3.02: Cell Types and Chromosome Number In an organism, there are somatic cells and there are sex cells. o Somatic cells are all of the body’s cells that are not sex cells. o Sex cells are sperm and egg. Somatic cells have the diploid number of chromosomes, which means one set from the mother and one set from the father. o Diploid = full set (2n)

Cell Types and Chromosome Number Humans have 46 chromosomes in their somatic cells, 23 from Mom and 23 from Dad, that are paired together. These paired chromosomes are called homologous, meaning they are similar in size, shape, and genetic content.

Cell Types and Chromosome Number Gametes (sex cells, egg or sperm) contain only one set of chromosomes. Since a gamete contains only one set of chromosomes, it is said to be haploid. o Haploid = half set (n) The fusion of two haploid gametes is called fertilization and results in a zygote. A zygote is a fertilized egg cell, the first cell of a new individual.

3.02: Meiosis Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes or spores. Meiosis involves 2 divisions of the nucleus – Meiosis I and Meiosis II. Before meiosis begins, the DNA in the original cell is replicated (just like before mitosis).

Meiosis  Meiosis I (kind of like mitosis, but with pairs of chromosomes instead of singles): 1.Prophase I : the chromosomes become visible, the nuclear envelope dissolves, and homologous chromosomes pair up forming a tetrad—2 chromosomes joined together. Crossing-over occurs when portions of a chromatid on one chromosome are broken and exchanged with the same chromatid portions on the homologous chromosome.

Meiosis 2.Metaphase I : pairs of homologous chromosomes are moved by the spindle to line up at the middle of the cell. 3.Anaphase I : The homologous chromosomes separate and each is pulled to the opposite poles of the cell by the spindle fibers. The chromosomes do not separate at the centromere as in mitosis, each chromosome is still composed of 2 chromatids. 4.Telophase I : Individual chromosomes gather at each of the poles of the cell, then the cytoplasm divides (cytokinesis) forming 2 new cells.

Meiosis  Meiosis II (looks just like mitosis, but the chromosomes are genetically different than the parent cell because of crossing over): 5.Prophase II : a new spindle forms around the chromosomes. 6.Metaphase II : chromosomes line up along the middle and are attached at their centromeres to spindle fibers. 7.Anaphase II : centromeres divide, chromatids move to opposite poles of the cell. 8.Telophase II: nuclear envelope forms around each set of chromosomes, the cell undergoes cytokinesis.

Meiosis I

Meiosis II

Meiosis The end result of meiosis is 4 haploid cells from 1 diploid cell. Each new cell has half the genetic material as the original cell.