Which process[es] (replication, transcription, translation) would be affected by

Question

Which process[es] (replication, transcription, translation) would be affected by the following?

More than one process may be correct for each situation (when the success of one process is dependent on the success of a prior one), or none may be correct.

a. (2) Addition of an RNAi molecule to cells.

b. (2) Inversion of an enhancer sequence.

c. (2) Incorporation of an early stop codon.

d. (2) Mutation within a promoter sequence.

e. (2) Mutation within an origin sequence.

f. (2) Incorporation of a silent mutation.

Explanation / Answer

a. RNAi or RNA interference has an ability of gene silencing, by inhibiting expression of genes. The two major forms included in RNA interference are:

1) Micro RNA or miRNA, 2) Small interfering RNA or siRNA.

These forms of RNAi have specific complementary sequences, by which they can bind to mRNA sequences. This leads to increased or decreased function of mRNA.

Thus, the process of translation is affected.

b. Enhance comprises of 50 – 1500 bp sequences on DNA molecules. Enhancers allow the binding of transcription factors, referred to as enhancer binding proteins. The enhancer may be situated, upstream downstream or within the gene to be expressed. Enhancers operate as “cis acting regulatory elements”, that can promote the activity of the promoter, in a position independent manner. This is proved by observing that, enhancers may be functional at variable distance, closer distance (of 200 bp) or further away (even 25kb).

Enhancers can also function independent of the orientation. Though the promoter sequence requires to be 5’ to 3’orientation, the enhancer can function in same or opposite orientation, relative to the target gene.

Thus, inversion of enhancer, will not affect the process of transcription.

c. Stop codon or termination codon are triplet nucleotides, designated in mRNA as:

1. UAG -amber, 2.UAA-ochre, 3. UGA-opal.

Stop codons signals for the termination in protein synthesis or translation, by terminating elongation of polypeptide chain.

Thus, insertion of stop codon will affect translation.

d. Transcription is the process by which DNA sequence is copied to mRNA., by enzyme RNA polymerase. Initiation of transcription involves, formation of initiation complex. This involves binding of transcriptional factors on specific sequence on promoter, that determines the binding of RNA polymerase, to form transcriptional initiation complex.

Promoter sequences are located near the start sites of transcription, upstream of the DNA sequence to be transcribed, in 5’ -3’ (sense strand). Thus, promoter plays an important role in gene transcription, by affecting the formation of transcriptional initiation complex. This would affect the mRNA generation. So, translation process may be affected indirectly due to unavailability of the specific mRNA transcript, that would normally lead to translation.

Thus, mutation in promoter, will affect transcription, consequently translation will also be affected.

e. Origin sequence is a specific sequence in DNA (or RNA in some organisms like viruses), at which replication is initiated. It is called, the origin of replication or replication origin. Origin sequence is essential for the interaction of replication proteins with DNA, to initiate replication.

Thus, mutation in origin sequence will affect replication.

f. Silent mutation is mutation in the coding region of a gene, which changes the codon, but do not change the amino acid sequence (due to ambiguous nature of codons, some codon coding for same amino acids).

Thus, silent mutation will not affect the process of replication, transcription or translation.

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