MEIOSIS MEIOSIS

KEY CONCEPT Gametes have half the number of chromosomes that body cells have. MEIOSIS : CELL TYPES

 You have Body cells and Gametes  Body cells are also called somatic cells.  Germ cells develop into gametes.  Germ cells are located in the ovaries and testes.  Gametes are sex cells: egg and sperm.  Gametes have DNA that can be passed to offspring. MEIOSIS : ALL ABOUT CHROMOSOMES Your cells have autosomes & sex chromosomes • Your body cells have 23 pairs of chromosomes. – Homologous pairs of chromosomes have the same structure and genetic material. – For each homologous pair, one chromosome comes from each parent. • Chromosome pairs 1-22 are autosomes. • Sex chromosomes, X and Y, determine gender in mammals. MEIOSIS : DIPLOID AND HAPLOID

 Diploid (2n) cells have two copies of every chromosome.  Body cells are diploid.  Half the chromosomes come from each parent.  In humans this means we have 2 sets of 23 chromosomes (total of 46) MEIOSIS : DIPLOID AND HAPLOID

 Haploid (n) cells have one copy of every chromosome. – Gametes are haploid. – In humans gametes have 22 autosomes and 1 sex chromosome. (23 total) MITOSIS REVIEW  Goal of Mitosis is Chromosome number must be maintained. • Mitosis and meiosis are types of nuclear division that make different types of cells with different number of chromosomes. • Mitosis makes more diploid cells. • Number of Chromosomes in parent is Equal to the number of chromosomes in offspring.

INTRODUCTION TO MEIOSIS

 Meiosis makes haploid cells from diploid cells.  This happens because cells go through TWO rounds of division during meiosis.

 Meiosis reduces chromosome number and creates genetic diversity.  Meiosis Occurs in Sex cells  Meiosis Produces gametes.

MEIOSIS DETAIL KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells. homologous chromosomes Meiosis I and Meiosis II  Meiosis I and meiosis II each have four phases, similar to those in mitosis.  Biggest difference is the how the DNA organizes for each division.  In Meiosis I Pairs of homologous chromosomes form tetrads.

sister sister chromatids chromatids MEIOSIS DETAIL KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

 Meiosis I pairs of homologous chromosomes (tetrads) separate.  Homologous chromosomes are similar but not identical genetically  In Meiosis II SISTER CHROMATIDS separate. MEIOSIS I DETAILS

• Meiosis I occurs after DNA has been replicated. • Meiosis I divides homologous chromosomes in four phases. Prophase I Metaphase I Anaphase I Telophase I 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. CROSSING OVER !!!!!

They do the genetic TANGO! PROPHASE I (PICTURE)

Homologous Chromosomes (called Tetrads) are held together by a synapse METAPHASE I

Tetrads line up along the equator

Spindle fibers attach to the centromeres of each chromosome in the tetrad. ANAPHASE I The cell begins lengthening. The spindle fibers pull the homologous chromosomes apart and toward opposite ends of the cell. Sister chromatids attached at centromeres move together Disjunction Occurs – process of separation There are ½ as many chromosomes as in the original cell but the chromosome is double stranded.

TELOPHASE I

Movement of homologous chromosomes continues until there is a haploid set at each pole Each chromosome = linked sister chromatids At each pole, now, there is a complete haploid set of chromosomes (but each chromosome still has two sister chromatids). Nuclear membranes reforms. A cleavage furrow appears. The cell separates into 2 daughter cells. TELOPHASE I IMPORTANT NOTE

IN BETWEEN MEIOSIS I and

MEIOSIS II

there is no INTERPHASE!!!! PROPHASE II

• Meiosis I results in two haploid (N) cells. • Each cell has half the number of chromosomes as the original cell. • Spindle Reforms • Sister Chromatids are present METAPHASE II

 The Sister Chromatids line up .along the equator (middle) of the cell similar to metaphase in mitosis ANAPHASE II

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

• Nuclear membranes form around each set of chromosomes at opposite ends of the cell • Spindle fibers break apart • Cell undergoes cytokinesis • End result is four HAPLOID cells with genetically unique information.

COMPARE AND CONTRAST • Meiosis differs from mitosis in significant ways. – Meiosis has two cell divisions while mitosis has one. – In mitosis, homologous chromosomes never pair up. – Meiosis results in haploid cells; mitosis results in diploid cells.

GAMETOGENESIS Haploid cells develop into mature gametes.

• Gametogenesis is the production of gametes. • Gametogenesis differs between females and males. In Males: SPERMATOGENESIS – Sperm become streamlined and motile. – Sperm primarily contribute half of the DNA to an embryo.

GAMETOGENESIS In Females: Oogenesis – Eggs contribute half of DNA, all of the cytoplasm, and all organelles to an embryo. – During meiosis, the egg gets most of the contents; the other cells form polar bodies. – So only one functional gamete per cycle of meiosis