1. CELL CYCLE
Meiosis

2. Homologous Chromosomes
Homologous chromosomes contain DNA that codes for the same genes.
In this example, both chromosomes have all the same genes in the same locations (represented with colored strips), but different “versions” of those genes (represented by the different shades of each color).
Sister chromatids are exact replicas.
Homologous chromosomes are not

3. Chromosome Pairing Humans have 46 chromosomes arranged in 23 pairs
22 of the 23 pairs are homologous, meaning that the 2 chromosomes in each pair are similar with regard to shape, size, banding pattern, etc.
The 23rd pair is homologous in females but not in males

5. Haploid vs. Diploid
The body cells of every species have a characteristic number of chromosomes
Diploid: any cell that contains two complete sets of chromosomes (2n)
Haploid: a cell with only one complete set of chromosomes; these are also known as sex cells or gametes (n)

6. Haploid vs. Diploid (cont.)
Gametes must be haploid
If they weren’t, the chromosome number would double = 92; =184
Even one extra chromosome can cause serious defects

7. Meiosis
To keep the number of chromosomes stable from generation to generation, haploid cells are produced by meiosis
Meiosis is a type of cellular reproduction in which the number of chromosomes is reduced by half, so that the daughter cells are haploid
Haploid gametes combine to create a diploid offspring which grows by mitosis into an adult
As an adult, the offspring will produce haploid gametes

8. Phases of Meiosis
First, the cell replicates its DNA during interphase; this is followed by meiosis
Meiosis is divided into 2 parts:
Meiosis I
Meiosis II
In some organisms, these phases are divided by interkinesis
In other organisms, meiosis I is immediately followed by meiosis II

9. Meiosis I Prophase I Chromosomes condense
Each homologous pair becomes attached to one another forming a TETRAD [recall that tetrads do not form in mitosis]
Tetrads exchange parts of their chromatids in a process called crossing over

10. Meiosis I (cont.) Metaphase I Anaphase I Telophase I
Spindle fibers attach to the chromosomes
Anaphase I
Spindle fibers move homologous chromosomes to extreme ends of the cell
Telophase I
Nuclear membranes form

11. Interkinesis The cytoplasm divides creating 2 cells
Each chromosome in the haploid cells consists of 2 sister chromatids

12. Meiosis II Prophase II Metaphase II Spindle fibers form again
Chromosomes condense
Metaphase II
Spindle fibers line up the chromosomes in the center of the cell

13. Meiosis II (cont.) Anaphase II Telophase II
Sister chromatids are separated and pulled to opposite poles of the cell
Telophase II
Nuclei form
Spindle fibers break down

14. Cytokinesis Just as in mitosis, meiosis is followed by cytokinesis
Cytokinesis produces 4 haploid cells
Each cell contains one copy of one complete set of chromosomes (n)

15. Male Cytokinesis
In male animals, the cell divisions that occur after meiosis I and II are equal
4 sperm cells are created

16. Female Cytokinesis
In female animals, cytokinesis divides the cytoplasm unevenly so that one cell receives almost all of the cytoplasm
Only one cell, called the ovum, is formed; the other 3 cells, called polar bodies, die