Your DNA stays inside the nucleus, yet proteins are built outside it, in the cytoplasm. The original cannot leave. So how does the message get out? A copy is made and sent. Making that copy is transcription.
1. Copying DNA into RNA
Transcription is copying. The process of copying the chemical information stored in a DNA molecule into an RNA molecule, the mRNA, is transcription. In HIV, reverse transcription occurs.
Transcription happens in the nucleus. The mRNA it makes passes through the nuclear pore into the cytoplasm. There, making protein from RNA is called translation.
The two strands of DNA have different names. The 5'→3' strand is the sense or coding strand. The 3'→5' strand is the antisense or template strand.
RNA polymerase II uses double-stranded DNA as its template and makes single-stranded mRNA. The mRNA is an exact copy of the DNA, except that U stands where T would.
2. What it needs, and where it starts
Transcription needs five things: DNA as the template; RNA polymerase; free ribonucleotide triphosphates, that is ATP, GTP, CTP and UTP; the energy released when the triphosphates break; and helper proteins.
Transcription starts at the cap site. 7-methylguanosine joins the 5' end of the pre-RNA and caps it. This is capping, putting on the hat.
A short stretch after the cap is never translated. It is the leader sequence. After it, the first regulatory codon AUG signals the start of translation, and a stop codon signals its end. One sequence in Figure 1.34 reads AUG CCG UAU GCU UAU GCU CCU UAA UCA: AUG at the start, UAA near the end.
3. The three stages
In a eukaryote, transcription runs in three stages: initiation, elongation and termination.
Initiation begins at the promoter. Every gene has a promoter in the DNA, a special sequence in the gene's regulatory part. Transcription factors bind it first, and then RNA polymerase joins.
In a prokaryote this step is direct: RNA polymerase binds the promoter itself. Eukaryotes have three kinds of RNA polymerase, prokaryotes one.
Bound to the promoter, RNA polymerase first unwinds at least 20 base pairs. The strand that carries the gene is the template. The other is the complementary strand, and it is not transcribed. Transcription proceeds in the 5'-3' direction, and the start site and end site are fixed beforehand.
In elongation, RNA polymerase adds nucleotides according to base pairing. It moves along the template strand from 3' to 5'.
A template sequence ATTCGA gives UAAGCU in the RNA. The RNA is antiparallel to the template but like the complementary strand, with U for T. Set the bases yourself below.
sequence copierset the template
Sequence copier
You choose the bases. Nothing goes anywhere; the notebook is yours.
When the open part has been transcribed, RNA polymerase opens the next part ahead and re-winds the part behind it at the same time. The energy comes from breaking phosphate bonds.
Transcription ends when RNA polymerase reaches the termination site, a fixed base sequence in the DNA. DNA replication has proofreading and repair; transcription has neither. Now pull the two processes apart.
sort themtap a chip, then pick a bin
Which process?
Pick up a chip, then drop it in a bin. Ask yourself: what is being built, DNA or RNA?
Right. Next one.
Not yet. Ask which process can fix its mistakes, and in which one U stands in place of T.
All in place. In replication DNA polymerase builds DNA, errors are fixed, and a primer's 3'-OH is needed. In transcription RNA polymerase builds RNA, U goes in place of T, and there is no repair.
4. From pre-mRNA to mRNA
The mRNA made first is pre-mRNA, and it is longer than the final mRNA. Special processing turns it into the final mRNA. In a prokaryote the final mRNA forms directly, and translation starts at once.
The parts of a gene that are never translated are introns, the intervening sequences. The parts that are translated are exons, the expressed sequences.
Cutting the introns out of the mRNA and joining the exons is splicing. It takes place in the spliceosome, which is made of proteins and snRNA, small nuclear RNA.
Splice it yourself. First guess whether the final mRNA will be longer or shorter.
splicingguess, then splice
From pre-mRNA to mRNA
Three exons and two introns are for the drawing; a real gene has its own count.
The transcription and processing of mRNA happen in the nucleus. Translation happens in the cytoplasm.
A feel for the speed: an E. coli gene needs only 10 seconds to transcribe an mRNA of 1000 nucleotides.
The part of the genome that continuously gives one RNA molecule is the transcription unit, with three parts: the promoter, the start point and the end point. The DNA part holding all the information for one polypeptide chain is a gene, or cistron.
Here is the whole process in one picture, in the manner of the book's Figure 1.34. Tap the nine points.
Figure 1.34Tap a point
Three stages
RNA polymerase. It makes the RNA. It binds the promoter, unwinds the DNA, then moves along the template adding nucleotides.
Promoter. Where it starts. Transcription factors sit here first, and then RNA polymerase.
Start site. The beginning. This point is fixed beforehand.
Template strand. It is copied. The strand that carries the gene is the template.
Complementary strand. It is not copied.
Termination site. The end. Transcription stops when the polymerase reaches this fixed base sequence.
Pre-mRNA. The first RNA. It is longer than the final mRNA because it still carries introns.
Splicing. Introns removed. In the spliceosome the introns are cut out and the exons joined.
Nuclear pore. The final mRNA goes out this way into the cytosol, where translation happens.
All nine seen. Which one is not copied? The complementary strand.
Drawn by hand after the book's figure, not to scale.
Now put the steps in their order.
in orderPut the steps in order
Order of transcription
What happens first of all? Who sits on the promoter?
Yes. Then?
Not yet. Start first, then elongation, then processing last.
Right order. Transcription factors, RNA polymerase, unwinding, elongation, termination, then splicing, then the trip to the cytosol.
5. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
If the sequence of one helix of DNA is ACTG, the base sequence of the mRNA will be:
TCGA has T, but mRNA has no T; it has uracil (U).
Right. From the template ACTG, A pairs to U, C to G, T to A and G to C, so the mRNA is UGAC.
CGTA has T and its pairing does not fit either.
TGAC starts with T; in mRNA U pairs opposite A, not T.
For the second, recall the stages.
check yourselfpick one answer
Question 2
During transcription, while mRNA is made:
i. Uracil joins opposite adenine ii. Thymine joins opposite adenine iii. Guanine joins opposite cytosine
i and ii: ii is wrong, since thymine is not in RNA.
Right. Uracil pairs opposite adenine and guanine opposite cytosine, not thymine.
ii and iii: ii is wrong.
Not all three, because ii is wrong.
In the last, watch the strands and the bases.
check yourselfpick one answer
Question 3
What are the nucleotides of DNA that do not form codons called?
Right. The part that forms no codons and is not translated is the intron.
The recon is the unit of recombination.
Glycogen is a stored carbohydrate, not part of a gene.
The cistron is the unit of gene function.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhat is transcription? What occurs in HIV?
AnswerCopying the chemical information stored in DNA into an RNA (mRNA) molecule. In HIV, reverse transcription occurs.
QuestionWhich strand is sense and which antisense?
Answer5'→3' strand: sense (coding). 3'→5' strand: antisense (template).
QuestionHow does the mRNA sequence match the DNA?
AnswerRNA polymerase II uses double-stranded DNA as template to make single-stranded mRNA; an exact copy with U for T.
QuestionName the three stages of transcription in a eukaryote.
AnswerInitiation, elongation, termination.
QuestionIf the template strand reads ATTCGA, what is the mRNA?
AnswerA→U, T→A, T→A, C→G, G→C, A→U, so UAAGCU.
QuestionWhat are introns and exons?
AnswerIntrons: never-translated parts (intervening sequences). Exons: translated parts (expressed sequences).
QuestionWhat is splicing? Where does it occur?
AnswerCutting introns out of mRNA and joining the exons; in the spliceosome (proteins and snRNA).
Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.4.