Transcription

How does DNA provide the information required by a cell to function properly? DNA or genes give instructions for the synthesis of proteins, which constitute most of the substances in our body, such as enzymes, hormones, antibodies etc. Transcription is the first step in the synthesis of proteins from DNA. Let’s learn about this process in detail.

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Transcription Unit

The central dogma of molecular biology is as follows:

DNA → RNA → protein

Transcription is the copying of genetic information from one DNA strand into RNA by RNA polymerase. Like replication, it is also governed by the principle of complementarity. However, unlike replication, only one DNA strand is copied to RNA in transcription. Why is this?

Firstly, if both DNA strands act as templates, it will result in two different RNA sequences and in turn give rise to two different proteins. This complicates the transfer of genetic information. Secondly, the two RNA sequences produced will be complementary to each other and produce a double-stranded RNA. This will prevent RNA from being translated into protein. Now, let’s look at the different regions of a transcription unit in DNA.

1. Structural Gene

The two DNA strands within the structural gene have different names. Since RNA polymerase catalyzes polymerization in only one direction 5′ → 3′, the strand with 3′ → 5′ polarity becomes the template strand. The other strand with 5′  → 3′ polarity is displaced during transcription and is called the coding strand even though it does not code for anything. In a transcription unit, the promoter and terminator regions lie on either side of the structural gene.

2. Promoter

It is a DNA sequence located towards the 5′ end (upstream) of the coding strand. It is the binding site for RNA polymerase and is the site that tells the polymerase to start transcription. Additionally, the presence of the promoter defines the template and coding strand in a transcription unit.

3. Terminator

It is a DNA sequence located towards the 3′ end (downstream) of the coding strand. It provides the stop signal and defines the end of transcription. Additional regulatory sequences may be present upstream or downstream of the promoter.

The Gene

The functional unit of inheritance is a gene. Although genes are located on DNA, it is difficult to define a gene in terms of DNA sequence. A DNA sequence that codes for tRNA (transfer RNA) or rRNA (ribosomal RNA) is also a gene.

A cistron is a segment of DNA that codes for a polypeptide (a polymer of amino acids). A cistron can be polycistronic (mostly in prokaryotes and bacteria), i.e. it can code for several proteins. It can also be monocistronic (mostly in eukaryotes) i.e. it codes for a single protein. The monocistronic genes in eukaryotes consist of coding sequences called exons and intervening sequences called introns. Exons appear in mature or processed RNA whereas introns do not.

Types Of RNA

There are three major types of RNAs in bacteria:

• Messenger RNA (mRNA) – It provides the template to make protein.
• Transfer RNA (tRNA) – It reads the genetic code and transfers amino acids for protein synthesis.
• Ribosomal RNA (rRNA) – It has a structural and catalytic role in protein synthesis.

The Process Of Transcription

Transcription has the following steps:

• Initiation: Here, RNA polymerase binds to the promoter region and transiently binds to the ‘initiation factor’ to initiate transcription.
• Elongation: This is the step where the RNA strand starts getting longer. RNA polymerase “walks” along one strand of DNA. For every nucleotide recognized on the DNA template, it adds a complementary RNA nucleotide to the growing RNA transcript.
• Termination: Transcription stops once the RNA polymerase reaches the terminator region. At this region, the RNA transcript and the RNA polymerase, both fall off. RNA polymerase transiently associates with the ‘termination factor’ to stop transcription.

Complexity In Eukaryotic Transcription

In bacteria, since the mRNA does not need to be processed and since transcription and translation occur in the same cell compartment, the two processes can occur simultaneously. Also, the RNA Polymerase catalyzes transcription of all kinds of RNA. Eukaryotes, however, differ and show two main complexities. There are 3 types of RNA polymerases –

• RNA Polymerase I that transcribes rRNA.
• Type II that transcribes a precursor of mRNA – heterogenous nuclear RNA (hnRNA).
• RNA Polymerase III that transcribes tRNA and small nuclear RNAs (snRNA).

The primary transcript in eukaryotes is non-functional since it contains exons and introns. It undergoes splicing, a process that removes introns and joins the exons together in a specific order. The precursor hnRNA undergoes additional processing called capping and tailing.

An unusual nucleotide is added to the 5′ end of hnRNA during capping. In tailing, 200-300 adenylate residues are added to the 3′ end of hnRNA. This fully processed hnRNA, called mRNA is now transported out of the nucleus for translation.

Solved Example For You

Q1: If the sequence of the coding strand is 5′-TACGTACGTACGTA-3′, what is the sequence of the transcribed RNA?

Sol: The answer is 5′-UACGUACGUACGUA-3′. RNA has the same sequence as the coding strand, except in place of thymine it has uracil.