Meiosis
takes place in the germ cells to generate male and female gametes.to reduce the number of chromosomes to the haploid number of 23 chromosome.
Meiosis
Meiosis I:Similar to mitosis, including the replication of the DNA amount to (2N).
In contrast to mitosis, the homologous chr were paired in meiosis I by a process called synapsis forming 22 pairs of doubled structured chr (except the 2 sex chr). At the final phase of meiosis I each pair of chr will be separated into each of the daughter cells.
Therefore each daughter cell contains haploid 23 doubled structured chr number with N amount of DNA.
Each chromosome shows two cromatids
Crossover
In meiosis Iinterchange of segments between the chromatids of a pair of doubled structured chromosome.
The point of interchange is temporarily united, therefore during the separation of the homologus pair in the anaphase of meiosis I an X-like structure is formed called the chiasma formatiom
Crossover
the interchanged segments break and exchange the positions on the homologus pair after the anaphase.
most frequent between genes that are far apart on a chromosome.
Genetic variability is enhanced through
crossover, which redistributes genetic material
random distribution of homologous chromosomes to the daughter cells.
Crossover
Meiosis II
No replication of the DNA occur in this stepResult in two daughter cells each of them containing haploid 23 single structured chr number with half amount of DNA.
Each chromosome is derived from a chromatid that results from splitting of the centromere of the doubled structured chr.
Mitosis
Meiosisoccurs in somatic and germ cells.
Occurs in germ cells only.
Does not show crossover.
Crossover seen
The resulting daughter cells contain diploid 46 chr and normal amount of DNA.
The daughter cells contain haploid 23 chr and half DNA amount.
Mitosis
Meiosis I
• no pairing of homologus chr.
pairing occurs.
• does not show crossover.
crossover seen.
• resulting daughter cells contain diploid 46 single structured chr number and normal amount of DNA.
resulting daughter cells contain haploid 23 double structured chr number and normal amount of DNA.
• occurs in somatic and germ cells.
occurs in germ cells only.
Mitosis
Meiosis IIthe resulting daughter cells contain diploid 46 single structured chr number and normal (N) amount of DNA.
the resulting daughter cells contain haploid 23 single structured chr number and half amount of DNA.
occurs in somatic and germ cells.
occurs in germ cells only.
• Meiosis I
Meiosis IIthe resulting daughter cells contain haploid 23 double structured chr number and normal amount of DNA.
the resulting daughter cells contain haploid 23 single structured chr number and half (1/2N) amount of DNA.
crossover seen.
does not show crossover.
pairing of homologus chr seen.
the paired homologus chr are separated.
Polar Bodies
During meiosis , primary oocyte gives rise to four daughter cells, each with 22+1 X chromosomes, only one develops into a mature gamete (the mature oocyte); the other three, the polar bodies, which degenerate during subsequent development.
Similarly, one primary spermatocyte gives rise to four daughter cells; two with 22+1 X chromosomes and two with 22+1 Y chromosomes, all these four develop into mature gametes (spermatozoa).
Polar Bodies
Prenatal changes:
PGC oogonia cluster of cellsOogenesis
mitosis
3rd month
Prenatal changes:
PGC oogonia cluster of cellsSome arrested in prophase M I and form primary oocyte
At the 5th month they reach 7 million
Then cell death occur even for the pri oocyte
At the 7th month degeneration of most of the oogonia
Each remaining pri oocytes surrounded by a single layer of flat follicular epithelial cells forming the primordial follicle
) Near birth all primary oocyte started prophase M I but enter diplotene stage by OMI ((secreted by fol cells))
at birth pri oocyte number = 600,000 to 800,000.
Oogenesis
mitosis3rd month
Prenatal changes:Oogenesis
Postnatal changes:
primary oocytes degenerate during childhoodAbout 200,000 - 400,000 oocytes reach puberty
Fewer than500 oocytes will be ovulated
At puberty, 15-20 primordial follicles begin maturation in each monthly ovarian cycle, usually only one reach maturity and the other degenerate
Stages of maturation:
• primary (preantral) follicle
• Secondary (antral) follicle the longest
• Preovulatory (graafian ) follicle (37 hr before ovulation)
Oogenesis
Postnatal changes:Oogenesis
Postnatal changes:
Oogenesis
Postnatal changes:
At maturity, the secondary follicle may be 25 mm in diameter or more.A surge in luteinizing hormone (LH) induces the preovulatory growth phase
The primary oocyte of the mature follicle stops the diplotene stage and continues the meiosis I forming two daughter cells
The secondary oocyte enters the meiosis II, and ovulation occurs at the metaphase of meiosis II
Oogenesis
Postnatal changes:Oogenesis
It involves postnatal changes only, each cycle after puberty last about 74 days
childhood, PGC are localized in the sex cords together with the supporting(Sertoli)
cells
Spermatogenesis:
Shortly before puberty the sex cord canalized forming the seminiferous tubules.PGC Form spermatogonial stem cells.
spermatogenesis regulated by:
• LH ---Leydig cells ---testosterone production--- Sertoli cells to encourage spermatogenesis.
• FSH: binding to Sertoli cells stimulates testicular fluid production and synthesis of intracellular androgen receptor proteins
Also the primordial germ cells differentiate into spermatogonia which are of two types:
Spermatogenesis:
• Type A:• that divide by mitosis forming more spermatogonia cell reserve.
• Type B:
• - Derived from the last division of type A.
• - divide to form the primary spermatocytes (enter prolonged prophase meiosis I (22 days)) resulting in the formation of secondary spermatocytes, these secondary spermatocytes undergo meiosis II forming the haploid spermatids
Spermatogenesis:
Spermatogenesis:
• The spermatids undergo series of spermiogenesis changes to form the sperms, these changes include:
1.formation of the acrosomal sac
2.condensation of the nucleus.
3.shedding of most of the cytoplasm.
4.formation of the sperm neck, middle piece and tail.
Spermatogenesis:
The spermatogenesis involves clonal changes by incomplete cytokinesis; in which a chain of differentiating cells derived from one spermatogonia are connected by cytoplasmic bridges. Only mature sperms are separated from this chain leaving a residual bodies.
Spermatogenesis:
Clinical Correlate