Turner College & Career High school  2015 meiosis Turner College & Career High school  2015

Characteristics of living things Living things are made of cells. Living things grow and develop.

Meiosis Meiosis involves two successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) cells, each with half of the genetic material of the original cell. Through the mechanisms by which paternal and maternal chromosomes segregate, and the process of crossing-over, genetic variation is produced in the haploid cells.

Chromosome In body cells vs. gametes? Body (Somatic) Cells: Diploid (2N) - have 2 complete sets of chromosomes and genes. Human diploid number is 46. Gametes (Sex Cells): Haploid (N) - have a single set of chromosomes and genes. Sperm cells and ova are gametes.

Chromosomes If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA). Most organisms are diploid. Humans have 23 pairs of chromosomes… therefore humans have 46 total chromosomes… The diploid number for humans is 46 (46 chromosomes per cell).

Homologous Chromosomes Pair of chromosomes (maternal & paternal) that are similar in shape and size. Tetrads (homologous pairs) carry genes controlling the same inherited traits. Each locus (position of a gene) is in the same position on each homologue. Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes

Homologous Chromosomes A homologous pair consists of 4 chromatids called a Tetrad. Locus for Eye Color Locus for Enzyme Locus for Hair Color Maternal Paternal

Meiosis is reduction division The process by which gametes, with half the number of chromosomes, are produced. During meiosis diploid cells are reduced to haploid cells Diploid (2n)  Haploid (n) If meiosis did not occur the chromosome number in each new generation would double… the offspring would die.

Meiosis is reduction division The chromosome number is cut in half by the separation of homologous chromosomes in diploid cells. Meiosis involves two distinct divisions called Meiosis I and Meiosis II.

Meiosis I (4 phases) Cell division that reduces the chromosome number by one-half. Four phases: Prophase I Metaphase I Anaphase I Telophase I

Prophase I Longest and most complex phase. 90% of the meiotic process is spent in Prophase I Chromosomes condense. Synapsis occurs: homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids). The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Chiasma are the sites of crossing-over.

Prophase I: Synapsis & Crossing-Over

Prophase I Chromosomal crossover (or crossing over) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes. It occurs during prophase I of meiosis in a process called synapsis.

Metaphase I Shortest phase Tetrads align on the metaphase plate. Independent assortment occurs: Orientation of homologous pair to poles is random. Produces variation

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

Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

Prophase II Same as prophase in mitosis.

Metaphase II

Anaphase II

Telophase II Cytokinesis occurs. Remember: four haploid daughter cells produced. Gametes = Sperm or Egg

What are the results of meiosis? 4 haploid (N) cells. Genetically different from each other & the original cell.

MEIOSIS I

MEIOSIS II Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells.

Differences between Mitosis & Meiosis? Mitosis produces 2 identical diploid (2N) cells. Meiosis produces 4 different haploid (N) cells.

MEIOSIS PROCESS OF REDUCTION DIVISION Homologous Chromosomes Interphase Meiosis I Daughter Cells I Meiosis II

Summarize the events of meiosis.

Human cells 2N=46. How many chromosomes are in a…? Sperm Cell? 23 chromosomes. Gamete is haploid (N). Egg Cell? White Blood Cell? 46 chromosomes. Body cell is diploid (2N).