Transcription is the process cells use to copy information from DNA into RNA. It is the first major step in gene expression, allowing the instructions stored in a gene to be used without changing the original DNA. In eukaryotes, transcription happens in the nucleus, while translation happens later in the cytoplasm.
Understanding transcription helps explain how cells control which proteins they make and when they make them.
During transcription, RNA polymerase binds near a gene at a promoter, opens a small region of DNA, and builds an RNA strand using one DNA strand as a template. The RNA grows by base pairing, with A pairing with U in RNA and C pairing with G. After termination, the RNA transcript is released, and in eukaryotic cells it is processed by adding a 5 prime cap, adding a poly-A tail, and removing introns by splicing.
These processing steps help protect the RNA and prepare it to be translated into protein.
Understanding Biology: Transcription in Detail
A gene has two DNA strands, but only one serves as the template for a particular RNA molecule. The other strand is called the coding strand because its sequence closely matches the RNA sequence. The main difference is that RNA uses uracil where DNA uses thymine.
This distinction helps students check their work when they write an RNA sequence from DNA. First identify which strand is the template and its direction.
Then build the complementary RNA bases in the correct order. A sequence can look correct in its base pairs yet be backwards, which changes the message.
RNA polymerase does more than attach to DNA. It creates a short opened area called a transcription bubble, exposing the bases needed for copying. As it moves, DNA behind the enzyme joins together again.
The new RNA strand separates from the DNA soon after it is made. Cells use many proteins to help polymerase find the right starting point. These proteins can increase or reduce transcription by affecting access to a gene.
DNA is wrapped around proteins called histones in eukaryotic cells. Tightly packed DNA is usually harder to transcribe, while loosened DNA is easier for the cell machinery to reach.
The end of transcription is not identical in every organism or for every type of RNA. In bacteria, some RNA molecules contain sequences that make the RNA fold into a shape that stops the polymerase. In eukaryotic cells, the RNA is cut after a signal near the end of the gene.
The polymerase then leaves the DNA. This control matters because a cell must make complete RNA molecules of the right length.
If transcription starts at the wrong place or ends too soon, the resulting protein instructions may be incomplete. Cells reduce such mistakes through careful recognition of DNA signals and through later checks on RNA.
RNA processing can greatly expand what one gene can do. During splicing, different groups of coding sections, called exons, may be joined in different combinations. This is called alternative splicing.
It allows one gene to produce RNA instructions for related proteins with different jobs. Nerve cells, muscle cells, and skin cells can therefore use the same DNA but make very different sets of proteins. Problems with splicing are linked to some inherited diseases and cancers.
When learning this topic, keep the stages separate. First the DNA sequence is copied into an initial RNA transcript.
Next the transcript is modified and selected sections are joined. Only after these steps can a mature messenger RNA usually guide protein production.
Key Facts
- Transcription copies DNA into RNA, not protein.
- RNA polymerase builds RNA in the 5 prime to 3 prime direction.
- RNA base pairing rules during transcription are A to U, T to A, C to G, and G to C.
- The DNA template strand is read by RNA polymerase in the 3 prime to 5 prime direction.
- Initiation begins when RNA polymerase binds to a promoter near the start of a gene.
- In eukaryotes, pre-mRNA processing includes a 5 prime cap, a poly-A tail, and splicing to remove introns.
Vocabulary
- Transcription
- Transcription is the process of making an RNA copy of a gene from a DNA template.
- RNA polymerase
- RNA polymerase is the enzyme that opens DNA and links RNA nucleotides together during transcription.
- Promoter
- A promoter is a DNA sequence where RNA polymerase and other proteins bind to begin transcription.
- Transcription bubble
- A transcription bubble is the small opened region of DNA where RNA polymerase reads the template strand and makes RNA.
- Splicing
- Splicing is the process that removes introns from pre-mRNA and joins exons together.
Common Mistakes to Avoid
- Confusing transcription with translation is wrong because transcription makes RNA from DNA, while translation makes a protein from mRNA.
- Writing thymine in the RNA strand is wrong because RNA uses uracil instead of thymine.
- Using the coding DNA strand as the direct template is wrong because RNA polymerase reads the template strand, while the RNA sequence usually matches the coding strand except U replaces T.
- Forgetting RNA processing in eukaryotes is wrong because pre-mRNA usually must receive a 5 prime cap, a poly-A tail, and splicing before it becomes mature mRNA.
Practice Questions
- 1 A DNA template strand has the sequence 3 prime TAC GGA CTT 5 prime. What RNA sequence is produced from it?
- 2 An mRNA contains 900 nucleotides after splicing. If every 3 nucleotides form one codon, how many codons are present?
- 3 Explain why a mutation in a promoter can reduce the amount of mRNA made from a gene even if the protein-coding sequence is unchanged.