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Nucleic acids, genomes and protein synthesis definitions

Every term defined in this unit, lesson by lesson, in the same words the lessons use, with a link to the notes where you will see it used.

29 terms

DNA and RNA structure

Nucleotide: A phosphate group, a pentose sugar and a nitrogenous base joined together.
Polynucleotide: A chain of nucleotides joined by phosphodiester bonds between the sugar of one and the phosphate of the next.
Phosphodiester bond: The covalent bond joining the phosphate group of one nucleotide to the sugar of the next, formed by condensation.
Purine: A base built from two fused rings: adenine and guanine.
Pyrimidine: A base built from a single ring: cytosine, thymine and, in RNA, uracil.
Antiparallel: The two strands of a DNA molecule run in opposite directions: the 5′ end of one lies alongside the 3′ end of the other.
Complementary base pairing: Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three, so the sequence of one strand fixes the sequence of the other.
Double helix: Two antiparallel polynucleotide strands wound into a spiral, bases inside and backbones outside.
Messenger RNA (mRNA): A single-stranded copy of one gene, made in the nucleus and carrying codons to a ribosome in the cytoplasm.
Transfer RNA (tRNA): A small folded RNA of about 80 nucleotides with an anticodon at one end and a binding site for a specific amino acid at the other.
Ribosomal RNA (rRNA): RNA which, with proteins, forms the two subunits of a ribosome and catalyses the formation of peptide bonds.

Semi-conservative replication: Replication in which the two strands of the parent molecule separate and each acts as a template, so every daughter molecule contains one original strand and one newly synthesised strand.
Template strand: A strand of DNA whose base sequence determines the sequence of the new strand built against it.
Mutation: A change in the base sequence of DNA.
Substitution: A mutation in which one base is replaced by another.
Deletion: A mutation in which a base is lost; within a coding region, losing a number of bases not divisible by three shifts the reading of every triplet that follows.
DNA precipitation: The recovery of DNA from a filtered tissue extract as visible fibres, by layering ice-cold ethanol on top; the DNA, insoluble in cold ethanol, forms at the boundary and is spooled onto a rod.

Codon: A sequence of three bases in mRNA that specifies one amino acid or a stop signal.
Degenerate: Describing a code in which most amino acids are specified by more than one triplet.
Non-overlapping: Describing a code in which each base is part of one triplet only and triplets are read in sequence.
Gene: A sequence of DNA bases coding for a polypeptide or a functional RNA.
Exon: A section of a gene that is represented in the mature mRNA.
Intron: A non-coding section of a gene, transcribed into pre-mRNA and then removed before the mRNA leaves the nucleus.
Genome: The complete set of DNA in a cell, including the DNA that does not code for polypeptides.
Pre-mRNA: The immediate product of transcription in a eukaryote, containing both exons and introns.
Splicing: The removal of introns from pre-mRNA and the joining of exons to produce mature mRNA.

Ribosome: An organelle of ribosomal RNA and protein, in two subunits, which holds mRNA and tRNA in position while a polypeptide is assembled.
Anticodon: A triplet of bases on tRNA, complementary to a codon on mRNA.
Post-translational modification: Changes made to a polypeptide after translation, such as cutting out sections or adding carbohydrate or lipid groups, carried out in the endoplasmic reticulum and the Golgi apparatus. Folding is not one of them: it begins on the ribosome and is finished in the endoplasmic reticulum.