Review
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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.
Meiosis
• Remember:
Meiosis
happens
in my
ovaries
Mitosis
“happens
in my
toesies”
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
Sperm or egg cells
Original cell
The purposes of Meiosis:
1. Reduce the total number of chromosomes by
half so that only ONE chromosome from each
pair is inherited from each parent.
2. Produce variation in offspring by:
a) randomly “sorting” (pairing up) the homologous
chromosomes when the gametes meet.
b) exchanging (crossing over) homologous
chromosome arms
Meiosis
Original cell
•Diploid: pairs of
homologous chromosomes
Crossing over occurs here
Sperm or egg cells
•Haploid single
chromosomes
Fertilization
Sperm
Egg
Gametes:
(haploid sex cells)
Zygote:
(diploid embryo)
Fertilized egg
Meiosis Overview
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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).
Comparison of meiosis and
mitosis
• Draw and label each step on your
worksheet.
• Fill in the concept map.
Meiosis animation
Mitosis animation