The steps of MEIOSIS Meiosis occurs only in the sex cells (the ovaries or the testes) Gametes (sperm or eggs) have half the genetic information of a somatic (body) cell Meiosis is the process that takes a sex cell (diploid) and turns it into four gametes (haploid) In humans, meiosis takes 46 chromosomes (23 pairs) in one cell and gives four haploid gametes (23 chromosomes each)

INTERPHASE uncondensed chromatin nuclear membrane

EARLY PROPHASE I bivalent 1. the chromatin condenses and is visible as thin 2-stranded bivalents 2. -the nuclear membrane disappears

MID-PROPHASE I homologous chromosome pair 1. -the chromosomes continue to condense and are now visible as 4-stranded tetrads 2. -the tetrad consists of two similar chromosomes, one from each parent, called homologous chromosomes 3. centriole duplicates centriole

Meiosis I – The Breakdown -All of the Chromosomes here have already been duplicated before meiosis begins. - The Big Chromosomes here represent one homologous pair - The little chromosomes are also one homologous pair of a different chromosome. -The black chromosomes came from one parent, the grey from the other.

LATE-PROPHASE I chiasmata 1. pieces of the homologous pair break and exchange segments with other strands in crossing over 2. the structures are now called chiasmata as many are cross-shaped

METAPHASE I 1. centrioles move to the poles 2. spindle fibres attach to the centromeres 3. homologus pairs are pulled to the cell equator by the contracting spindle fibres

ANAPHASE I 1. spindle fibres contract pulling apart the homologus pairs 2. double-stranded chromosomes are pulled to opposite poles

TELOPHASE I 1. the homologus pair of chromosomes are fully separated 2. the sister chromatids remain together 3. the nuclear membrane re-appears 4. the centrioles and spindle fibres disappear

Meiosis II 1. Meiosis II proceeds just like mitosis, but with half as many double-stranded chromosomes The final product is 4 non-identical cells each with single-stranded chromosomes due to independent assortment [see pg 135]

Meiosis II – The Breakdown 1. Independent assortment refers to the way that chromosomes line up on the Metaphase II plate. 2. Note how each of the cells below has one copy of the larger chromosome and one copy of the smaller. These cells are haploid because they have one of each. NOT because of THIS difference 

Terms for cells: diploid [2N]: 2 sets of information or chromosomes haploid [1N]: 1 set of information or chromosomes all cells are diploid prior to meiosis at the end of meiosis I, the two sets of information or chromosomes has been reduced to one set [diploid  haploid]

ASSIGNMENT Using the just learned terms complete the Chromosome Numbers Worksheet Then, using the just completed handout and pg 134-135 to assist you complete the MITOSIS and MEIOSIS assignment.