• When it comes to IBDP Biology, it occurs in all organisms which reproduce sexually.
• It creates variation as offspring are created from a unique sperm and egg.
• It starts with cells containing homologous chromosomes
• These divide to form gametes:
  • Contain one chromosome from each homologous pair
  • These are randomly distributed
  • Immense scope for genetic variety
• Involves two divisions:
  • Chromosomes replicate
  • First division homologous pairs separate
  • Second division sister chromatids separate
• Number of chromosomes is reduced:
  • From the diploid number 2n
  • To the haploid number n
• Gametes fuse at fertilisation:
  • Zygote forms
  • Diploid number is restored
  • Enables chromosome number to remain constant from generation to generation

When it comes to IBDP Biology,

Interphase

1. Cell grows, and organelles start to replicate.

Meiotic division I:

1. Each chromosome forms a pair of chromatids, DNA is replicated so there are 4 copies of each gene.
2. Homologous pairs of chromosomes pair up.
3. Chromatids form chiasmata (the join between two chromatids).
4. Nuclear membranes break down.
5. Spindle fibres form.
6. Homologous pairs attach perpendicularly to spindle at equator.
7. Sections of genes are swapped between chromatids (crossing over)
8. Chromosomes pulled apart to opposite poles
9. 2 new nuclei form.
10. Cell divides

Meiotic division II:

1. Centrioles migrate, nuclear membrane breaks down and spindles form.
2. Sister chromatids attach to spindle perpendicularly at equator.
3. Chromatids pairs are pulled apart.
4. Nuclear membranes form and cells divide
5. There are now 4 cells, each one contains a unique set of individual chromosomes, and so each gene is present once.

When it comes to IBDP Biology,

• Gametes are genetically different as a result of different combinations of maternal and paternal chromosomes.
• This occurs through:

1. Random positioning of the pairs of homologous chromosomes (independent segregation)
2. Crossing over exchanges alleles between homologous chromosomes.
3. Random separation of chromosomes allows independent assortment of alleles.
4. Random directional separation of chromatids in meiosis II.

• At fertilisation there is random recombination of gametes.

This is the end of this topic

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