Sex and genetics: Gametogenesis

Meiosis leads to haploid gametesThe process of meiosis involve in two steps: meiosis I and meiosis II.. Meiosis produces haploid gametes... Prophase I with special chromosome pairingLept

Sex and genetics: Gametogenesis

Cell Biology Course

Hà Nội -21/03/2013

PhD Nguyen, Lai Thanh

Dept of Cytoloty, Histology, Embyology and Biophysics

 Benefits of sex?

 Meiosis?

 Gametogenesis?

Asexual reproduction generate the offsprings genetically identical to parent

Sexual reproduction rise the diversity

Benefits of sex

 The simple genetic resourse of asexual reproductive organisms.

 The diversity of genetic in sexual reproductive organisms

 Different, atractive in males and females?

The differents between males and females

 In appearance

 In behavior

 The most important is the gametes

they produce

 They fascinate the other to lead

the integration of two types of

gametes.

Males produce sperms – the motile gametes.

Females produce eggs – the nonmotile gametes

Sperm and egg can merge together in fertilization process

to form zygote

From one-cell structure of zygote, the individual will take

shape with specific characteristics inherited from parents

and his own features distinct from others What is the

mystery?

Why gametes is important?

Meiosis leads to haploid gametes

The process of meiosis

involve in two steps:

meiosis I and meiosis II

All the particularities of

meiosis occur in meiosis I,

mainly in prophase I

Meiosis produces haploid gametes

Prophase I with special chromosome pairing

Leptotene

During this stage, individual

chromosomes begin to

condense into long strands

within the nucleus

The two homologous

chromosomes become "zipped"

together, forming the

synaptonemal complex, a

reference to the four sister

chromatids so-call tetrad

Synaptonemal Complexes are meiosis specific

Synaptonemal complexes

(SCs) are conserved,

meiosis-specific structures

This structure play a central

role in synapsis of homologous

chromosomes and

chromosome segregation

A short section of a typical synaptonemal complex

Pachytene (pachynema)

During the pachytene stage each paired chromosome (bivalent) becomes shorter and thicker than in earlier substages and splits into two sister chromatids except at the region of the centromere

During this process, an exchange of genetic material between non-sister chromatids of each tetrad occurs in most meiocytes

The synaptonemal complex degrades and homologous chromosomes separate from one another a little but remain tightly bound at chiasmata, the regions where crossing-over occurred

In mammalian fetal all developing oocytes stop at this stage before birth

 The bivalents of chromosomes arrange randomly on the

Kinetochore

Whole chromosomes of each

homologous pair (tetrad) separate

into each pole of dividing cells

Anaphase I: paires of chromosomes separate

Anaphase I

 Two sets of haploid choromosomes

complete the migration to the two poles

of cell.

 Nuclear envelope may be reconstituted

(in a few species).

Telophase I: paires of chromosomes separate

 No new chromosome replication

occurs.

 The centrioles duplicate.

 The spindle apparatus forms (and

nuclear envelope breaks down, in

some cases).

Prophase II: chromosomes remain condense

Prophase II

 The formation of spindle apparatus is

completed.

 Single chromosomes align on the

metaphase plate.

 The kinetochores of the sister

chromatids attached to microtubules

coming from opposite poles.

Metaphase II: Single chromosomes lign on the equatorial plate

Anaphase II: dissociation of sister chromatids

 The centromeres separate, Sister

chromatids move to opposite poles

of the cell.

 A nuclear envelope forms around

each set of chromosomes.

 Cytokinesis takes place producing

four daughter cells with haploid set

of chromosomes.

Telophase II: formation of four haploid cells

Telophase II

Prosess of meiosis

Meiosis may go wrong

Down Syndrome

Function of sex chromosome

Gametogenesis

At month 7th of pregnant period, there are about 7

million of oocytes and all are arrested at diplotene stage

of meiosis I Oogenesis pauses in a half way when the the girl is born

Oogenesis

In the male spermatogenesis

occurs from puberty to old age

The spermatozoa produced

from spermatogonium

Spermatogenesis

Not as oogenesis, the number

of spermatogonium are

maintained because they can

self renew

Spermatogenesis

Spermiogenesis

The following are the major events that occur during spermatogenesis:

Formation of Acrosome (from the Golgi) at the front end of the sperm

Formation of flagellum results due to elongation of microtubules (9+2 axoneme) from distal centriole at the

base of the nucleus

Nuclear Shaping & Condensation occurs due to changes in chromatin packaging (change from somatic

histones to sperm-specific protamines)

Rearrangement of organelles (e.g., mitochodria, centrioles); Mitochondria fuse and form spiral around

anterior portion of axoneme of flagellum in the midpiece region

Shedding of the Residual Body Most of the cytoplasm is lost; sperm becomes more streamlined.

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