Chromosomes and Chromosome Number Human body cells have 46 chromosomes Each parent contributes 23 chromosomes Homologous chromosomes -one of two paired chromosomes, one from each parent

Homologous Chromosomes Same length Same centromere position Carry genes that control the same inherited traits One came from Dad, & one came from Mom!

Why Meiosis? An organism produces gametes to maintain the same number of chromosomes from generation to generation, and also introduce and maintain variation within the population. We will discuss later, what happens when there is an error in meiosis. Mutations and Evolutionary events

Compare diploid and haploid number Diploid: cell that contains two of each kind of chromosome (2N) Body cells(somatic cells) are diploid Haploid: cell with only one kind of chromosome (N) Gametes (sex cells) are haploid

Meiosis I The sexual life cycle in animals involves meiosis. Meiosis produces gametes. When gametes combine in fertilization, the number of chromosomes is restored. This is often called alternation of generations, the cycling between haploid and diploid organisms (cells)

Meiosis I Prophase I- each chromosome pairs with its corresponding homologous chromosome to form a tetrad(4 chromatids) The nuclear envelope breaks down. Spindles form. *Crossing-over- chromosomes will switch some genes. This gives us genetic variation. Crossing-over is more common the farther it is located from the centromere. Remember map units in Sordaria

Crossing Over

Metaphase I Homologous pairs of chromosomes, still in their tetrad, line up together at the middle of the cell Chromosome centromeres attach to spindle fibers.

Anaphase I homologous chromosomes separate (the tetrad seperates) & move to opposite ends. (centromeres do not split)

Telophase I & Cytokinesis spindle disappears and the cell divides Meiosis I results in 2 daughter cells, each with the same number of chromosomes as the original cell however sister chromatids are still attached to each other

Meiosis II spindle apparatus reforms and the During the second meiotic division, nothing is replicated, everything is going to be just divide Prophase II spindle apparatus reforms and the chromosomes condense, while the nuclear membrane dissolves again

Metaphase II chromosomes still attached at the centromere line up at the equator (middle)

Anaphase II sister chromatids are pulled apart at the centromere & move to opposite poles

Telophase II (includes cytokinesis) when the 2 daughter cells divide into 4 new daughter cells, each new daughter cell has 23 chromosomes. This is half the number of the original parent cell

Difference in meiosis In males: spermatogenesis In females: oogenesis 4 mature sperm Males begin to produce sperm after puberty, produced constantly until death; meiosis II occurs immediately after meiosis I Much smaller than egg May have X or Y chromosomes Have flagella to move In females: oogenesis 1 mature egg, 3 polar bodies which break down Women born with all eggs they will have, meiosis I occurs before birth, meiosis II occurs once a month Much larger Have all X chromosomes Has no method of movement

Meiosis/gamete production Females have XX as their 23rd pair of chromosomes Males have XY which are nonhomologous Since all eggs are X, father determines the sex of the child since the sperm may be X or Y

Meiosis 23 + 23 = 46 chromosomes There are 2 divisions of meiosis- Meiosis I and Meiosis II. It must be this way in order to end up with the sex cells (sperm & egg) only having 23 chromosome. Meiosis occurs in the testes of the male and is called spermatogenesis (4 viable sperm) And in the ovaries of the female it is called oogenesis. ( produces 1 egg and 3 polar bodies) 23 + 23 = 46 chromosomes

Mitosis/Meiosis Mitosis Cell division producing identical somatic cells Results in 2 daughter cells One division Daughter cells have same number of chromosomes (2N) Meiosis Cell division producing unique gametic cells Results in 4 daughter cells Two divisions Daughter cells have half the number of chromosomes (N)

How cells divide:Mitosis vs. meiosis Cell division cell type # of daughter # of cells chromosomes Mitosis somatic(body) 2 46(diploid) Meiosis gametes(sex) 4 23(haploid) www.pbs.org How cells divide:Mitosis vs. meiosis