DNA, genes and chromosomes

DNA in prokaryotic cells

• short

• circular

• not associated with proteins

DNA in eukaryotic cells

• in nucleus

• long

• linear

• associated with histones (proteins)

What is a chromosome

A DNA molecule and its associated proteins form a chromosome

What is a homologous pair/chromosome?

Two chromosomes that carry the same genes but not necessarily the same alleles of the genes

What is an allele?

One of a number of alternative forms of a gene

In eukaryotic cells what also contains DNA apart from the nucleus?

• mitochondria

• chloroplasts

short, circular and not associated with proteins

What is a gene?

A base sequence of DNA that codes for:

• the amino acid sequence of a polypeptide

• a functional RNA (including ribosomal RNA and tRNAs)

What is a locus

A genes fixed position on a particular DNA molecule

What is a triplet

• a sequence of three DNA bases

• codes for a specific amino acid

Genetic code - universal

each triplet codes for the same amino acid in all organisms

Genetic code - non-overlapping

each base in the sequence is read once only

Genetic code - degenerate

most amino acids are coded for by more than one triplet

Exons and introns

• in eukaryotes, much of the nuclear DNA does not code for polypeptides

• there are non-coding multiple repeats of base sequences between genes

• within a gene, only some sequences - exons - code for amino acid sequences

• exons are separated by one or more non-coding sequences - introns