Transcripts Gene Expressions

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transcripts
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a) Eukaryotes may regulate gene expression at any number of levels.
Imagine that transcripts of gene A are found only in the developing liver and transcripts of gene B are found only in the developing eye. List a general type of cis-regulatory sequence difference that could explain the expression difference between the two genes.

b) List one possible mechanism by which a gene may give rise to differently sized mature mRNAs in different tissues.

c) List 2 ways in which gene expression may be regulated after a mature transcript is produced ( after addition of a 5' cap and poly (A) tail and removal of introns).

Solution:

a) Eukaryotes may regulate gene expression at any number of levels.
Imagine that transcripts of gene A are found only in the developing liver and transcripts of gene B are found only in the developing eye. List a general type of cis-regulatory sequence difference that could explain the expression difference between the two genes.

1. Cis-regulatory elements are linked to the gene they regulate, although not necessarily in close proximity. The two types of cis-regulatory elements are promotors and enhancers. The promotor binds RNA polymerase, which is the enzyme that catalyzes polymerization of the ribonucleotides to form RNA in the process called transcription. However, other proteins, called transcription factors, must bind in the promotor region before transcription can occur. The polypeptide product of the Pax6 gene is a transcription factor, which no doubt binds with the promotor of one or more genes involved in eye development.

Another class of cis-regulatory elements is called enhancers. As the name suggests they modulate gene expression. Enhancers are often located more remotely relative to their target gene than is the promotor. Regulation of gene expression, is accomplished by transcription factors which "press" the on-off" buttons and enhancers that function as the "volume controls" for specific genes at specific times. Again, the precision of the control system comes from the specificity of chemical properties of proteins, and the enormous diversity of living forms (now living and that have lived in the past) results from the huge number of permutations that could be generated by such a system-it is in effect limitless.

2. RE1/NRSE a Common cis-Regulatory Sequence for ChAT and VAChT Genes:

Choline acetyltransferase (ChAT), the biosynthetic enzyme of acetylcholine, and the vesicular acetylcholine transporter (VAChT) are both required for cholinergic neurotransmission. These proteins are encoded by two embedded genes, the VAChT gene lying within the first intron of the ChAT gene. In the nervous system, both ChAT and VAChT are synthesized only in cholinergic neurons, and it is therefore likely that the cell type-specific expression of their genes is coordinately regulated.

It has been suggested that a 2336-base pair ...