The mismatch repair system can detect and fix incorrectly inserted nucleotides t

Question

The mismatch repair system can detect and fix incorrectly inserted nucleotides that have escaped detection by DNA Polymerase's proofreading ability. This repair system finds mismatches and removes DNA on the newly synthesized strand of DNA. How does the mismatch repair machinery determine which strand is the newly synthesized strand? Below is an illustration of an mRNA. Is it a prokaryotic or eukaryotic mRNA? Initiation of transcription of this mRNA requires that RNA Polymerase be bound by which of the following proteins? snRNPs Sigma Factor General transcription factors A Ribosome On the above diagram, label the following: Open Reading Frame (ORF), 5'UTR, 3'UTR.

Explanation / Answer

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Part 1: DNA mismatch repair:The DNA mismatch is indeed identified and repaired by DNA polymerases in the cells. Mechanistically, the principle of this mismatch repair is very easy. There are two copies of genes present in a normal somatic cell. DNA replication takes place on both of these copies. This means that at a given point of time, there would be 4 copies of DNA present in a cell for a given gene. However, since this would require a lot of volume inside the nucleus, the replication of these two copies is slightly unsynchronized. Thus, at a given point of time, one copy of the allele would have been replicated while the other would remain normal unreplicated. This gives a chance to the DNA-repair mechanism of the cell to read the newly synthesized DNA and match its sequence similarity to the other non-replicated copy and identify any mismatch. If any mismatch is identified on the strand, it would be excised and repaired by the repairing enzymes taking the unreplicated copy as a reference. Thus, strand replication and repair both take place in the cells with high fidelity and accuracy.

Part 2: Shine-Dalgarno sequence is a characteristic conserved sequence of the prokaryotic DNA which helps the RNA polymerase to identify and bind to the promoter site of the DNA. This conserved and consensus sequence is located upstream the start codon of the gene nearly 8-10 base pairs.

Part 3: Choice b, sigma factor (the prokaryotic RNA transcription occurs by binding of RNA polymerase to sigma factor. The sigma factor remains an important transcription factor which spans the length of transcribind DNA along with RNA polymerase and thus helps the RNA polymerase to correctly identify and bind to the appropriate binding sites)

Part 4: The order of various parts of a newly synthesized RNA contain a 5' region which is not translated but provides structural stability to it. After it, the open reading frame of mRNA occurs which is actually tranlsated into the required amino acid sequence. Lastly, the 3' UTR is located after the stop codon which provides structural stability and helps in appropriate release of the mRNA template from the translation machinery. Thus, the labelling would be in the below given order:

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