Chapter 11.4 (Pg. 323-329): Meiosis

Chromosome Number - Is how many chromosomes a cell has - Is represented by the letter N - Can be either haploid or diploid Changes during S phase of interphase, and during cell division

Chromosome Number - Diploid A cell has two copies of each chromosome Is written as 2N These two copies are homologous One set came from each parent Have the same genes in the same locations Diploid cells are created by mitosis

Chromosome Number - Haploid A cell has one copy of each chromosome Is written as N Most often occurs in gametes Haploid cells are created by meiosis Haploid sperm can combine with a haploid egg to make a diploid zygote, or fertilized egg

Phases of Meiosis Occur in two stages, Meiosis I and Meiosis II Each contains prophase, metaphase, anaphase, telophase, and cytokinesis Interphase occurs before Meiosis I, but not before Meiosis II

Meiosis I Interphase DNA is copied during S phase of Interphase Organelles and molecules are copied to prepare for cell division

Meiosis I - Prophase I - Spindle forms - Nuclear membrane disappears - Chromosomes condense - Homologous chromosomes pair up - Forms a tetrad Homologous chromosomes cross over Chromatid on one chromosome swaps a section with another This creates genetic variation

Meiosis I - Metaphase I - Homologous chromosomes line up across the center of the cell

Meiosis I - Anaphase I Spindle fibers pull each homologous chromosome pair towards opposite ends of the cell

Meiosis I Telophase I and Cytokinesis A nuclear membrane forms around each cluster of chromosomes Spindle disappears The cell membrane pinches in to separate the cytoplasm into two haploid daughter cells

Meiosis II - Prophase II Chromosomes condense Nuclear membranes disappear Spindles form

Meiosis II Metaphase II Chromosomes line up along the middle of the cell

Meiosis II - Anaphase II Individual chromatids move to opposite sides of the cell

Meiosis II Telophase II and Cytokinesis Nuclei re-form The cell membrane pinches in to separate the cytoplasm into four genetically unique haploid daughter cells

Differences Between Mitosis and Meiosis Makes genetically identical cells Makes diploid cells Maintains the original chromosome number Makes two daughter cells Uses only one cell division No pairing of homologous chromosomes Only separates sister chromatids No crossing over occurs Used for many cellular processes Makes genetically unique cells Makes haploid cells Reduces the chromosome number by half Makes four daughter cells Uses two cell divisions Pairs homologous chromosomes together Separates homologous chromosomes, then sister chromatids Crossing over occurs Only makes gametes for reproduction

Linked Genes Alleles of different genes tend to be inherited together from one generation to the next when those genes are located on the same chromosome Can be shown with a gene map Indicates the location of each gene on a chromosome