DNA, genes and chromosomes

DNA in prokaryotes

• Prokaryotes possess a single, circular chromosomal DNA molecule

  • It is sometimes referred to as a nucleoid

• They also usually have one or more plasmids

  • Plasmids are very small circular DNA molecules which contain a few genes

  • Plasmids are more accessible for proteins required for gene expression and therefore contain genes that are required often, quickly and/or in emergencies

  • The genes for antibiotic resistance are often found in plasmids

• The DNA within prokaryotic cells is not associated with any proteins

  • Eukaryotic DNA associates with histone proteins

DNA in eukaryotic cells

• In the nucleus of eukaryotic cells, DNA molecules are very long, linear

• Together, a DNA molecule and its associated proteins form a chromosome

  • The main proteins present are the large, positively charged globular proteins called histones. Their role is to organise and condense the DNA tightly so that it fits into the nucleus

  • The other proteins are enzymes used in copying and repairing the DNA

chromosome structure

• The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of

• Chromatids are organised into sections, which is a series of genes

• The ends of the chromatids in chromosomes are ‘sealed’ with protective structures called telomeres

Mitochondrial and chloroplast DNA

• similar to the DNA of prokaryotes, in that it is:

  • short,

  • circular and

  • not associated with protein

where is mitochondrial DNA located

matrix of the mitochondria

where is mitochondrial DNA located

in the stroma

what is a gene

A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule.

A length of DNA that codes for a single polypeptide or protein

mRNA

the base sequences on messenger RNA molecules are used by ribosomes to form polypeptide chains

tRNA

amino acids are carried to the ribosome by transfer RNA molecules

rRNA

ribosomal RNA molecules form part of the structure of ribosomes

what do the genes in DN molecules do

The genes in DNA molecules, therefore, control protein structure and as a result, protein function as they determine the exact sequence in which the amino acids join together during protein synthesis

what is a locus

the position of a gene on a chromosome

what is an allele

each gene can exist in 2 or more slightly different forms

alleles

Different alleles of a gene have slightly different nucleotide sequences, but they still occupy the same position (locus) on the chromosome

the triplet code

• The DNA nucleotide base code found within a gene is a three-letter, or triplet, code

• Each triplet codes for one amino acid

• There are four bases (A, G, C, T), so there are 64 different triplets possible (4³), yet there are only 20 amino acids that commonly occur in biological proteins

features of the triplet code

• degenerative

• non-overlapping

• universal

degenerative

This results in multiple triplets coding for the same amino acids

non-overlapping

each DNA base is only read once and is only used for one codon

universal

meaning that almost every organism uses the same code

non coding DNA

DNA that doesn’t code for any amino acid

introns

non coding pieces of DNA, sit between exons

exons

coding regions of DNA

what happens to introns before pre-mRNA

Before the pre-mRNA exits the nucleus, the non-coding sections (introns) are removed, and the coding sections (exons) are joined together in a process called splicing