C-value paradox, Summaries of Genetics

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C-value paradox

M.Sc. Microbiology, 2nd^ Semester MCB 202 : Genetics and Gene regulation Gr. A: Fundamental Genetics

by Dr. Suman Kumar Halder

• In 1952, Watson and Crick proposed that DNA is a double helix which is known to have alternative forms.

The Genome

• The organization of total sum of genetic information (or genome) of an organism is in the form of double-stranded DNA.
• In many viruses and prokaryotes, the genome is a single linear or circular molecule. In eukaryotes, the nuclear genome consists of linear chromosomes (usually as a diploid set) and the mitochondrial and chloroplast (in plants) genomes are small circular DNA molecules.
• The DNA is converted from relaxed to supercoiled by DNA specific enzymes.
• In general, genome size increases with organism's complexity.

• The term C-value refers to the amount,

in picograms, of DNA contained within

a haploid nucleus (e.g. a gamete) or one half

the amount in a diploid somatic cell of

a eukaryotic organism.

C-Value

C-Value paradox

• C-Value Paradox refers to the observation that

genome size does not uniformly increase with

respect to perceived complexity of organisms,

for example vertebrate with respect to

invertebrate animals, or "lower" versus

"higher" vertebrate animals. For examples

that some Amphibians have more than 10-fold

more DNA than do Mammals, including

humans.

• The C-value enigma or C-value paradox is the complex puzzle surrounding the extensive variation in nuclear genome size among eukaryotic species. At the center of the C-value enigma is the observation that genome size does not correlate with organism complexity; for example, some single-celled protists have genomes much larger than that of humans
• The term " C-value enigma " represents an update of the more common but outdated term "C-value paradox“ (Thomas 1971), being ultimately derived from the term "C-value" (Swift 1950) in reference to haploid nuclear DNA contents. The term was coined by Canadian biologist Dr. T. Ryan Gregory of the University of Guelph in 2000/2001.

• There is in fact no "paradox." Evolution does not proceed in a linear manner, nor is there a linear succession of organisms from "lower" to "higher."
• Despite differences in DNA content, the number of genes in any vertebrate genome is roughly similar. Also, plant and amphibian genomes in particular are frequently polyploid , in which the chromosome number undergoes doubling to two-, four, or eight-fold without a radical change to the form of the organisms.
• However the discovery of a large amount of non-coding DNA lead to the concept of C-DNA value or C-Value paradox and variation is surprisingly so vast it is called C- DNA value paradox.
• The paradox or the enigma is between the C-value and the gene numbers. Elucidation of noncoding DNA and noncoding but functional RNA can resolve this.