HW11 key - Homework 11 1(3 pts In Drosophila mutations at...

Homework 11
1. (3 pts) In Drosophila , mutations at four genes known as the male-specific lethals ( msl )
cause male-specific lethality during the larval stages. Analysis has shown that male-
specific lethality is caused by defects in the process of dosage compensation.
Dosage compensation in Drosophila is a process by which the single X chromosome of a
male is transcribed at twice the rate of either of a female’s two X chromosomes).
Examination of chromosomes in normal males and females shows that:
1. The X chromosome is less tightly condensed in males than in females.
2. The X chromosome of a male is enriched in an acetylated form of histone H4.
3. The protein products of each of the four msl loci are primarily associated with the X
chromosome of the male but not that of the female.
Examination of chromosomes in each of the four types of mutant msl males reveals that
their X chromosome remains about as tightly condensed as an X chromosome in females,
and that the absence of any single MSL-protein in a male results in the other three MSL-
proteins not associating with the X chromosome.
a. Based on your understanding of the relationship between chromatin structure and gene
transcription, how might a decrease in chromatin condensation be associated with dosage
compensation?
The acetylation of histone can destabilize chromatin packaging. As the DNA associated
with nucleosomes becomes more accessible to transcription factors, the generally
repressive action of histone on transcription is overcome and the rate of transcription is
increased. In this case, the four MSL-proteins and an acetylated histone H4 appear to be
necessary to decondensed the chromatin structure of the X chromosome of the male.