen bonds
        • deoxyribose sugar phosphate group and organic base are combined as a result of condensation reaction.
      • DNA is made up of two nucleotides.
        • They are joined together by hydrogen bonds formed between certain bases.
    • Mitosis
      • Interphase- cell is actively synthesis proteins, chromosomes are visible, Prior to division, DNA replicates.
        • prophase- in which the chromosomes become visible and the nuclear envelope disappears.
          • metaphase- in which the chromosomes arrange themselves at the centre of the cell.
            • Anaphase- in which each of the two threads of a chromosome (chromatid) migrates to an opposite pole.
              • telophase- in which the nuclear envelope reforms.
    • The Cell Cycle
      • Interphase- which occupies most of the cell cycle, and is sometimes known as the resting phase because no division takes place. It is divided into three parts:
        • First growth (G1) phase, when proteins from which cell organelles are synthesised are produced.
        • synthesis (S) phase, when DNA is replicated.
        • Second growth (G2) phase, when the organelles grow and divide and energy stores are increased.
      • Nuclear division, when the nucleus divides into (mitosis) or four (meiosis)
      • Cell Division, which follows nuclear division and is the process by which the whole cell divides into two or four.
    • Cancer
      • metastasis is the spread of cancer cells through blood/lymph
      • Treatments: chemotherapy radiation surgery.
      • Damaged Cells
        • Divide uncontrollably to form a tumour
          • Benign- non-canerous
          • malignant- cancerous
      • proto-oncogenes (control normal cell) when mutated form oncogenes causing cell to rapidly divide.
      • tumour suppressor genes -slow down cell division unless mutation occurs/


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