Test: Transcription (December 16) - NEET MCQ

mRNA has methylated region at the 5' terminus. It functions as a cap for attachment with ribosome. Cap is followed by an initiation codon (AUG) either immediately or after a small non-coding leader region. Then there is coding region followed by termination codons (UAA, UAG OF UGA). After termination codon there is a small non-coding trailer region and poly A area at the 3' terminus. These leader and trailer regions are called UTRs (Untranslated regions). Cap at 5' end and poly-A tail at 3' end also serves to protect mRNA from enzymatic decay.

The RNA polymerase enzyme binds to a specific site called promoter and initiates transcription. The promoter region determines which DNA strand is to be transcribed. Thus, a promoter region has RNA polymerase recognition site and RNA polymerase binding site .

mRNA strand is complementary to one of the DNA strands i.e., template strand. In RNA, uracil is present instead of thymine which is complementary to adenine. Cytosine and guanine are also complementary to each other. Hence, the sequence of bases in transcript would be CGAAUCCGUU.

At 5' end of hnRNA, a cap is formed by modification of GTP into 7-methyl guanosine or 7_m(G_ppp). This process is called capping and it protects the mRNA from degradation by nucleases and also.

Eukaryotes have three RNA polymerases. RNA polymerase I is located in the nucleolus and transcribes for RNAs (28S, 18S and 5.8S) whereas RNA polymerase II is localised in the nucleoplasm and used for hnRNA, mRNA and RNA polymerase Ill is localised in the nucleus, possibly the nucleolar-nucleoplasm interface and transcribes for tRNA, 5S, RNA and nRNAs.

In eukaryotes, the coding sequences or expressed sequences are defined as exons. Exons appear in mature or processed RNA. The exons are interrupted by introns, they do not appear in mature or processed RNA.

When a particular gene codes for a mRAN strand it is said to be monocistronic or monogenic. When several genes (cistrons) are transcribed into a single mRNA molecule, it is described as polycistronic or polygenic. Prokaryotic mRNA are polycistronic. Their translation begins even during their synthesis. They are short-lived and hardly undergo any processing, hence they do not possess a poly A tail at 3 end. Eukaryotic mRNA are monocistronic. Their translation begins only when the transcription has been completed. Thus, they are more stable than the prokaryotic mRNA. The transcribed RNA (hnRNA) undergoes processing (capping, methylation and polyadenylation) to form the mRNA molecule.

As eukaryotes possess compartmentalization of cell organelles, therefore, transcription occurs in nucleus and translation occurs in the cytoplasm in eukaryotes.

As transcription proceeds the initiation and elongation steps are mediated by the RNA Polymerase enzyme (DNA dependent RNA polymerase). As a result, the RNA transcript is synthesized as a single-stranded structure and is released when the Rho factor binds to the enzyme RNA polymerase to terminate the transcription. 

The primary mRNA transcript is longer and localised into the nucleus, where it is also called heterogeneous nuclear RNA (hnRNA) or pre-mRNA. At the 5' end of hnRNA, a cap (consisting of 7-emthyl guanosine triphosphate or 7 mG) and a tail of poly A (Adenylate residues) at the 3' end are added. These processes are respectively called as capping and tailing. The cap is a chemically modified molecule of guanosine triphosphate (GTP). The primary mRNA are made up of two types of segments, non-coding introns and the coding exons. The introns are removed by a process called RNA splicing and the exons are joined in a defined order.