DNA & Gene Expression

• DNA Replication is semi-conservative — each new DNA molecule keeps one original strand and builds one brand-new strand.
• Helicase unwinds/unzips the double helix; DNA polymerase builds new strands from free nucleotides; DNA ligase joins fragments together.
• Because DNA polymerase only works 5'→3', one new strand (leading) is made continuously, the other (lagging) is made in chunks called Okazaki fragments.
• Meselson & Stahl's nitrogen-isotope experiment proved replication is semi-conservative, not conservative.
• The genetic code is a triplet code: 3 DNA bases = 1 codon = 1 amino acid. It is non-overlapping, degenerate, and universal.
• A gene is a sequence of DNA bases that codes for one polypeptide (Crick's Central Dogma: DNA → RNA → Protein).
• Transcription (in the nucleus): the antisense/template DNA strand is copied into a single-stranded mRNA molecule by RNA polymerase.
• Translation (in the cytoplasm, at the ribosome): tRNA molecules match their anticodons to mRNA codons, delivering amino acids that are joined by peptide bonds into a polypeptide.

Why does DNA need to be copied at all?

Think of DNA as the master instruction manual for building and running a cell. Before a cell splits into two (mitosis), both new "daughter" cells need their own complete, identical copy of that manual — otherwise one daughter cell would be missing pages of instructions. So before division, the cell doubles its DNA. This copying process is called replication, and it happens during the S phase of interphase (i.e. before the cell is actually dividing — not during mitosis itself).

Semi-conservative replication — the "half kept, half new" rule

DNA is a double helix — picture it like a zip, with two strands running antiparallel to each other, held together by hydrogen bonds between complementary bases (A–T and C–G). When it's time to replicate:

1. The helix is "unzipped" down the middle — the two original strands separate.
2. Each of these two original strands is used as a template to build a brand new complementary strand alongside it.
3. You end up with two complete double helices — but each one is made of one old (original) strand + one new strand.

This is why it's called semi-conservative: "semi" = half. Half of each new DNA molecule (one whole strand) is conserved from the original, and the other half is freshly synthesised.

Why does it matter that one original strand is kept?

genetic continuity

The machinery of replication, step by step