Protein Synthesis
Discover how your cells read the DNA code to build proteins -- the molecular machines that carry out nearly every function in living organisms.
From DNA to Protein: The Central Dogma
The central dogma of molecular biology describes the flow of genetic information in a cell: DNA → RNA → Protein. DNA stores the instructions, messenger RNA (mRNA) carries a copy of those instructions out of the nucleus, and ribosomes read the mRNA to assemble a chain of amino acids into a protein.
The Central Dogma
DNA
Double helix in the nucleus
mRNA
Transcription in the nucleus
Protein
Translation at ribosomes
Key idea: A gene is a section of DNA that codes for one polypeptide (protein chain). The human genome contains roughly 20,000 protein-coding genes.
Transcription
Transcription is the process of copying a gene's DNA sequence into a messenger RNA (mRNA) molecule. It occurs in the nucleus and is catalysed by the enzyme RNA polymerase.
Steps of Transcription
1. Initiation
RNA polymerase binds to the promoter region on DNA
2. Elongation
RNA polymerase reads the template strand 3' → 5' and builds mRNA 5' → 3'
3. Termination
RNA polymerase reaches a terminator sequence and releases the mRNA
4. mRNA Processing
Introns removed, exons spliced; 5' cap and poly-A tail added
Base pairing rule in RNA: In mRNA, uracil (U) replaces thymine (T). So A pairs with U, and C pairs with G. If the DNA template strand reads 3'-TACGGA-5', the mRNA will read 5'-AUGCCU-3'.
Translation
Translation is the process of reading the mRNA code to assemble a polypeptide chain of amino acids. It occurs at ribosomes in the cytoplasm (or on the rough endoplasmic reticulum). Transfer RNA (tRNA) molecules deliver the correct amino acids.
Codon
A sequence of three mRNA bases (e.g. AUG) that codes for one amino acid. There are 64 possible codons.
tRNA
A cloverleaf-shaped RNA with an anticodon that pairs with the mRNA codon, carrying the matching amino acid.
Ribosome
The cellular machine that reads mRNA and joins amino acids via peptide bonds. Has an A-site, P-site and E-site.
Steps of Translation
1. Initiation: The small ribosomal subunit binds to the mRNA at the start codon (AUG). The first tRNA (carrying methionine) occupies the P-site. The large subunit joins.
2. Elongation: A tRNA with a matching anticodon enters the A-site, a peptide bond forms between amino acids, and the ribosome shifts along the mRNA by one codon. This repeats as the polypeptide grows.
3. Termination: The ribosome reaches a stop codon (UAA, UAG or UGA). A release factor enters, the polypeptide is freed, and the ribosome disassembles.
Key Vocabulary
Codon
A three-nucleotide sequence on mRNA that specifies a particular amino acid or a stop signal during translation.
RNA Polymerase
The enzyme that synthesises mRNA by reading the DNA template strand during transcription.
Anticodon
A three-base sequence on tRNA that is complementary to a specific mRNA codon, ensuring the correct amino acid is added.
Polypeptide
A chain of amino acids linked by peptide bonds. One or more polypeptides fold into a functional protein.
Worked Examples
A DNA template strand reads 3'-TACAAAGCG-5'. What is the mRNA sequence?
Step 1: Apply complementary base pairing (A→U, T→A, C→G, G→C).
Step 2: T→A, A→U, C→G, A→U, A→U, A→U, G→C, C→G, G→C.
Answer: The mRNA sequence is 5'-AUGUUUCGC-3'.
The mRNA sequence is AUG-UUU-CGC. What amino acids does this code for?
Step 1: Divide the mRNA into codons: AUG | UUU | CGC.
Step 2: Use the codon table: AUG = Methionine (start), UUU = Phenylalanine, CGC = Arginine.
Answer: The amino acid sequence is Met-Phe-Arg.
Explain why a mutation that changes one DNA base could alter protein function.
Step 1: A single base change (point mutation) alters the mRNA codon produced during transcription.
Step 2: The altered codon may code for a different amino acid (missense mutation) or a premature stop codon (nonsense mutation).
Step 3: A different amino acid can change the protein's 3D shape and therefore its function, as seen in sickle cell anaemia where a single base change in the haemoglobin gene substitutes valine for glutamic acid.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Where does transcription take place in a eukaryotic cell?
Question 2
What is the start codon that initiates translation?
Question 3
Which molecule carries amino acids to the ribosome during translation?
Question 4
In RNA, which base replaces thymine (T) found in DNA?
Question 5
Which of the following is a stop codon?
Key Concepts Summary
- ●The central dogma describes the flow of information: DNA → mRNA → Protein.
- ●Transcription copies a gene into mRNA in the nucleus, catalysed by RNA polymerase.
- ●Translation occurs at ribosomes where mRNA codons are read and tRNA delivers amino acids.
- ●AUG is the start codon; UAA, UAG and UGA are stop codons.
- ●A single base mutation can change the amino acid sequence, potentially altering protein structure and function.