1. Sexual Reproduction and Genetics Section 1- Meiosis
Chapter 10
Sexual Reproduction
and Genetics
Section 1- Meiosis

2. Chromosomes and Chromosome Number
Section 1 - Meiosis
Sexual Reproduction and Genetics
Chromosomes and Chromosome Number
Human body cells have 46 chromosomes.
Each parent contributes 23 chromosomes.
Homologous chromosomes—one of two paired chromosomes, one from each parent.

4. Chromosomes and Chromosome Number
Section 1 - Meiosis
Sexual Reproduction and Genetics
Chromosomes and Chromosome Number
Homologous Chromosomes:
Same length.
Same centromere position.
Carry genes that control the same inherited traits.

5. Haploid and Diploid Cells
Section 1 - Meiosis
Sexual Reproduction and Genetics
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.

7. The sexual life cycle in animals involves meiosis.
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis
The sexual life cycle in animals involves meiosis.
Meiosis produces gametes.
When gametes combine in fertilization, the number of chromosomes is restored (back to 2n – diploid).

9. Meiosis is a reduction division.
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis
Meiosis is a reduction division.
In other words, it reduces the chromosome number by half through the separation of homologous chromosomes.
In humans - from 46 chromosomes to 23.
Meiosis involves two consecutive cell divisions called meiosis I and meiosis II.

11. Chromosomes replicate. Synthesis of proteins.
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis I
Interphase
Chromosomes replicate.
Synthesis of proteins.

12. Meiosis I Prophase I Chromosomes coil and condense.
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis I
Prophase I
Chromosomes coil and condense.
Pairing of homologous chromosomes occurs.
Each chromosome consists of two chromatids.
The nuclear envelope breaks down.
Centrioles moves toward poles.
Spindles form and bind to centromeres.

13. Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis I
Prophase I
Crossing over produces exchange of genetic information.
Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes.

15. Homologous chromosomes line up at the equator of the cell.
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis I
Metaphase I
Homologous chromosomes line up at the equator of the cell.

16. Meiosis I Anaphase I Homologous chromosomes separate and move to
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis I
Anaphase I
Homologous chromosomes
separate and move to
opposite poles of the cells.
Chromosome # is reduced from 2n to n when homologous chromosomes separate.

17. The spindles break down. Chromosomes uncoil and form two nuclei.
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis I
Telophase I
The spindles break down.
Chromosomes uncoil and form two nuclei.
The cell divides.

18. apparatus forms and the chromosomes condense.
Section 1 – Meiosis
Sexual Reproduction and Genetics
Meiosis II
Prophase II
A second set of phases
begins as the spindle
apparatus forms and the
chromosomes condense.

19. Meiosis II Metaphase II A haploid number of chromosomes line up
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis II
Metaphase II
A haploid number of
chromosomes line up
at the equator.

20. Meiosis II Anaphase II The sister chromatids are pulled apart at the
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis II
Anaphase II
The sister chromatids are
pulled apart at the
centromere by spindle fibers and
and move toward the opposite poles of the cell.

21. Meiosis II Telophase II The chromosomes reach
Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis II
Telophase II
The chromosomes reach
the poles, and the nuclear
membrane and nuclei
reform.

22. Section 1 - Meiosis
Sexual Reproduction and Genetics
Meiosis II
Cytokinesis results in four haploid cells, each with n number of chromosomes.

25. Meiosis videos

26. The Importance of Meiosis
Section 1 - Meiosis
Sexual Reproduction and Genetics
The Importance of Meiosis
Meiosis consists of two sets of divisions.
Produces four haploid daughter cells that are not identical.
Results in genetic variation.

27. Section 1 - Meiosis
Sexual Reproduction and Genetics
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.

29. Law of Independent Assortment
Section 2 - Meiosis
Sexual Reproduction and Genetics
Law of Independent Assortment
Random distribution of alleles occurs during gamete formation.
Genes on separate chromosomes sort independently during meiosis.
Each allele combination is equally likely to occur.

30. Law of Independent Assortment
Section 2 - Meiosis
Sexual Reproduction and Genetics
Law of Independent Assortment
Two pairs of homologous chromosomes can be arranged in two different ways at meiosis I of meiosis.
These arrangements occur randomly, with approximately half the meiotic cells oriented one way and the other half oriented the opposite way.

31. Independent Assortment
Section 2 – Mendelian Genetics
Sexual Reproduction and Genetics
Independent Assortment

33. Section 2 - Meiosis
Sexual Reproduction and Genetics
Crossing Over
During prophase I, nonsister chromatids of homologous chromosomes exchange pieces of genetic material.
As a result each homologue no longer entirely represents a single parent.

34. Section 2 - Meiosis
Sexual Reproduction and Genetics
Crossing Over
Free ends of one maternal and one paternal chromatid wrap around each other at one or more points.
Crossover allows the paired maternal and paternal chromosomes to exchange genetic material.

35. Sexual Reproduction and Genetics
Mitosis vs. Meiosis

38. Take out your book and copy Table 1 from page 275.
Sexual Reproduction and Genetics
Mitosis vs. Meiosis
Mitosis vs. Meiosis
Take out your book and copy Table 1 from page 275.
Mitosis
Meiosis