Describe the structure of DNA and explain how transcription and translation produce proteins.

What this dot point is asking

The structure of DNA

DNA (deoxyribonucleic acid) is a polymer made of repeating units called nucleotides. Each nucleotide has three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases, adenine (A), thymine (T), cytosine (C), or guanine (G). Nucleotides join through sugar-phosphate bonds to form a strand with a sugar-phosphate backbone.

Two strands wind around each other into a double helix. The strands are held together by hydrogen bonds between bases following complementary base pairing: A pairs with T (two hydrogen bonds) and C pairs with G (three hydrogen bonds). The two strands run antiparallel, meaning one runs 5 prime to 3 prime and the other 3 prime to 5 prime. This complementary, antiparallel arrangement allows DNA to be copied accurately during replication.

Transcription

Gene expression is the process of using the information in a gene to build a product. The first stage is transcription, which happens in the nucleus of eukaryotic cells.

1. The enzyme RNA polymerase binds to a promoter region at the start of a gene.
2. The DNA double helix unwinds and the two strands separate over the gene.
3. RNA polymerase moves along the template (antisense) strand and assembles a complementary strand of messenger RNA (mRNA) using free RNA nucleotides.
4. RNA uses the base uracil (U) in place of thymine, so an A on the template pairs with U in the mRNA.
5. The completed mRNA detaches. In eukaryotes the pre-mRNA is processed: a cap and tail are added and non-coding introns are removed by splicing, leaving the coding exons.

The mRNA then leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm.

Translation

Translation reads the mRNA and builds a polypeptide. The genetic code is read in groups of three bases called codons. Each codon specifies one amino acid, and the code is described as degenerate because most amino acids are coded for by more than one codon. There is a start codon (AUG) and there are stop codons that end translation.

1. The ribosome binds to the mRNA at the start codon.
2. Transfer RNA (tRNA) molecules carry specific amino acids. Each tRNA has an anticodon, three bases that are complementary to a codon.
3. A tRNA whose anticodon matches the codon binds, delivering its amino acid.
4. Peptide bonds form between adjacent amino acids, building a chain.
5. The ribosome moves along codon by codon until it reaches a stop codon, then the finished polypeptide is released.

The polypeptide folds into a three-dimensional shape, which determines the protein function. This flow of information, DNA to RNA to protein, is often called the central dogma of molecular biology.

Gene regulation

Cells do not express every gene all the time. Regulation controls which genes are switched on, when, and how strongly, which is why a nerve cell and a skin cell carry the same DNA but behave differently. Regulatory proteins called transcription factors bind to control regions of DNA and either promote or block RNA polymerase. Environmental signals, hormones, and the cell type all influence which genes are active. This differential gene expression underlies cell specialisation in multicellular organisms.

Mutations

A mutation is a change in the DNA base sequence. A substitution swaps one base for another and may be silent (no change to the amino acid), missense (a different amino acid), or nonsense (creates an early stop codon). Insertions or deletions that are not multiples of three cause a frameshift, which shifts how every following codon is read and usually disrupts the whole protein. Mutations are the ultimate source of new alleles and therefore of variation.