Introduction

What is Genomic DNA?

Genomic DNA, often abbreviated to gDNA, is the total genetic information of an organism. The genomes of almost all organisms are DNA, the only exceptions being some viruses that have RNA genomes. Genomic DNA molecules are generally large, and in most organisms are organized into DNA–protein complexes called chromosomes.

The size, number of chromosomes and structure of genomic DNA varies between different organisms (see table Genomic DNA and chromosome data by organism).

Structure of genomic DNA

  • Viruses: Viral DNA genomes are relatively small and can be single- or double-stranded, linear or circular.
  • Bacteria: Most bacteria have a single, circular chromosome.
  • Eukaryotes: In eukaryotic organisms, most genomic DNA is located within the nucleus (nuclear DNA), where it is organized into multiple linear chromosomes of different sizes. Eukaryotic cells also contain genomic DNA in the mitochondria. In plants and lower eukaryotes, genomic DNA is located in the chloroplasts. This organelle DNA is usually circular and present in multiple copies.

All other organisms have double-stranded DNA genomes.

Unraveling the structure and function of genomic DNA holds great potential for understanding gene regulation and disease mechanisms, and this is set to drive discoveries in the genomics field. These studies rely on successful sample disruption for extraction of genomic DNA and require a good knowledge of DNA extraction technologies and how these vary for different types of organisms.

Genomic DNA and chromosome data by organism
Organism Base pairs per haploid genome Molecular weight of genome (daltons) Number of chromosomes
SV40 5243 3.4 x 106 -
F174 5386 3.5 x 106 -
Adenovirus 2 35,937 2.3 x 107 -
Lambda 48,502 3.2 x 107 -
Escherichia coli 4.7 x 106 3.1 x 109 x = 1
Saccharomyces cerevisiae 1.5 x 107 9.8 x 109 2x = 32
Dictyostelium discoideum 5.4 x 107 3.5 x 1010  x = 6
Arabidopsis thaliana 7.0 x 107 4.6 x 1010 2x = 10
Caenorhabditis elegans 8.0 x 107 5.2 x 1010 2x = 12
Drosophila melanogaster 1.4 x 108 9.1 x 1010 2x = 8
Gallus domesticus (chicken) 1.2 x 109 7.8 x 1011 2x = 78
Mus musculus (mouse) 2.7 x 109 1.8 x 1012 2x = 40
Rattus norvegicus (rat) 3.0 x 109 2.0 x 1012 2x = 42
Xenopus laevis 3.1 x 109 2.0 x 1012 2x = 36
Homo sapiens 3.3 x 109 2.1 x 1012 2x = 46
Zea mays 3.9 x 109 2.5 x 1012 2x = 20
Nicotiana tabacum 4.8 x 109 3.1 x 1012 2x = 48

Genomic DNA and genes: Coding sequence, introns and expression control

Genomic DNA contains genes, which are discrete regions that encode a protein or RNA. Each gene includes not only the coding DNA sequence, but also the regulatory elements that control gene expression. Nuclear eukaryotic genes also contain noncoding regions called introns. The number of genes varies widely between different organisms, reflecting the complexity of their genomes.

Coding versus noncoding DNA: Eukaryotes, small coding fraction and repeats

Coding DNA represents only a small fraction of eukaryotic genomic DNA. The majority of the DNA is noncoding, much of which consists of repetitive sequences. While some noncoding DNA has structural and regulatory functions, the function of most of this DNA remains unknown.

Ploidy: Copies of genes in cells

 The number of copies of each genetic locus present in a cell is called ploidy. Ploidy varies across organisms and has important biological implications.

  • Eukaryotic somatic (body) cells: Usually diploid, having two sets of homologous chromosomes (one from each parent).
  • Germ (reproductive) cells: Haploid with only one copy of each chromosome.
  • Prokaryotes: Typically haploid, meaning they have a single copy of each chromosome.
  • Polyploid organisms: Some plants are polyploid, e.g., modern wheat, which is hexaploid (six copies of each chromosome).
Next DNA extraction technologies
Previous