MEIOSIS Making Haploid n Gametes Meiosis Cell division

MEIOSIS: Making Haploid (n) Gametes

Meiosis • Cell division that reduces the chromosome number by one-half. • Diploid (2 n) haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis I and meiosis II). II http: //vcell. ndsu. nodak. edu/animations/meiosis/01_ meiosis 01. htm

Meiosis: • Sex cells are gametes (sperm or egg) • Gametes have half the # of chromosomes • Meiosis occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis

Spermatogenesis n=23 human sex cell sperm n=23 2 n=46 haploid (n) diploid (2 n) n=23 meiosis II

Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres • Centriole pairs also replicate.

Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

Meiosis I (four phases) • Four phases: phases a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • Synapsis occurs: homologous chromosomes come together to form a tetrad • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids

Homologous Chromosomes • Pair of chromosomes (maternal and paternal) paternal that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus (position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes

Homologous Chromosomes eye color locus hair color locus Paternal Maternal

Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata • Crossing over: over segments of nonsister chromatids break and reattach to the other chromatid • Chiasmata (chiasma) are the sites of crossing over

Crossing Over - variation nonsister chromatids chiasmata: site of crossing over Tetrad variation

Sex Chromosomes XX chromosome - female XY chromosome - male

Prophase I spindle fiber aster fibers centrioles

Metaphase I • The spindle fibers, or microtubules, attach to the centromere of each chromosome. • Tetrads align on the metaphase plate • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation

Metaphase I OR metaphase plate

Question: • In terms of Independent Assortment how many different combinations of sperm could a human male produce?

Answer • Formula: 2 n • Human chromosomes: 2 n = 46 n = 23 • 223 = ~8 million combinations

Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres

Anaphase I

Telophase I In telophase I, I the spindle fibers break down, the nuclear membrane reforms, and the chromosomes return to an uncondensed state. • Cytokinesis occurs and two daughter cells are formed.

Telophase I

Meiosis II • No interphase II (or very short - no more DNA replication) replication • Remember: Meiosis II is similar to mitosis

Prophase II • Chromosomes condense once again, the nuclear membrane breaks down and a spindle apparatus begins to form in each of the daughter cells.

Metaphase II • Spindle fibers attach to the centromeres of each sister chromatid, and the chromosomes align at the equator of each cell. • The alignment of the sister chromatids is completely random. metaphase plate

Anaphase II • The cells elongate and the sister chromatids separate and are pulled to opposite ends of the cells. • The sister chromatids are now considered chromosomes.

Telophase II • Chromosomes uncoil, new nuclear membranes form and the spindle fibers are broken down. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg

Telophase II

Variation • Important to population as the raw material for natural selection. • Question: What are three sexual sources of genetic variation?

Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type.

Sexual Reproduction: • If a haploid (n) male gamete and a haploid (n) female gamete combine, they form a new diploid (2 n) zygote which can go on to become an embryo.

Fertilization • The fusion of a sperm and egg to form a zygote • A zygote is a fertilized egg n=23 egg sperm n=23 2 n=46 zygote