DNA, Genes and Chromosomes

Structure of prokaryotic DNA

Single circular DNA molecule; DNA not associated with proteins; may contain plasmids

Plasmid

Small circular DNA molecule containing a few genes; often carries antibiotic resistance genes

Structure of eukaryotic DNA

Long linear DNA molecules associated with histone proteins to form chromosomes

Chromosome

DNA molecule associated with proteins

Histone protein

Protein that organises and condenses DNA so it fits inside the nucleus

Nucleosome

DNA wrapped around a histone protein

Chromatin

Tightly coiled DNA-protein complex that makes up chromosomes

Chromatid

One of two identical replicated strands of a chromosome joined at the centromere

Telomere

Protective structure at the end of a chromosome that maintains genetic stability

Homologous chromosomes

Chromosomes carrying the same genes at the same loci but not necessarily the same alleles

Gene

A sequence of DNA bases that codes for a polypeptide or functional RNA

Locus

The position of a gene on a chromosome

Allele

An alternative form of a gene

Genome

The complete set of DNA in a cell

Proteome

The full range of proteins produced by a cell

Three features of the genetic code

Degenerate; universal; non-overlapping

Degenerate genetic code

More than one triplet code can code for the same amino acid

Advantage of degenerate code

A point mutation may not alter the amino acid sequence

Universal genetic code

The same triplet codes for the same amino acid in all organisms

Why universal code supports evolution

It provides evidence for a common ancestor

Non-overlapping genetic code

Each base is read only once as part of one codon

Start codon

The first codon that initiates translation

Stop codon

Codon that does not code for an amino acid and terminates translation

Intron

Non-coding section of DNA removed during splicing

Exon

Coding section of DNA that remains in mature mRNA