Gametogenesis Flashcards
(26 cards)
How many rounds of meiotic division are necessary to result in germ cells? How much DNA is present?
- two rounds of meiotic division
- half the normal amount of DNA (haploid cells)
Describe the general process of meiotic division to produce germ cells?
- pairing of chromosomes
- chiasma formation (exchange of genetic material between homologous chromosomes)
- pulling apart of double-structured chromosomes (two chromosomes separate from each other)
- anaphase of first meiotic division
- –> two separate cells each containing 23 double-structured chromosomes (cells resulting from first meiotic division; each cell has one of the homologous chromosomes)
- each of these new cells goes through a second meiotic division resulting in two cells each (total 4) containing 23 single chromosomes (sister chromatids separate, one into each cell)
Describe the process of oogenesis
- every month about 15-20 primary oocytes undergo maturation, each within a structure called the follicle
- oocyte grows in size due to increased cytoplasmic volume and becomes surrounded by a layer of glycoproteins called the zona pellucida
- oocytes are arrested in first meiosis prior to birth and only complete first meiosis as they progress towards ovulation (after puberty)
- in ovulation, second meiosis begins
- second meiosis is completed only when the ovum has been fertilized
- the entire process generates a single ovum (egg cell) plus two (or three) nonviable polar bodies, all with 22+X chromosome complement
- only one ovum is released during each cycle of ovulation
What is the zona pellucida?
- the layer of glycoproteins surrounding the growing oocyte
- secreted by surrounding follicular (granulosa) cells
When does second meiosis occur for oocytes?
starts in ovulation, but only completed when the ovum has been fertilized
A single ovum (egg cell) contains how many chromosomes?
22 + X chromosome complement
*one precursor cell (primary oocyte) gives rise to one ovum
What is the antrum?
- as the follicle around the oocyte grows in size, spaces appear between the granulosa cells
- this space is called the antrum
- the antral follicle is the entire structure (follicle around oocyte)
What is the overall consequence of spermatogenesis?
beginning at puberty, germ cells in the male undergo standard meiotic divisions to produce four spermatids, which later become mature sperm cells (spermatozoa)
*the complete process generates 4 sperm cells, two 22+X and two 22+Y
Where does production of mature spermatozoa occur?
within the seminiferous tubes of the testes
(spermatogonium starts at basement membrane and as the cell develops and matures it makes it’s way to the lumen of the tubule: spermatogonium –> primary spermocyte –> secondary spermocyte –> spermatids –> spermatozoa — all are surrounded by Sertoli cells)
What provides nutrients and protection to the developing sperm cells?
Sertoli cells that surround the developing sperm cells
*sertoli cells also phagocytize most of the mature sperm cell cytoplasm
What are the major causes of infertility in males?
- absence of sperm
- reduction in sperm number
- production of defective sperm
*normal sperm count approx 20-40 million per mL
What are the major causes of infertility in females?
- failure of oocyte maturation or release
- blockage of egg transport into the fallopian tubes
What is Polycystic Ovary Syndrome (POS)?
a very common disease that results in increase in male hormone levels (in women) with a corresponding inhibition of oocyte maturation and release
What are three chromosome abnormalities?
- trisomies
- sex chromosome copy number variation
- chromosomal variations
Chromosome abnormalities:
Trisomies
- abnormal meiotic divisions can produce either extra or missing chromosomes in an ovum or sperm
- missing copies of chromosomes 1-22 appear to be invariably lethal
- however, extra copies of a chromosome may lead to viable trisomies (especially of chromosome 21 –> Down’s syndrome; less frequently of chromosome 18 and 13, both 13 and 18 trisomies die soon after birth)
Trisomy 21
Down’s syndrome
1 in 1000 births
approx 75% of concepti with trisomy 21 die in embryonic or fetal life
approx 85% of those born live to age 1
approx 50% can be expected to live to age 50+ years
Trisomy 13
Patau’s syndrome
1 in 5000 births
- involves multiple abnormalities, many of which are not compatible with life
- more than 80% of children with trisomy 13 die in the first month, some live to their teens
Trisomy 18
1 in 5000 births
50% of infants with trisomy 18 do not survive beyond the first week of life
some have survived to teenage years, but with serious medical and developmental problems
All trisomies, but those of 21 13 and 18, result in what?
they are all embryonic lethal
Chromosome abnormalities:
What are two types of diseases of sex chromosome copy number variation?
- Turner Syndrome (single copy of the X chromsome in femalex, XO instead of XX)
- Klinefelter syndrome (additional copy of the X in males, XXY instead of XY)
*sex chromosome copy variations can be extra copies or absence of copies; fairly common and rarely lethal
Turner Syndrome
- single copy of the X chromosome in females (XO instead of XX)
- most severe sex chromosome abnormality
- symptoms: short stature, extended neck folds, reduced sexual development/secondary female characteristics, sterility due to abnormal ovary development)
Klinefelter Syndrome
- additional copy of X chromosome in males (XXY instead of XY)
- one copy of X inactivated
- medical consequences are rather minor and many individuals may remain undiagnosed
- in sever cases symptoms include: slightly enhanced female characteristics (ex: higher voice, breast enlargement, reduced body hair, and sterility due to low testosterone levels), and impaired testicular development
Chromosome abnormalities:
What is an example of chromosomal rearrangements?
Robertsonian Translocations
Robertsonian Translocations
- most common of all chromosomal abnormalities, 1 in 1000 births
- in live births, translocations are almost always observed for the acrocentric chromosomes 13, 14, 15, 21, and 22 (acrocentric chromosomes have their centromeres very near one end)
- the two long arms fuse together to form a single large chromosome (with a centromere near the middle) and the two short arms also fuse (usually the short fusion is rapidly lost)
- when the translocations occur between acrocentric chromosomes, the individuals have no health difficulties
- slightly increased rate of infertility/miscarriage for person carrying Robertsonian translocations, probably due to problems during meiosis
- Robertsonian translocations may occur for the other chromosomes, and these can cause a range of genetic defects including mental retardation and delayed growth
- in most sever cases, the result is embryonic or fatal death
