The structure of DNA
DNA (deoxyribonucleic acid) is a long polymer of nucleotides. Each nucleotide has three parts: a deoxyribose sugar, a phosphate group and a nitrogenous base.
Purines (two rings): adenine (A) and guanine (G).
Pyrimidines (one ring): cytosine (C) and thymine (T). RNA has uracil (U) instead of thymine.
In 1953, Watson and Crick proposed the double helix model, using X-ray diffraction data from Maurice Wilkins and Rosalind Franklin:
Two antiparallel strands (one runs 5′→3′, the other 3′→5′) coil around a common axis.
The sugar–phosphate backbone is on the outside, and the bases pair up inside.
A pairs with T (two hydrogen bonds), and G pairs with C (three hydrogen bonds).
Chargaff's rule: in double-stranded DNA, A = T and G = C, so the purines equal the pyrimidines.
In eukaryotes, DNA is wrapped around histone proteins to form nucleosomes, which coil further into chromatin.
How we know DNA is the genetic material
Griffith (1928): heat-killed virulent (S-strain) pneumonia bacteria could transform live harmless (R-strain) bacteria into virulent ones. Some "transforming principle" was passing between them.
Avery, MacLeod and McCarty (1944): showed that the transforming principle is DNA. Enzymes that destroyed DNA stopped transformation, while enzymes that destroyed protein or RNA did not.
Hershey and Chase (1952): grew bacteriophages with radioactive phosphorus (³²P, which labels DNA) or radioactive sulphur (³⁵S, which labels protein). Only the ³²P label entered the bacteria and appeared in new viruses. DNA is the genetic material.
DNA is better suited than RNA as genetic material because it is more stable (no reactive 2′-OH group, and it uses thymine instead of uracil). RNA, being more reactive, was probably the first genetic material, and it still acts as a catalyst in some reactions.
Replication: semi-conservative
Each strand serves as a template for a new strand, so each new DNA molecule has one old strand and one new strand. Meselson and Stahl (1958) proved this using heavy nitrogen (¹⁵N) and density-gradient centrifugation.
The double helix is unwound, opening a replication fork.
DNA polymerase adds nucleotides only in the 5′→3′ direction.
The leading strand is made continuously. The lagging strand is made in short pieces called Okazaki fragments, which are joined by DNA ligase.
In eukaryotes, replication happens during the S phase of the cell cycle.
Transcription: DNA to RNA
Transcription copies the information of one DNA strand into RNA, using the enzyme RNA polymerase.
A transcription unit has a promoter (where RNA polymerase binds), the structural gene, and a terminator.
There are three main types of RNA:
RNA | Role |
|---|---|
mRNA (messenger) | carries the coded instructions for a protein |
tRNA (transfer) | brings amino acids to the ribosome; reads codons with its anticodon |
rRNA (ribosomal) | structural and catalytic part of the ribosome |
In eukaryotes, the primary transcript is processed. Non-coding introns are removed and coding exons are joined (splicing), a cap is added at the 5′ end, and a poly-A tail is added at the 3′ end.
The genetic code
The code is a triplet: three bases (a codon) specify one amino acid.
There are 4³ = 64 codons. 61 code for amino acids, and 3 are stop codons (UAA, UAG, UGA).
AUG codes for methionine and also acts as the start codon.
The code is degenerate (most amino acids have more than one codon), unambiguous (each codon codes for only one amino acid), non-overlapping, and nearly universal.
Translation: RNA to protein
Translation takes place on ribosomes. tRNA molecules, each carrying a specific amino acid, pair their anticodons with the codons on mRNA. The ribosome joins amino acids with peptide bonds. A part of the large ribosomal RNA itself catalyses this reaction, so it acts as a ribozyme. Translation stops when a stop codon is reached.
Regulation of gene expression: the lac operon
In E. coli, the lac operon controls the enzymes needed to use lactose.
When lactose is absent, a repressor protein binds to the operator and blocks transcription.
When lactose is present, it acts as an inducer: it binds the repressor and inactivates it, so the genes are transcribed.
Genes are therefore switched on only when they are needed.
The Human Genome Project
The Human Genome Project (1990–2003) sequenced the human genome: about 3 × 10⁹ base pairs, with an estimated 20 000 to 25 000 genes. Less than 2% of the genome codes for proteins.
DNA fingerprinting compares repetitive DNA sequences (VNTRs), which differ between individuals. It is used in forensic science and paternity testing.