MEIOSIS

MEIOSIS

• Occurs in plants and

animals reproducing

sexually.

• In the course of

gametogenesis or

maturation of gametes.

• Reduction of chromosomes

to haploid conditions.

• Two successive divisions

but chromosomes divide

only once.

Meiosis

• a type of cell division that occurs in specialized cells of the ovaries and testes

• at sexual maturity

– The ovaries and testes produce haploid gametes by meiosis

Figure 13.5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

Sperm

Cell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploid

zygote

(2n = 46)

Mitosis and

development

Multicellular diploid

adults (2n = 46)

MEIOSIS

• Consists of two successive nuclear divisions:

a. First meiotic division (Meiosis I)

- reduction division

- involved reduction in the number of chromosomes

- diploid to haploid

- 46 chromosomes – 23 chromosomes

b. Second meiotic division(MeiosisII)

- equational division

- no reduction in the number of chromosomes

Each of the two divisions has the same four stages:

prophase, metaphase, anaphase and telophase

• In animals

– meiosis occurs during gamete formation

– gametes are the only haploid cells

Gametes

Figure 13.6 A

Diploid

multicellular

organism

Key

MEIOSIS FERTILIZATION

n

n

n

2n 2n Zygote

Haploid

Diploid

Mitosis

(a) Animals

Homologous Chromosomes

- carry genes for the same traits

- same length

- similar pattern

Figure 13.4 Describing chromosomes

in a diploid cell at metaphase - in a

eukaryote

Key

Maternal set of

chromosomes (n = 3)

Paternal set of

chromosomes (n = 3)

2n = 6

Two sister chromatids

of one replicated

chromosome

Two nonsister

chromatids in

a homologous pair

Pair of homologous

chromosomes

(one from each set)

Centromere

Meiosis

sexual reproduction

haploid, diploid, gamete

homologous chromosome

two stages of meiosis

independent assortment

crossing-over

Synapsis

- a process where the

homologous chromosomes line

up side by side

- process of linking of

homologous chromosomes

Tetrad

- also called bivalent

- a thick (4) structure being

composed of (2) homologous

chromosomes

Crossing over

- nonsister chromatid of (2)

homologous chromosomes in a

tetrad exchange DNA segments

Chiasmata

- point at which 2 non-sister

chromatids intertwine

Figure 13.8 The Meiotic Division

of an Animal Cell

Centrosomes

(with centriole pairs)

Sister

chromatids

Chiasmata

Spindle

Tetrad

Nuclear

envelope

Chromatin

Centromere

(with kinetochore)

Microtubule

attached to

kinetochoreTetrads line up

Metaphase

plate

Homologous

chromosomes

separate

Sister chromatids

remain attached

Pairs of homologous

chromosomes split upChromosomes duplicate Homologous chromosomes

(red and blue) pair and exchange

segments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Meiosis

two stages of meiosis

Prophase I

• Nuclear membrane disintegrates or breaks up

• Synapsis

• Crossing over

Metaphase I

• Nuclear membrane disappeared

• Spindle apparatus is formed

• Homologous chromosomes line up at the center of the metaphase plate

Anaphase I

• Homologoues separate and begin to move toward each pole

Telophase I

• similar with telophase in mitosis

• daughter cells have one chromosome from each homologous pair

• transition to the second nuclear division is called interkinesis

TELOPHASE I AND

CYTOKINESISPROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND

CYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavage

furrow Sister chromatids

separate

Haploid daughter cells

forming

During another round of cell division, the sister chromatids finally separate;

four haploid daughter cells result, containing single chromosomes

Two haploid cells

form; chromosomes

are still double

Meiosis

two stages of meiosisFigure 13.8 The Meiotic Division

of an Animal Cell

Meiosis II

• resemble an ordinary mitotic division except that the number of

chromosomes has been reduced by half

• Prophase II, Metaphase II, Anaphase II,Telophase II and

Cytokinesis

• 4 new cells with haploid chromosomes

Comparison between Mitosis and Meiosis

Mitosis Meiosis

- Formation of two daughter - formation of four daughter cells

cells (2n) (n)

- Cells divide only once for - two nuclear divisions resulting

every cell division cells with haploid number of

chromosomes

- Chromosomes duplicate - chromosomes form pairs

but do not pair which are segregated into (2)

daughter cells of the first

division

- in the second division

homologous chromosomes

split and form 4 haploid cells

- occurs in somatic/body cells - occurs in gametes

Meiosis in relation to Gametogenesis

• Maturing gametes divide

by meiosis and undergo

series of development and

transformation

through“gametogenesis”

two types:

1. spermatogenesis

- male gametogenesis

- testis

2. oogenesis

- female

- ovary

Cross-section of frog’s

testis Seminiferous tubules

showing spermatogenesis

Spermatogenesis in frog’s testis

Oogenesis

Oogonia

oogonium

Primary

oogonium/oocyte

Secondary oogonia

Spermatogenesis

spermatogonia

Primary spermatocytes

Secondary spermatocytes

Spermatozoa