Review

1
Review

• How many chromosomes do you have in each cell?
• How many sets of your chromosomes do you have (1,
  2, 3, 4, more?)
• How many of each type of chromosome is inherited
  from your mom?
• How many of each type of chromosome is inherited
  from your dad?
• How many sex chromosomes do you have.

2
Meiosis

• Remember

Mitosis happens in my toesies
Meiosis happens in my ovaries
3
If parents have two copies of each chromosome,
why and how do they pass on only one to their
offspring?

• MEIOSIS is the process of cell division that
  decreases the number of chromosomes to make sperm
  or egg cells

4
The purposes of Meiosis

• Reduce the total number of chromosomes by half so
  that only ONE chromosome from each pair is
  inherited from each parent.
• Produce variation in offspring by
• randomly sorting (pairing up) the homologous
  chromosomes when the gametes meet.
• exchanging (crossing over) homologous chromosome
  arms

5
Meiosis

• Original cell
• Diploid pairs of homologous chromosomes

Crossing over occurs here

• Sperm or egg cells
• Haploid single chromosomes

6
Fertilization
Sperm
Egg
Gametes (haploid sex cells)
Zygote (diploid embryo)
Fertilized egg
7
Meiosis Overview

• Each pair of chromosomes first replicates to form
  sister chromatids.
• The cell then divides twice without any further
  replication of DNA.
• The first division separates the homologues
  (Meiosis 1).
• The second division separates the 2 sister
  chromatids (Meiosis 2).

8
Comparison of meiosis and mitosis

• Draw and label each step on your worksheet.
• Fill in the concept map.

Meiosis animation
Mitosis animation