The DNA is the genetic material in all eukaryotes from the simplest to the complex ones. The size of genome is constant for any species and can be considered as a characteristic. But nature has proved that complexity of the organism does not have a direct relationship with the genome size. This phenomenon known as C value paradox have been the object of study of many researchers.
For example, the amphibians and humans have fairly similar genome size of 109 bp. When considering this with lungfish (1011), there is a remarkable difference. Some fish, mollusks and amphibians have larger genomes than human beings. Even within the same class of organisms, there is a great deal of variation of genome size. For example, the genome of common house fly ( 8* 108 bp) is almost six times larger than its close relative fruitfly ( 1.4 *108 bp).
Generally, the minimum genome size in each phyla tend to increase along with the complexity of the phyla. C-value is the total amount of DNA in the haploid genome of the organism and is used to characterize the species. Only a small variation of 2% is found within the phyla of birds, reptiles and mammals. But plants, insects, arthrpods, fish and algae exhibit remarkable variations even to the tune of 5000 fold within the phyla even though their function and metabolic activities are similar. Even the body size, function and efficiency of the organism remain same among the members of the phyla.
The term C value paradox was first used by Thomas in 1971 to denote the absence of definite relationship between the organism complexity and genome size. He described it as a paradox since the exact mechanism which determined the genome size was not known at that time.
So how do we account for this paradox?
Researchers have used techniques such as DNA: DNA hybridization, DNA: RNA hybridization and DNA microarrays to decipher the reasons for the occurrence of C value paradox.
The findings targeted mainly to the large amount of non coding DNA; i.e DNA which does not code for proteins. In vertebrates for example, 90 percent of the total genome accounts for this non coding DNA and has been found to be the major cause of variation of genome size among related members of the same phyla.
Variations in the rate of spontaneous loss of these non coding DNAs have been found to be the reason for difference in haploid genome in Drosophilia. These non coding DNA regions were found to be having highly repetitive sequences.
Based on the repetitiveness, there are different types of DNA sequences found in eukaryotes.
a. Single copy DNA
b. Introns - non coding portions of genome.
c. Pseudogenes which are the result of mutations occurring in a duplicated gene.
The C value phenomenon has been explained in many ways and numerous hypotheses have been proposed to satisfy the reasons. Genome size has been found to be correlated with the size of the cell and nucleus of organisms.
There are several forces which cause the genome to grow.
b. Errors in DNA replication like bulk gene modifications and strand slippage.
The most common hypothesis is the junk DNA hypothesis which assumes that the above mentioned methods accumulate without the forces of selection. According to this hypothesis, these forces might result in a linear increase in genome size.
This has not been always the case and there were extremes of exponential growth. This type of increase in size could be explained by the selfish gene hypothesis which gives a selective advantage to the transposable elements to pass the effect of deleterious mutations. The genome selective benefit hypothesis proposes the selective effect of non coding DNA on the genome.
There are less known hypotheses such as nucleoskeletal hypothesis. This is based on the minimum requirement of cell size for maintaining the metabolism, cell division etc. This in turn requires the nucleus to be of optimal size which acts as the selective force. The non coding regions of DNA were also thought to be responsible for preventing recombination in the coding regions of DNA thus preserving the genome's essential function.
The latest in these theories is those which are based on genetic programming. In addition to the incorporation of junk DNA, transposons and errors of DNA replication, these theories also consider the evolutionary computation. It is based on the principle that all organisms tend to proceed in course of evolution which results in an increase of fitness. However, the inclusion of fitness in models for explaining evolution needs to be further defined and improved to satisfy the questions posed by C- value paradox.
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