4.3 Molecular Genetics & Biotechnology

Why This Matters

Molecular genetics explains how the information in DNA becomes the proteins that build and run a cell. Diploma questions test base pairing, the steps of replication and protein synthesis, the effects of mutations, and the logic of common lab techniques. Precise rules — especially base pairing and reading codons in triplets — are easy marks if memorized exactly.

The central dogma summarizes the flow of information: DNA → RNA → protein. DNA is the stored blueprint, RNA is the working copy, and protein is the functional product. Keep this direction in mind as you work through each step below.

DNA Structure

DNA (deoxyribonucleic acid) is a double helix of two strands. Each nucleotide has three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. The sugar-phosphate backbones run antiparallel.

Bases pair by complementary base pairing:

• Adenine (A) pairs with Thymine (T) — two hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) — three hydrogen bonds.

So if one strand reads 5'-ATGC-3', its complement reads 3'-TACG-5'.

Purines (A, G) are double-ring bases that always pair with single-ring pyrimidines (T, C). This keeps the helix a uniform width and is why Chargaff's rule holds: in any DNA sample, %A = %T and %G = %C.

Semiconservative Replication

Before a cell divides, DNA must copy itself. Replication is semiconservative: the double helix unzips, and each original strand acts as a template for a new complementary strand. Every daughter molecule therefore has one old strand and one new strand.

Key enzymes:

• Helicase unwinds and separates the two strands.
• DNA polymerase adds complementary nucleotides to each template strand following A-T and G-C rules.
• Ligase seals fragments together.

Because base pairing is exact, replication is highly accurate, ensuring both daughter cells receive identical genetic information. The classic Meselson-Stahl logic confirms this: after one round, every DNA molecule is a hybrid of one parental and one new strand — never two fully old or two fully new strands. Replication must finish before mitosis or meiosis so each daughter cell inherits a complete genome.

Transcription and Translation

Protein synthesis has two stages:

Transcription (in the nucleus) copies one gene into a strand of mRNA (messenger RNA). RNA uses uracil (U) instead of thymine, so DNA's A pairs with RNA's U. RNA polymerase builds the mRNA from the DNA template.

Translation (at the ribosome) reads the mRNA in groups of three bases called codons. Each codon specifies one amino acid. tRNA (transfer RNA) carries an amino acid and has an anticodon that base-pairs with the mRNA codon. The ribosome links the amino acids into a polypeptide until it reads a stop codon.

The Genetic Code and a Worked Example

The genetic code is read in non-overlapping triplets. The start codon AUG codes for methionine; three stop codons (UAA, UAG, UGA) end translation.

Worked example — trace DNA to protein for the template strand 3'-TAC GGA-5':

1. Transcription: mRNA = 5'-AUG CCU-3' (A-U, T-A, C-G, G-C pairing).
2. Codons: AUG and CCU.
3. Translation: AUG = Methionine (start), CCU = Proline.

The tRNA anticodons would be UAC and GGA. Reading frame matters: shifting even one base changes every codon downstream.

Mutations and Biotechnology

Gene mutations change the DNA sequence:

• Point (substitution) mutations swap one base. These can be silent (no amino acid change), missense (different amino acid), or nonsense (creates a stop codon).
• Frameshift mutations (insertions or deletions) shift the entire reading frame, usually altering every codon after the change and severely disrupting the protein.

Biotechnology manipulates genes using:

• Restriction enzymes — cut DNA at specific sequences, leaving "sticky ends."
• Gel electrophoresis — separates DNA fragments by size (smaller travel farther).
• PCR (polymerase chain reaction) — amplifies tiny DNA samples.
• Recombinant DNA — combines DNA from two sources, used to make GMOs and products like insulin.

A typical workflow: a restriction enzyme cuts both a human gene and a bacterial plasmid at the same site, the sticky ends are joined by ligase, and the recombinant plasmid is inserted into bacteria that mass-produce the protein.

Gel electrophoresis is also the basis of DNA fingerprinting, comparing fragment patterns between individuals for identity or paternity testing — a frequent diploma application question.