How much DNA is too much DNA?

In this Step we will cover some of the common terminology used to describe large genomes.

DNA content varies dramatically from one organism to the next. For example, although viruses have some of the smallest genomes of only a few thousand letters (Hepatitis B virus has about 3,000 bp (base pairs)) a Japanese flower plant called Paris japonica has the largest genome ever sequenced with 150 billion (150,000,000,000 !!!!) bp.

In the graph below we can see the relationship between number of genes and genome size in viruses, bacteria (prokaryotes) and eukaryotes. Number_of_genes-03.png.

As genomes get larger and larger, it becomes impractical to denominate them by the thousand, so the community has adopted the following abbreviations:

1,000 bases = 1 kb

1,000,000 bases = 1 Mb

1,000,000,000 bases = 1 Gb

Therefore, a viral genome of 3,000 bases can be expressed as a 3 kb genome.

Bacterial genomes have a range of sizes and can be contained on one or many chromosomes and these can be circular or linear. Regardless of their shape, bacterial genomes show a high correlation between the genome size and number of genes contained in the genome. This means that the larger the genome size, the more genes it will contain.


Acknowledgement: “Genome size vs number of genes.svg” by Francesca Short. Reproduced from Estevezj is licensed under Attribution-Share Alike 3.0 Unported. This plot can be made from scratch. R code is included in the linked page.

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Bacterial Genomes: Accessing and Analysing Microbial Genome Data

Wellcome Genome Campus Advanced Courses and Scientific Conferences