Biology 30 · Molecular genetics
DNA up close
The genetics unit showed how traits are inherited. This one zooms in: how DNA copies itself, how its code becomes a protein, what happens when a letter changes, and how scientists cut, copy and sort it.
DNA structure and replication
The idea: DNA is two strands of nucleotides held together by paired bases. To copy it, the strands separate and each one is used as a template.
Each nucleotide has a phosphate, a deoxyribose sugar and a base. Sugar and phosphate form the backbone; the bases pair across the middle: A with T and C with G.
Chargaff's rule
Because the bases pair, A = T and C = G in any DNA sample. If 30% is adenine, then thymine is also 30%, leaving 40% split evenly: 20% guanine and 20% cytosine.
Replication, step by step
- Helicase unwinds the helix and separates the strands.
- DNA polymerase adds matching nucleotides to each template strand.
- It can only build in one direction, so one strand (leading) is made continuously and the other (lagging) is made in short Okazaki fragments.
- DNA ligase joins the fragments.
Every new molecule keeps one original strand and gets one new one — that is why replication is called semi-conservative.
Protein synthesis
The idea: transcription copies a gene into mRNA in the nucleus; translation reads the mRNA three bases at a time at a ribosome to build a protein.
| Transcription | Translation | |
|---|---|---|
| Where | Nucleus | Ribosome |
| Makes | mRNA from a DNA template | A chain of amino acids from mRNA |
| Key players | RNA polymerase | mRNA codons, tRNA anticodons, ribosomes |
- mRNA processing: in eukaryotes, non-coding introns are cut out and the exons are joined before the mRNA leaves the nucleus.
- Codons: each three-base mRNA codon means one amino acid. AUG is the start codon; UAA, UAG and UGA are stop codons.
- tRNA: carries one amino acid and has an anticodon that pairs with the codon. The anticodon for AUG is UAC.
- Redundancy: 64 codons but only 20 amino acids, so many amino acids have several codons.
Watch out: writing T in any RNA sequence. RNA uses U instead of T, in both codons and anticodons.
Mutations
The idea: how much a mutation matters depends on what it does to the codons — change nothing, change one amino acid, stop the protein, or scramble everything after it.
| Type | What happens | Example |
|---|---|---|
| Silent | A substitution, but the codon still means the same amino acid | GAA → GAG (both glutamic acid) |
| Missense | A substitution changes one amino acid | Sickle cell anemia |
| Nonsense | A substitution creates a stop codon, cutting the protein short | UAC → UAA |
| Frameshift | An insertion or deletion shifts the reading frame, changing every codon after it | Usually a non-working protein |
Mutations can happen by chance during replication or be caused by mutagens such as UV light, X-rays and some chemicals. They are also the original source of every new allele — without them, there would be no variation for evolution to act on.
Biotechnology
The idea: four tools do most of the work — enzymes that cut DNA, enzymes that join it, a way to copy it, and a way to sort it by size.
| Tool | What it does |
|---|---|
| Restriction enzymes | Cut DNA at specific sequences, often leaving "sticky ends" |
| DNA ligase | Joins pieces of DNA together |
| PCR | Makes millions of copies of a DNA segment by repeated heating and cooling |
| Gel electrophoresis | Separates DNA fragments by size |
Making human insulin in bacteria
- Cut the human insulin gene and a bacterial plasmid with the same restriction enzyme, so their sticky ends match.
- Join them with DNA ligase, making recombinant DNA.
- Put the plasmid into bacteria. As they multiply, they read the human gene and make insulin.
The same toolkit underlies DNA fingerprinting in forensics, genetically modified crops, gene therapy research and newer gene-editing tools like CRISPR — each of which raises its own ethical questions.
Watch out: PCR copies DNA; gel electrophoresis sorts it. Questions often put one in place of the other.