Do Now 2.7 Objectives: 1. Define crossing over 2.Describe the process of meiosis and the cells it produces. 3.Define the Law of Independent Assortment Task: Horses: 2n = 64; Donkey: 2n = 62 Mule = offspring from male donkey & female horse Mule 2n = ??? Mule n = ???

Meiosis Makes Gametes Meiosis basics: – CHROMOSOME NUMBER REDUCED BY HALF 2n  n – Occurs only in specialized cells in gonads (testes & ovaries) – Produces sperm (spermatogenesis) and eggs (oogenesis) – Includes 2 cycles of cell division with NO INTERPHASE.

Let’s assume we have an organism where 2n=4 2n = 4

Now the cells have only one set of chromosomes n = 2 Haploid n = 2 Meiosis I: 1 chromosome from each homologous pair goes to each cell

What is the n of the gametes? n = 2 1n = n All 4 Are Haploid Meiosis II: The chromatids of each chromosome separate during division. n = 2

Important Notes In meiosis I, chromosomes begin with two chromatids and END with two chromatids. – Chromosomes do not divide during meiosis I, a homologous pair of chromosomes divides. In meiosis II, each chromosome with two chromatids divides. – The cells begin with n chromosomes that have 2 chromatids each. – The resulting cells (gametes) have n chromosomes with 1 chromatid each

Are those gametes from the previous example the only outcome? NO!

The homologous chromosomes can line up differently during…. 2n = 4 Metaphase I

n = 2 Meiosis I products: note different combination n = 2

n n Meiosis II: note the gametes now have a different combination of chromosomes

The Law of Independent Assortment Homologous chromosomes separate randomly during meiosis It does not matter which chromosome that each gamete gets, as long as it gets one of each type! In other words, when you produce gametes, each sperm or egg cell has a 50% chance of carrying the chromosome #1 you got from mom, and a 50% chance of carrying the chromosome #1 you got from dad. The chromosome #2 in a gamete has NOTHING to do with what version of #1 it has.

So what’s the Point of Meiosis? Produces haploid gametes for reproduction. Mixes up parental chromosomes to make new combinations of genes. END RESULT: lots of genetic variation and diversity

But Wait, There’s More! We’ve already seen how meiosis makes new combinations of genes, but there’s even more mixing that happens! A process called crossing-over creates NEW chromosomes UNIQUE from those you got from your parents!

Crossing over: homologous chromosomes exchange genes during Metaphase I. The chromosomes in the middle are NEW combinations of genes that did not previously exist!

Recap Meiosis makes haploid gametes Meiosis I separates homologous chromosomes, meiosis II splits chromosomes. Chromosomes follow the Law of Independent Assortment Crossing over during meiosis leads to new chromosomes that have new combinations of information