Lecture 20: The Sexual Cycle, Meiosis, and the Value of Sex

Question 1

Single-celled organisms usually reproduce by ______ fission.

Answer 1

• Binary

Question 2

Haploid = ?
Diploid = ?

Answer 2

• n

- 2n

Question 3

What happens in Meiosis I?

Answer 3

• Prophase I, DNA coils tightly and individual chromosomes become visible
• Homologous pairing
• Synapsis
• Metaphase I, nuclear membrane disintegrates
• Microtubules form spindle
• Centrioles move to opposite poles
• Each pair of homologous chromosomes line up
• Spindle fibres attach to one side
• Anaphase I, microtubules shorten and pull apart, taking both sister chromatids
• Each pole ends up with a complete haploid set of chromosomes
• Telophase I, nuclear membrane reforms
• Each daughter nucleus contains two sister chromatids for each chromosome

NOT IDENTICAL

Question 4

Mitosis vs. Meiosis

Answer 4

• Mitosis:
• Chromosomes align independently
• No chiasmata
• Centromeres on metaphase plate
• Chromatids disjoin
  2n –> 2n
• Meiosis I:
• Homologous chromosome synapse
• Chiasmata (crossing over)
• Chiasmata on metaphase plate
• Chromatids disjoin
• 2n –> n

Question 5

What occurs in Meiosis II?

Answer 5

• Prophase II
• Nuclear envelope breaks down, new spindle forms
• Metaphase II
• Spindle fibres bind to both sides of centromeres
• Anaphase II, spindle fibres contract, sister chromatids move towards opposite poles
• Telophase II, nuclear envelope reforms around sets of daughter chromosomes

NOT IDENTICAL

Question 6

n chromosomal pairs = _^n gametes

Answer 6

• 2