Sec 4 Biology Molecular Genetics notes

Inside the nucleus, DNA is wound into chromosomes — one long DNA molecule (with associated proteins) per chromosome. A gene is a specific stretch of that DNA: a sequence of nucleotides that codes for one polypeptide. Genes are the units of inheritance — they pass unchanged (or occasionally mutated, chapter 7) from parents to offspring, and the suite of genes an organism carries is what its DNA says.

Keep the hierarchy straight in exam language: DNA is the molecule; a gene is a section of DNA with one job (one polypeptide); a chromosome is a package of many genes — a thread of DNA long enough to carry hundreds of recipes.

DNA is a double helix — two strands twisted around each other like a twisted ladder. Each strand is a chain of nucleotides, and each nucleotide has three parts: a sugar, a phosphate, and one of four bases. Sugar and phosphate form the ladder’s side-rails; the bases pair across the middle like rungs. The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G), and they pair by strict rule: A always pairs with T; C always pairs with G — complementary base pairing.

The rule is what makes DNA copyable. Because each base dictates its partner, every strand carries all the information of the other: read one strand, and you can write the other exactly. When DNA replicates, the two strands separate and each becomes a template for a perfect new partner strand — which is how genetic information is copied so faithfully into every new cell.

A gene’s bases are read in sequence, and that sequence specifies a polypeptide — the chain of amino acids that folds into a protein. The genetic code works in threes: each group of three bases in the gene stands for one amino acid in the chain, so a gene of 450 bases can code for a polypeptide of up to 450 ÷ 3 = 150 amino acids. Change the base sequence and you can change the amino acid sequence — and therefore the protein’s shape and function. This is why genes are the units of inheritance: what is inherited is the instruction for each polypeptide the organism needs.

At O-Level the journey needs no biochemistry of transcription or translation — only the logic: base sequence → amino acid sequence → polypeptide → protein function. Enzymes (chapter 3 of Sec 3), haemoglobin, insulin: every protein the syllabus names begins as a gene’s base sequence.

Because the genetic code is universal — the same triplets mean the same amino acids in every living thing — a gene transplanted between species still reads correctly. Gene transfer cuts a chosen gene out of one organism and inserts it into another’s DNA; the recipient is a transgenic organism. The classic case: the human insulin gene is inserted into the DNA of bacteria. The bacteria cannot tell the gene is human — they transcribe and translate it exactly as one of their own, secreting genuine human insulin, which is harvested from the fermenter.

The chain of the process, in order: identify and isolate the human insulin gene → insert it into a bacterial DNA carrier (a plasmid) → return the plasmid to bacterial cells → grow the bacteria in fermenters, where they multiply and express the gene → harvest and purify the insulin. Because bacteria reproduce so fast, huge quantities of medicine are grown in tanks rather than extracted from animals.

Benefits: medicines like human insulin are mass-produced cheaply and reliably, without animal extraction, in essentially unlimited quantity; gene transfer can improve crops — resistance to pests or richer nutrients; and the same techniques underpin gene therapy research. Ethical considerations: is it right to alter organisms’ genomes at all; possible long-term effects on ecosystems of released transgenic organisms; welfare of engineered animals; questions of ownership and fairness — who owns a gene, and who can afford the products; and the worry that engineered traits might spread unintentionally to wild populations.

Exam answers should weigh both sides: state a benefit, state a concern, and where asked, give a reasoned position. "Genetic engineering is good/bad" earns nothing alone — the reasoning is the answer.