Start for Free

Biology guide

DNA replication and protein synthesis

How DNA copies itself, how genes become proteins through transcription and translation, and how to read a codon chart, with practice problems.

DNA is the instruction manual for every living thing, and two processes put it to work: replication copies the whole manual before a cell divides, and protein synthesis reads one instruction at a time to build a protein. This guide walks through both, then shows you how to solve the codon problems that appear on almost every biology test.

The structure that makes copying possible

DNA is a double helix: two strands twisted around each other. Each strand is a chain of nucleotides, and each nucleotide has a sugar (deoxyribose), a phosphate and one of four bases: adenine (A), thymine (T), guanine (G) or cytosine (C). The bases pair across the strands by hydrogen bonds, and the pairing is strict: A with T and G with C. Because each strand determines its partner, either strand can serve as a template for rebuilding the other.

The strands also run in opposite directions, described as 5′ to 3′ and 3′ to 5′. This detail matters because the copying enzyme can only add nucleotides to one end.

DNA replication, step by step

  1. Unwinding: the enzyme helicase breaks the hydrogen bonds and opens the helix, creating a replication fork.
  2. Priming: primase lays down a short RNA primer, because DNA polymerase cannot start a new chain from nothing.
  3. Building: DNA polymerase adds matching nucleotides in the 5′ to 3′ direction. On the leading strand it works continuously toward the fork. On the lagging strand it works away from the fork in short pieces called Okazaki fragments.
  4. Finishing: the RNA primers are replaced with DNA, and DNA ligase seals the gaps between fragments.

Replication is semiconservative: each new double helix contains one original strand and one new strand. The Meselson and Stahl experiment in 1958 confirmed this using heavy and light nitrogen isotopes.

Protein synthesis part 1: transcription

Transcription happens in the nucleus. RNA polymerase binds to a gene's promoter, unwinds that section and builds a strand of messenger RNA (mRNA) complementary to the template strand. RNA uses uracil (U) instead of thymine, so A in the DNA template pairs with U in the mRNA. In eukaryotes the new mRNA is processed: non-coding introns are removed, exons are spliced together, a cap is added to the front and a poly-A tail to the end. Then the mRNA leaves the nucleus.

Protein synthesis part 2: translation

Translation happens at ribosomes. The ribosome reads the mRNA three bases at a time; each triplet is a codon. Transfer RNA (tRNA) molecules carry amino acids, and each tRNA has an anticodon that pairs with a matching codon. Translation starts at the start codon AUG (which codes for methionine), adds one amino acid per codon and stops at a stop codon (UAA, UAG or UGA). The chain of amino acids then folds into a working protein.

Worked example: from DNA to amino acids

Given the DNA template strand 3′ TAC GGT CAT ATT 5′:

  1. Write the mRNA by pairing each base (A→U, T→A, G→C, C→G): 5′ AUG CCA GUA UAA 3′.
  2. Read the codons with a codon chart: AUG = methionine (start), CCA = proline, GUA = valine, UAA = stop.
  3. The protein fragment is Met–Pro–Val.

Always check that you used U, not T, in the mRNA, and that you started reading at AUG.

Mutations and why they matter

  • Substitution: one base swapped for another. It may change one amino acid (missense), create an early stop codon (nonsense) or change nothing (silent), because several codons code for the same amino acid.
  • Insertion or deletion: adding or removing bases shifts the reading frame if the number is not a multiple of three. A frameshift scrambles every codon after it, which usually ruins the protein.

Sickle cell disease comes from a single substitution in the hemoglobin gene that changes glutamic acid to valine, which shows how one letter can matter.

Common mistakes

  • Using T in mRNA. RNA has uracil.
  • Reading codons from the wrong end or not starting at AUG.
  • Mixing up transcription (DNA to RNA, in the nucleus) and translation (RNA to protein, at ribosomes).
  • Pairing the mRNA codon with the DNA coding strand instead of the template strand. The mRNA matches the coding strand, with U in place of T.

Practice questions

  1. What is the complementary DNA strand to ATG CCT GA?
  2. Transcribe the template strand TAC AAA CCG ATT into mRNA.
  3. Why does a deletion of one base usually do more damage than a substitution?
  4. What does semiconservative replication mean?

Answers: (1) TAC GGA CT. (2) AUG UUU GGC UAA, which translates to Met–Phe–Gly–stop. (3) It shifts the reading frame, changing every codon after it. (4) Each new DNA molecule keeps one original strand and gains one new strand.

Last reviewed 2026-10-23 by the BrainBoost team.

More study guides