RNA Types and the Process of Transcription
Transcription is the process by which the information in a gene is copied from DNA into a molecule of RNA, and the main types of RNA are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). In brief: the enzyme RNA polymerase reads the template strand of a gene and builds a complementary RNA strand, using uracil (U) in place of thymine (T). In eukaryotes the RNA is processed before it leaves the nucleus. The mRNA is then used to make a protein by translation.
Genes do not make proteins directly. The flow of information is from DNA to RNA to protein, a principle known as the central dogma of molecular biology. This guide explains the link between genes and proteins, the types and roles of RNA, the three stages of transcription, RNA processing in eukaryotes, works through a transcription example and corrects common mistakes.
How are genes linked to proteins?
A gene is a sequence of DNA that carries instructions for making a protein, or in some cases a functional RNA. The proteins determine the structure and function of cells, so genes influence traits. DNA stays in the nucleus in eukaryotes, while proteins are made on ribosomes in the cytoplasm. RNA carries the message from the nucleus to the ribosomes.
The central dogma, proposed by Francis Crick in 1958, states that information flows from DNA to RNA to protein: DNA → (transcription) → RNA → (translation) → protein. There are exceptions, such as retroviruses, which copy RNA into DNA using the enzyme reverse transcriptase.
What are the types of RNA?
RNA (ribonucleic acid) differs from DNA in having the sugar ribose, the base uracil (U) instead of thymine, and in being usually single-stranded.
- Messenger RNA (mRNA): a copy of a gene. It carries the genetic information from the DNA to the ribosome. Every three bases (a codon) code for one amino acid. mRNA is made in different lengths, depending on the gene, and is short-lived: it lasts from minutes to hours.
- Transfer RNA (tRNA): a small molecule, about 75 to 90 nucleotides, folded into a cloverleaf shape. One end carries a specific amino acid, and the other has a three-base anticodon that pairs with a codon on mRNA. tRNA brings the correct amino acid to the ribosome.
- Ribosomal RNA (rRNA): the main component of ribosomes, together with proteins. It makes up most of the cell’s RNA, about 80 percent. It helps to catalyze the formation of peptide bonds.
Other types of RNA, such as microRNA and small interfering RNA, regulate the activity of genes.
How does transcription work?
Only one strand of the DNA double helix, the template (antisense) strand, is read in a gene. The other strand, the coding (sense) strand, has the same sequence as the RNA, except that T is replaced by U. Transcription has three stages.
- Initiation: RNA polymerase binds to a region called the promoter, which is next to the start of the gene. In eukaryotes, proteins called transcription factors must bind first. The DNA unwinds locally, and a transcription bubble is formed.
- Elongation: RNA polymerase moves along the template strand in the 3′ to 5′ direction and builds the RNA in the 5′ to 3′ direction. It adds RNA nucleotides complementary to the template: A pairs with U, T with A, C with G and G with C. Unlike DNA polymerase, RNA polymerase does not need a primer.
- Termination: RNA polymerase reaches a terminator sequence, releases the RNA transcript and detaches from the DNA. The DNA helix closes again.
How is RNA processed in eukaryotes?
In eukaryotes, the first transcript (pre-mRNA) is modified before it leaves the nucleus.
- 5′ cap: a modified guanine nucleotide is added at the 5′ end, protecting the RNA and helping the ribosome to attach.
- Poly-A tail: a chain of about 100 to 250 adenine nucleotides is added to the 3′ end, which protects the RNA from degradation.
- Splicing: most eukaryotic genes contain introns, non-coding regions that are removed, and exons, coding regions that are joined. Alternative splicing allows one gene to give several different proteins. Humans have about 20 000 genes but make many more different proteins.
Bacteria have no nucleus or introns, and so translation can start while the mRNA is still being made.
RNA types compared
| Type | Shape | Function | Where it works |
|---|---|---|---|
| mRNA | Linear | Carries the genetic code from a gene | Nucleus to cytoplasm |
| tRNA | Cloverleaf | Brings amino acids to the ribosome | Cytoplasm |
| rRNA | Folded with proteins | Builds ribosomes, forms peptide bonds | Ribosomes |
Worked example: transcribing a gene
A short section of a gene has the following sequence on the template strand, written 3′ to 5′: 3′-TACGGATCC-5′. Find the mRNA sequence and the coding strand.
- Pair each template base with its RNA complement: T→A, A→U, C→G, G→C, G→C, A→U, T→A, C→G, C→G.
- mRNA: 5′-AUGCCUAGG-3′. It is written 5′ to 3′ in the same left-to-right order, because the template is written 3′ to 5′.
- Coding strand: it has the same sequence as the mRNA, but with T in place of U: 5′-ATGCCTAGG-3′. It is also the complement of the template: 3′-TACGGATCC-5′ pairs with 5′-ATGCCTAGG-3′.
- Codons: split the mRNA into groups of three: AUG, CCU, AGG. AUG is the start codon.
A common trap is that if the question gives the coding strand, the mRNA is the same sequence with T replaced by U, but if it gives the template strand, the mRNA is its complement. The check: the mRNA must contain no T, and it must have the same length as the template. The next article explains how the codons are translated into amino acids.
How was transcription discovered?
Francis Crick proposed the central dogma in 1958. In 1961, François Jacob and Jacques Monod proposed that a short-lived “messenger” RNA carries information from DNA to the ribosomes, and Sydney Brenner and others demonstrated this the same year. Severo Ochoa and Arthur Kornberg discovered how nucleic acids are made in the 1950s. The discovery of introns and splicing in 1977, by Richard Roberts and Phillip Sharp, led to the 1993 Nobel Prize in Physiology or Medicine. Mainstream use of mRNA as a vaccine technology, in the 2020s, followed decades of work on RNA stability and delivery.
Which mistakes are common?
Mistake 1: Using thymine in RNA
RNA contains uracil in place of thymine. A on the template pairs with U in the mRNA.
Mistake 2: Saying that both DNA strands are transcribed
In a gene, only the template strand is used as the template for RNA.
Mistake 3: Mixing up transcription and translation
Transcription makes RNA from DNA. Translation makes protein from mRNA.
Mistake 4: Confusing the mRNA sequence with the template sequence
The mRNA is complementary to the template, and identical (apart from U for T) to the coding strand.