1. Chromosomes and Meiosis

2. Chromosomes
DNA coils around histones to form nucleosomes, which coil to form chromatin fibers
The chromatin fibers supercoil to form chromosomes that are visible in the metaphase stage of mitosis.
Formed during prophase

3. Chromosomes and Chromosome Numbers
Human body cells have 46 chromosomes
Each parent contributes 23 chromosomes
Homologous chromosomes—one of two paired chromosomes, one from each parent
Same length
Same centromere position
Carry genes that control the same inherited traits

4. Haploid and Diploid Cells
An organism produces gametes to maintain the same number of chromosomes from generation to generation.
Human gametes contain 23 chromosomes.
A cell with n chromosomes is called a haploid cell.
A cell that contains 2n chromosomes is called a diploid cell.

5. Meiosis The sexual life cycle in animals involves meiosis.
Meiosis produces gametes.
When gametes combine in fertilization, the number of chromosomes is restored.

6. Stages of Meiosis
Reduces the chromosome number by half through the separation of homologous chromosomes
Involves two consecutive cell divisions called meiosis I and meiosis II

7. Stages of Meiosis Interphase Prophase II Metaphase II Anaphase II
Chromosomes replicate
Metaphase II
Anaphase II
Chromatin condenses
Telophase II
Prophase I
Cytokinesis
Metaphase I
Anaphase I
Telophase I

8. Interphase Prophase I Chromosomes replicate Chromatin condenses
Pairing of homologous chromosomes occurs
Each chromosome consists of two chromatids
The nuclear envelope breaks down.
Spindles form
Crossing over occurs

9. Crossing Over in Prophase I
Crossing over produces exchange of genetic information
Crossing over—chromosomal segments (Genes) are exchanged between a pair of homologous chromosomes

10. Metaphase I
Chromosome centromeres attach to spindle fibers
A diploid number (46) of homologous chromosomes line up at the equator (center of the cell)
Anaphase I
Homologous chromosomes separate and move to opposite poles (ends) of the cell

11. Telophase I The spindles break down Nuclear envelope reforms
Chromosomes uncoil and form two nuclei
The cell divides

12. Prophase II Metaphase II
A second set of phases begins as the spindle apparatus forms and the chromosomes condense
Nuclear envelope breaks down
Metaphase II
Spindles attach to centromere of each sister chromatid
A haploid (23) number of chromosomes line up at the equator

13. Anaphase II Telophase II
The sister chromatids are pulled apart at the centromere by spindle fibers
Sister chromatids move toward the opposite poles of the cell
Telophase II
The chromosomes reach the poles
The nuclear envelope reform
4 nuclei are created
Spindle fibers break down

14. Cytokinesis
Cytokinesis results in four haploid cells, each with n number of chromosomes
Cells divide into 4 haploid daughter cells
They are not identical
Can be sperm or egg cells, depending on the gender of the diploid original cell

15. Meiosis Summary Meiosis consists of two sets of divisions
Produces four haploid daughter cells that are not identical
Results in genetic variation