6.6 Meiosis and Genetic Variation

KEY CONCEPT - Independent assortment and crossing over

during meiosis result in genetic diversity.

EQ – What is meiosis and how does it contribute to genetic

diversity (reproductive variability)?

SB1.a. Explain the role of cell organelles for both prokaryotic & eukaryotic cells,

including the cell membrane, in maintaining homeostasis and cell reproduction.

SB2.c. Using Mendel’s laws, explain the role of meiosis in reproductive variability.

6.6 Meiosis and Genetic Variation

6.6 Meiosis and Genetic Variation

• Meiosis I:

• occurs after DNA has been replicated.

• divides homologous chromosomes in four phases.

6.2 Meiosis

tetrad

Prophase IChromosomes

condense, homologous

chromosomes begin to

pair up, nuclear

envelope breaks down,

spindle fibers form

Metaphase ISpindle fibers align

homologous

chromosomes along the

cell equator

Anaphase IHomologous

chromosomes separate

to opposite sides of

cell, sister chromatids

remain attached

together

Telophase ISpindle fibers fall

apart, nuclear

membrane may

form again, cell

undergoes

cytokinesis

6.6 Meiosis and Genetic Variation

• Meiosis II:

• divides sister chromatids in four phases.• DNA is not replicated between meiosis I and meiosis II.

6.2 Meiosis

Prophase IINuclear envelope

breaks down if

necessary, spindle fibers

form

Metaphase IISpindle fibers align

chromosomes along the

cell equator

Anaphase IIChromatids separate

to opposite sides of

cell

Telophase IINuclear membranes

form around

chromosomes,

chromosomes begin to

uncoil, spindle fibers

fall apart, cytokinesis

occurs

6.6 Meiosis and Genetic Variation

• Meiosis differs from mitosis in significant ways.

– Meiosis has two cell divisions while mitosis has one.

– In mitosis, homologous chromosomes never pair up.

– Meiosis results in haploid cells; mitosis results in diploid

cells.

6.2 Meiosis

6.6 Meiosis and Genetic Variation

KEY CONCEPT - Independent assortment and crossing over

during meiosis result in genetic diversity.

EQ – What is meiosis and how does it contribute to genetic

diversity (reproductive variability)?

SB1.a. Explain the role of cell organelles for both prokaryotic & eukaryotic cells,

including the cell membrane, in maintaining homeostasis and cell reproduction.

SB2.c. Using Mendel’s laws, explain the role of meiosis in reproductive variability.

6.6 Meiosis and Genetic Variation

Sexual reproduction creates unique combinations of

genes.

• Sexual reproduction creates unique combination of genes.

– independent assortment of chromosomes in meiosis

– random fertilization of gametes

• Unique phenotypes (expressed traits) may give a

reproductive advantage to some organisms.

6.6 Meiosis and Genetic Variation

Fertilization

• Is random

• Increases unique combinations of genes

• In humans, the chance of getting one combination of

chromosomes from any one set of parents is 1 out of 223

x 223 (1 out of over 64 trillion) combinations

46

4646

23

23

23

23

1. 2. 3.

6.6 Meiosis and Genetic Variation

Meiosis

• Chromosomes independently assort in meiosis

• Increases unique combinations of genes (genetic variation)

• Homologous chromosomes pair randomly along the cell

equator

• In human cells, about 223 (8 million) different combinations of

chromosomes could result

6.6 Meiosis and Genetic Variation

Crossing over during meiosis increases genetic diversity.

• Crossing over is the exchange of chromosome segments

between homologous chromosomes.

– occurs during prophase I of meiosis I

– Creates new combinations of genes

– Recombined chromosomes are a combination of both

the mother and the father

6.6 Meiosis and Genetic Variation

• Chromosomes contain many genes.

– The farther apart two genes are located on a

chromosome, the more likely they are to be separated

by crossing over.

– Genes located close together on a chromosome tend to

be inherited together, which is called genetic linkage.

6.6 Meiosis and Genetic VariationConstruct a Crossing Over Model

1st: 1. Thread 2 colored marshmallows onto 2 toothpicks.

2. Tie the toothpicks together with a twist tie.

3. Thread 2 white marshmallows onto 2 toothpicks.

4. Tie the toothpicks together with a twist tie.

• The marshmallows represent the genetic information supplied by each parent.

• Each toothpick pair represents one duplicated chromosome (2 sister

chromatids) of a homologous pair – homologous chromosomes.

• The twist tie represents the centromere.

2nd: 1. Bring the pairs of homologous chromosomes together as in prophase I

(tetrad).

2. Model crossing over by exchanging one colored for one white marshmallow.

3. Line up the homologous chromosomes side-by-side as in metaphase I.

4. Separate the pairs as in anaphase I.

5. Then, separate the sister chromatids into individual chromosomes as in

anaphase II of Meiosis II.

Draw the process represented in your model. Label the phase of Meiosis

and the structures.

Answer this question: How is the genetic material that is exchanged during

crossing over alike and how is it different?