Transcription

Transcription is the process of making an RNA molecule from a DNA template. It's the first step in gene expression — the cell's way of creating a working copy of a gene that can leave the nucleus and be used to build a protein.

Key Takeaways

• Helicase unwinds the DNA double helix and topoisomerase relieves the resulting tension; RNA polymerase reads the template (antisense) strand to build RNA, while the coding (sense) strand matches the RNA product except for T-versus-U.

• Transcription begins at the promoter region, often marked by a TATA box, with help from transcription factors; RNA Polymerase II synthesizes mRNA (Pol I makes rRNA, Pol III makes tRNA and some rRNA).

• Synthesis starts at the plus one site and produces raw hnRNA, read 3' to 5' on the template and built 5' to 3'.

• hnRNA is processed into mature mRNA through three steps: splicing (spliceosome removes introns, joins exons), 5' capping (7-methylguanosine, aids ribosome recognition and stability), and 3' polyadenylation (poly-A tail, protects RNA and assists nuclear export).

• Alternative splicing lets a single gene produce multiple mRNA — and protein — versions by including or skipping different exons.

Opening the DNA Double Helix

Before transcription can begin, the DNA double helix has to be opened up. Helicase handles this job, unwinding the DNA and separating the two strands. As the DNA unwinds, another enzyme, topoisomerase, works ahead of the fork to relieve tension and prevent the DNA from supercoiling on itself.

Template Strand vs. Coding Strand

Once the two strands are separated, only one of them is actually used to build RNA.

The template strand (also called the antisense strand) is the one RNA polymerase reads to synthesize RNA. The other strand, the coding strand (or sense strand), isn't used directly — but because it has the same sequence as the resulting RNA (just with T instead of U), it's a useful reference for predicting the RNA product.

The Promoter, the TATA Box, and Transcription Factors

Transcription begins at a specific DNA sequence called the promoter region, found upstream of the gene — the site where RNA polymerase binds to begin the process. A common element within eukaryotic promoters is the TATA box, a DNA sequence rich in adenine and thymine that helps position RNA polymerase correctly at the start site.

RNA polymerase doesn't act alone, though. In eukaryotes, it needs help from transcription factors — proteins that help RNA polymerase recognize the promoter and bind tightly to the DNA, making sure everything is properly aligned before transcription starts.

The Three RNA Polymerases

Eukaryotic cells contain three RNA polymerases, but only one of them makes mRNA.

RNA Polymerase II is the one that matters most for mRNA synthesis — the focus of this subtopic.

From Start Site to hnRNA

Transcription begins at a defined location called the plus one site, also known as the transcription start site. From there, RNA polymerase reads the DNA template strand in the 3' to 5' direction, while building the new RNA strand in the 5' to 3' direction.

The immediate product is a strand of heterogeneous nuclear RNA (hnRNA) — the raw, unprocessed form of RNA made in the nucleus. Before it can function as messenger RNA, hnRNA needs to go through several processing steps.

Processing hnRNA into Mature mRNA

Posttranscriptional processing happens in the nucleus and is essential to make mRNA stable, functional, and ready to be exported to the cytoplasm for translation. Three major modifications occur.

Splicing. hnRNA includes both exons and introns. Exons are the coding regions — the parts that actually contain instructions for building a protein. Introns are noncoding regions that need to be removed. Splicing is carried out by a large molecular machine called the spliceosome, made of small nuclear RNAs (snRNAs) and proteins. The spliceosome recognizes specific sequences at intron boundaries and catalyzes intron removal, joining the exons into a continuous coding sequence. Once splicing is complete, the mRNA contains only exons.

5' capping. A special molecule called 7-methylguanosine is added to the 5' end of the RNA — the 5' cap. This modification serves two purposes: it helps the ribosome recognize the mRNA during translation, and it protects the RNA from degradation by enzymes while it's still inside the cell.

3' polyadenylation. At the 3' end of the RNA, a long stretch of adenine nucleotides — roughly 100 to 250 — is added by a dedicated enzyme, forming the poly-A tail. Like the 5' cap, the poly-A tail protects the RNA from degradation, and it also assists with exporting the mRNA from the nucleus into the cytoplasm.

Alternative Splicing and Protein Diversity

There's one more important concept worth highlighting here: alternative splicing. A single hnRNA transcript can be spliced in different ways to produce multiple versions of mature mRNA — meaning a single gene can yield different proteins depending on which exons are included or skipped. This allows for significant protein diversity without requiring additional genes, a mechanism that's especially powerful in complex organisms like humans, where it helps maximize the information encoded in a relatively limited genome.

Common MCAT Mistakes

• Mixing up the template and coding strands. RNA polymerase reads the template (antisense) strand, not the coding (sense) strand. The coding strand is only useful as a reference because it matches the RNA product (with T instead of U) — it's never the strand actually being read.

• Forgetting the reading/building direction rule. RNA polymerase reads the template strand 3' to 5' while building the new RNA strand 5' to 3'. Reversing this is a common error under exam pressure.

• Assuming all three RNA polymerases make mRNA. Only RNA Polymerase II makes mRNA. Pol I makes rRNA, and Pol III makes tRNA and some rRNA — mixing these up is an easy way to miss a straightforward recall question.

• Confusing exons and introns. Exons are the coding regions that stay in the mature mRNA; introns are the noncoding regions removed by the spliceosome. After splicing, the mature mRNA contains only exons.

MCAT-Style Concept Check

Question: A researcher isolates a strand of hnRNA before it has undergone any posttranscriptional processing. Which of the following correctly describes what still needs to happen before this molecule can function as mature mRNA?

• A) The molecule needs a poly-A tail added to its 5' end and a 7-methylguanosine cap added to its 3' end

• B) The molecule needs introns removed by the spliceosome, a 5' cap added, and a poly-A tail added to its 3' end

• C) The molecule needs exons removed by RNA Polymerase I before it can be exported from the nucleus

• D) The molecule is already functional mRNA and requires no further processing

Answer: B

Explanation: Raw hnRNA must go through three processing steps before it becomes mature mRNA: splicing (the spliceosome removes introns and joins the exons into a continuous coding sequence), 5' capping (7-methylguanosine is added to the 5' end to aid ribosome recognition and protect against degradation), and 3' polyadenylation (a poly-A tail of roughly 100–250 adenines is added to the 3' end to protect the RNA and assist nuclear export). Option A is wrong because it reverses which end gets the cap versus the tail. Option C is wrong because it's the spliceosome that removes introns (not exons), and RNA Polymerase I makes rRNA, not mRNA processing machinery. Option D is wrong because unprocessed hnRNA cannot yet function as mRNA.

FAQ

What's the difference between the template strand and the coding strand?

The template strand (antisense strand) is the one RNA polymerase actually reads to build RNA. The coding strand (sense strand) isn't read directly, but it has the same sequence as the RNA product except with thymine instead of uracil, making it useful for predicting the RNA sequence.

Which RNA polymerase makes mRNA?

RNA Polymerase II. RNA Polymerase I makes rRNA and is active in the nucleolus, while RNA Polymerase III makes tRNA and some rRNA. RNA Polymerase II also makes snRNA, which assists in splicing.

What are the three steps that turn hnRNA into mature mRNA?

Splicing (the spliceosome removes introns and joins exons), 5' capping (7-methylguanosine is added to the 5' end), and 3' polyadenylation (a poly-A tail of about 100–250 adenines is added to the 3' end). All three make the mRNA stable, functional, and ready for export to the cytoplasm.

How does alternative splicing increase protein diversity?

A single hnRNA transcript can be spliced in different ways, including or skipping different exons to produce multiple distinct mature mRNA molecules from one gene. This lets one gene encode multiple different proteins without requiring additional genes.