Molecular Genetics

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1Summary 重點摘要

Chromosomes are DNA plus protein, and a gene is the DNA segment encoding a protein; the double helix is held by base pairing (A-T, C-G) and copies itself semi-conservatively; transcription writes the gene into mRNA (U instead of T), translation reads codons at the ribosome to join amino acids, and gene mutations change proteins — sickle cell anaemia is the classic case.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Chromosome, gene and nucleic acidChromosomes appear threadlike under the light microscope and consist of DNA and proteins; DNA is the nucleic acid that stores genetic information, and the segments encoding proteins are genes. The hierarchy runs from large to small: chromosomes contain genes, and genes are made of DNA. Humans carry some twenty-odd thousand genes across 23 pairs of chromosomes. For example, think of a chromosome as a complete cookbook, a gene as one recipe in it, the protein as the dish cooked from it, and the trait as the dish on the table.
2. Step 2: The DNA double helixDNA's unit is the nucleotide, assembled from a phosphate, deoxyribose and a nitrogenous base; there are four bases — A, T, C and G. Long chains of nucleotides form two antiparallel strands wound into a double helix, held together in the middle by base pairing: two hydrogen bonds join A to T and three join C to G. For example, the handrails of the spiral ladder are alternating phosphates and sugars, the rungs are paired bases, and the order of the rungs is the very text of the genetic message.
3. Step 3: Semi-conservative replicationBefore a cell divides, its DNA must be copied exactly. In semi-conservative replication the double strand unzips, each parental strand serving as a template on which complementary nucleotides are laid down by the A-T, C-G rules, producing two identical daughter molecules, each keeping one old strand and carrying one new one — hence semi-conservative. For example, unzip a zipper and fit new teeth against the old ones to get two identical zippers; because the pairing rule is fixed, information passes on faithfully generation after generation.
4. Step 4: Transcription — from DNA to mRNAThe first step of gene expression is transcription, in the nucleus: one strand of the gene serves as the template for building a single-stranded mRNA that copies out the message. RNA contains no thymine (T) — uracil (U) takes its place — so template A pairs with U, T with A, C with G and G with C. For example, the template sequence TAC yields the mRNA codon AUG. Once made, the mRNA leaves the nucleus through a nuclear pore, carrying instructions to the cytoplasm.
5. Step 5: Translation — from mRNA to proteinTranslation happens at the ribosome: the ribosome travels along the mRNA reading three bases at a time — one codon; the matching transfer RNA (tRNA) arrives carrying its amino acid and pairs its anticodon with the codon, and the amino acids are joined one by one into a polypeptide that folds into a working protein. For example, the codons AUG CCU specify two amino acids; the ribosome works like a card reader moving along, and tRNAs like trolleys delivering exactly the right parts to the assembly line.
6. Step 6: Gene mutation and sickle cell anaemiaA gene mutation changes base pairs within a gene, often swapping a single amino acid in the protein. Sickle cell anaemia is the textbook case: one base change in the haemoglobin gene replaces one amino acid, altering the protein's shape; at low oxygen the red blood cells collapse into sickle forms, prone to bursting and blocking capillaries, with sharply reduced oxygen carriage. For example, in malaria regions, heterozygotes carrying one mutant allele resist malaria better and survive more often — natural selection keeps the allele around, linking straight into the next lesson on evolution.
7. Step 7: Chromosome mutation, mutagens and the meaning of mutationMutation goes beyond single bases: a chromosome mutation changes the number or structure of whole chromosomes — people with Down syndrome carry three copies of chromosome 21, leading to characteristic developmental features. Mutations arise spontaneously or are induced by mutagens, commonly radiation such as ultraviolet light and X-rays, plus chemicals in tobacco smoke. For example, sunburn from long exposure reflects UV damage to the DNA of skin cells — everyday induced mutation. Most mutations are neutral or harmful, but they create new alleles, supplying the raw material for variation and evolution.

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8Recap Recap

• Chromosomes are DNA plus protein, genes are the protein-coding segments of DNA, and the three nest from large to small.
• DNA is a double helix: nucleotides of phosphate, deoxyribose and base, two antiparallel strands held by A-T and C-G hydrogen bonds.
• Semi-conservative replication gives every daughter molecule one parental strand, so information passes faithfully down the generations.
• Gene expression has two steps: transcription into mRNA in the nucleus (U for T), then translation at the ribosome, where tRNA joins amino acids by codon.
• Changing the base sequence can change the protein: one base makes sickle cell anaemia; chromosome mutations include Down syndrome; mutation is the raw material of evolution.