1.2 Nucleic acids: genetic, functional and structural roles in the cell
The first role that one immediately thinks about for nucleic acids is that of an inherited genetic material, principally in the form of DNA. In some cases, the inherited genetic material is RNA instead of DNA. For example, almost 60% of all characterised viruses have RNA genomes and these are more common in plant viruses than in animal viruses. There is considerable variation in the amount of genetic material present within organisms (Table 1). RNA genomes tend to be smaller than DNA genomes, ranging between 10 and 30 kilobases (kb) in size (but rarely larger), compared to over several hundreds of kilobases for DNA viruses. The most likely reason for this limitation in the size of RNA genomes is that RNA replication is considerably more prone to errors than is DNA replication (over one million times more so) and therefore the hazard of lethal mutations occurring in RNA is much higher.
What advantage might a high error rate in replication of its genome confer on a virus?
Within any population of a virus, the high rate of replication error will create considerable genetic variation due to the misincorporation of individual bases. Variation arising from such mutations will contribute to the overall evolution of the virus within a host.
If we examine the contents of the cell, we will see that the nucleic acids that comprise an organism's inherited genome (usually DNA) are commonly the minority nucleic acid within a cell. For example, nucleic acids represent approximately 12% by mass of a typical bacterial cell, compared to 16% for protein; but only 15% of this nucleic acid component (i.e. less than 2% of the cell mass) is chromosomal DNA; the rest is RNA of various types. Similarly, if you consider a typical human cell, the genome of 6.2 × 109 base pairs (bp) in each diploid cell equates to around 6 pg of DNA, whereas the same cell contains 20–30 pg of RNA, depending upon the level of its transcriptional activity. The bulk of this RNA (80%) comprises ribosomal RNAs (rRNAs), which make up the translation machinery, approximately 15% are small functional RNAs, such as transfer RNAs (tRNAs), and the remaining 5% is messenger RNA (mRNA).
You will encounter several cases where we discuss absolute masses of macromolecules within the cell, such as in ‘pg’. The relationship between the units you will encounter and 1 gram (1 g) is as follows:
1 μg (microgram) = 10−6 g;
1 ng (nanogram) = 10−9 g;
1 pg (picogram) = 10−12 g.
Other sources of nucleic acids within the cell are mitochondria and, in plant cells, chloroplasts. These organelles contain their own DNA genomes and RNA transcribed from them. The contribution of mitochondria or chloroplasts to the total nucleic acid content of any one cell depends upon their number, which can vary considerably depending upon the cell's metabolic activity. In mammalian cells, a mitochondrion can contain 10,000 copies of the 16 kb mitochondrial genome (mtDNA) and in some cells mitochondrial DNA can represent as much as 0.1% of the cellular DNA.
Many bacteria and some fungi, in addition to their genomic DNA, carry naturally occurring circular DNAs called plasmids, which are easily exchanged between individual cells within a population. Many antibiotic-resistance genes are carried on plasmids, a feature that is now exploited for genetic manipulation techniques where plasmid DNA is manipulated in vitro and re-introduced into cells.
Table 1 The genome content and size of various organisms.
|Organism*||Genome size in number of base pairs (DNA) or nucleotides (RNA)**|
|Rhinovirus 1A||RNA: 7200 nt|
|HIV||RNA: 9700 nt|
|Adenovirus 2||35 900|
|Epstein–Barr virus (EBV)||1.7 × 105|
|Escherichia coli||4.7 × 106|
|Saccharomyces cerevisiae (a budding yeast)||1.25 × 107|
|Trypanosoma brucei (a protoctist)***||1.27 × 107|
|Schizosaccharomyces pombe (a fission yeast)||1.4 × 107|
|Dictyostelium discoideum (a slime mould)||3.4 × 107|
|Caenorhabditis elegans (a nematode worm)||9.7 × 107|
|Arabidopsis thaliana (wall cress)||1.0 × 108|
|Drosophila melanogaster (fruit-fly)||1.7 × 108|
|Fugu rubripes (puffer fish)||3.6 × 108|
|Danio rerio (zebra fish)||1.7 × 109|
|Mus musculus (house mouse)||2.7 × 109|
|Homo sapiens (human)||3.1 × 109|
|Rattus norvegicus (brown rat)||3.1 × 109|
|Xenopus laevis (African clawed toad)||3.1 × 109|
|Zea mays (maize)||3.9 × 109|
|Nicotiana tobacum (tobacco)||4.8 × 109|
* Viruses are not considered to be free-living organisms. Those listed here are included for purposes of comparison.
** Genome sizes (other than those of viruses and E. coli) are the haploid genome size.
*** Estimate only: the nuclear genome also contains numerous mini-chromosomes with varying ploidy.