CELL CYCLE Meiosis Homologous Chromosomes n n Homologous

CELL CYCLE Meiosis

Homologous Chromosomes n n Homologous chromosomes contain DNA that codes for the same genes. In this example, both chromosomes have all the same genes in the same locations (represented with colored strips), but different “versions” of those genes (represented by the different Sister chromatids are exact replicas. shades of each color). Homologous chromosomes are not

Chromosome Pairing Humans have 46 chromosomes arranged in 23 pairs n 22 of the 23 pairs are homologous, meaning that the 2 chromosomes in each pair are similar with regard to shape, size, banding pattern, etc. n The 23 rd pair is homologous in females but not in males n

Haploid vs. Diploid The body cells of every species have a characteristic number of chromosomes n Diploid: any cell that contains two complete sets of chromosomes (2 n) n Haploid: a cell with only one complete set of chromosomes; these are also known as sex cells or gametes (n) n

Haploid vs. Diploid (cont. ) Gametes must be haploid n If they weren’t, the chromosome number would double 46 +46 = 92; 92 + 92 =184 n Even one extra chromosome can cause serious defects n

Meiosis n n To keep the number of chromosomes stable from generation to generation, haploid cells are produced by meiosis Meiosis is a type of cellular reproduction in which the number of chromosomes is reduced by half, so that the daughter cells are haploid Haploid gametes combine to create a diploid offspring which grows by mitosis into an adult As an adult, the offspring will produce haploid gametes

Phases of Meiosis First, the cell replicates its DNA during interphase; this is followed by meiosis n Meiosis is divided into 2 parts: n ¨ Meiosis II In some organisms, these phases are divided by interkinesis n In other organisms, meiosis I is immediately followed by meiosis II n

Meiosis I n Prophase I ¨ Chromosomes condense ¨ Each homologous pair becomes attached to one another forming a TETRAD [recall that tetrads do not form in mitosis] ¨ Tetrads exchange parts of their chromatids in a process called crossing over

Meiosis I (cont. ) n Metaphase I ¨ Spindle fibers attach to the chromosomes n Anaphase I ¨ Spindle fibers move homologous chromosomes to extreme ends of the cell n Telophase I ¨ Nuclear membranes form

Interkinesis The cytoplasm divides creating 2 cells n Each chromosome in the haploid cells consists of 2 sister chromatids n

Meiosis II n Prophase II ¨ Spindle fibers form again ¨ Chromosomes condense n Metaphase II ¨ Spindle fibers line up the chromosomes in the center of the cell

Meiosis II (cont. ) n Anaphase II ¨ Sister chromatids are separated and pulled to opposite poles of the cell n Telophase II ¨ Nuclei form ¨ Spindle fibers break down

Cytokinesis Just as in mitosis, meiosis is followed by cytokinesis n Cytokinesis produces 4 haploid cells n Each cell contains one copy of one complete set of chromosomes (n) n

Male Cytokinesis n In male animals, the cell divisions that occur after meiosis I and II are equal 4 sperm cells are created

Female Cytokinesis In female animals, cytokinesis divides the cytoplasm unevenly so that one cell receives almost all of the cytoplasm n Only one cell, called the ovum, is formed; the other 3 cells, called polar bodies, die n