Monday, March 19,2012  OBJECTIVES: 1. What is Meiosis? 2. Why is it needed? 3. Outline and order the steps. 4. Explain the events of each step. 5. Identify the exclusive events and explain their importance. 6. Explain the outcome of meiosis. 7. Distinguish between spermatogenesis and oogenesis.

Brainteaser  What needs an answer, but doesn't ask a question?

Lets Review! Cell Cycle (IPMATC) Mitosis

Prophase? Metaphase? Anaphase? Telophase?

MEIOSIS What is it? Making sex cells or gametes (sperm & eggs). Who does it? All sexually reproducing organisms. Where does it happen? In males- in the testes. In females- in the ovaries. Why does it happen? So when organisms reproduce they retain their species’ chromosome number. Why is a different process needed?  1) Chromosome # needs to be cut in half  2) Creates genetic diversity (helps evolution of species)

MEIOSIS The process by which one diploid cell creates four haploid cells  Diploid Cell (2n) – a cell that contains both sets of homologous chromosomes  Haploid Cell (n) – a cell that contains one set of homologous chromosomes  Homologous Chromosome – corresponding (matching) chromosomes from a female and male parent.

Remember, humans have 23 pairs of chromosomes - 46 total. Pairs 1-22 are said to be Homologous chromosomes (they go together because they have same traits but they are not identical.) Gene for eye color Gene for hair color

The goal is to make….haploid sex cells FatherMother Spermatogenesis Oogenesis 3 polar bodies 1 viable egg 4 viable sperm

MEIOSIS HAS TWO STAGES: Meiosis 1 and Meiosis 2

Meiosis 1 This is the stage where the chromosome number is halved Includes 4 stages: Prophase 1 Metaphase 1 Anaphase 1 Telophase 1

Prophase 1 is similar in some ways to prophase in mitosis. Chromosomes condense. Spindle fibers appear. Nucleus and nucleolus disappear.

THIS PROCESS IS CALLED SYNAPSING LINED UP HOMOLOGUES ARE CALLED TETRADS UNLIKE in mitosis, homologous chromosomes line up next to each other during prophase

THIS IS CALLED “CROSSING OVER” When homologous chromosomes hang out so close to each other, they sometimes swap parts!

WHY IS THIS IMPORTANT FOR A SPECIES’ LONG-TERM SURVIVAL? Crossing over increases genetic diversity

Independent Assortment Independent assortment produces 2 n distinct gametes, where n = the number of unique chromosomes. That’s a lot of diversity simply by this mechanism alone. In humans, n = 23 and 2 23 ≈ 8,000,0000.

During metaphase 1, tetrads line up on a metaphase plate.

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis.

MEIOSIS MEIOSIS I  Anaphase I – Spindle fibers pull the homologous chromosomes toward opposite ends of the cell.

MEIOSIS MEIOSIS I  Telophase I - Meiosis I results in 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell.

In telophase 1, two daughter cells are formed. They are NOT identical! (Why?)

After Meiosis 1… Homologous chromosomes have been separated. Two non-identical daughter cells have been formed. The chromosome number has been cut in half.

WHY NOT? Chromosomes are NOT duplicated again between Meiosis 1 and Meiosis 2.

Meiosis 2 is similar to mitosis Two identical daughter cells are formed from each of the cells created in Meiosis 1 Includes 4 Stages: – Prophase 2 – Metaphase 2 – Anaphase 2 – Telophase 2

Meiosis II

Prophase 2: centrioles replicate, spindle reforms and chromosomes move toward the metaphase plate. Metaphase 2: sister chromatids lined up on the metaphase plate. Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell. Telophase 2 and Cytokinesis: nuclei form at either pole and both cells each are divided into two very different daughter cells – giving rise to 4 genetically different haploid cells - 4 gametes.

All together now…

Meiosis and mitosis are two different reproductive processes. What happens ONLY during meiosis? A. replication of organelles. B. disappearance of the nucleolus. C. crossing over occurs. D. complete breakdown of the nuclear membrane.

Which statement is correct? A. Meiosis is a way to reproduce, but mitosis is not. B. Meiosis is a way to create diversity, but mitosis is not. C. During mitosis, chromosomes are copied, but during meiosis chromosomes double. D. During mitosis, chromosome numbers double, but during meiosis chromosome numbers remain constant.