Genome size varies greatly within and between organisms, with no clear relationship to complexity. Junk DNA partially solves the “C-value conundrum.” Genome size is measured by weight and base pairs, with notable exceptions such as bacteria and plants having larger genomes than humans.
The size of the genome differs in different organisms for reasons not fully known to modern science. Genome size is in many cases related to complexity, but there are a number of notable exceptions. For example, some bacteria and many plant species have genomes larger than humans. A term often used interchangeably with genome size is “C-value”. This is an abbreviation of the word “constant,” a reference to the fact that genome size between individuals of the same species is approximately constant. The question of why some simple organisms have large genomes is called the “C-value conundrum” in biology.
The discovery of “junk DNA,” or noncoding DNA, in the early 1970s partially solved the C-value conundrum. Junk DNA does not code for proteins, and although there is recent evidence that it may regulate the way where genes turn on and off, does not contribute to the biological complexity of the portion of DNA that contains the actual genes. When junk DNA is taken into account, the number of genes in an organism correlates roughly with what we would intuitively call biological complexity.
The simplest answer to the question about the relationship between genome size and organism type is that there is no relationship. Genome sizes vary greatly even within the same category of organisms; for example, in animals there is a variance by a factor of 3,300, and in terrestrial plants by a factor of about 1,000, and among protists by as much as 300,000.
Genome size is measured in two ways: by weight, in picograms, and in base pairs, in millions of bases or megabases. The human genome contains about 3,000 megabases, but only 1.5% of the genome actually codes for true genes. The genome of a chicken contains approximately 1,300 megabases. A clam has about 3,200, as do mice. Some frogs grow to 6,500 megabases, more than double the size of the human genome. a ladybug has about 300 megabases. It’s impossible to guess the size of an organism’s genome just by looking around it, unless you already have prior knowledge of the answer.
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