Revision notes for Edexcel A A Level Biology DNA, RNA and protein synthesis. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
DNA, RNA and protein synthesis
What you'll learn
- How DNA and RNA are built from nucleotides.
- How base sequences in genes carry the information for proteins.
- How transcription and translation make a polypeptide.
- How mutations can alter protein structure and function.
The big picture: genes to proteins
Your cells contain DNA, which stores genetic information. Sections of DNA called genes contain instructions for making polypeptides: chains of amino acids that fold to form proteins.
Proteins then affect the cell’s structure and function. For example, a protein may be an enzyme, a membrane channel, a receptor, an antibody, or a structural molecule such as collagen.
Definition
Gene
A gene is a sequence of DNA bases that codes for a polypeptide or a functional RNA molecule.
Key Idea
The central idea
The order of bases in DNA determines the order of amino acids in a polypeptide, and the amino acid sequence affects the protein’s shape and function.
DNA is a polymer of nucleotides
DNA stands for deoxyribonucleic acid. It is a nucleic acid, which means it is a polymer made from nucleotide monomers.
Definition
Nucleotide
A nucleotide is a monomer made from three parts: a phosphate group, a pentose sugar, and a nitrogen-containing organic base.
In DNA, the sugar is deoxyribose. The four DNA bases are:
- A: adenine
- T: thymine
- C: cytosine
- G: guanine
Adjacent nucleotides in the same strand are joined by phosphodiester bonds, forming a sugar-phosphate backbone. The bases project inwards from the backbone.
DNA normally forms a double helix: two strands twisted around each other. The strands are antiparallel, meaning they run in opposite directions, one 5′ to 3′ and the other 3′ to 5′.
Complementary base pairing
DNA bases pair in a specific way:
- adenine pairs with thymine: A–T
- cytosine pairs with guanine: C–G
The paired bases are held together by hydrogen bonds. Hydrogen bonds are individually weak, but many together stabilise the DNA double helix.
Key Idea
Complementary base pairing
Because A always pairs with T, and C always pairs with G, one DNA strand can act as a template for making a complementary strand.
Example
Finding a complementary DNA strand
A DNA strand has the sequence 5′ ATG CCA TTA 3′. Work out the complementary strand.
- Apply the base-pairing rules to each base: A pairs with T, T pairs with A, G pairs with C, and C pairs with G.
- Match the bases in order: ATG CCA TTA becomes TAC GGT AAT.
- Remember the strands are antiparallel, so the complementary strand is written 3′ TAC GGT AAT 5′.
RNA is similar, but not identical
RNA stands for ribonucleic acid. It is also made from nucleotides, but it differs from DNA in three important ways:
- RNA contains ribose sugar, not deoxyribose.
- RNA uses uracil instead of thymine.
- RNA is usually single-stranded.
So in RNA, the bases are A, U, C and G.
There are several types of RNA, but the two you most need here are:
- mRNA, or messenger RNA: carries a copy of the genetic code from DNA to a ribosome.
- tRNA, or transfer RNA: carries amino acids to a ribosome during protein synthesis.
The genetic code
The genetic code is the set of rules that links base sequences to amino acid sequences.
A codon is a sequence of three bases on mRNA. Each codon codes for one amino acid or a stop signal. For example, AUG codes for methionine and also acts as a start codon.
Definition
Codon
A codon is a triplet of bases on mRNA that codes for a specific amino acid or a stop signal.
The genetic code is:
- triplet: three bases code for one amino acid
- non-overlapping: each base is read as part of only one codon
- degenerate: most amino acids have more than one codon
- nearly universal: the same codons code for the same amino acids in almost all organisms
Common Mistake
One codon does not always mean one unique amino acid rule
It is true that one codon codes for one amino acid or stop signal. But because the code is degenerate, one amino acid can be coded for by several different codons.
Protein synthesis has two main stages
Protein synthesis is the process by which cells make polypeptides using genetic information.
In eukaryotic cells, such as animal and plant cells:
- Transcription happens in the nucleus.
- Translation happens at ribosomes in the cytoplasm or on the rough endoplasmic reticulum.
Transcription: making mRNA from DNA
Transcription is the production of an RNA copy of a gene.
During transcription:
- The DNA double helix unwinds and the hydrogen bonds between bases break.
- One DNA strand acts as the template strand.
- Free RNA nucleotides line up by complementary base pairing.
- RNA polymerase joins the RNA nucleotides together.
- The mRNA molecule separates from the DNA.
- The DNA strands rejoin.
In RNA synthesis, A pairs with U, not T. So if the DNA template has A, the mRNA will have U.
Definition
Template strand
The template strand is the DNA strand used to make a complementary mRNA molecule during transcription.
Common Mistake
Template strand versus coding strand
The mRNA is complementary to the template strand. It is almost the same as the coding strand, except RNA has U where DNA has T.
Example
Transcribing from a DNA template
A DNA template strand has the sequence 3′ TAC CCA TTG ACC ATT 5′. Work out the mRNA sequence.
- Use RNA base-pairing rules: T pairs with A, A pairs with U, C pairs with G, and G pairs with C.
- Pair each triplet with its complementary mRNA codon: TAC gives AUG, CCA gives GGU, TTG gives AAC, ACC gives UGG, and ATT gives UAA.
- Write the mRNA in the 5′ to 3′ direction: 5′ AUG GGU AAC UGG UAA 3′.
RNA processing in eukaryotes
In eukaryotic cells, the first RNA molecule made is often called pre-mRNA. It contains:
- exons: sections that remain in the mature mRNA
- introns: sections that are removed
During splicing, introns are removed and exons are joined together. The mature mRNA can then leave the nucleus through a nuclear pore.
Definition
Splicing
Splicing is the removal of introns from pre-mRNA and the joining together of exons to form mature mRNA.
Prokaryotic cells, such as bacteria, do not have a nucleus, so transcription and translation can happen in the cytoplasm, often at the same time.
Translation: making a polypeptide from mRNA
Translation is the assembly of amino acids into a polypeptide using the codons on mRNA.
It happens at a ribosome, which holds the mRNA and helps join amino acids together.
A tRNA molecule has:
- an anticodon: three bases complementary to an mRNA codon
- a specific amino acid attached at the other end
Definition
Anticodon
An anticodon is a triplet of bases on a tRNA molecule that is complementary to a codon on mRNA.
During translation:
- The ribosome attaches to the mRNA.
- Translation begins at a start codon, usually AUG.
- A tRNA with a complementary anticodon binds to the codon.
- The amino acid carried by the tRNA is added to the growing polypeptide.
- Peptide bonds form between adjacent amino acids.
- The ribosome moves along the mRNA codon by codon.
- Translation stops at a stop codon, and the polypeptide is released.
Example
Translating an mRNA sequence
An mRNA molecule has the sequence 5′ AUG GGU AAC UGG UAA 3′. Use this information: AUG = methionine/start, GGU = glycine, AAC = asparagine, UGG = tryptophan, UAA = stop.
- Split the mRNA into codons from the 5′ end: AUG, GGU, AAC, UGG, UAA.
- Match each codon to its amino acid or signal: AUG gives methionine, GGU gives glycine, AAC gives asparagine, UGG gives tryptophan, and UAA is a stop signal.
- Stop codons do not add an amino acid, so the polypeptide sequence is methionine–glycine–asparagine–tryptophan.
From polypeptide to functional protein
A polypeptide is a chain of amino acids. The order of amino acids is the protein’s primary structure.
The primary structure affects how the chain folds. Folding gives the protein its three-dimensional shape, which is essential for function. For example:
- an enzyme’s active site must have the correct shape
- a membrane channel must fold correctly to transport ions
- a receptor must have the correct binding site
Key Idea
Sequence affects shape
A change in the DNA base sequence can change the mRNA codons, which can change the amino acid sequence, which can alter protein folding and function.
Mutations can change proteins
A mutation is a change in the base sequence of DNA. Mutations can happen spontaneously during DNA replication or be caused by mutagens such as ionising radiation or some chemicals.
Common types include:
- substitution: one base is replaced by another
- insertion: one or more bases are added
- deletion: one or more bases are removed
A substitution may be silent if the new codon still codes for the same amino acid. It may also be missense, causing a different amino acid, or nonsense, producing a stop codon.
Insertions and deletions can cause a frameshift if the number of bases added or removed is not a multiple of three. This changes how all later codons are read.
Example
Predicting a frameshift mutation
Original mRNA: 5′ AUG GAA UUU UGA 3′. A C is inserted after the start codon, giving 5′ AUG CGA AUU UUG A... 3′. Predict the likely effect.
- Split the original sequence into codons: AUG, GAA, UUU, UGA.
- Split the mutated sequence from the start codon: AUG, CGA, AUU, UUG, with leftover bases continuing beyond the shown sequence.
- Compare the codons after AUG: every following codon has changed, so the amino acid sequence after the start is likely to be very different.
- Conclude that this insertion causes a frameshift, which may produce a non-functional protein.
Tip
A quick sequence-checking habit
When working with sequence questions, always check whether you have been given DNA or RNA, and whether the DNA strand is the template strand or coding strand.
Exam technique
In the exam
- For transcription questions, use complementary pairing carefully: DNA template A gives RNA U, not T.
- For translation questions, read mRNA codons from the 5′ end in groups of three and stop at the stop codon.
- When explaining mutations, link all the way through: base sequence changes codons, codons may change amino acids, amino acid sequence affects folding, and folding affects protein function.
Self review
Check yourself
- Why is mRNA complementary to the DNA template strand but similar to the coding strand?
- What roles do mRNA, tRNA and ribosomes play in translation?
- Why can an insertion mutation have a larger effect than a substitution mutation?
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