SECTION A
Q.1 In eukaryotes RNA is nonfunctional, name the process that turns it functional?
Answers
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.
Transcription
Transcription unit [Source: Wikimedia Commons]
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.
Transcription
Transcription in bacteria [Source: Wikimedia Commons]
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.
Transcription
Post-transcriptional modifications in Eukaryotes [Source: Wikimedia Commons]
mark me as brilliant