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-IDENTICLE cells
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
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.
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 (thickens) Interphase Chromatin is the material chromosomes are made of.
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.
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 at the equator.
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 at the equator. Metaphase II Haploid- only ½ the chromosomes.
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.
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
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.
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
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.
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