Topic 10 - Genetic Evolution

• Meiosis


  1. DNA replication

  2. Prophase I

  3. Metaphase I

  4. Anaphase I

  5. Telophase I

  
• Meiosis


  • It is a form of nuclear division that results in the production of haploid cells from diploid cells
  • It produces gametes in plants and animals that are used in sexual reproduction
  • It has two successive divisions: meiosis I and meiosis II

  
• Where meiosis occurs


  • Testes of male animals
  • Ovaries of female animals
  • Anthers and ovaries of flowering plants

  
• Gametes produced by meiosis

  • Spermatozoa (sperm cells) in male animals
  • Ova (eggs) in female animals
  • Male plant gametes in pollen grains
  • Female plant gametes in ovules

  
• Bivalent


  Two homologous chromosomes aligned alongside each other

  
• Tetrad


  A bivalent composed of four chromatids
• Synapsis


  The alignment of homologous chromosomes

  
• Non-sister chromatids


  Chromatids originated from different parental chromosomes

  
• Crossing over


  1. Exchange of DNA material between non-sister homologous chromatids

  2. Catalysed by endonuclease and DNA ligase enzymes

  3. Forms chiasmata
• Chiasmata


  Crossing points where DNA strands remain attached after crossing over
• Crossing over produces new combinations of alleles on the chromosomes of the haploid cells

  
• Crossing over contributes greatly to intraspecific variation
• Meiosis I


  1. Prophase I

  2. Metaphase I

  3. Anaphase I

  4. Telophase I

  
• Meiosis I


  Homologous chromosomes separate

  
• Meiosis I is referred to as reduction division because homologous chromosomes separate and move to opposite poles

  
• Chiasma (chiasmata; plural)


  The point at which the crossing over of non-sister chromatids occurs
• Meiosis I


  1. Centrioles migrate to opposite poles and the spindle is formed

  2. Nuclear envelope breaks down and the nucleolus disintegrates

  3. Bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres

  4. Bivalents line up by independent assortment (random orientation)

  5. Homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle

  6. Centromeres do not split

  7. Chromosomes arrive at opposite poles

  8. Spindle fibres start to break down

  9. Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

  
• Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

  
• Meiosis I


  Reduction division, the number of chromosomes per cell is reduced by a factor of 2

  
• Meiosis II


  1. No interphase between meiosis I and meiosis II so the DNA is not replicated

  2. Chromosomes condense

  3. Spindle forms at a right angle to the old one

  4. Chromosomes line up in a single file along the equator of the spindle

  5. Centromeres divide and individual chromatids are pulled to opposite poles

  6. Nuclear membranes form around each group of chromosomes

  7. Cytoplasm divides as new cell surface membranes are formed creating four haploid cells

  
• Meiosis II is almost identical to the stages of mitosis

  
• Sister chromatids separate in meiosis (anaphase) II, but they are likely to be non-identical sister chromatids at this stage due to crossing over having happened in prophase I

  
• Independent assortment


  The production of different combinations of alleles in gamete cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I

  
• The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
• Formula to calculate number of different possible chromosome combinations

  2^n, where n corresponds to the number of chromosomes in a haploid cell

  
• For humans this is 2^23 which calculates as 8 388 608 different combinations
• Crossing over introduces an almost infinite amount of variation
• Explaining deviations from Mendelian ratios

  1. Careful observation and record keeping turned up anomalous data that Mendel's law of independent assortment could not account for

  2. Bateson and Punnett found seemingly anomalous ratios of offspring for which they could offer no explanation

  3. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies

  
• Linkage


  Unexpected patterns of inheritance caused by separate alleles being inherited together, from the same chromosome (an autosome)

  
• Mutations can occur within genes, leading to the production of new alleles and increased genetic variation
• Drawing chiasmata


  • Use two coloured pens/pencils to show chromosomes/chromatids of maternal or paternal origin
  • Draw each homologous chromatid as a long line
  • Stage 1: Synapsis - all 4 chromatids of a pair of homologous chromosomes align closely together
  • Stage 2: Cuts occur in the DNA of non-sister chromatids
  • Stage 3: Formation of chiasmata - the loose, cut ends of DNA re-form hydrogen bonds to complementary bases on a different chromatid, causing the overall chromosome shape to feature X-shapes at the chiasmata

  
• Unlinked genes


  Genes that an organism carries on separate chromosomes, not on homologous copies of the same chromosome

  
• Unlinked genes segregate independently as a result of meiosis

  
• Assortment of chromosomes


  The alignment of each bivalent independently of all the others in metaphase I of meiosis

  
• Segregation of chromosomes


  The separation of whole chromosomes to different poles of the cell in anaphase I, which determines which combinations of alleles end up in which gamete cells

  
• Linked genes (on the same chromosome) tend to be inherited together

  
• Unlinked genes in fruit flies (Drosophila melanogaster)

  • Gene for curly wings on chromosome 2
  • Gene for mahogany eyes on chromosome 3
• Unlinked genes in humans

  • Gene for trypsin (a stomach enzyme) on chromosome 7
  • Gene for human growth hormone on chromosome 17

  
• Assortment of chromosomes


  Their alignment in metaphase I of meiosis, where each bivalent assorts (aligns) itself independently of all the others

  
• Segregation of chromosomes


  How they get separated, governed by their pattern of assortment, where whole chromosomes are pulled to different poles of the cell in anaphase I