Cell Division

• Cytokinesis in plant cells

  A cell plate is laid down along the centre of the cell.Vesicles from the golgi apparatius coalesce at the cell plate to form the new cell wall.

  
• Cytokinesis in animal cells

  A cleavage furrow forms as the cell surface membrane invaginates, this splits the cell into two.

  
• Cytokinesis

  At the end of mitosis.Cell divides to form two daughter cells, each with an identical chromosome makeup to the parent cell.

  
• Telophase

  Sister chromatids are now at opposite poles of the cell and are now called chromosomes again.Chromosomes decondense and become less visible.Nucelolus reappears and the nuclear membrane reforms.Spindle disappears

  
• Anaphase


  Spindle fibres contract, the centromeres split and the pairs of chromatids are are pulled apart.

  
• Metaphase

  Chromosomes align along the equator, attached to the spindle fibres by their centromeres.

  
• Prophase

  Chromatin condenses and chromosomes become visible.Nucleolus disappears.In animal cells centrioles move to opposite poles and form the spindle.Nuclear membrane breaks down.Each chromosome appears as a pair of chromatids joined by a centromere.

  
• Stages of Mitosis

  ProphaseMetaphaseAnaphaseTelophase
• Male Karyotype


  An X and a Y chromosome

  
• Female karyotype

  Two X chromosomes

  
• Karyotype

  A diagram that shows the the chromosomes arranged in homologous pairs.In humans the longest chromosomes are called chromosome pair 1.

  
• Homologous Chromosomes

  Carry the same genes in the same sequence along their length.Are not identical as they may have different alleles (versions of the same gene)Homologous chromosomes are the same size and shape.

  
• Diploid Organisms


  Have chromosomes arranged in homologous pairs in the cells.

  
• Chromosomes

  DNA supported by special proteins called histones.

  
• When does the DNA content of a cell double and when does it return to normal?

  DNA doubles during the S phase and returns to normal levels during cytokinesis.

  
• How chemotherapy drugs work?

  1. Inhibit microtubule formation e.g vincristine stops the spindle fibres contracting (stops anaphase)2. Antimetabolites inhibit the S phase and prevent DNA synthesis. Fluorouracil inhibits the enzyme involved in making the nucelotides that contain the base thymine.
• What is the principle of chemotherapy?

  Using drugs to disrupt the cell cycle and stop the process of mitosis.
• What is cancer?

  Mitosis out of control.Cancer cells cannot enter G0.There is a breakdown in the ability of the checkpoints in the cell cycle to regulate cell division.
• Metaphase checkpoint

  A checkpoint at the end of metaphase in the cell cycle.Checks that all the chromosomes have attached properly to the spindle fibres.
• G2 Checkpoint

  At the end of G2, checks that DNA replication has been accurate and there is no DNA damage.If passed mitosis will begin.
• If G1 checkpoint requirements are not met?

  Cell enters a resting state called G0, they can remain here for a short time or permanently.
• If G1 checkpoint requirements are met?

  Cell enters S and DNA synthesis takes place.
• G1 Checkpoint

  At the end of G1 before SEnsures1. Cell is a critical size2.Enoough nutrients are present for the next stage.3. Necessary growth factors are in place4. There is no DNA damage
• Checkpoints in the cell cycle

  Check that the previous stages have been carried out correctly before the process can continue.
• G2

  Spindle proteins are made.Chromosomes are checked for error and repaired.More growth
• S

  DNA is duplicated.Chromosomes are now a pair of chromatids.Histone proteins form
• G1


  Organelles replicateThe cell grows in size
• What are the three stages of interphase?

  G1, S and G2

  
• What do G and S stand for in the cell cycle?

  G stnds for growth or gap and S stands for synthesis

  
• What does mitosis ensure?

  That the daughter cells produced during cell division have exactly the same chromosome makeup as the parent cell.
• What is the cell cycle?


  This is the cycle that includes growth, mitosis and the cell dividing in two (cytokinesis)

  
• How do organisms grow?

  The cells grow, duplicate their DNA and then divide in two
• Fertilisation

  Adds to variation as it is totally random which gamete from each parent is actually involved in fertilisation.

  
• Recombinants


  The two chromatids which have been involved in crossing over.

  
• Result of Crossing Over

  1. The two chromatids of the same chromosome are no longer genetically identical.2. Unique sequences of alleles are produced.

  
• Crossing Over

  In Prophase 1 in the bivalent 4 chromatids lie side by side.Two non-sister chromatids break and exchange sections with each otherThe points where the chromosomes cross over are called chiasmata.

  
• Independent Assortment

  Either of the two homologous chromosomes from a homologous pair can enter a gamete.It depends on the random nature of how the chromosomes align on the equator during metaphase 1Humans have 23 pairs of chromosomes so there are millions of possible chromosome combinations for any one gamete.

  
• How does Meiosis produce genetic variation?

  1. Independent Assortment.2. Crossing Over
• End of Meiosis 11


  Cytokinesis occurs and each of the 2 cells from meiosis 1 produces 2 new daughter cells.4 haploid genetically different cells are produced.
• Meiosis 11

  Prophase 11 follows cytokinesis but the spindle fibres are formed at right angles to the angle of division in meiosis 1.Each of the chromosomes is separated into its two chromatids.(very similar to mitosis)