Intro to Cytogenetics - RS Flashcards
Describe the chromosome content and DNA content in mitosis (somatic cell division):
Chromosomes 2N -> 2N -> 2N. Because the chromatids are held together and appear as one structure there is no increase in the number of chromosomes although the DNA has doubled.
DNA: 2C -> 4C -> 2C
Describe the chromosome content and DNA content in meiosis (germ cell division):
Chromosomes: 2N -> 2N -(anaphase 1)-> N -> N. The chromosome recombine in prophase 1 and then undergo reductional division in anaphase 1. The centromere do NOT uncouple in anaphase 1.
DNA: 2C -> 4C -(A1)-> 2C -(A2)-> C.
What are the final chromosome numbers if nondisjunction occurs in anaphase 1?
N+1, N+1, N-1, N-1. Two disomic and two nullosomic cells.
What are the final chromosome numbers if nondisjunction occurs in anaphase 2?
N+1, N-1, N, N. 1 disomic and 1 nullosomic cell. 2 normal cells.
Spermatogenesis is pretty straight forward and happens like normal meiotic divisions, but oogenesis is different. How is it different?
The outcome is different. The first meiotic division results in two unequal daughter cells. Both receive equal DNA but one receives most of the cytoplasm (secondary oocyte). Division occurs again with the same outcome, an egg with most of the cytoplasm and another polar body. Thus in oogenesis the original oocyte gives rise to only one functional gamete.
Describe the timing of oogenesis:
It begins in the developing fetus. By the 3rd month of gestation the primary oocytes are present. These cells reach dictyotene (prophase 1) by birth and remain there until ovulation! At ovulation meiosis 1 finishes, but meiosis 2 only occurs if fertilization does.
In females when is meiosis completed?
Not until fertilization
What does fertilization trigger?
The final division of the egg; the secondary polar body is separated.
What happens when the sperm penetrates the egg?
The sperm head breaks down releasing the male pronucleus. Fusion of the male and female pronuclei will result in zygote formation.
Is the male or female development the default pathway?
Female development. TDF (testis determining factor) and SRY (sex determining region of the Y) are present in males.
What are the mullerian ducts? What are the wolffian ducts?
Proliferation of the mullerian ducts forms a female. Proliferation of the wolffian ducts produces a male. (note once sex is determined the loss of the chromosomes is clinically irrelevant).
How is sex determined?
Due to genes on the X, Y, and autosomes. Occurs very early.
Why do men survive with only 1 X?
X inactivation
What is the lyon hypothesis?
1X is inactivated in somatic tissue (dosage compensation). This happens 3 to 7 days after fertilization. Inactive X chromosomes are called barr bodies and there will be 1 active x per cell. This results in somatic mosaicism
What happens to a damaged X chromosome during inactivation?
It may be preferentially inactivated. Non-random X inactivation. This could lead to a female expressing an X-linked recessive trait since she only has 1 X being used in all cells.
