Question

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

a) Expression of Sex lethal gene is responsible for sex determination in Drosophila. This gene is present on the X chromosome and the amount of gene product determine the formation of female progeny by dosage compensation. Its splicing enzyme, which helps in gene regulation and expression of functional protein.

(i) in female, the specific translational factors (enzymes) or protein specifically binds to RNA. Sxl transcribed in female or ON in female embryo and specifically acts on the transformer mRNA product to splice it. This will result in the downstream regulation of genes and splicing the transcribed products for the functional product (Sxl protein) formation for female somatic differentiation. These proteins help to determine the fate of the developing embryo. In case female or XX embryo, Sxl protein activates through the tra (transformer) gene- tra and tra 2 to regulate the alternative splicing of dsx (doublesex) gene, that results in female development.

(ii) Sxl gene splicing product results in the formation: late female-specific, late male-specific and early female-specific transcribed product and promoter region for the development of the embryo to adult. In Drosophila germ line cell gene expression is regualted by the soma and cell-autonomous signalling, during the blastodermal stage in early soma. In female pair of X-chromosome have Sxl gene in ON condition, Sxl protein moves to the gonads and autonomously induces the primordial germ cells (PGCs) to develop the embryo in the female body. In the case of a male, the gene products for splicing is absent because of the lack of X chromosome pair.  During the later life cycle the pathway is turned ON or active in female, however, in case of female its remain turned off and produce male-specific Dsx proteins for sex determination in somatic cells.

(b) RNA modification in eukaryotes- After transcription of unmatured pre RNA to form functional mRNA, rRNA, tRNA and so on, different modifications occur such as polyadenylation/5' methylguanosine cap a Splicing.

In case of polyadenylation poly A (adenine) tail is added at the 3'OH group and at the 5' site methylguanosine cap is added to protect the enzymatic breakdown of transcribed products and to make mature mRNA for translation.

In splicing noncoding (introns) sequence are removed from the transcribed gene product (exons sequence array), to make product-specific coding RNA. This will result in formation of two different product form the same gene.

(c) Genome is packed in the formation chromatin form consist of the nucleosome. Nucleosome consist of DNA (acidic) and histone protein (basic), which result in formation condensed structure of the chromosome.

(i) acetylation or deacetylation of Histone protein- it results in the addition of acetyl group to the histone protein that results in weak interaction with the DNA and the formation of open chromatin region called euchromatin for the significant binding to the specific protein of transcription product for gene expression.

(ii) Methylation- Histone protein methylation results due to the addition of the methyl group to the specific amino acid present in histone protein. this will result in a change in nucleosome structure and gene expression regulation.