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?

1 2 3 4

5 6 7

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 beHomologous 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

Father Mother

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 toprophase in mitosis.

Chromosomes condense.Spindle fibers appear.Nucleus and nucleolus disappear.

THIS PROCESS ISCALLED SYNAPSING

•LINED UPHOMOLOGUES ARECALLED TETRADS

UNLIKE in mitosis, homologouschromosomes line up next to each

otherduring prophase

THIS IS CALLED “CROSSING OVER”

When homologous chromosomes

hang out so close to each other,

they sometimes swap parts!

WHY IS THIS IMPORTANTFOR A SPECIES’

LONG-TERM SURVIVAL?

Crossing over increasesgenetic diversity

Independent Assortment•Independent assortment produces 2n 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 223 ≈ 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 areformed. 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 NOTduplicated again betweenMeiosis 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

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.