Gametogenesis

Question 1

How many rounds of meiotic division are necessary to result in germ cells? How much DNA is present?

Answer 1

• two rounds of meiotic division

- half the normal amount of DNA (haploid cells)

Question 2

Describe the general process of meiotic division to produce germ cells?

Answer 2

• 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)

Question 3

Describe the process of oogenesis

Answer 3

• 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

Question 4

What is the zona pellucida?

Answer 4

• the layer of glycoproteins surrounding the growing oocyte

- secreted by surrounding follicular (granulosa) cells

Question 5

When does second meiosis occur for oocytes?

Answer 5

starts in ovulation, but only completed when the ovum has been fertilized

Question 6

A single ovum (egg cell) contains how many chromosomes?

Answer 6

22 + X chromosome complement

*one precursor cell (primary oocyte) gives rise to one ovum

Question 7

What is the antrum?

Answer 7

• 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)

Question 8

What is the overall consequence of spermatogenesis?

Answer 8

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

Question 9

Where does production of mature spermatozoa occur?

Answer 9

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)

Question 10

What provides nutrients and protection to the developing sperm cells?

Answer 10

Sertoli cells that surround the developing sperm cells

*sertoli cells also phagocytize most of the mature sperm cell cytoplasm

Question 11

What are the major causes of infertility in males?

Answer 11

• absence of sperm
• reduction in sperm number
• production of defective sperm

*normal sperm count approx 20-40 million per mL

Question 12

What are the major causes of infertility in females?

Answer 12

• failure of oocyte maturation or release

- blockage of egg transport into the fallopian tubes

Question 13

What is Polycystic Ovary Syndrome (POS)?

Answer 13

a very common disease that results in increase in male hormone levels (in women) with a corresponding inhibition of oocyte maturation and release

Question 14

What are three chromosome abnormalities?

Answer 14

• trisomies
• sex chromosome copy number variation
• chromosomal variations

Question 15

Chromosome abnormalities:

Trisomies

Answer 15

• 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)

Question 16

Trisomy 21

Answer 16

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

Question 17

Trisomy 13

Answer 17

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

Question 18

Trisomy 18

Answer 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

Question 19

All trisomies, but those of 21 13 and 18, result in what?

Answer 19

they are all embryonic lethal

Question 20

Chromosome abnormalities:

What are two types of diseases of sex chromosome copy number variation?

Answer 20

• 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

Question 21

Turner Syndrome

Answer 21

• 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)

Question 22

Klinefelter Syndrome

Answer 22

• 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

Question 23

Chromosome abnormalities:

What is an example of chromosomal rearrangements?

Answer 23

Robertsonian Translocations

Question 24

Robertsonian Translocations

Answer 24

• 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

Question 25

When the translocations occur between acrocentric chromosomes, the individuals have no health difficulties. Why is this?

Answer 25

because on these short segments that are lost there are tRNA genes, we have so many copies of these tRNA genes that losing these ones doesn’t harm us

Question 26

In what chromosomes do Robertsonian translocations have no health difficulties?

Answer 26

acrocentric chromosomes:
13, 14, 15, 21, 22
*slightly increased rate of infertility/miscarriage for person carrying Robertsonian translocations, probably due to problems during meiosis