1. Meiosis: The Big Picture
Like mitosis, all DNA copied in 2n cell to make 4n cell which
then divides to make 2n cells again BUT
The resulting cells are NOT identical b/c of how chromosomes line up
Those 2n cells divide AGAIN to make n (haploid) cells 2n
“2n” 2n 2n +
Mitosis n
“n” 2n
“2n” n
“n” n n
Meiosis

2. Important Terminology:
Sister chromatids: identical copies of same chromosome
Homologous pairs of chromosomes: pairs of sister chromatids
one set from mom, one set from Dad, both the same chrom. #
In (cross) sexual reproduction: chromosomes come from one of
two parents so chromosomes can be “paternal” or “maternal”
Draw:
sister chromatids of maternal chromosome 2.
... of 10.
…homologous pairs of each chromosome.
=maternal
chrom 2
chrom 10
=paternal

3. Important Terminology:
Sister chromatids: identical copies of same chromosome
Homologous pairs of chromosomes: pairs of sister chromatids
one set from mom, one set from Dad, both the same chrom. #
In (cross) sexual reproduction: chromosomes come from one of
two parents so chromosomes can be “paternal” or “maternal”
=maternal
chrom 2
=maternal
chrom 10
= sister chromatids
=paternal
chrom 2
=paternal
chrom 10
= homologous pair
= sister chromatids
= homologous pair

4. (2n) (“2n”) Meiosis involves TWO cell divisions:
both resulting in reduction of chromosome copy by half
Prior to Meiosis:
DNA replication
Draw what happens next (immediately).
(2n)
(“2n”)
Homologous pair
of chromosomes
Sister chromatids
(copies of a
replicated chromosome)

7. maternal #3
paternal #3
Recall in Mitosis, homologs lined up but not
side by side as a pair and therefore
did not separate
like they do in Meiosis
paternal #3
maternal #3

8. Clicker: Homologous pairs of chromosomes align during meiosis
Clicker: Homologous pairs of chromosomes align during meiosis. Do homologous chromosomes pair up in mitosis?
Yes No 8

9. Clicker: Homologous pairs of chromosomes align during meiosis
Clicker: Homologous pairs of chromosomes align during meiosis. Do homologous chromosomes pair up in mitosis?
Yes No 9

10. Crossing Over between similar chromosomes
during Meiosis
Early in meiosis (before alignment in replicated 2n cell)
Homologous chromosome pairs come together
Similar regions at the molecular level attract one another.
The non-sister chromatids can cross-over (swap pieces of their chromosomes)
Usually at least 1 per homologous pair of replicated chromosomes
Produces new allele combinations which may have different expression/regulation
Produces variation beyond the 8.4 million possible with independent assortment

11. A Closer Look at Three Key Events in Prophase of Meiosis I.
Sister chromatids
Centromere
Chromosomes
1. Replication, during interphase.
One homolog
Synaptonemal complex
2. Synapsis, during prophase I.
Second homolog
Non-sister chromatids
3. Crossing over, during prophase I.
Figure 12.9
A Closer Look at Three Key Events in Prophase of Meiosis I.
Protein complex
Crossing over usually
occurs at least once
in each non-sister
chromatid; two of four
chromatids are shown
crossed over here

12. Sexual vs Asexual Reproduction
Mitosis is the mechanism of asexual reproduction in eukarya (only
Genetic variation in daughter cells is by random mutation
during DNA replication)
Self-fertilization is 1 mechanism of sexual reproduction:
one organism produced 2 different types of gametes that join
and form new organism (sponges and many snails)
~More variation than reproduction by mitosis b/c self-fertilization involves meiosis to form gametes so still get variation due to independent assortment and crossing over (in addition to random mutation)
Sexual reproduction between 2 organisms allows for the MOST
genetic variation: random mutation, independent assortment,
crossing over AND mixing of genes from two parents.

13. * * * * * * * * * * * * * * * * Sexual vs Asexual Reproduction
But, not equal: sexual reproducing organisms have offspring with greater
genetic diversity. So, if a “bad” mutation arises, it will be inherited by all offspring of asexual parents b/c no mixing of DNA between two individuals.
Mitosis: * * * * * * * + 2n 4n
Two 2n daughter cells *
Meiosis: * * * * * *
Meiosis I
Meiosis II * *
gametes + 2n 4n
homologous
pairs align