12 min readPathway sheet1-mark MCQ, a 2-mark question on template and coding strands, a 3-mark question on RNA processing, and a 5-mark answer on transcription; also a NEET favourite

Central dogma and transcription

The central dogma says genetic information flows from DNA to RNA to protein. In transcription, RNA polymerase copies one DNA strand of a transcription unit into RNA.

Central dogma (Francis Crick)DNA → mRNA → protein; in some viruses, such as retroviruses, RNA → DNA (reverse transcription)

Transcription unitPromoter (upstream), structural gene, terminator (downstream), defined on the coding strand

Bacterial RNA polymerase helpersσ (sigma) factor for initiation; ρ (rho) factor for termination

Eukaryotic RNA polymerasesI: rRNAs (28S, 18S, 5.8S); II: hnRNA (mRNA precursor); III: tRNA, 5S rRNA, snRNAs

The central dogma

DNA also copies itself by replication. In some viruses, such as retroviruses, information flows from RNA back to DNA by reverse transcription.

Template strand, coding strand and the transcription unit

Transcription copies the information in one strand of DNA into RNA. Base pairing is the same as in DNA, except that adenine pairs with uracil. Only a segment of DNA, and only one strand of it, is copied. If both strands were copied, two different RNAs (and proteins) would come from one stretch. Also, the two complementary RNAs would pair into double-stranded RNA, which cannot be translated. The strand that is read is the template strand (3′→5′); RNA is built 5′→3′. The other strand is the coding strand (5′→3′): it has the same sequence as the RNA, with T in place of U. A transcription unit has three parts: a promoter, the structural gene and a terminator. By convention, the promoter lies upstream (towards the 5′ end of the coding strand), and the terminator lies downstream (towards its 3′ end).

Writing the mRNA from a gene

Problem

The coding strand of a gene reads 5′-ATGCCAGTTTAA-3′. Write (a) the template strand and (b) the mRNA, with their polarity. (c) What does the mRNA code for?

Working

The template is complementary and antiparallel to the coding strand.
Pair A with T and G with C: template = 3′-TACGGTCAAATT-5′.
mRNA is complementary to the template, so it has the coding strand's sequence.
Replace T with U: mRNA = 5′-AUGCCAGUUUAA-3′.
Codons: AUG CCA GUU UAA = start (methionine), proline, valine, stop.

Answer

(a) 3′-TACGGTCAAATT-5′. (b) 5′-AUGCCAGUUUAA-3′. (c) Met-Pro-Val, then stop.

Transcription in bacteria

1
Initiation: RNA polymerase, helped by the σ (sigma) factor, binds the promoter and starts transcription.
2
Elongation: the enzyme opens the helix and adds ribonucleoside triphosphates, following base pairing with the template. Only a short stretch of new RNA stays bound to it.
3★
Termination: at the terminator, helped by the ρ (rho) factor, the new RNA and the RNA polymerase fall off.
4
A single RNA polymerase makes all three kinds of RNA: mRNA, tRNA and rRNA.
5
Bacterial mRNA needs no processing. With no nuclear membrane, translation can begin before transcription is complete.

★ key step

Transcription and processing in eukaryotes

1
Three RNA polymerases in the nucleus share the work: I makes rRNAs (28S, 18S, 5.8S); III makes tRNA, 5S rRNA and snRNAs; II makes hnRNA, the precursor of mRNA.
2
Eukaryotic genes are split genes: coding exons are interrupted by non-coding introns. hnRNA contains both.
3
Splicing: introns are removed and exons are joined in a set order.
4
Capping: an unusual nucleotide, methyl guanosine triphosphate, is added to the 5′ end.
5
Tailing: 200 to 300 adenylate residues are added to the 3′ end, without a template.
6★
The processed hnRNA, now mRNA, leaves the nucleus for translation.

★ key step

Exons vs introns

ExonsExpressed sequences: they stay in the mature, processed mRNA and code for the protein.

IntronsIntervening sequences: they are removed by splicing and do not appear in mature mRNA. Split genes are thought to be an ancient feature, and splicing is seen as a trace of the RNA world.

A cistron is a DNA segment coding for one polypeptide. Eukaryotic genes are mostly monocistronic; bacterial genes are mostly polycistronic.

In real life

The TB drug rifampicin works by blocking bacterial RNA polymerase, so the germ cannot transcribe its genes and stops growing.

Exam angle

2 marks: “Why is only one strand of DNA transcribed in a given segment?” If both were copied, one segment would give two different RNAs and proteins, making the machinery complicated. Also the two complementary RNAs would pair to form double-stranded RNA, which cannot be translated.

3 marks: “Name the three RNA polymerases of eukaryotes and what each makes. What are capping and tailing?” I: rRNAs (28S, 18S, 5.8S); II: hnRNA; III: tRNA, 5S rRNA, snRNAs. Capping: methyl guanosine triphosphate added at the 5′ end. Tailing: 200 to 300 adenylate residues added at the 3′ end.

5 marks: “Describe transcription in bacteria. How is it different in eukaryotes?” Transcription unit; RNA polymerase with σ binds promoter; elongation; ρ at terminator; one polymerase; coupled with translation. Eukaryotes: three polymerases; hnRNA; splicing, capping, tailing; mRNA leaves nucleus.

Remember it

RNA polymerase I, II, III make r, m(hn), t RNA: 'Read My Text'.