The central dogma of molecular biology describes the flow of genetic information: DNA is copied (replication), read into RNA (transcription), and then decoded into protein (translation). Codons on mRNA are matched by anticodons on tRNA, which deliver the correct amino acids to the ribosome. Before mRNA leaves the nucleus it is processed (capped, tailed, and spliced) to become a mature transcript.
Difficulty: Introductory Prerequisites: Basic understanding of DNA structure and base pairing (A-T, G-C).
This material sits at the heart of introductory biology: how cells turn the instructions stored in DNA into functional proteins. If you understand base pairing and the double-helix structure of DNA, you have enough to follow along. Everything here builds toward a single question: how does a four-letter nucleotide alphabet encode the twenty amino acids that make up every protein in your body? The answer runs through transcription, RNA processing, and the genetic code itself.
Central dogma
The principle that genetic information flows from DNA to RNA to protein. In simple terms, DNA is the master blueprint, RNA is the working copy, and protein is the finished product.
Transcription
The process of copying a gene's DNA sequence into a complementary RNA molecule, carried out by RNA polymerase. Think of it as photocopying one page of a recipe book.
mRNA (messenger RNA)
The RNA molecule that carries the genetic message from the nucleus to the ribosome. In simple terms, it is the portable copy of the gene that the protein-building machinery reads.
RNA polymerase
The enzyme that reads the DNA template strand and synthesises a complementary RNA strand during transcription. It moves along the gene in the 3' to 5' direction on the template, building RNA 5' to 3'.
Pre-mRNA (primary transcript)
The initial RNA copy made in the nucleus before any processing. It contains both coding regions (exons) and non-coding regions (introns).
Exon
A segment of the pre-mRNA that is kept in the mature mRNA and codes for part of the protein. Think of exons as the sentences you keep when editing a draft.
Intron
A segment of the pre-mRNA that is removed (spliced out) during RNA processing. These are the parts that get cut away before the message is sent to the ribosome.
5' cap
A modified guanine nucleotide added to the 5' end of the mRNA. It protects the mRNA from degradation and helps the ribosome recognise where to start reading.
Poly-A tail
A string of adenine nucleotides added to the 3' end of the mRNA after transcription. It stabilises the mRNA and aids its export from the nucleus.
Codon
A sequence of three nucleotides on mRNA that specifies a particular amino acid (or a stop signal). In simple terms, it is a three-letter word in the genetic language.
Anticodon
A sequence of three nucleotides on tRNA that is complementary to a codon on mRNA. It ensures the correct amino acid is delivered during translation.
tRNA (transfer RNA)
A small RNA molecule that carries a specific amino acid to the ribosome, matching its anticodon to the mRNA codon. Think of it as a delivery shuttle that reads the address (codon) and drops off the right package (amino acid).
Aminoacyl-tRNA synthetase
The enzyme that attaches the correct amino acid to its corresponding tRNA. Each of the 20 amino acids has its own synthetase, which is what keeps the genetic code accurate.
Start codon (AUG)
The codon that signals the beginning of translation. It always codes for methionine (Met) and is the first amino acid in nearly every new polypeptide.
Stop codons (UAA, UAG, UGA)
Codons that signal the end of translation. They do not code for any amino acid; instead, release factors bind and free the finished polypeptide.
Genetic code
The set of rules by which the 64 possible three-nucleotide codons map to 20 amino acids and three stop signals. The code is nearly universal across all life, and it is degenerate (redundant), meaning most amino acids are specified by more than one codon.
Replication (DNA → DNA): DNA polymerase copies the entire genome before a cell divides. Both strands of the double helix serve as templates, producing two identical copies.
Transcription (DNA → RNA): RNA polymerase reads one strand of a gene and builds a complementary RNA molecule. This happens inside the nucleus in eukaryotes.
Translation (RNA → Protein): Ribosomes read the mRNA and, with the help of tRNAs, assemble amino acids into a polypeptide chain. This happens in the cytoplasm.
The central dogma describes the default direction of information flow. Information moves from nucleic acid to protein, not the other way round. (Reverse transcriptase, found in retroviruses, copies RNA back into DNA, but this does not reverse the dogma's core claim about protein-to-nucleic-acid flow.)
Where it happens: In the nucleus (eukaryotes). Prokaryotes transcribe in the cytoplasm since they lack a nucleus.
Enzyme: RNA polymerase binds to the promoter region of a gene and unwinds a short stretch of DNA.
Template strand vs. coding strand: RNA polymerase reads the template strand (3' → 5') and synthesises the RNA transcript in the 5' → 3' direction. The resulting RNA sequence matches the coding (non-template) strand, except with uracil (U) in place of thymine (T).
Elongation: RNA polymerase adds complementary RNA nucleotides one at a time (A pairs with U, G pairs with C).
Termination: RNA polymerase reaches a termination signal and releases the newly made RNA transcript (the primary transcript, or pre-mRNA in eukaryotes).
Key detail for exams: the mRNA sequence is complementary to the template strand and identical to the coding strand (with U replacing T).
Before the primary transcript leaves the nucleus, three modifications turn it into mature mRNA:
5' capping: A modified guanine nucleotide is added to the 5' end. This protects the mRNA from enzymatic degradation and is recognised by the ribosome during translation initiation.
3' polyadenylation (poly-A tail): A chain of 100 to 250 adenine nucleotides is added to the 3' end. The poly-A tail stabilises the transcript and helps export it through nuclear pores.
Splicing: Introns (non-coding sequences) are removed and exons (coding sequences) are joined together by the spliceosome, a complex of small nuclear RNAs and proteins.
The mature mRNA, now carrying only exon sequences flanked by its cap and tail, exits the nucleus and enters the cytoplasm for translation.
Alternative splicing: the same pre-mRNA can be spliced in different ways, including or excluding certain exons, which allows one gene to code for multiple protein variants. This is one reason the human proteome is far larger than the number of human genes.
Triplet code: Each codon is a group of three consecutive nucleotides on mRNA. With four possible bases at each position, there are 4 × 4 × 4 = 64 possible codons.
20 amino acids + stops: Of the 64 codons, 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA). AUG is the universal start codon and codes for methionine.
Degeneracy (redundancy): Most amino acids are specified by more than one codon. For example, leucine has six codons. This redundancy tends to occur in the third position of the codon (the "wobble" position), which is why a single-nucleotide change there often has no effect on the protein.
Reading frame: Codons are read in a fixed frame starting from the start codon (AUG). There is no punctuation between codons. A single insertion or deletion shifts the entire reading frame downstream (a frameshift mutation).
How anticodons work:
Each tRNA molecule has a three-nucleotide anticodon that base-pairs with a complementary codon on the mRNA.
The pairing is antiparallel: the mRNA codon is read 5' → 3', and the anticodon pairs 3' → 5'.
Example from the diagrams: the mRNA codon UGG pairs with the anticodon ACC on a tRNA carrying tryptophan (Trp). The codon UUU pairs with anticodon AAA on a tRNA carrying phenylalanine (Phe). The codon GGC pairs with anticodon CCG on a tRNA carrying glycine (Gly).
Wobble pairing: The third base of the codon (and first base of the anticodon) can form non-standard base pairs, which is why fewer than 61 different tRNAs are needed to read all 61 sense codons.
No mathematical formulas for this topic, but you should be able to reproduce or label these key diagrams from memory:
Central dogma flow: DNA → (replication) → DNA; DNA → (transcription, RNA polymerase) → RNA; RNA → (translation, ribosome) → Protein.
Transcription and processing overview: DNA in nucleus → RNA polymerase produces pre-mRNA → 5' cap and poly-A tail added → introns spliced out → mature mRNA exits nucleus.
Translation at the ribosome: mRNA threaded through ribosome, read 5' to 3'; tRNAs with anticodons arrive at the A site, amino acids are linked into a growing polypeptide chain, used tRNAs exit from the E site.
Codon table: You should be able to use a standard codon table (organised by first, second, and third base) to look up any codon's amino acid. Practise reading it both ways: codon → amino acid, and amino acid → possible codons.
Understanding transcription is the basis for how mRNA vaccines work: synthetic mRNA is delivered into cells, where ribosomes translate it into a target protein (such as the SARS-CoV-2 spike protein) that trains the immune system. The genetic code's near-universality is also what makes genetic engineering possible: a human gene inserted into a bacterium will be transcribed and translated using the same codon-to-amino-acid rules.
Students often confuse the template strand with the coding strand. The mRNA matches the coding strand (with U instead of T). RNA polymerase reads the template strand.
"Degenerate" in the context of the genetic code means redundant (multiple codons for one amino acid). It does not mean the code is flawed or degraded.
The 5' cap and poly-A tail are added during RNA processing, not during transcription itself. They are post-transcriptional modifications.
Introns are removed from the RNA, not from the DNA. The gene in the DNA still contains its introns after transcription.
⚠️ Be able to trace the flow: given a DNA template strand, write the mRNA sequence, then identify the codons and the amino acids they encode.
⚠️ Know all three RNA processing steps (capping, polyadenylation, splicing) and where each occurs.
⚠️ Understand why the genetic code is described as "degenerate" or "redundant" and how wobble base pairing relates to this.
⚠️ Distinguish between replication, transcription, and translation: which enzyme, which template, which product, and which cellular compartment.
True or false: RNA polymerase reads the coding strand of DNA during transcription. (False, it reads the template strand.)
Fill in the blank: The three post-transcriptional modifications to eukaryotic mRNA are the ______, the ______, and ______. (5' cap, poly-A tail, splicing of introns.)
True or false: There are (61) codons that code for amino acids and (3) stop codons.
Fill in the blank: The codon AUG codes for the amino acid ______ and serves as the ______ codon. (Methionine; start.)
True or false: Introns are physically removed from the DNA after transcription. (False, they are removed from the pre-mRNA during RNA processing; the DNA is unchanged.)
Q: A DNA template strand reads 3'-TACGGCAATCCC-5'. What is the mRNA sequence?
A: 5'-AUGCCGUUAGGG-3'. (Each DNA base is replaced by its RNA complement: T→A, A→U, C→G, G→C.)
Q: Why is the genetic code described as "degenerate"?
A: Because most amino acids are encoded by more than one codon. For example, leucine can be coded by six different codons. The redundancy is concentrated at the third (wobble) position.
Q: Name the three modifications made to eukaryotic pre-mRNA and state the purpose of each.
A: (1) A 5' cap protects the mRNA and aids ribosome binding. (2) A poly-A tail stabilises the transcript and assists nuclear export. (3) Splicing removes introns so that only exon sequences remain in the mature mRNA.
Q: What is the role of aminoacyl-tRNA synthetase?
A: It attaches the correct amino acid to its corresponding tRNA molecule, ensuring that each tRNA delivers the right amino acid during translation. There is a specific synthetase for each of the 20 amino acids.
Q: Explain how alternative splicing increases protein diversity.
A: Different combinations of exons can be retained or removed during splicing of the same pre-mRNA. Each combination produces a distinct mature mRNA, which is translated into a different protein variant. This means one gene can give rise to multiple proteins.
This material connects directly to translation and protein synthesis (covered in the companion study notes), where the mRNA produced here is decoded at the ribosome. It also ties into mutation, because any change to the DNA sequence can alter the mRNA codons and therefore the protein product. Gene regulation, covered later in most courses, builds on transcription by asking when and how much a gene is transcribed.
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