Question

Genomic control: What is the aspect or type of gene expression control event described;

a) The methylation of cytosines in a DNA promoter region prevent transcription factors from binding at a promoter. And acetylation of histones to make a region of DNA more available for transcription are both forms of this type of control:

b) Xenopus laevis, normally contains about 500 copies of ribosomal RNA genes, but during oogenesis that number expands to about 2 million due to egg development during fertilization:

c) Presence of enhancer or silencer regions of the DNA in the viscinity of a promoter region are known as:

d) Random DNA excisions bring together a unique combination of V, D and J segments of the genes that code for antibody heavy chains

Answer 1

a) DNA methylation is biological process by which methyl groups are added to DNA molecule.methylation can change the activity of DNA segment without changing the sequence.when located in agene promoter DNA methylation typically acts to repress gene transcription.In mammals DNA methylation is essential for normal development and is associated with a no. of key processes including genomic imprinting, X- chromosome inactivation, repression of transposable elements,aging and carcinogenesis.DNA methylation can directly interfere with transcription factor binding in some cases.in other cases DNA methylation does not inhibit trabscription factor binding directly,but transcriptional repression occurs nevertheless.

histone acetylation and deacetylation are essential parts of gene regulation.these reactions are typically catalyzed by enzymes with histone acetyl transferase (HAT). acetylation is a process where acetyl functional grp. is transferred from one molecule to another.it is used in gene regulation.

both methylation and acetylation have opposite effects.methylation used for gene repression while acetylation used for gene regulation.

b) oogenesis in xenopus levis can be divided into 6 stages based on anatomy of developing oocyte.in xenopus the genes for ribosomal RNA are selectively amplified during the early stages of oogenesis and give rise to over 1000 extrachromosomal nuclei.these oocyte nucleoli are unique in that they contain very high copies of r RNA genes.

there may be coordinate expression of ribosomal protein and r RNA gens during oogenesis.ribosomal protein m RNA attains maximal steady srage level in stage II oocyte concomitant with the onset of vitellogenesis.approx. 50 % of ribosomal protein mRNAs are associated with polysomes throughout oogenesis,resulting in constant rate of ribosomal proteins in stage III through stage VI.in contrast, the polysomal to non polysomal distribution of bulk poly A + RNA increases during oogenesis,resulting in 5-8 fold stimulation of rate of overall protein synthesis.that is why no. is expands to upto 2 million.

c)  insulator/ transcription factors in this both enhancer/silencer regions are present.it is useful in gene on-off mechanisms.

d) a system is required to ensure that DNA rearrangement takes place at the correct lacations relative to V,D,J gene segment coding region.joins must be regulated such that V gene segment joins to D or J and not to another V.DNA rearrangements are in fact guided by conserved noncoding DNA sequences that are found adjcsant to the points at which recombination takes place.the reaction that recmbines V,D and J segments involves both lymphocyte specific and ubiquitious DNA modifying enzymes.recombination signal sequence (RSS) helps in unique combination of V,D, J segements.RAG proteins are required for V,D,J recombination.due to this generation of antibody diversity occurs.