Meiosis

• Meiosis is nuclear division that results in four cells that have half the DNA (n) of the original organism (2n).
• The purpose of meiosis is to produce gametes which can then take place in sexual reproduction.
• Meiosis allows for the fusing of two haploid nuclei to restore the full number of chromosomes and make a diploid nucleus.
• Meiosis introduces genetic variation by producing a new unique organism with a different combination of alleles from each parent.
• The whole of the genome is split up into sections called chromosomes.
• When chromosomes replicate (duplicate) a clone of the original is produced. These are held together at a centromere and form two sister chromatids. This is still considered to be one chromosome.
• Diploid cells have pairs of chromosomes - maternal and paternal
• In a diploid cell, the pairs of chromosomes are said to be homologous.
• Homologous chromosomes in diploid cells are not clones of each other as one is from each parent.
• Homologous chromosomes in diploid cells pair up in meiosis I. The genes on these chromosomes are alleles of each other. They code for the same protein, but these could be different versions (e.g. blue eyes (b) or brown eyes (B)).
• 𝙄𝙉𝘿𝙀𝙋𝙀𝙉𝘿𝙀𝙉𝙏 𝘼𝙎𝙎𝙊𝙍𝙏𝙈𝙀𝙉𝙏:

  • in humans, there are 23 pairs of chromosomes, and these are randomly assigned to each side (pole) of the cell during meiosis I.
  • alleles on separate chromosomes are independently assorted from each other, and different combinations of alleles are present in each gamete.
• The process of independent assortment is random and 2^23 varieties of gametes can be produced. With two gametes forming each new organism, this means that there can be 70 trillion different possible combinations of fertilised egg cells.
• Independent assortment is the basis of variation in higher organisms (eukaryotes) and explains why no two people on the planet are identical.
• The position of the chromosome in metaphase (north or south pole) will affect which daughter cell it ends up in.
• What are chromatids?

  Two chromosomes of each type (one from each parent).
• What are gametes?

  Haploid cells used for sexual reproduction.
• What are homologous chromosomes?

  A matching pair of chromosomes, one inherited from each parent.
• What is a haploid cell?

  A cell with half the normal chromosome number of a diploid cell (only one copy of each chromosome).
• What is a centromere?

  Where two sister chromatids are joined together.
• What is a diploid cell?

  A cell formed when chromosomes are replicated.
• What is independent assortment?

  The random arrangement of homologous chromosome pairs in Metaphase 1 and 2 is independent of each other and results in genetic variation.
• What does bivalent mean?

  Associated pair of homologous chromosomes.
• What is crossing over?

  Bivalent chromosomes become entangled and exchange sections of DNA, forming recombinant chromatids and providing genetic variation.
• What is the chiasma?

  The place on a chromatid where crossing over has happened.
• What are alleles?

  Different versions of the same gene.
• What happens in interphase?

  Growth and DNA replication occur before meiosis.
• What occurs during Meiosis 1?

  Reduction happens when pairs of homologous chromosomes are separated into different cells.
• What happens in Prophase 1?

  • chromosomes condense
  • the nuclear envelope disintegrates
  • the nucleolus disappears
  • the homologous chromosomes pair up
  • spindle fibres start to form
  • crossing over occurs
• What happens in Metaphase 1?

  • the homologous chromosome pairs line up along the cell equator
  • the orientation of each pair is random
  • maternal/paternal chromosomes can end up facing either pole
  • this is independent assortment
• What happens in Anaphase 1?

  • the homologous chromosomes are pulled to opposite poles of the cell by the spindle fibres
  • the chromatids stay joined together
  • sections of DNA that crossed over now break and rejoin
  • results in an exchange of DNA
• What happens in Telophase 1?

  • the chromosomes assemble at each pole of the cell
  • the nuclear membrane reforms
  • chromosomes uncoil
  • cytokinesis occurs
  • the cell divides into two new cells
  • these new cells are haploid
• What is cytokinesis?

  The division of the cytoplasm and its contents to make two new daughter cells.
• What happens during Meiosis 2?

  The pairs of chromatids in each daughter cell are separated, forming two more cells. Four haploid daughter cells are produced in total.
• What happens during Prophase 2?

  • chromosomes (consisting of two chromatids which are no longer identical) condense
  • the nuclear envelope breaks down
  • spindle formation begins
• What happens during Metaphase 2?

  • the chromosomes attach to the spindle by their centromere
  • the chromatids of each chromosome are randomly arranged on the equator
  • this means there is independent assortment again
• What happens during Anaphase 2?

  • the centromeres divide
  • the chromatids of each chromosome are pulled to opposite poles of the cell by the spindles
• What happens during Telophase 2?

  • nuclear envelopes form around each of the four haploid nuclei
  • the chromosomes uncoil and form chromatin again
  • the nucleolus becomes visible
  • cytokinesis occurs
  • genetically different haploid cells are formed
• What is meiosis?

  Nuclear division that results in four genetically different daughter cells.
• What is chromatin?

  Uncondensed DNA in a complex with histones.
• What is a centriole?

  Made of microtubules and where the spindle fibres are formed from.