Meiosis: Gamete Meiosis: Gamete FormationFormation

GametesGametes

What is a gamete?What is a gamete?

• Sex cell (egg and sperm)Sex cell (egg and sperm)• Haploid (n)Haploid (n)• Carries the DNA that will be used in Carries the DNA that will be used in

a offspring.a offspring.

WHERE ARE THESE SEX CELLS HELD?WHERE ARE THESE SEX CELLS HELD?(the pictures you’ve all been waiting for)(the pictures you’ve all been waiting for)

•Ovules in the ovary of the plant.

•Pollen in the pollen sacs of the anther

•Eggs are in the ovaries

•Sperm are in the scrotum

How are they made?How are they made?

• Process called Process called MeiosisMeiosis

• Unlike in mitosis, in meiosis the cell Unlike in mitosis, in meiosis the cell goes through goes through twotwo divisions divisions

– REDUCTION DIVISION called Meiosis IREDUCTION DIVISION called Meiosis I

– MITOTIC DIVISION called Meiosis IIMITOTIC DIVISION called Meiosis II

Let’s Take a LookLet’s Take a LookMEIOSIS I (Reduction Division):

There are 4 stages (Interphase happens right before)

Prophase I

Metaphase I

Anaphase I

Telophase I

Hmmm, where have you seen this before?

Interphase IInterphase I

Cell is growingCell is growingDNA is being DNA is being

replicatedreplicated

Prophase IProphase I Chromatin condense to become chromosomesChromatin condense to become chromosomes Nuclear membrane and nucleolus disappearNuclear membrane and nucleolus disappear HomologousHomologous chromosomes pair up chromosomes pair up CROSSING OVERCROSSING OVER occurs (exchange of genes) occurs (exchange of genes)

-- More on this later!-- More on this later!

Metaphase IMetaphase IPairs of homologous chromosomes Pairs of homologous chromosomes

move to the centermove to the centerThe pairs stay together!The pairs stay together!

Anaphase IAnaphase IHomologous pairsHomologous pairs of chromosomes move of chromosomes move

AWAY from one anotherAWAY from one another (The sister chromatids are still together)(The sister chromatids are still together)

Independent AssortmentIndependent Assortment of chromosomes of chromosomes (this means you have a mix of each of your (this means you have a mix of each of your parent’s chromosomes in each cell)parent’s chromosomes in each cell)

Telophase ITelophase I Chromosomes gather at poles and get 2 new cellsChromosomes gather at poles and get 2 new cells Chromosome number is cut in half Chromosome number is cut in half Cells start to form new nuclear membranesCells start to form new nuclear membranes BUT…Each cell still has too much DNA (sister BUT…Each cell still has too much DNA (sister

chromatids have not separated)chromatids have not separated)

What exactly is this What exactly is this crossing over?crossing over?

Homologous pairs of chromosomes Homologous pairs of chromosomes exchange sections of DNA exchange sections of DNA

This increases chance of gentetic This increases chance of gentetic varietyvariety

Occurs during Prophase IOccurs during Prophase I

A view of crossing over:A view of crossing over:HOMOLOGOUS PAIRSHOMOLOGOUS PAIRS

Meiosis II: Mitotic DivisionMeiosis II: Mitotic Division

This division is also broken down into 4 This division is also broken down into 4 StagesStages

-Prophase II-Prophase II

-Metaphase II-Metaphase II

-Anaphase II-Anaphase II

-Telophase II-Telophase II

• These stages are similar to MitosisThese stages are similar to Mitosis

MEIOSIS : An OverviewMEIOSIS : An Overview

Mieosis II (cont’)Mieosis II (cont’)

• Mieosis II is similar to Mitosis Mieosis II is similar to Mitosis except:except:

1.1. Chromosomes do not replicate Chromosomes do not replicate again.again.

2.2. The final product is 4 haploid cells The final product is 4 haploid cells (gametes) instead of 2 diploid (gametes) instead of 2 diploid cells (somatic cellscells (somatic cells