Meiosis Modified by Liz LaRosa 2011

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-IDENTICAL cells

Chromosomes and Chromosome Number Sexual Reproduction and Genetics 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

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. A cell that contains 2n chromosomes is called a diploid cell.

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.

Chromosomes replicate. Sexual Reproduction and Genetics Meiosis Meiosis I Interphase Chromosomes replicate. Chromatin condenses. Interphase

Pairing of homologous chromosomes occurs. Sexual Reproduction and Genetics 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.

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.

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 the middle of the cell.

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

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.

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

A haploid number of chromosomes Sexual Reproduction and Genetics Meiosis Meiosis II Metaphase II A haploid number of chromosomes line up in the middle. Metaphase II

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

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

Division of the cytoplasm Sexual Reproduction and Genetics Meiosis Meiosis II Cytokinesis Division of the cytoplasm Results in four haploid cells, each with n number of chromosomes. Cytokinesis

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 Results in genetic variation

Meiosis Provides Variation Sexual Reproduction and Genetics 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.

Mitosis vs Meiosis