Genet 302 lec 35 & 36
What is heterochromatin?
A compact chromatin state with low or no gene activity
Where is heterochromatin often located in the nucleus?
At the edge of the nucleus
Where is heterochromatin commonly located on chromosomes in Drosophila?
Around centromeres
What DNA sequence is common in heterochromatin? what else is found in it and is it required to live with?
Satellite DNA repeats. some rRNA genes and male flies can live w/o the y chromosome that is fully heterochromatin (blue)
How does heterochromatin differ from euchromatin?
Heterochromatin is more compact and less active (no genes) and only needed during mitosis, while euchromatin is less dense and more active
What is a histone writer?
An enzyme that adds a histone modification mark
What writer was emphasized for heterochromatin in this lecture?
SU(VAR)3-9
What kind of enzyme is SU(VAR)3-9?
A histone methyltransferase
What histone mark does SU(VAR)3-9 make?
H3K9 methylation
What is a histone reader?
A protein that recognizes and binds a histone modification mark
What reader was emphasized for heterochromatin in this lecture?
HP1
How does HP1 recognize the heterochromatin mark?
Its chromodomain binds H3K9me
What does HP1 do after binding H3K9me?
It compacts chromatin
What is a histone reader-writer complex?
A complex in which a reader binds an existing mark and also interacts with a writer that adds the same mark nearby
What reader-writer complex was emphasized in lecture 35?
HP1 with SU(VAR)3-9
How do HP1 and SU(VAR)3-9 work together?
HP1 binds H3K9me and also binds SU(VAR)3-9, which can methylate nearby histones
How are histone modification marks made in heterochromatin?
A writer such as SU(VAR)3-9 adds H3K9 methylation to histones
How are histone modification marks maintained after DNA replication?
Old marked histones help recruit reader-writer complexes, which place the same mark on nearby new histones
How are old and new histones distributed after DNA replication in this lecture?
Randomly
What is the basic maintenance cycle for heterochromatin marks?
Old mark recruits reader-writer complex, writer adds new mark, then the mark can recruit another reader-writer complex
How are heterochromatin marks confined instead of spreading forever?
Opposing euchromatin marks form boundaries that block spread
What euchromatin writer was emphasized as opposing heterochromatin?
JIL-1
What modification does JIL-1 add?
H3S10 phosphorylation
What chromatin state is associated with H3S10ph?
Euchromatin
Can H3 have both H3K9me and H3S10ph at the same time?
No
Why does mutual exclusivity between H3K9me and H3S10ph help confine heterochromatin?
Because incompatible marks prevent the heterochromatin state from spreading into euchromatin
What is stage 1 in the evolution of chromosome-based sex determination?
A single autosomal gene determines sex
What is stage 2 in sex chromosome evolution?
The unneeded allele mutates and disappears
What is stage 3 in sex chromosome evolution?
The chromosome trapped in one sex accumulates mutations, degenerates, and becomes a sex chromosome
What are sex chromosomes? is the degenerate chromsome always smaller than its counterpart?
Chromosomes not present in two copies in both sexes. sometimes, in mammals y is smaller in dros its larger
In mammals, what gene takes control of sex determination in stage 1?
SRY
In mammals, which chromosome degenerates and becomes the Y chromosome?
The SRY+ chromosome
In Drosophila, what gene takes control of sex determination in stage 1?
sis
In Drosophila, which chromosome degenerates and becomes the Y chromosome?
The sis− chromosome
In birds, what gene takes control of sex determination in stage 1?
DMRT1
In birds, which chromosome degenerates and becomes the W chromosome? what gender is this?
The DMRT1− chromosome. the female has ZW so the W only has oogenis genes
What are the present-day sex chromosome systems emphasized in this lecture?
XY in mammals, XY in Drosophila, ZW in birds, and XO in grasshoppers
In mammals, where is the sex-determining gene located?
On the Y chromosome
In Drosophila, where is the sex-determining gene located?
On the X chromosome
In birds, where is the sex-determining gene located?
On the Z chromosome
Why can the degenerate chromosome sometimes be smaller than its partner?
Because degeneration can involve loss of genes and DNA, but this is only sometimes true
If the original autosome had a gene needed for spermatogenesis, where will it end up?
Only on the Y chromosome
Why would a spermatogenesis gene be retained on the Y chromosome?
Because it is only needed in males
Why are grasshoppers XO instead of XY in this lecture?
The degenerated Y was lost because it had neither the sex-determining gene nor spermatogenesis genes
Why are sex chromosomes independent of autosomes in this lecture?
A sex chromosome-autosome translocation would be lethal
How do houseflies get half female and half male offspring in the stage 1 autosomal system?
One parent produces gametes with or without the male-determining autosomal allele, so half the offspring inherit it and are male and half do not and are female
How do mammals get half female and half male offspring?
The father produces half X sperm and half Y sperm, while the mother produces X eggs
How does Drosophila get half female and half male offspring in this lecture’s model?
One parent produces gametes with X or Y, giving half XX and half XY offspring
How do birds get half female and half male offspring?
The male is ZZ and makes only Z sperm, while the female is ZW and makes half Z eggs and half W eggs
How do grasshoppers get half female and half male offspring?
The female is XX and makes only X eggs, while the male is XO and makes half X sperm and half sperm with no sex chromosome
If an organism is stage 1 with one autosomal sex-determining gene, what should it look like by stage 3?
One homolog with the sex-determining allele becomes trapped in one sex, degenerates, and becomes a sex chromosome
If an organism is stage 3 with XY or ZW chromosomes, what did it look like in stage 1?
It originally had a pair of autosomes with a sex-determining gene on one homolog and the alternative allele on the other