It was so stressed out
these days because I was officially trapping in one of the “Kick My Ass”
genetics and genomics course. It is my first time to realize that graduate
school is going to be tough because of the course titled “ADVANCED”. Anyway, I
can make it fun to learn couple of decent genetic findings, as well as many
terminologies that I’ve never heard before. So I set up my mind on summarizing
the things I learned about genetics for this new post.
Dosage Compensation
It refers to a
hypothetical mechanism that balances the expression of X-linked genes between
males and females. Dosage compensation varies in different orgasm, but it is
ubiquitous in eukaryotes. In human and other mammals, males (XY) express normal
level of X-linked genes while in females (XY) one of X chromosome is
inactivated. This was found by Ohno S in 1959, by showing two different
X-chromosomes of the mammal cells; one is like autosome and the other is
condensed and heterochromatic. On the contrary, in Drosophila, it is the female
who dominates the gene dosage, so the males double the expression of genes on
X-chromosome. As for hermaphrodite C. elegans, both X-chromosomes are somewhat
repressed. The following picture shows that Xist RNA is coating only one
X-chromosome, which indicates the inactivation of another.
How the Y-chromosome is
maintained in human evolution?
Y-chromosome is the most
unique, funky and charismatic guy in human males’ karyotype. It stands on its
own, being without a homolog to pair with. It is scared by X-chromosome, which
may try hard to invade Y by homologue recombination. Y has to make its own way
for spermatogenesis. During meiosis, palindrome structures in male-specific
region of Y (MSY) are maintained by gene conversion. These palindromes play a
big role in protecting Y-chromosome from exchanging sequence with X-chromosome
and the subsequent loss of function.
Lange et al investigated
the unexpected consequence by maintaining palindromes in MSY and the genetic
etiology for Turner’s syndrome. Their model is based on the isodicentric (idic
Y) generated during the recombination of sister chromatids of Y by crossover
pathway. Idic Y would then lost during the process of spermatogenesis of
father’s germline cells, giving a daughter with 45, X0 karyotype. The daughter
is diagnosed as Turner syndrome.
This picture shows how idic Y is produced in their
model of Turner syndrome
References
Ohno, S. (1969).
Evolution of sex chromosomes in mammals. Annual Review of Genetics, 3(1),
495. doi:10.1146/annurev.ge.03.120169.002431
Lange, J. (2009). Isodicentric Y chromosomes and
sex disorders as byproducts of homologous recombination that maintains
palindromes. Cell, 138(5), 855. doi:10.1016/j.cell.2009.07.042
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