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Overview of Meiosis• Meiosis is a form of cell division that leads to the

production of gametes.

• Gametes: egg cells and sperm cells (reproductive)

• -contain half the number of chromosomes of an adult body cell

• Adult body cells (somatic cells) are diploid (2n), containing 2 sets of chromosomes.

• Gametes are haploid (n), containing only 1 set of chromosomes.

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Meiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the

answers lie in

meiosis.

Meiosis does two things -

1) Meiosis takes a cell with two sets of every

chromosome (diploid) and makes cells with a

single set of every chromosome (haploid).

This is a good idea if you’re going to combine

two cells to make a new organism. This trick

is accomplished by halving chromosome

number.

In meiosis, one diploid cells produces four

haploid cells.

Why do we need meiosis?

• Meiosis is necessary to halve the number

of chromosomes going into the sex cells

Why halve the chromosomes in gametes?

• At fertilization the male and female sex

cells will provide ½ of the chromosomes

each – so the offspring has genes from

both parents

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2) Meiosis scrambles the specific forms of

each gene that each sex cell (egg or sperm)

receives.

This makes for a lot of genetic diversity. This

trick is accomplished through independent

assortment and crossing-over.

Genetic diversity is important for the evolution

of populations and species.

Meiosis

Parent cell –

chromosome pair

Chromosomes

copied

1st division - pairs split

2nd division – produces

4 gamete cells with ½

the original no. of

chromosomes

Meiosis

Meiosis – mouse testes

Parent cell

4 gametes

1st division

2nd division

The Stages of Meiosis:

• aka: Reduction Division

Meiosis I : Separates

Homologous Chromosomes

• Interphase

– Each of the chromosomes replicate

– The result is two genetically identical

sister chromatids which remain

attached at their centromeres

Prophase I

• Chromosomes condense

• Homologous pairs fasten together (synapsis) in a group of four called a tetrad.

• Crossing Over occurs.

• Crossing Over is the exchange of “like for like” segments of homologous chromosomes.

• Crossing over: genetic recombination between non-sister chromatids

• What do the arrows represent?

• What do the “x”’s of the same size and shape

represent?

• What do the different colors represent?

• What is happening in this picture?

Another Way Meiosis Makes Lots of Different

Sex Cells – Crossing-Over

Crossing-over multiplies the already huge number of different gamete

types produced by independent assortment.

Metaphase I

• Homologous pairs of chromosomes

line up in the “middle of the cell”

Independent Assortment• Occurs in metaphase 1

• Homologous chromosomes independently line

up on either side of the equator of the cell

• Results in random “shuffling” genes.

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Anaphase I

• Homologous chromosomes move to

opposite sides of the cell

– Sister chromatids remain attached

Telophase I/ Cytokinesis I

• The cytoplasm divides,

• 2 Genetically different haploid cells

• Concludes Meiosis I - Reduction

Division

Cytokinesis

• Occurs simultaneously with telophase I

– Forms 2 daughter cells

• Plant cells – cell plate

• Animal cells – cleavage furrows

Figure 13.7 The stages of meiotic cell division: Meiosis I

Meiosis II : Separates sister chromatids

• Very similar to mitosis (starts with haploid cells)

• NO INTERPHASE BETWEEN MEIOSIS I AND II !

Prophase II

• Chromosomes condense

• Nuclear envelope breaks down

• Spindle fibers form

Metaphase II

• Chromosomes line up in the middle

of the cell (just like mitosis)

Anaphase II

• The sister chromatids separate and

move toward opposite sides of the

cell

– (just like mitosis)

Telophase II and Cytokinesis

• Nuclei form at opposite poles of the

cell and cytokinesis occurs

• Results in four haploid (n) daughter

cells

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of

Different Sex Cells (Gametes) –

Independent Assortment

Independent assortment produces 2n

distinct gametes, where n = the number

of unique chromosomes.

That’s a lot of diversity by this

mechanism alone.

In humans, n = 23 and 223 = 6,000,0000.

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

Boy or Girl? The Y Chromosome “Decides”

X chromosome

Y chromosome

Boy or Girl? The Y Chromosome “Decides”

Meiosis – division error

Chromosome pair

Meiosis error - fertilization

Should the gamete with the

chromosome pair be fertilized

then the offspring will not be

‘normal’.

In humans this often occurs

with the 21st pair – producing

a child with Downs Syndrome

21 trisomy – Downs Syndrome

Can you see the

extra 21st

chromosome?

Is this person male

or female?

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