Meiosis Chapter 7 Section 1

Mitosis vs. Meiosis Meiosis: Mitosis: occurs because the cell gets too big and for healing. produces genetically identical diploid daughter cells. Meiosis: occurs in the production of gametes. produces haploid daughter cells.

Somatic vs. Gametes Somatic Cells – They are all the cells in the body except for sperm and eggs Always diploid! Produced by mitosis. Gametes – In males – sperm/pollen In females – eggs Always haploid! Produced by meiosis.

Gametes Each sperm and egg cell contains half the normal number of chromosomes – essential to sexually reproducing organisms. When a egg is fertilized by the sperm, the offspring inherits one chromosome for each homologous pair from each parent. It gains the normal number of chromosomes.

Overview of Meiosis G1 Phase Meiosis produces gametes through two stages of cell division. Meiosis begins with a cell that has the normal number of chromosomes. Due to the cell having completed the S (synthesis) phase of the cell cycle, the chromosome has replicated DNA. After S Phase G2 Phase

Overview of Meiosis Second reduction Meiosis I: separation of homologous chromosomes – with each of the 2 new daughter cells get one chromosome from each pair. Meiosis II: each chromosome is separated into two chromosomes, so that each of the four daughter cells get half the normal number of chromosomes First reduction Unreplicated chromosomes

Phases of meiosis During meiosis, two cell divisions – divided into 2 stages – stages I and II. Each stage, the cell undergoes the same phases as mitosis: prophase, metaphase, anaphase and telophase.

phases of Meiosis Prophase I: Nuclear envelope breaks down. Homologous chromosomes pair up Crossing over occurs.

Crossing-over Crossing-over occurs in Prophase I after the homologous chromosomes pair up. DNA strands from one chromosome join with the other, breaking off and piecing themselves back together. END RESULT: each chromosome contains pieces of DNA from its homolog

Result of Crossing-Over Each homolog has a slightly different set of alleles. Crossing-over shuffles the genetic material so that each sister chromatid has a different combination of alleles. Instead of the two original allele combinations, there are now four

Result of crossing-over Crossing over makes it possible for offspring to inherit unique combinations of alleles. This increases the genetic variation in the gene pool of a species.

phases of Meiosis Metaphase I: Spindles attach to the centromeres of the homologues. Homologous chromosomes align at the equator.

phases of Meiosis Anaphase I: Spindles shorten Homologous chromosomes separate

phases of Meiosis Telophase I: Cytokinesis occurs. Two cells have formed. At the end of telophase I, one parent cell has been divided into two new daughter cells with half the number of chromosomes – haploid cell.

phases of Meiosis Prophase I Prophase II: Chromosomes condense Spindle fibers form Prophase II

phases of Meiosis Metaphase I Metaphase II: Spindles attach to the centromeres of the sister chromatids. Chromosomes align at the equator. Metaphase II

phases of Meiosis Anaphase I Anaphase II: Spindle fibers shorten. Sister chromatids separate. Anaphase II

phases of Meiosis Telophase I Telophase II: Chromosomes begin to unwind and stretch out. Nuclear envelope reforms. Cytokinesis occurs. Telophase II At the end of telophase II, two daughter cells have been divided into four new daughter that are haploid.

Review Time! Mitosis Meiosis I Meiosis II Number of daughter cells   Mitosis Meiosis I Meiosis II Number of daughter cells Chromosome number Crossing-over occurs (yes/no) Sister chromatids separate Daughter cells genetically identical to parent cell Daughter cells genetically identical to each other