DNA is the molecule that stores all the genetic instructions needed to build and run a living organism. Its elegant double-helix structure encodes information in the sequence of four chemical bases, and cells read this code through a two-stage process — transcription, then translation — to manufacture every protein the organism needs.
What is the structure of DNA?
DNA (deoxyribonucleic acid) is a double-stranded molecule coiled into a double helix. Each strand consists of a chain of nucleotides, where every nucleotide contains:
- A deoxyribose sugar (5 carbons)
- A phosphate group
- One of four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), or Cytosine (C)
The two strands are held together by hydrogen bonds between complementary bases. The base-pairing rules are:
| Base on one strand | Complementary base on other strand |
|---|---|
| Adenine (A) | Thymine (T) |
| Thymine (T) | Adenine (A) |
| Guanine (G) | Cytosine (C) |
| Cytosine (C) | Guanine (G) |
This complementary base pairing (A-T and G-C) is the key to both DNA replication and protein synthesis. The sugar-phosphate backbones form the sides of the "ladder" and the base pairs form the "rungs."
DNA is packaged around proteins called histones and coiled tightly to fit inside the nucleus of every cell. The full set of DNA in a human cell contains about 3 billion base pairs, encoding approximately 20,000 genes.
What is a gene?
A gene is a specific sequence of bases on a DNA molecule that codes for a particular protein (or functional RNA). The sequence of bases in a gene acts like an instruction manual: every three consecutive bases (a codon or triplet) codes for a specific amino acid. The order of codons in a gene determines the order of amino acids in the resulting protein, and the sequence of amino acids determines the protein's shape and function.
The genetic code is nearly universal — the same codon means the same amino acid in almost every organism on Earth, from bacteria to humans.
What is transcription?
Transcription is the first stage of protein synthesis. It occurs in the nucleus and produces a molecule of messenger RNA (mRNA) from the DNA template.
Steps in transcription
- The double helix unwinds and the hydrogen bonds between base pairs break in the region of the gene to be copied.
- An enzyme called RNA polymerase moves along one strand of the DNA (the template strand), reading it in the 3′ to 5′ direction.
- RNA polymerase assembles complementary RNA nucleotides (using the base-pairing rules, but with uracil (U) replacing thymine in mRNA):
- DNA A → mRNA U
- DNA T → mRNA A
- DNA G → mRNA C
- DNA C → mRNA G
- The mRNA strand is released. In eukaryotic cells it is processed and leaves the nucleus through a nuclear pore.
The result is a single-stranded mRNA molecule whose sequence directly mirrors the coding strand of the DNA (except U replaces T).
What is translation?
Translation is the second stage, in which the mRNA sequence is decoded to build a chain of amino acids (a polypeptide) at a ribosome in the cytoplasm.
Steps in translation
- The mRNA attaches to a ribosome.
- Transfer RNA (tRNA) molecules carry specific amino acids. Each tRNA has a three-base anticodon that pairs with a complementary codon on the mRNA.
- The ribosome moves along the mRNA, reading each codon in turn.
- The tRNA carrying the matching amino acid slots in, and the ribosome joins the amino acid to the growing chain using a peptide bond.
- The tRNA leaves, the ribosome shifts to the next codon, and the process repeats until a stop codon is reached.
- The completed polypeptide is released and folds into its functional three-dimensional shape.
Worked example — decoding a short mRNA sequence
mRNA sequence: AUG — UUU — GGC — UAA
| Codon | Amino acid |
|---|---|
| AUG | Methionine (start codon) |
| UUU | Phenylalanine |
| GGC | Glycine |
| UAA | STOP — polypeptide released |
The resulting tripeptide is: Met – Phe – Gly
How does a mutation affect protein synthesis?
A mutation is a change in the base sequence of DNA. Even a single base change (a substitution) can alter the codon and therefore the amino acid incorporated at that position. If the new amino acid is in a critical part of the protein — for example, in an enzyme's active site — the protein may no longer function. However, because the genetic code is degenerate (most amino acids are coded for by more than one codon), some substitutions are silent and have no effect on the protein.
Frequently asked questions
What is the difference between DNA and RNA?
DNA (deoxyribonucleic acid) is double-stranded, contains deoxyribose sugar, and uses the bases A, T, G, C. It stores the genetic code permanently in the nucleus. RNA (ribonucleic acid) is single-stranded, contains ribose sugar, and uses A, U, G, C (uracil instead of thymine). Different types of RNA have different roles: mRNA carries the code from nucleus to ribosome; tRNA brings amino acids to the ribosome; ribosomal RNA (rRNA) forms part of the ribosome structure itself.
Why is complementary base pairing important for protein synthesis?
Complementary base pairing ensures the mRNA sequence produced during transcription is an accurate copy of the gene sequence, and then that tRNA anticodons pair precisely with mRNA codons during translation. If bases paired randomly, the wrong amino acids would be inserted, producing non-functional proteins. The strict A-U, G-C pairing rules also allow accurate DNA replication, ensuring genetic information is faithfully copied every time a cell divides.
What happens if a mutation changes the start codon?
The start codon (AUG) signals the ribosome where to begin translation and also codes for methionine, the first amino acid. If a mutation changes AUG to another codon, the ribosome cannot initiate translation at the correct point. The result is typically either no protein at all or a non-functional truncated protein, because translation cannot begin properly. This type of mutation usually has severe consequences for the organism.
Where exactly does protein synthesis take place in a cell?
Transcription occurs in the nucleus, where the DNA is located. The mRNA produced then exits through nuclear pores into the cytoplasm. Translation occurs on ribosomes, which may be free in the cytoplasm (producing proteins for use inside the cell) or attached to the rough endoplasmic reticulum (producing proteins destined for export from the cell, such as insulin, or for incorporation into the cell membrane).
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