1. Meiosis

2. Homologous Chromosomes
Homologous chromosomes contain the same genes, one from each parent
Ex: Two copies of chromosome 3, one from mother and one from father, form a homologous pair.
Homologous pair after DNA replication: L D
This paternal chromosome 3 carries instructions for light hair
This maternal chromosome 3 carries instructions for dark hair L D
Both have the gene for hair color, but the color itself may be different!

3. Chromosome Number Diploid Haploid
All eukaryotic cells can be categorized as either…
Diploid
Contain homologous pairs of chromosomes (di = two sets)
All body cells except sperm and egg are diploid
In humans, diploid chromosome number is 46 (23 pairs)
Can be written as 2n
Haploid
Contain only one of each chromosome (hap = half as much)
Only sperm and egg are haploid
In humans, haploid chromosome number is 23 (no pairs, only singles)
Can be written as n

4. How many homologous pairs?
How many chromosomes? 3 6
How many homologous pairs? 3
Haploid or Diploid?
Haploid
Diploid
2n or n? n 2n

5. Meiosis Overview Meiosis is the creation of sex cells, or gametes.
It occurs in the testes of males and the ovaries of females.
It results in eggs (females) and sperm (males).

6. Mitosis vs. Meiosis Mitosis Meiosis
Produces identical copies of somatic cells (body cells)
Responsible for growth and healing
Produces 2 diploid daughter cells
Meiosis
Produces unique gametes – no two are the same!
Responsible for making egg and sperm
Produces 4 haploid daughter cells

7. Phases of Meiosis Mitosis Meiosis
Like mitosis, meiosis also follows the PMAT phases
Unlike mitosis, meiosis goes through them twice (meiosis I and meiosis II)
Mitosis
Prophase
Metaphase
Anaphase
Telophase
Meiosis
Prophase I
Metaphase I
Anaphase I
Telophase I & cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II & cytokinesis

8. Meiosis I
In meiosis I, a reduction division occurs: chromosome number is halved from 2n to n (46 to 23)
In metaphase I, homologous pairs line up at the equator
In anaphase I, the pairs are separated
Meiosis I

9. Animation
Meiosis II
Meiosis II looks identical to mitosis, except two cells are becoming four
Meiosis II

10. Crossing Over
When homologous chromosomes pair during meiosis I, crossing over may occur
Chromosomes come into contact, crossed sections are exchanged
Produces brand new combinations of genes

11. 46 total 92 total 46 total in each 23 total in each
Chromosomes replicate
Homologous chromosomes pair up
1st division – homologous chromosomes separate
2nd division – sister chromatids separate

12. “Fun” Facts
Females are born with as many eggs as they need- Around 1 million. About 40,000 will survive to puberty and develop one at a time each month.
Males can produce sperm until they die, nearly 100 million per day, on average. Sperm live a maximum of five days.
It takes only one sperm to fertilize one egg.

13. Critical Thinking Practice Diversity in Sexually Reproducing Species
For an organism with four chromosomes, the diagrams of metaphase I below are equally likely. Using two different colors, draw the daughter cells each of these would produce at the end of meiosis I.
How does this randomization of maternal and paternal chromosomes contribute to diversity in sperm and egg?
Name one other source of variation during meiosis. OR ? ? ? ?