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

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Presentation transcript:

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

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

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

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.

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.

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

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.

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.

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

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

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

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

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

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

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

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 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