Molecular biology: DNA replication, repair, mutations is a core MBBS — Pre-Clinical (Phase 1) concept in MBBS. This guide explains what it is, walks through a fully worked example, and lists the key equations you need — with a short quiz to test yourself.

Key equations and worked example

One full turn of the DNA helix spans 10 base pairs and rises 3.4 nm, so each base pair adds 0.34 nm. A human chromosome with 100 million base pairs would stretch about 3.4 cm if unwound — yet it packs into a nucleus only micrometres wide, wound around histone proteins. A always pairs with T (2 hydrogen bonds) and G with C (3 hydrogen bonds) — which is why the two strands are complementary.

• <code>1 helical turn = 10 base pairs = 3.4 nm</code>
• <code>Rise per base pair = 0.34 nm</code>
• <code>A = T (2 H-bonds); G ≡ C (3 H-bonds)</code>
• <code>Chargaff&#39;s rule: [A] = [T], [G] = [C]</code>

Molecular biology: DNA replication, repair, mutations in detail

Molecular biology: DNA replication, repair, mutations is one of the central ideas in MBBS — Pre-Clinical (Phase 1), and it appears in MBBS curricula under Biochemistry. It is worth learning deeply because it connects to so many other topics in this section.

DNA is a double helix of two antiparallel strands: each strand is a chain of nucleotides (sugar + phosphate + base), and the strands are held together by hydrogen bonds between complementary bases — adenine with thymine, guanine with cytosine. The sequence of bases encodes proteins in triplets (codons). During replication the strands separate and each acts as a template, which is why complementarity guarantees faithful copying.

For exams, the pattern is predictable: first a definition or statement of the result, then a direct numerical application of one of the equations above, then a "why" question — why the formula takes that form, or what changes when a variable is doubled or halved. The worked example and quiz below cover exactly that progression.

Quick self-check:

• Q: Which bases pair together in DNA, and how many hydrogen bonds hold each pair?<br />A: A–T with 2 hydrogen bonds; G–C with 3 hydrogen bonds.
• Q: Why is DNA replication so accurate?<br />A: Each strand is a template: complementarity (A–T, G–C) forces the new strand to copy the sequence exactly.
• Q: What is a codon?<br />A: A triplet of bases on mRNA that codes for one amino acid (e.g. AUG = methionine, the start codon).
• Q: How long is one full turn of the helix?<br />A: 3.4 nm, containing exactly 10 base pairs.