1. Meiosis
Modified by Liz LaRosa 2011

2. Human body cells have 46 chromosomes
Sexual Reproduction and Genetics
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
Sexual Reproduction and Genetics
Homologous: pair of chromosomes line up with the same genes in the same locations. One set comes from the mother and the other set from the father.
Meiosis – Homologous Chromosomes
Chromosomes and Chromosome Number
Same length
Same centromere position
centromeres
Carry genes that control the same inherited traits
One is from mother, one is from father = a matching set

4. Haploid and Diploid Cells
Sexual Reproduction and Genetics
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.
A cell with n chromosomes is called a haploid cell.
Haploid – contains ½ the number of chromosomes as a diploid cell
A cell that contains 2n chromosomes is called a diploid cell.

5. The sexual life cycle in animals involves meiosis.
Sexual Reproduction and Genetics
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.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Interphase
Chromosomes replicate.
Chromatin condenses (thickens)
Interphase
Chromatin is the material chromosomes are made of.

7. Pairing of homologous chromosomes occurs.
Sexual Reproduction and Genetics
Meiosis
Homologous: pair of chromosomes line up with the same genes in the same locations. One set comes from the mother and the other set from the father.
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. Crossing over produces exchange of genetic information.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Prophase I
Crossing over produces exchange of genetic information.
Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes.

9. Chromosome centromeres attach to spindle fibers.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Metaphase I
Chromosome centromeres attach to spindle fibers.
Metaphase I
Homologous chromosomes line up at the equator.

10. Homologous chromosomes separate and move
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Anaphase I
Homologous chromosomes separate and move
to opposite poles of the cell.
Anaphase I

11. The spindles break down.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
Telophase I
The spindles break down.
Telophase I
Chromosomes uncoil and form two nuclei.
The cell divides.

12. A second set of phases begins
Sexual Reproduction and Genetics
Meiosis
Meiosis II
Prophase II
A second set of phases begins
as the spindle apparatus forms and the
chromosomes condense.
Prophase II

13. A haploid number of chromosomes
Sexual Reproduction and Genetics
Meiosis
Meiosis II
Metaphase II
A haploid number of chromosomes
line up at the equator.
Metaphase II
Haploid- only ½ the chromosomes.

14. The sister chromatids are
Sexual Reproduction and Genetics
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
Chromatid: one copy of the duplicated chromosome.

15. The chromosomes reach the poles, and
Sexual Reproduction and Genetics
Meiosis
Meiosis II
Telophase II
The chromosomes reach the poles, and
the nuclear membrane and nuclei reform.
Telophase II

16. Sexual Reproduction and Genetics
Meiosis
Meiosis II
Cytokinesis results in four haploid cells, each with n number of chromosomes.
Cytokinesis
Cytokinesis is the physical process of cell division.

17. The Importance of Meiosis
Sexual Reproduction and Genetics
Meiosis
The Importance of Meiosis
Meiosis consists of two sets of divisions
Produces four haploid daughter cells that are not identical (contains only ½ of chromosomes)
Results in genetic variation

18. Meiosis Provides Variation
Sexual Reproduction and Genetics
Meiosis
4 gametes = 4 sex cells either 4 sperm or 4 eggs
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.

19. Mitosis vs Meiosis Replication of Body Cells Replication of Sex Cells:
2n = diploid cell with all pairs of chromosomes
1n = haploid cell with only ½ the number of crhomosomes
Mitosis vs Meiosis
Replication of Body Cells
Replication of Sex Cells:
Egg & sperm
4 sperm or 4 egg sex cells produced. All are genetically different