10.1.3 Meiosis I

Homologous chromosomes separate in meiosis I

Prophase I

• DNA has already replicated and condenses and becomes visible as chromosomes
• Each chromosome consists of two sister chromatids joined together by a centromere
• The chromosomes are arranged side-by-side in homologous pairs
  • A pair of homologous chromosomes is called a bivalent
• As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
• In this stage centrioles migrate to opposite poles and the spindle is formed
• The nuclear envelope breaks down and the nucleolus disintegrates

Metaphase I

• The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
• The bivalents line up by independent assortment (random orientation)

Anaphase I

• The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
• The centromeres do not split

Telophase I

• The chromosomes arrive at opposite poles
• Spindle fibres start to break down
• 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 is reduction division

• Meiosis I is referred to as reduction division because homologous chromosomes separate and move to opposite poles of the cell.
• Therefore, the number of chromosomes per cell is reduced by a factor of 2

The different stages of meiosis I in an animal cell

• During metaphase I, an event occurs that is an important source of genetic variation in the gametes formed by meiosis
• Independent assortment is 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
  • This random alignment is sometimes referred to as random orientation
• In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
  • Each pair can be arranged with either chromosome on top, this is completely random
  • The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
• The homologous chromosomes are then separated and pulled apart to different poles during anaphase
• The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
• To work out the number of different possible chromosome combinations the formula 2ⁿ can be used, where n corresponds to the number of chromosomes in a haploid cell
• For humans this is 2²³ which calculates as 8 388 608 different combinations
  • This may seem like a lot of combinations, but by contrast, crossing over introduces an almost infinite amount of variation

Independent assortment of homologous chromosomes leading to different genetic combinations in daughter cells

NOS: Careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for

• Gregor Mendel (of pea plant fame) devised the law of independent assortment in 1866
• In this, he states that characteristics are inherited completely independently of others
• This was refined by later scientists to state that the allele that gets sorted into a particular gamete cell is not influenced by the allele received for another gene
• In many cases, this holds true, and can be observed in Mendelian ratios of offspring in certain crossing experiments
  • Mendel observed a 9:3:3:1 ratio in many dihybrid crosses of sweet pea plants
• As a result, Mendel's findings were not challenged until the early 20th century, when Bateson and Punnett found seemingly anomalous ratios of offspring
• For which they could offer no explanation
• These became named as non-Mendelian ratios because they did not follow the pattern as predicted by Mendel

Bateson and Punnetts' experiment (1905)

• Also working with sweet peas, two pairs of alleles were identified (Dominant alleles in capital letters)
  • Flower colour:
    • P = purple, p = red
  • The shape of pollen grains:
    • R = long, r = round
• Their first cross involved pure-bred purple-flowered plants with long pollen grains (PPRR) with pure-bred red-flowered plants with round pollen grains (pprr)
• As expected, this cross resulted in 100% purple-flowered plants with long pollen grains, with the double-heterozygous genotype PpRr in the F₁ generation
• However, when individuals from the F₁ generation were crossed with each other, Bateson and Punnett would have expected a 9:3:3:1 mix of phenotypes, in line with Mendel's law of independent assortment

Modelling the expected ratios using a Punnet square

• Expected phenotype ratios for the second generation:
  • 9 purple flowers, long pollen grains
  • 3 purple flowers, round pollen grains
  • 3 red flowers, long pollen grains
  • 1 red flowers, round pollen grains

Actual Results from Bateson and Punnetts' Experiment

• In the F₂ generation of offspring, there were...
  • Very many of the grandparent phenotype (purple-flowered, long pollen grains) produced
  • Many of the grandparent phenotype (red-flowered, round pollen grains) produced
  • Very few (but not zero) other phenotypes (purple/round or red/long) produced
• This appeared as an anomaly to Bateson and Punnett, though they did not find an explanation

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

• Later in the 20th century (approx. 1910-1940), an American biologist, Thomas Hunt Morgan, put forward an explanation for the results previously observed by Bateson and Punnett
• From his findings, he devised the theory of linkage
• Morgan worked on fruit flies (Drosophila melanogaster) thanks to their ability to reproduce quickly, in large numbers, in a small physical space and with observable heritable characteristics like eye colour and wing shape
• In fact, Morgan bred them selectively for many years to develop a range of phenotypes by natural mutation
• Morgan's work identified sex-linked characteristics from genes carried on the X or Y chromosomes that determine gender
• This work led Morgan to develop the idea of autosomal linkage
• This is where unexpected patterns of inheritance are caused by separate alleles being inherited together, from the same chromosome (an autosome)
• Morgan then elaborated his work by developing the theory of crossing over
  • As a way of accounting for unexpected (recombinant) genotypes