Cell division

Cell division

1. The role of mitosis

   • division of nucleus into two genetically identical daughter nuclei
   • all DNA in nucleus replicated before mitosis
     • during interphase, before mitosis
     • single DNA molecule to two chromatids
   • one chromatid to each daughter
   • Uses
     • embryonic development
     • growth
     • tissue repair
     • asexual reproduction

2. Interphase

   • active phase in metabolic processes
   • many processes in nucleus and cytoplasm
   • DNA replication in nucleus
   • protein synthesis in cytoplasm
   • numbers of mitochondria in cell increase due to growth and division
   • plant cells and algae
     • chloroplasts increase
     • cellulose formation and added to cell wall by vesicles
   • phases
     • G1
       • some never progress past this stage
         • enter Gn
     • S
       • replication of genetic material in nucleus
     • G2

3. Supercoiling of chromosomes

   • condensation of chromosomes
     • pack chromosomes into shorter structures
     • occurs during prophase
   • two chromatids move to opp. poles
   • DNA molecule
     • diameter less than 5µm
     • length 50,000 µm
   • repeatedly coiling of chromosomes to make them shorter and wider
   • assisted by histones and enzymes

4. Phases of mitosis

   • Prophase
     • condensation of chromsomes
     • breakdown of nucleolus
     • microtubules grow from MTOC to form spindles
     • nuclear membrane breaks down at the end
   • metaphase
     • microtubules grow more and attach to centromeres
     • chromatids attach to microtubule
     • microtubules shorten; if correct, chromosomes stay at equator
   • anaphase
     • each centromere divided, allowing sister chromatid pairs to separate
     • spindle microtubules pull them to poles
     • 2 genetically identical nuclei produced
   • telophase
     • chromatids reach poles; called chromosomes
     • chromosomes pulled into tight groups near MTOC at each pole
     • nuclear membrane reforms around chromosomes
     • chromosomes uncoil; nucleolus formed
     • usually enters interphase again

5. Cytokinesis

   • occurs after mitosis
   • in plants
     • vesicles fuse at equator to form tubular structure
     • tubular structures fuse to form two membranes across equator
     • division of cytoplasm
     • pectin and other substances deposited between 2 membranes by exocytosis
       • forms middle lamella; will link walls
     • cellulose deposited to center by exocytosis
     • each daughter cell builds own wall adjacent to equator
   • in animals
     • plasma membrane pulled inwards around cell equator to form cleavage furrow
       • using ring of contractile protein at equator
       • actin and myosin
       • pinched off into daughter cells
   • process of cell division

6. Cyclins and the control of the cell cycle

   • cyclins - group of proteins
   • ensure tasks are performed at correct time
   • bind to enzymes cyclin-dependent kinases
   • enzymes become active and attach phosphate groups to other proteins in the cell
     • triggers protein to carry out specific task
   • stage progresses only if cyclin reaches threshold concentration
   • diagram in supplementary

7. Tumor formation and cancer

   • mutagens, oncogenes and metatstasis involved
   • tumor - abnormal group of cells
   • benign tumor - adhere to each other and unlikely to spread and cause harm
   • carcinomas/ malignant tumors - detach, move elsewhere, create secondary tumor, life threatning
     • cause cancer; diverse causes
   • carcinogens - cancer causing chemicals and agents
   • all (chemical and high energy radiation) mutagens are carcinogenic
     • cause gene mutation which can cause cancer
     • X-rays & short ray UV light
   • mutations - random changes in base sequence
   • oncogenes - genes that cause cancer after mutation
     • usually involved in control of cell cycle and division
     • several mutations must occur at the same time for tumor cell
   • metastasis - movement of tumor cells from primary tumor to develop secondary tumor in other body parts
   • primary tumor - original group of tumor cells