Meiosis

• Meiosis

  • Cell division to make gametes (sperm and ova)
  • Sperm and ova have half the normal number of chromosomes (23)
  • This makes them haploid (n)
  • Occurs in the testes and ovaries and four cells are produce
• Chromosomes

  • The process of meiosis involves two nuclear divisions but the chromosomes only duplicate once
  • Interphase: prior to undergoing meiosis, where the cell grows and the DNA is replicated. This DNA replication occurs in the same way as it does in mitosis.
  • First division: the homologous pairs separate and 2 daughter cells form with 23 chromosomes, each itch 2 chromatids.
  • Second division: the chromatids separate, resulting in 4 daughter cells with 23 chromosomes, each with 1 chromatid.
• Steps in meiosis

  • Meiosis has similar steps to mitosis, but with 2 divisions instead of one.
  • Meiosis 1 = IPMATC
  • Meiosis 2 = 2 PMATC
• Interphase 1

  • Prior to meiosis the cell grows and undergoes DNA replication
  • Duplicated chromosomes are called sister chromatids and are joined to each other at the centromere.
• Prophase 1

  • Chromosomes become visible as long threads
  • Chromosomes move so that members of a pair lie along side each other lengthwise
  • Chromosomes then shrink and thicken as DNA becomes more coiled
  • Each chromosome pair appears as 4 strands (a tetrad)
  • A spindle (forms) ores stretching between the poles of the cell.
• Prophase 1 - Crossing Over

  • Chromosomal material is exchanged between the 2 pairs of sister chromatids = crossing over
  • This increases the variation in offspring s the sister chromatids are no longer identical = recombination
• Metaphase 1

  • The tetras line up randomly on the equator of the cell
  • This is random assortment
  • It increases variation as the gametes produced have a different combination of chromosomes
• Anaphase 1

  • The homologous chromosomes (in the form if sister chromatids joined at the centromere) migrate to opposite sides of the cell.
• Telophase 1

  • Chromosomes decondense and nuclear membranes reform
  • Cytokinesis occurs (cytoplasm divides) reducing 2 daughter cells.
• Prophase 2

  • Chromosomes condense (in the form of sister chromatids joined at the centromere), nuclear membrane dissolves
  • New spindle forms
  • No crossing over.
• Metaphase 2

  • Chromosomes line up randomly n the equator of the cell (random assortment)
  • Spindle attaches
• Anaphase 2

  • The sister chromatids are pulled apart and move to the opposite end of the cell.
• Telophase 2

  • Cytokinesis occurs
  • Nuclear membrane reforms
  • 2 haploid daughter cells produced (from each parent cell)
• Random/independent assortment

  • Homologous pairs of chromosomes separate at random during the first meiotic division
  • The way one pair of chromosomes separate is unaffected by the way any of the other pairs separate
  • The total number of possible chromosomes is 2^23 which is approximately 8.4 million
  • This occurs in both sperm and eggs so at fertilisation the combination of genes is completely random.
• Crossing over

  • During pro haste 1 the chromatids may cross, break and exchange segments.
  • This is called crossing over and the point where 2 chromatids cross is called a chiasma
  • Recombination - new combination of alleles along the chromosome as a result of crossing over. 
• Non-disjunction

  • Occurs when one or more of the chromosome pairs or chromatids fail to separate when the cell divides
  • This results in one daughter cell receiving an extra chromosome and the other lacking that chromosomes.
  • After fertilisation the zygote will have either 47 or 45 chromosomes instead of the normal 46.
  • This can result in severe and distinctive birth defects and miscarriage often occurs early in the pregnancy.
• Non-disjunction - Trisomy

  • Trisomy occurs when an extra chromosome is inherited
  • Partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes
  • One common type of trisomy is Down syndrome (trisomy 21). 
• Non-disjunction - Monosomy

  • Monosomy is when an individual is missing a chromosome
  • Like trisomy it usually results in severe malformations and often miscarriage
  • Partial Monosomy occurs when only part of a chromosome is missing