Transcription & RNA Processing

You’ll be able to

Describe how RNA polymerase synthesizes mRNA from a DNA template strand
Apply base pairing (A–U, G–C) to transcribe a template strand into mRNA
Explain the three RNA-processing steps that convert pre-mRNA into mature mRNA

From DNA to a working message

A gene stays safely in the nucleus, so to use it the cell first copies it into a portable messenger RNA (mRNA). This is transcription. RNA polymerase binds a DNA region called the promoter, unwinds the helix, and reads one strand — the template (antisense) strand — building an mRNA that is complementary to it. The other DNA strand, the coding (sense) strand, matches the mRNA sequence (except that RNA carries uracil (U) wherever DNA would have thymine). Like DNA polymerase, RNA polymerase builds only in the 5′→3′ direction, but it needs no primer.

Initiation, elongation, termination

Transcription runs in three stages. In initiation, RNA polymerase (with the help of transcription factors in eukaryotes) recognizes the promoter — often marked by a TATA box — and clamps onto the DNA. In elongation, the polymerase moves along the template strand, adding complementary RNA nucleotides and re-zipping the DNA helix behind it. In termination, the polymerase reaches a terminator sequence, releases the finished RNA transcript, and lets go of the DNA.

Transcription base pairing

DNA A → RNA U · DNA T → RNA A · DNA G → RNA C · DNA C → RNA G

RNA has no thymine — wherever the DNA template has adenine, the new mRNA carries uracil (U), not thymine (T).

Processing: capping, tailing, splicing

In eukaryotes the first transcript is a pre-mRNA that must be processed before it can leave the nucleus. Three edits happen. A protective 5′ cap (a modified guanine) is added to the front, and a poly-A tail (a string of adenines) is added to the back — both help the mRNA survive and get exported and translated. Most importantly, splicing removes the non-coding introns ("intervening" sequences) and joins the coding exons ("expressed" sequences) together, carried out by a complex called the spliceosome. Because a cell can splice the same pre-mRNA in different ways (alternative splicing), one gene can yield several different proteins.

Worked example

A gene’s template strand reads 3′–T A C G G G C T T–5′. Transcribe it into mRNA, written in the correct 5′→3′ orientation.

1.Read the template 3′→5′ and pair each base for RNA: T→A, A→U, C→G, G→C, G→C, G→C, C→G, T→A, T→A.
2.Remember RNA uses uracil (U) in place of thymine, so the A on the template gives a U in the mRNA.
3.The mRNA is antiparallel to the template, so it runs 5′→3′ across from the 3′→5′ template, giving 5′–A U G C C C G A A–3′.

Answer: The mRNA is 5′–A U G C C C G A A–3′ (complementary and antiparallel to the 3′–T A C G G G C T T–5′ template).

Checkpoint

A DNA template strand reads 3′–G C A–5′. What is the corresponding mRNA codon, written 5′→3′?

Watch out

The mRNA sequence matches the coding strand (with U for T) and is complementary to the template strand. Mixing up which strand is read is the classic transcription trap — RNA polymerase always reads the template strand and builds the coding-strand sequence.

Checkpoint

During RNA processing, what happens to introns and exons before the mature mRNA leaves the nucleus?

On the exam

Alternative splicing is a favorite "how can one gene make many proteins?" answer. By keeping different combinations of exons, a single pre-mRNA yields multiple distinct mRNAs — a major reason humans have far more proteins than genes.

Answer the 2 checkpoints as you read.