Bellringer If normal cells have 46 chromosomes, how many chromosomes do you think gametes (sex cells) have and why?

Meiosis Division of Sex Cells

Meiosis A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Diploid – 2 sets of chromosomes Haploid – 1 set of chromosomes Homologous – chromosomes that each have a corresponding chromosome from the opposite sex parent

Meiosis

Meiosis Principle of genetics requires 2 things 1. Each organisms must inherit a single copy of every gene from each of it’s “parents.” 2. When an organisms produces gametes, those two sets of genes must be separated from each other so that each gamete contains just one set of genes.

Meiosis Stages Meiosis usually involves 2 distinct stages Meiosis I Meiosis II

Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromosomes in a tetrad. The pairing of homologous chromosomes is the key to understanding meiosis. Crossing-over may occur here Crossing-over is when chromosomes overlap and exchange portions of their chromatids.

Prophase I

Metaphase I Spindle fibers attach to the chromosomes Chromosomes line up in the middle in pairs

Metaphase I

Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell.

Anaphase I

Telophase I & Cytokinesis Nuclear membranes form. The cell separates into 2 cells.

Telophase I

Prophase II Meiosis I results in two haploid (N) cells. Each cell has half the number of chromosomes as the original cell.

Prophase II

Metaphase II The chromosomes line up similar to metaphase in mitosis.

Metaphase II

Anaphase II Sister chromatids separate and move to opposite ends of the cell.

Anaphase II

Telophase II Meiosis II results in 4 haploid cells.

Telophase II

Gamete Formation In males, meiosis results in 4 sperm cells In females, meiosis results in 1 egg cell and three polar bodies, which are not used in reproduction.

Mitosis vs Meiosis Mitosis Meiosis Results in 2 Diploid Cells (2N) 4 Haploid Cells (N) Cells are Genetically Identical Genetically Different Occurs in Somatic (Body) Cells Sex Cells

Cell Cycle Regulation Cyclin regulate the timing of the cell cycle in eukaryotic cells. Internal regulators Proteins that respond to events inside the cell. External regulators Proteins that respond to events outside the cell Directs cells to speed up or slow down the cell cycle. Growth factors stimilate the growth and division of cells. Uncontrolled cell growth can lead to cancer Cancer: a disorder in which some of the body’s own cells lose the ability to control growth. They do not respond to the signals that regulate the growth of most cells. Most cancer cells have a defect in p53 gene, which normally stops the cell cycle until all chromosomes have been properly replicated.