Lesson 9: Meiosis and Gametogenesis

What is sexual reproduction?

Mixing genomes from two different progenitors.

It is based on 2 phenomena.

What are the 2 phenomena of sexual reproduction

1. Cells are divided by meiosis (gametogenesis) which reduces by half the chromosome number (2n => n) (diploid→haploid)

2. Cell fusion: during fertilization two different genomes are joined. Haploid gametes unit to form a diploid zygote (n + n => 2n)

What is the meiosis and what is its purpose?

Special type of cell division necessary for sexual reproduction in eukaryotes:

diploid cell nucleus is divided generating four haploid nuclei.

▪ Ensures genetic variability: genetic recombination

- DNA fragments are exchanged between homologous chromosomes producing a different genetic combination in each gamete

Characteristics

It consists of two sequential nucleus divisions and there is no genetic material duplication.

▪ Meiosis begins with one diploid cell containing two copies of each chromosome (one from the mother and one from the father) and ends with 4 haploid cells

Hoe many meiotic divisions happen during the meiosis?

2

What happens in the first meiotic divison?

•The homologous chromosomes are separated.

•Two daughter nuclei are generated, each with a pair of homologue chromosomes (haploid nuclei).

• Genetic variability:

•Chromosomes are randomly distributed

•Genetic recombination

What happens in the second meiotic divison?

• Sister chromatids are distributed among the daughter nuclei.

• Four haploid cells containing one copy of each

chromosome (a sister chromatid) (haploid nuclei).

What are the steps of the meiosis?

Why is the Prophase 1 special and what happens during the phase?

-longest phase of meiosis

-DNA is exchanged between homologous chromosomes in a process called homologous recombination.

What are the stages of the Prophase 1?

a) leptotene

b) zygotene

c) pachytene

d) diplotene

e) diakinesis.

What happens during Leptotene?

-Start of the Formation of Meiotic spindle

-Two sister chromatids are so tightly bound that they are indistinguishable to each other

-chromosomes condensed and bind to the nuclear membrane at their telomeres

What happens during Zygotene?

Synapsis: Homologous chromosomes line up with each other into homologous chromosomes pairs

→ bivalent or tetrad chromosomes: paired chromosomes

Synapsis is facilitated by the synaptonemal complex:

protein structure that closes in a zipper-like fashion, ensuring proper pairing

What happens during Pachytene?

- Chromosomal crossing-over occurs (homologous recombination):

→ non-sister chromatids may exchange segments over regions of homology.

- This recombination occurs through recombination nodules: multienzymatic complexes that appear at the exchange sites (chiasmata or recombination points)

- Recombination implies that the DNA double helix is broken and lately fused again.

What happens during Diplotene?

-synaptonemal complex degraded and homologous chromosomes start seperating

-homologous chromosomes remain tightly boudn at chiasmata

-Meiosis may be paused: Human fetal oogenesis

→ all developing oocytes get to this stage and stop before birth (7. month of embryonic development) and remain until puberty

What happens during Diakenesis?

-chromosomes condense further - first point in meiosis where the four parts of the tetrads are actually visible

-chiasmatas clearly visible

-closely resembles prometaphase

First Meiotic divison

Prophase 1

Prometphase 2: similar to Mitosis

Metaphase 1: Homologous pairs aligned along equatorial plane

Anaphase 1: seperation of homologous pairs

Telophase 1: two haploid nuclei

Similarities of Meiosis and Mitosis

➢ Formation of Chromosomes.

➢ The spindle apparatus (microtubules of cytoskeleton) orchestrates their movements.

➢ Nucleus and cytoplasm are divided.

Differences between Meiosis and Mitosis

Mitosis:

→ Dna replication always followed by 1 cell divison

→ 2 diploid daughter cells are formed

→ short period of time

→ Each chromosomes is independent

→ each daughter cell has exactly same genetic informaion that the progenitor cell

Meiosis:

→ DNA replication always followed by 2 cell divisons

→ 4 haploid daughter cells are formed

→ can be paused or sustained for long periods

→ Homologous chromosomes paired forming bivalent chromosomes

→ genetic recombination occurs

GAMETOGENESIS

What are the 2 types of cells in adults?

Somatic cells and Germ cells

Somatic cells (every cell except germ cells)

-form the body of multicellular organisms

-cells that make up all the tissues and organs of an individual

Germ cells

-originate early in developement and located in gonads (ovaries and testes)

-include gametes and their specfiic precurser cells

-form mature gametes (egg and sperm)

-repdroduction of organism

Germ cells and haploid gamete formation:

Sexual reproduction, Specialized cells and biological significance

• Sexual reproduction: carried out by diploid organisms, in which each cell contains two copies of the chromosomes, inherited from each of the progenitors

• Specialized cells involved in sexual reproduction: gametes (sperm and eggs). They are haploid cells generated by meiosis, that will fuse (sexual reproduction).

• Biological significance: once they fuse (fertilisation), two haploid gametes (from two organisms) will form a new cell, in which both genomes are shuffled and generate a diploid genome of a new organism (zygote).

➢ The activated egg (fertilization) can give rise to a

complete new individual within a matter of days or weeks (mitosis).

Where does Gametogenesis occur?

→ occurs in the gonads (ovaries and testes)

What are the types of Gametogenesis?

Male gamete synthesis: spermotogenesis

→ Initiated during puberty (10-14 years old)

Female Gamete synthesis: Oogenesis

→ during fetal development, maturation arrested at the first stage of meiotic divison (prophase 1)

→ no further developement until sexual maturity

→ during 3rd month of fetal developement it is possible to become mature female gametes

What are the types of gametes?

Sperm cell

→ small and motile

→ ready for the propagation of paternal genes

Oocyte

→ large and immotile

→ ensures the survival of maternal genes

→ huge ammount of material

→ protective cover

What is the function of gametes?

Ensure the transmission of the genes they carry and form a zygote