AP Syllabus focus:
‘Messenger RNA molecules carry genetic information copied from DNA in the nucleus to ribosomes in the cytoplasm, specifying polypeptide sequences.’
Messenger RNA is the cell’s disposable working copy of genetic instructions. It links the stable information in nuclear DNA to protein-building ribosomes, enabling cells to produce specific polypeptides when and where needed.
Overview: what mRNA does in information flow
Cells manage genetic information through a directional pathway: DNA stores instructions, and mRNA carries a gene’s sequence information to the site of protein synthesis.
Messenger RNA (mRNA): A single-stranded RNA molecule that carries a gene’s nucleotide sequence information from DNA to ribosomes for polypeptide production.
Key idea from the syllabus: mRNA molecules carry genetic information copied from DNA in the nucleus to ribosomes in the cytoplasm, specifying polypeptide sequences.
From DNA to mRNA: the message is copied
In eukaryotes, DNA remains protected inside the nucleus, so the cell makes an RNA copy of a gene that can leave the nucleus.
Eukaryotic pre‑mRNA is initially synthesized with introns and exons, then processed into mature mRNA by splicing out introns and joining exons. The figure also labels the 5′ cap and 3′ poly‑A tail, two modifications that stabilize mRNA and help it function as a transportable message. Source
What “copied from DNA” means
A gene’s information is encoded in the order of nucleotides in DNA.
That information is transferred to mRNA as an RNA nucleotide sequence that corresponds to the gene’s instructions (with U in RNA in place of T).
The critical outcome is an mRNA sequence that preserves the “readout” needed to assemble amino acids in the correct order.
Why the cell uses mRNA instead of exporting DNA
DNA is long and fragile at the chromosome level, and it must remain available for all genes and for future copying.
mRNA is temporary and gene-specific, letting the cell produce only the proteins it needs at a given time.
Multiple mRNA molecules can be made from the same gene, increasing protein output without altering DNA.
Transport: nucleus to cytoplasm
The syllabus emphasizes location: DNA in the nucleus and ribosomes in the cytoplasm. This spatial separation is central to information flow in eukaryotic cells.
Movement of mRNA to ribosomes
After an mRNA transcript is produced in the nucleus, it is delivered to the cytoplasm.
In the cytoplasm, mRNA associates with ribosomes, the molecular machines that read the message.
This transport step ensures nuclear DNA is not physically involved at the site of polypeptide assembly.
How mRNA specifies a polypeptide sequence
mRNA “specifies” a polypeptide because its nucleotide order is interpreted as instructions for amino acid order during translation.
Codon: A three-nucleotide sequence in mRNA that corresponds to a specific amino acid or a stop signal during polypeptide synthesis.
Reading the message
Ribosomes read mRNA in groups of three nucleotides (codons).
This diagram shows a ribosome translating an mRNA by matching each codon with a tRNA anticodon at the A, P, and E sites. It emphasizes how codon-by-codon reading drives peptide-bond formation and elongation of the polypeptide in a specific order. Source
The sequence of codons determines the sequence of amino acids in the polypeptide.
This codon table summarizes the standard genetic code by mapping each three-base mRNA codon (using A, U, G, C) to its corresponding amino acid or stop signal. It makes explicit how changing the codon sequence in mRNA can change the resulting polypeptide’s primary structure. Source
A change in mRNA codon order changes the amino acid order, which can change protein structure and function.
What “polypeptide” refers to in this context
A polypeptide is a chain of amino acids linked by peptide bonds.
Many functional proteins are one polypeptide or multiple polypeptides assembled together; mRNA provides the primary sequence for each polypeptide chain.
Properties of mRNA that support its role
mRNA is designed for information transfer rather than long-term storage.
Key functional features (conceptual)
Single-stranded: accessible for ribosome reading.
Short-lived: allows rapid adjustment of protein production by making or degrading the message.
Gene-specific: each mRNA corresponds to instructions for a particular polypeptide sequence.
Common points of confusion (high-yield clarifications)
mRNA does not “become” protein; it is an information carrier read by ribosomes.
The nucleus-to-cytoplasm flow is a hallmark of eukaryotes; the syllabus statement specifically frames copying from nuclear DNA to cytoplasmic ribosomes.
The information content is in sequence order, not in the chemical amount of RNA.
FAQ
Binding depends on molecular recognition between ribosomal components and sequence/structural features on the mRNA.
In eukaryotes, initiation factors help position the ribosome correctly before peptide synthesis begins.
Stability is influenced by sequence elements and proteins that bind the mRNA.
Cell conditions can accelerate degradation to rapidly reduce a protein’s production.
In many cases, multiple ribosomes can translate the same mRNA at once, producing many copies of the same polypeptide.
Some mRNAs can also encode more than one product depending on how translation is initiated.
Because it is transient, cells can quickly increase or decrease protein output by changing mRNA abundance.
This control can be faster than changing DNA-level information.
Common approaches include hybridisation-based methods and sequencing-based methods.
They quantify transcript abundance to infer which genes are actively being expressed under specific conditions.
Practice Questions
State the role of mRNA in the flow of genetic information in a eukaryotic cell. (2 marks)
mRNA carries genetic information copied from DNA (1)
mRNA travels from nucleus to ribosomes in the cytoplasm for polypeptide synthesis / specifies polypeptide sequence (1)
Explain how the nucleotide sequence of an mRNA molecule determines the amino acid sequence of a polypeptide. (5 marks)
mRNA is read by a ribosome during translation (1)
mRNA is read in triplets called codons (1)
each codon specifies an amino acid (or stop signal) (1)
the order of codons determines the order of amino acids in the polypeptide (1)
therefore altering the mRNA sequence can alter the polypeptide’s amino acid sequence (1)
