Biology (university) · Part 04
DNA, transcription and translation
How DNA is built and copied, and how a gene becomes a working protein.
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- DNA structureTwo strands of nucleotides twisted into a double helix. Each nucleotide has a sugar, a phosphate and a base: A, T, G or C. A pairs with T and G pairs with C. The strands run in opposite directions (antiparallel).
- Semi-conservative replicationDNA copies itself by unzipping. Each old strand is used as a template for a new strand, so every new double helix has one old and one new strand.
- Lagging strandDNA polymerase only builds a new strand in one direction (5' to 3'). On one template strand it builds continuously (leading strand). On the other it builds short pieces called Okazaki fragments (lagging strand), joined by ligase.
- Central dogmaInformation in cells usually flows from DNA to RNA to protein. DNA is transcribed into RNA, and RNA is translated into protein.
- TranscriptionMaking an RNA copy of a gene. RNA polymerase binds a promoter, unwinds the DNA and builds messenger RNA (mRNA). In RNA, uracil (U) replaces thymine (T).
- RNA splicingIn eukaryotes the first RNA copy contains introns (non-coding stretches) and exons (coding stretches). Splicing removes the introns and joins the exons. A cap and a poly-A tail are also added.
- Genetic codeGroups of three bases on mRNA (codons) specify an amino acid. 64 codons cover 20 amino acids, plus start (AUG) and stop signals. The code is redundant: several codons can code for the same amino acid.
- TranslationBuilding a protein by reading mRNA. A ribosome moves along the mRNA, and transfer RNA (tRNA) molecules bring in the amino acids that match each codon through their anticodon.
- Point mutationA change in one DNA base. A silent mutation leaves the amino acid the same. A missense mutation changes it. A nonsense mutation makes a stop codon too early.