Meiosis

Question 1

What process happen to the chromosomes before meiosis?

Answer 1

• Interphase
• Happens before meiosis and involves key events needed for successful cell division
• In S phase the DNA is replicated, chromosomes contain two identical DNA strands
• The genetically identical strands are sister chromatids
• Chromatids separate during Meiosis 2, becoming independent chromosomes (haploid)

Question 2

What would happen if the DNA would not be replicated?

Answer 2

• Prior to meiosis, DNA is replicated, the second meiotic division (meiosis 2) would not be needed
• The duplication of chromatids is important for potential genetic recombination to occur (more variation)

Question 3

What is independent assortment?

Answer 3

• The random and independent way each pair of homologous chromosomes lines up at the metaphase plate during meiosis 1
• The inheritance of one gene is independent to the inheritance of any other gene

Question 4

How does independent assortment work?

Answer 4

• During metaphase 1, homologous chromosomes line up at the equator as bivalents in one of two arrangements:
• Maternal copy left and paternal copy right OR maternal copy right and paternal copy left
• The orientation of each pair is random
• The allele on one chromosome has an equal chance of being paired with any allele on another chromosome
• Their inheritance is independent of one another
• The genes must however be located on different chromosomes

Question 5

What is synapsis?

Answer 5

• The pairing of homologous chromosomes
• The homologous pairs of replicated chromosomes align closely before condensation during prophase 1
• The connected chromosomes are bivalent (4 chromatids)
• The chromosomes are connected by a protein-RNA complex called synaptonemal complex
• Autosomes always undergo synapsis during meiosis

Question 6

What is chiasmata?

Answer 6

• The point of exchange between two non-sister chromatids where crossing over and exchange of genetic material occurred
• Chiasmata (chiasma) hold the homologous chromosomes together as a bivalent until anaphase 1
• The formation of chiasmata between non-sister chromatids can result in the exchange of alleles
• Multiple chiasmata may form along the same pair of homologous chromosomes

Question 7

How do you draw diagrams of chiasmata after crossing over?

Answer 7

• They are X-shaped
• They form as a result of crossing over, show exchange of genetic material
• Use different colours

Question 8

What is crossing over?

Answer 8

• The exchange of genetic material, this results in new allele combinations on the chromosomes
• The DNA is exchanged between non-sister homologous chromatids
• Chromosomes with genetic material from both maternal and paternal chromosomes are recombinant
• Crossing over results in new combinations of alleles in haploid cells, increases genetic diversity of potential offspring

Question 9

What happens during crossing over?

Answer 9

• Breaking covalent bond in the backbone of both strands of the DNA double helix at identical positions of the non-sister chromatids
• The helices switch sides
• Positions of chromatids are switched
• Genetic information is swapped from the point of exchange onward

Question 10

What is the difference between sister and non-sister chromatids?

Answer 10

• Non-sister: They have the same genes at the same loci but different alleles
• Sister: have the same alleles as well as the same genes, since they are identical copies