MEIOSIS

Objective:

Students know that meiosis is an early step of sexual

reproduction (2a)

Students know that gametes are produced during

meiosis

Interphase I

• Chromosomes replicate (S phase).

Meiosis I (four phases)

• Cell division that reduces the chromosome

number by one-half.

• four phases:

a. prophase I

b. metaphase I

c. anaphase I

d. telophase I

Prophase I

• Chromosomes condense.

• Synapsis occurs: homologous chromosomes

come together to form a tetrad (or a pair).

Prophase I - Synapsis

Homologous chromosomes

sister chromatids sister chromatids Tetrad

Crossing Over

• Crossing over (variation) may occur

between nonsister chromatids

• Crossing over: segments of nonsister

chromatids break and reattach to the other

chromatid.

Crossing Over - variation

nonsister chromatids

chiasmata: site

of crossing over variation

Tetrad

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.

Sex Chromosomes

XX chromosome - female XY chromosome - male

Prophase I

centrioles spindle fiber

aster

fibers

Metaphase I

• Tetrads align in the middle.

• INDEPENDENT ASSORTMENT OCCURS:

1. Orientation of homologous pair to poles is random.

2. Variation

3. Formula: 2n

Example: 2n = 4

then n = 2

thus 22 = 4 combinations

Metaphase I

metaphase plate

OR

metaphase plate

Question:

• In terms of Independent Assortment -

how many different combinations of

sperm could a human male produce?

Answer

• Formula: 2n

• Human chromosomes: 2n = 46

n = 23

• 223 = ~8 million combinations

Anaphase I

• Homologous chromosomes separate and

move towards the poles.

• Sister chromatids remain attached at their

centromeres.

Anaphase I

Telophase I

• Each pole now has haploid set of

chromosomes.

• Cytokinesis occurs and two haploid daughter

cells are formed.

Telophase I

Meiosis II

• No interphase II

(or very short - no more DNA replication)

• Remember: Meiosis II is similar to mitosis

Prophase II • same as prophase in mitosis

• Spindle and centrioles form

Metaphase II

• same as metaphase in mitosis

• Chromosomes line up in a single line

along the equator

metaphase plate metaphase plate

Anaphase II

• same as anaphase in mitosis

• sister chromatids separate

Telophase II

• Same as telophase in mitosis.

• Nuclei form.

• Cytokinesis occurs.

• Remember: four haploid daughter cells produced.

gametes =

sperm by spermatogenesis

or egg (ova) by oogenesis

Telophase II

Meiosis

2n=4

sex cell

diploid (2n)

n=2

n=2

meiosis I

n=2

n=2

n=2

n=2

sperm

haploid (n)

meiosis II

Variation

• Important to population as the raw

material for natural selection.

• Question:

What are the three sexual sources of

genetic variation?

Answer:

1. crossing over (prophase I)

2. independent assortment (metaphase I)

3. random fertilization

Remember: variation is good!

Question:

• A cell containing 20 chromosomes (diploid)

at the beginning of meiosis would, at its

completion, produce cells containing how

many chromosomes?

Answer:

• 10 chromosomes (haploid)

Sources

• www.ursulinehs.org/powerpoint/meiosi

s.ppt

• www.biology4teachers.com/Cell%20Divi

sion/MEIOSIS.ppt

• http://www.iteachbio.com/Life%20Scien

ce/LifeFunctionsandTheCell/Meiosis.ppt