Question

18) Which of the following describes enhancers?

		They may contain binding sites for both activators and repressors
		They regulate gene expression in prokaryotes
		They can only be located upstream of the coding sequence of a regulated gene.
		they only work if an insulator is located between the enhancer and its regulated gene
		all of the above

20)

A single-base change occurs in the 5' splice site of a gene, resulting in a loss-of-function mutation. Scientists have found a suppressor mutation: a mutation in a second gene that counteracts the first mutation. Which of the following is most likely to suppress the original loss-of-function phenotype?

		A mutation in the 3' splice site
		A mutation in the intron
		A mutation in an snRNP
		A mutation in a tRNA

Answer 1

18. They regulate gene expression in prokaryotes.

Enhancer sequences are regulatory DNA sequences that, when bound by specific proteins called transcription factors (activators), enhance the transcription  of an associated genes. These regions, called enhancers, are not necessarily close to the genes they enhance. They can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or may be thousands of nucleotides away. They can regulate gene expression in both eukaryotes and prokaryotes.

20. Mutation in tRNA

Here reversion and suppressor mutation had occured supressor mutation .

Reversion occurs when the effects of one mutation are counteracted by a second mutation. Reversion refers specifically to phenotype. Rarely, the original base sequence is restored. These are called true revertants. Usually, a second base change cancels the effect of the first base change. In this case, the revertants are called second-site revertants. The second mutation is called a suppressor mutation. A mutation that produces a mutant tRNA molecule that inserts an amino acid in response to a nonsense codon, thus continuing a protein that would otherwise have been terminated.