Meiosis Sex cell gamete division egg and sperm

Meiosis Sex cell (gamete) division= egg and sperm n Products= 4 cells all genetically different n How is this different from mitosis? n Mitosis=2 identical cells! (clones) n Genes are located on the chromosomes. n

Chromosome number Homologous Chromosome – The chromosomes that make up a pair, one chromosome from each parent. n Diploid= 2 n, the # of chromosomes contained in “body” cells n Gamete – sex cells that have half the number of chromosomes. n Haploid – n, the # of chromosomes found in sex cells (gametes) n

Diploid Vs. Haploid n Why do sex cells have to be haploid? n They must have half the # of chromosomes to produce a viable gamete (23 mom+ 23 dad= zygote)

Quick Thinking… If a haploid cell in a frog is 13, what is the diploid cell? n 26 n If the muscle cell of a dog has 78 chromosomes, what does it’s egg cell have? n 39 n

Here’s Some More…. n n n n If Diploid= 8 Then Haploid= 4 If Haploid=70 Then Diploid= 140 If Diploid= 46 Then Haploid= 23 If Haploid = 56 Then Diploid= 112

Phases of Meiosis – cell division that reduces the # chromosomes, referred to as “reduction division”. n Meiosis I: n – Interphase – chromosomes replicate (DNA) – Prophase I – Metapase I – Anaphase I – Telophase I

Prophase I same as mitosis PLUS… tetrads -2 homologous chromosomes pair up. n Crossing over occurs =each tetrad “swaps” genes; provides for genetic variation n

Crossing Over!!!

Metaphase I n Same as mitosis, but the chromosomes line up as PAIRS (tetrads) at the plate

Anaphase I n Same as mitosis, except the pairs are separated, not the chromosome!!

Telophase I Same as mitosis, except… n DNA does not unwind, it stays in chromosome form for the next set of division n 2 cells not identical… why? n Do the 2 daughter cells undergo interphase II?

NO INTERPHASE II n n n The DNA stays wound for round 2 of meiosis for one reason. We now have 2 diploid chromosomes, we must divide again to reach the haploid state. We do not replicate the DNA again…

Can you guess the next stages? Prophase II n Metaphase II n Anaphase II n Telophase II n Cytokinesis n All look similar to mitosis, sisters get split in Anaphase II like mitosis!!! n

Prophase II Chromosomes condense. n Spindles form in each new cell. n Spindle fibers attach to chromosomes n

Metaphase II Centromeres of chromosomes line up randomly at the equator of each cell

Anaphase II Centromeres split n Sister chromatids separate and move to opposite poles n

Telophase II 4 nuclei form around chromosomes. n Spindles break down. n Cell divides n

End Result…Finally! n n Sperm= 4 viable haploid (23) sperm Egg= 1 viable egg (23), three polar bodies

Meiosis = Genetic Variation 1) 2) 3) Independent Assortment -all pairs separate independently -gives approx. 8 million variations Crossing Over (prophase I) -adds recombination -limitless variations Random Fertilization - gametes from independent organisms squares the variations (64 million)