Meiosis and Chromosomal Crossing Over

Study Notes

Homologous chromosomes undergo crossover, resulting in genetic variability through the exchange of chromatid segments.
Chiasma formation marks the point of crossover during meiotic prophase.
Meiosis consists of two divisions: Meiosis I produces 2 secondary gametocytes (2N), and Meiosis II results in 4 gametes (1N).
Main goal is to produce haploid gametes (sperm and egg) reducing chromosomes from diploid (46) to haploid (23).
Spermatogenesis (male) and oogenesis (female) are specific processes within meiosis for gamete formation.

Characterized by a 45, X karyotype and occurs only in females.
Commonly results from paternal nondisjunction, especially in older fathers.
Phenotypes include absence of ovaries, short stature, webbed neck, and broad chest with widely spaced nipples.
Considered the only monosomy compatible with life.

Caused by a partial deletion of the short arm of chromosome 5.
Symptoms include a distinct cat-like cry, microcephaly, intellectual disability, and congenital heart defects.

Starts at puberty with the development of secondary sex characteristics.
Spermatogonia transform into spermatozoa under hormonal influence.
Regulated by luteinizing hormone (LH) from the anterior pituitary gland, stimulating testosterone production.
Infertility defined as failure to achieve clinical pregnancy after 12 months of unprotected intercourse, affecting 10-15% of couples.

Primordial germ cells (PGCs) differentiate into oogonia during fetal development.
By the 5th month of gestation, germ cells peak at approximately 7 million before undergoing degeneration.
Oocytes arrest during prophase of meiosis I to form primary oocytes, with further development occurring at puberty.

Granulosa cells surrounding the oocyte form the cumulus oophorus in maturing follicles.
Graafian follicles can grow to over 25 mm in diameter and are crucial for ultrasound assessment during infertility evaluations.
Upon ovulation, meiosis I completes, resulting in unequal daughter cells; meiosis II pauses in metaphase.
LH surge during ovulation facilitates the maturation of the follicle.

Capacitation is a conditioning period for sperm in the female reproductive tract, lasting around 7 hours.
Acrosome reaction involves the sperm binding to the zona pellucida, initiated by ZP3 and ZP4 enzymes.
Fertilization restores diploid chromosome number, determines embryo sex (XX or XY), and initiates cleavage.

Sperm penetration into the zona pellucida is tightly regulated to prevent polyspermy.
Fusion of oocyte and sperm membranes leads to cortical and zona reactions, ensuring only one sperm fertilizes the oocyte.

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