AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/6/6-3)
Unit 6 · Topic 6.3
6.3 Transcription and RNA Processing
Transcription is the first step in using a gene: RNA polymerase reads one DNA strand and builds a complementary RNA. In eukaryotes, the new mRNA is then processed, getting a 5′ cap and a poly-A tail and having its introns removed, and alternative splicing lets one gene make several different proteins.
Key terms
- RNA polymerase
- template strand
- introns and exons
- 5′ cap (GTP cap)
- poly-A tail
- alternative splicing
The three main kinds of RNA
Genes are used by first copying them into RNA. Different RNAs do different jobs, and each one's job depends on its base sequence and its shape.
- Messenger RNA (mRNA) carries the gene's information from DNA (in the nucleus of eukaryotes) to ribosomes in the cytoplasm.
- Transfer RNA (tRNA) molecules each carry a specific amino acid and have an anticodon, a three-base sequence that pairs with a matching codon on the mRNA. They deliver amino acids in the right order during translation.
- Ribosomal RNA (rRNA) is a main structural and functional part of ribosomes, the machines that build proteins.
How transcription works
RNA polymerase binds to DNA at a promoter, a sequence just before the gene, and separates the two strands there. Only one strand, the template strand, is read for a given gene. RNA polymerase reads the template 3′ → 5′ and builds RNA 5′ → 3′, adding nucleotides that pair with the template: A in the template gives U in RNA, T gives A, G gives C, and C gives G. Unlike DNA polymerase, RNA polymerase doesn't need a primer.
The other DNA strand, the coding strand (also called the non-template strand), isn't read. It has the same sequence as the mRNA, written 5′ → 3′, except that it has T where the RNA has U. That's a handy shortcut: if you're given the coding strand, swap T for U to get the mRNA.
mRNA processing in eukaryotes
In eukaryotes, transcription happens in the nucleus and makes a pre-mRNA, which is modified by enzymes before it leaves:
- A modified guanine nucleotide is added to the 5′ end. Textbooks call this the 5′ cap; AP materials call it the GTP cap. It helps the ribosome recognize the mRNA and also protects it.
- A long chain of adenine nucleotides, the poly-A tail, is added to the 3′ end. It makes the mRNA more stable so it lasts longer in the cytoplasm.
- Introns, the noncoding stretches inside the gene, are cut out, and the exons, the parts that will be expressed, are spliced together.
Alternative splicing
Exons don't have to be joined the same way every time. In alternative splicing, a cell includes some exons and leaves out others, producing different mature mRNAs, and so different proteins, from the same gene. This is one reason humans can make far more kinds of proteins than they have genes, and it's one way different cell types use the same gene differently.
Prokaryotes don't do this processing. Their genes usually have no introns, and because there's no nucleus, ribosomes can start translating an mRNA while it's still being transcribed.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
From template strand to mRNA
A gene's template strand reads 3′-TACGGATTCACT-5′. Write the mRNA and the coding strand, with ends labeled.
Show the solutionHide the solution
- Step 1: RNA polymerase reads the template 3′ → 5′ and builds RNA 5′ → 3′, so the mRNA starts across from the template's 3′ end.
- Step 2: Pair each template base: T→A, A→U, C→G, G→C, G→C, A→U, T→A, T→A, C→G, A→U, C→G, T→A.
- Step 3: mRNA: 5′-AUGCCUAAGUGA-3′.
- Step 4: The coding strand matches the mRNA but with T instead of U: 5′-ATGCCTAAGTGA-3′.
Answer: mRNA 5′-AUGCCUAAGUGA-3′; coding strand 5′-ATGCCTAAGTGA-3′.
- Example 2
Counting splice variants
A gene has four exons. Exons 1 and 4 are always kept, but exons 2 and 3 can each be included or skipped independently. How many different mature mRNAs can be made? List them.
Show the solutionHide the solution
- Step 1: Exon 2 has two options (in or out), and exon 3 has two options.
- Step 2: The choices are independent, so multiply: 2 × 2 = 4.
- Step 3: List them, keeping exon order: 1-2-3-4, 1-2-4, 1-3-4, 1-4.
- Step 4: Exons stay in their original order; splicing doesn't rearrange them.
Answer: 4 mRNAs: 1-2-3-4, 1-2-4, 1-3-4 and 1-4.
Common mistakes
- Saying RNA polymerase reads both DNA strands of a gene. Only the template strand is read for any one gene.
- Putting T in an RNA sequence. RNA uses U wherever DNA would have T.
- Thinking introns are kept and exons removed. Introns are removed (they 'intrude'); exons are expressed.
- Mixing up the jobs of the cap and tail. In AP terms, the poly-A tail adds stability and the 5′ (GTP) cap helps the ribosome recognize the mRNA.
On the exam
- Expect sequence questions where you transcribe a template strand or identify which strand was the template; always label 5′ and 3′.
- Alternative splicing often shows up as an explanation for how one gene produces different proteins in different tissues.
Connected topics
Videos
Check yourself
4 questions on 6.3 Transcription and RNA Processing. Pick an answer to see if you got it, and why.
A segment of the DNA template strand reads 3′-TACGGATTC-5′. What mRNA is transcribed from it?
Humans have about 20,000 protein-coding genes but make many more kinds of proteins. Which process best explains this?
| Exon | Length (nucleotides) | In muscle mRNA? | In brain mRNA? |
|---|---|---|---|
| 1 | 150 | Yes | Yes |
| 2 | 90 | Yes | No |
| 3 | 120 | No | Yes |
| 4 | 210 | Yes | Yes |
Experimental data: the mature mRNA made from one gene was sequenced in muscle cells and in brain cells of the same animal. Introns separate the four exons in the gene.
A mutation in exon 3 creates a premature stop codon. Which cells would most likely make an altered protein from this gene?
A different mutation changes one base in the middle of the intron between exons 1 and 2, away from the sites where splicing happens. What is the most likely effect?
0 of 4 answered