Meiosis

Cells Divide, and Cells Die

Section 8.1

Your cells are rapidly dying and

being replaced.

Cell division produces a

continuous supply of replacement

cells.

Cell division also has a role in

reproduction, growth, and

development.

Cell Division• Cells divide to make more

cells. While all the other organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.

• Mitosis is normal cell division, which goes on throughout life in all parts of the body. Meiosis is the special cell division that creates the sperm and eggs, the gametes.

Two Types of Cell Division

Interact in the Sexual Life Cycle

Section 8.1

Each mature

individual produces

sex cells by another

form of cell division

called meiosis.

Figure 8.1Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Sex cells combine at

fertilization.

Two Types of Cell Division Interact

in the Sexual Life Cycle

Section 8.1

A fertilized egg, or

zygote, divides by

mitosis. The fetus

grows and develops

into a mature adult

consisting of countless

cells with identical

DNA.

Figure 8.1Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Why Meiosis?

• In order for each generation to have the

same number of chromosomes,

gametes must have half the number of

chromosomes (N) as a body cell (2N).

• Meiosis – a type of cell division that

produces four gametes, each with half

the number of chromosomes as a body

cell.

Homologous Chromosomes

• Paired chromosomes

with genes for the

same traits are called

homologous.

• Genes are arranged in

the same order on

homologous

chromosomes.

Human Chromosomes

P. 128

KARYOTYPE

Meiosis

• Meiosis occurs in two

separate divisions,

meiosis I and meiosis

II.

• Meiosis starts with one

2N cell and ends with

four N cells.

• Meiosis is necessary

for sexual

reproduction.

Meiosis cont.

Meiosis I

• Crossing Over – non-

sister chromatids of

homologous

chromosomes exchange

genetic material.

• Homologous

chromosomes are

separated into two new

cells.

Crossing

-Over

P. 131

Meiosis II

• Sister

chromatids are

separated

• Results in four

cells, each with

one copy of

each

chromosome.

Meiosis Provides Variation

• Meiosis makes cells that are not identical.

• Meiosis results in a variety of different

gametes.

• Gametes are randomly fertilized,

producing very large numbers of possible

unique offspring.

• Crossing over in Meiosis I adds more

variation.

9.2 Phases of Meiosis• The same four stages of mitosis occur

during meiosis.

– Prophase

– Metaphase

– Anaphase

– Telophase

• The four stages occur twice, once during

meiosis I and again during meiosis II.

Meiosis I Events

• Prophase I

• The spindle appears between the

separating chromosomes.

–Synapsis (tetrad formed)

– Crossing-over may occur.

• Metaphase I, tetrads aligned at the

spindle equator.

Meiosis I (cont.)

• Anaphase I- Homologous

chromosomes separate.

–Cell divides to become two

haploid (1N) cells.

• Telophase I, nuclear envelope

reforms

Meiosis I - Summary

P. 132

Meiosis II- Second Division

• Repeats Meiosis I with Sister

Chromatids separating to form

two more cells-

• Summary: Single diploid cell

undergoes meiosis to produce four

haploid gametes.

Meiosis II Summary

P. 133

9.3 Meiosis VS Mitosis

• Meiosis- two nuclear divisions

while Mitosis- one.

• Meiosis-four daughter nuclei (1N,

haploid)

Mitosis-only two. (2N, diploid)

• Both followed by cytokinesis.

9.3 Meiosis VS Mitosis (cont.)

• Meiosis- Daughter cells

genetically dissimilar to parent

cell.

• Mitosis- Daughter cells genetically

identical to parent cell.

Meiosis

VS

Mitosis

P. 135

Mistakes in Meiosis

• Nondisjunction –

failure of

homologous

chromosomes to

separate.

• Results in gametes

that are missing

chromosomes or

that have extra

chromosomes.

Down Syndrome• Down syndrome is trisomy (3-copies) of

chromosome 21.

• Down syndrome characteristics:

–Short stature

–Eyelid folds

–Stubby fingers

–Wide gap between the first and second toes.

–Fissured tongue

–Round head

–Palm crease (the Simian line)

–Mental Retardation

Down’s

Syndrome

P. 136

Mistakes in Meiosis cont.

• Monosomy – one

chromosome

instead of a pair.

• Triploidy – three

copies of ALL

chromosomes,

resulting from

complete

nondisjunction.