Genome size
Genome size refers to the total amount of genetic material found within an organism's cells and can vary dramatically among different species, exhibiting a range that spans 200,000-fold. Notably, this size does not correlate with the complexity of the organism, leading to what is termed the "C-value paradox." For example, the genome of the human is significantly smaller than that of several plants and even some simpler organisms. Prokaryotic genomes, typically smaller, can range from as few as 580,000 bases to several million, while eukaryotic genomes can reach extraordinary sizes, such as the 670 billion bases found in Amoeba dubia.
Most of the DNA within eukaryotic genomes is noncoding, often referred to as "junk" DNA, which does not contribute to protein coding. This leads to a variability in genome sizes that is not solely explained by the number of genes, as eukaryotes can contain large amounts of repetitive DNA. The organization of genomes also differs, with eukaryotic DNA structured into linear chromosomes, while prokaryotic DNA is generally circular and contained in a single chromosome. Understanding genome size and its implications can provide insights into developmental processes, evolutionary adaptation, and the intricate relationships between genetic material and organismal characteristics.
Genome size
SIGNIFICANCE:Genome size, the total amount of genetic material within a cell of an organism, varies 200,000-fold among species. Since the 1950s, it has been clear that there is no obvious link between an organism’s complexity and the size of its genome, although numerous hypotheses to explain this paradox exist.
Genome Sizes in Prokaryotes vs. Eukaryotes
Wide variation in genome size exists among species, from 580,000 bases in the bacterium Mycoplasma genitalium to 670 billion bases in the protist Amoeba dubia. In general, prokaryotic genomes are smaller than the genomes of eukaryotes, although a few prokaryotes have genomes that are larger than those of some eukaryotes. The largest known prokaryotic genome (10 million bases in the cyanobacterium Nostoc punctiforme) is several times larger than the genomes of parasitic eukaryotic microsporidia, with genome sizes of approximately 3 million bases. Within the prokaryotes, the archaea have a relatively small range of genome sizes, with the majority of species in the 1- to 3-million-base range, while bacterial species have been found with genomes differing by twentyfold.
Contrary to expectations, there is no obvious correlation between genome size and organismal complexity in eukaryotes. For example, the genome of a human is tenfold smaller than the genome of a lily, twenty-five-fold smaller than the genome of a newt, and two-hundred-fold smaller than the genome of an amoeba. In 2024, scientists discovered a very small species of fern that has fifty times the DNA of a human. Tmesipteris oblanceolata, which grows in the island nation of New Caledonia in the South Pacific, has a genome size of 160.45 Gbp, or gigabase pairs. The characteristic genome size of a species is called the C-value; the lack of relationship between genome size, number of genes, and organismal complexity has been termed the “C-value paradox.”
Reasons for Size Differences
The majority of DNA in most eukaryotes is noncoding. Previously known as “junk” DNA, this DNA (comprising up to 98.5 percent of some genomes) does not contain the coding sequences for proteins. The complexity of DNA can be characterized using a technique called reassociation kinetics. DNA is sheared into pieces of a few hundred bases, heated to denature into single strands, then allowed to renature during cooling. The rate of renaturation is related to the sequence complexity: DNA sequences present in numerous copies will renature more rapidly than unique DNA sequences. Unique DNA sequences usually represent protein-coding regions, whereas repetitive DNA generally does not encode traits. In many genomes, three types of DNA can be identified by reassociation kinetics: highly repetitive DNA, middle repetitive DNA, and unique DNA. Prokaryotes have little or no repetitive DNA. Among eukaryotes, the amounts of the three types of DNA varies. The share of the genome dedicated to genes is relatively constant, whereas the amount of repetitive DNA, 10-70 percent of the total, varies widely even within families of organisms. The existence of appears to account for the lack of correlation between genome size and complexity because complexity may be more directly related to number of genes, a number which does appear to have more correlation to organismal complexity.
The variation in the amount of repetitive DNA, even within families, may be related to the spontaneous rate of DNA loss. Small genomes may be small because they throw away very efficiently, whereas large genomes may be less able to weed out unnecessary DNA. Studies on several invertebrates support this hypothesis: Species within a family with large genomes have substantially lower spontaneous DNA losses.
Genome size does have a positive correlation with cell size and a negative correlation with cell division rate in a number of taxa. Because of these correlations, genome size is associated with developmental rate in numerous species. This correlation is not exact, however. For some organisms (particularly plants) with relatively simple developmental complexity, developmental rate is constrained by external factors such as seasonal changes, while for others (amphibians with time-limited morphogenesis) developmental complexity overwhelms the effects of developmental rate.
Differences in Chromosome Number
The genomes of eukaryotes are organized into sets of two or more linear DNA molecules, each contained in a chromosome. The number of chromosomes varies from two in females of the ant species Myrmecia pilosula to forty-six in humans to ninety-four in goldfish. These numbers represent the number of chromosomes. A genome that contains three or more full copies of the chromosome number is polyploid. As a general rule, polyploids can be tolerated in plants but are rarely found in animals. One reason is that the sex balance is important in animals, and variation from the diploid number results in sterility. Chromosome number appears to be unrelated to genome size or to most other biological features of the organism.
For most of the prokaryotes studied, the prokaryotic genome is contained in a single, circular DNA molecule, with the possible addition of small, circular, extrachromosomal DNA molecules called plasmids. However, some prokaryotes have multiple chromosomes, some of which are linear; and some prokaryotes have several very large plasmids, nearly the size of the bacterial chromosome.
Key terms
- C-valuethe characteristic genome size for a species
- chromosomea self-replicating structure, consisting of DNA and protein, that contains part of the nuclear genome of a eukaryote; also used to describe the DNA molecules comprising the prokaryotic genome
- genomethe entire genetic complement of an organism
- junk DNAa disparaging (and now known to be inaccurate) characterization of the noncoding DNA content of a genome
- reassociation kineticsa technique that uses hybridization of denatured DNA to reveal DNA classes differing in repetition frequency
- repetitive DNAa DNA sequence that is repeated two or more times in a DNA molecule or genome
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