1. Meiosis
How Gametes Have Only Half the Number of Chromosomes As Other Cells
Modified by Liz LaRosa 2011

2. Human body cells have 46 chromosomes
Meiosis - General Overview
Human body cells have 46 chromosomes
Each parent contributes 23 chromosomes
Takes place in sex cells (gametes): egg and sperm
One cell becomes 4 NON-IDENTICLE cells

3. Chromosomes and Chromosome Number
Meiosis – Homologous Chromosomes
Chromosomes and Chromosome Number
Same length
Same centromere position
Carry genes that control the same inherited traits
One is from mother, one is from father = a matching set

4. Haploid and Diploid Cells
Meiosis
Haploid and Diploid Cells
An organism produces gametes to maintain the same number of chromosomes from generation to generation.
Human gametes contain 23 chromosomes.

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

6. Chromosomes replicate.
Meiosis
Meiosis I
Interphase
Chromosomes replicate.
Chromatin condenses.
Interphase

7. Pairing of homologous chromosomes occurs.
Meiosis
Meiosis I
Prophase I
Pairing of homologous chromosomes occurs.
Each chromosome consists of two chromatids.
Prophase I
The nuclear membrane breaks down.
Spindles form.

8. Chromosome centromeres attach to spindle fibers.
Meiosis
Meiosis I
Metaphase I
Chromosome centromeres attach to spindle fibers.
Metaphase I
Homologous chromosomes line up at the MIDDLE/equator.

9. Meiosis
Meiosis I
Anaphase I
Homologous chromosomes separate and move AWAY to opposite poles of the cell.
Anaphase I

10. The spindles break down.
Meiosis
Meiosis I
Telophase I
The spindles break down.
Telophase I
Chromosomes uncoil and form TWO nuclei.
The cell divides.

11. begins as the spindle apparatus forms and the chromosomes condense.
Meiosis
Meiosis II
Prophase II
A second set of phases
begins as the spindle apparatus
forms and the
chromosomes condense.
(Notice that the DNA does not duplicate again.)
Prophase II

12. A haploid number of chromosomes
Meiosis
Meiosis II
Metaphase II
A haploid number of chromosomes
line up at the equator.
Metaphase II

13. The sister chromatids are
Meiosis
Meiosis II
Anaphase II
The sister chromatids are
pulled apart at the centromere by spindle
fibers and move toward the opposite poles
of the cell.
Anaphase II

14. The chromosomes reach the poles, and
Meiosis
Meiosis II
Telophase II
The chromosomes reach the poles, and
the nuclear membrane and nuclei reform.
Telophase II

15. Meiosis
Meiosis II
Cytokinesis results in four haploid cells, each with n number of chromosomes.
Cytokinesis

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

17. Meiosis
Meiosis Provides Variation
Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result.
Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.

18. Mitosis vs Meiosis