Question

Question 11 [20 marks / 20 minutes]

This question is about mechanisms of gene regulation in eukaryotes.

a) In Drosophila melanogaster, Sex-lethal (Sxl) controls somatic sexual development.

i. Describe the regulatory mechanism that leads to the presence of Sxl protein only in female

early embryos. [4 marks]

ii. Explain how this difference is maintained later in the life cycle [6 marks]

b) List two RNA modifications present in eukaryotes. Describe in detail the function of one of

these RNA modifications in a living cell. [6 marks]

c) Describe the function of two different DNA and/or histone modifications present in eukaryotes.

[4 marks]

Answer 1

1. Describe the regulatory mechanism that leads to the presence of Sxl protein only in female early embryos.

Ans1. Sex-lethal (Sxl) gene expression depends on the X:A signal, which is a main contributor in sex determination. It is in the form of 2X’2A in female and activated while in males its inactivated in the form of X;2A

sxl protein regulates alternative splicing of the sxl mRNA in early embryonic development of females but not in males. In female embryos the

Tra is active and regulate the expression of protein which via al;ternative splicing doubles the sex pre mRNA and adenylation at the poly A site and hence produce female specific Dsx protein. while due to absence of activation of Tra protein it is not expressed in males.

Following diagram is an elaboration of this mechanism.

ii. Explain how this difference is maintained later in the life cycle

In their life cycle

Ans. In later stage of life cycle the factors activating Sxl early promoter does not exists so the gender of the drosophila remains determined.

While the Sxl gene is transcribed in both males and females in later stages of life

b) List two RNA modifications present in eukaryotes. Describe in detail the function of one of

these RNA modifications in a living cell. [6 marks]

c) Describe the function of two different DNA and/or histone modifications present in eukaryotes.

Ans. Histone modification enhances the accessibility of the DNA for better transcription

It has an important function in eukaryotic chromatin remodeling where it improve specific enzyme activity.