The size of paradox in genome

The size of a genome ranges from few kilobases (Kb) in viruses to over 106 megabases (Mb) in higher plants. Even the humangenome is about 3000 Mb in size. In the 1950s a concept called ‘DNA constaiicij hypothesis’ was developed from the observations that nuclear DNA content in all the individuals within an animal species was nearly constant. Later observations revealed that the hypothesis is not universal. For example, while birds, reptiles or mammals conform well to the concept, large variation exists in DNA content of Salamander, Lungfishes and in many plant species. A term ‘C-value’ was coined by H. Swift in 1950 to describe the DNA content of a haploid cell. Once the C-values of different organisms were being determined, some puzzling information baffled the scientific community. It was observed that some of the simpler organisms have much larger genome than more complex organisms, a phenomenon termed as C-value paradox. Some Salamanders have C-value of more than 120 pg. while that of human genome is only 3.5 pg. Even some algae, many higher .Jants incIuding wheat, lilies and cucurbits have much higher C-value than that of human or primates. In the plant kingdom rice has a C-value of 0.5 pg, while that of Lilium is 35 pg. This directly confronts the theory that simpler organisms need fewer genes, while more complex organisms need much more genes. In other words, genome complexity is not related to the genome content,

or number of genes. Secondly, genome size of any organism is much more than the total size of genes present in the genome, implicating that some parts of genome are not expressed. The results of genome sequencing shows that human and mouse genome have approximately similar number of genes, yet humans are much advanced in the evolutionary scale. Another important observation from comparison of genome complexity of members of same species shows that some species have high within-species C-value diversity, although having same general level of phenotypic expression. The discovery of non coding DNA, which constitutes the bulk of the genome ended the paradox by pointing out that increase in genome content may not be co-linear with increase in number of genes. In fact most of the large genomes have higher amount of non coding repetitive DNA, often referred as junk DNA or more popularly ‘Selfish DNA’. However, this

finding does not end the paradox but raises a number of further questions including the role of the junk DNA that contains short and intermediate repetitive sequences, transposable elements, introns within genes and segmental duplications. For this reason, C-value paradox is now termed as C-value enigma, multiple

puzzles involving these questions. Even if genome complexity is considered related to gene number rather than C-value, further puzzle awaits to be solved as being observed from the results of genome sequencing projects. For example, compared to 25,000 genes in human, Drosophila contains 13,500 genes and rice contains 40,000-50,000 genes in their genomes. Even Arahidopsis has 25,000 genes in their genome. These organisms are extremely variable in their phenotypic complexity. Human being the most complex

representative of animal kingdom shares genes as many as Arahidopsis, about 11,500 more genes than Drosophila or just about 5000 more genes than simple nematode elegans. This new paradox which states that the genome complexity in many cases may not be related to gene number is popularly termed as

‘C-value paradox’ or ‘C-value enigma’. Possible role of transposable elements, genome wide duplications and elucidation of evolutionary pathways may help in solving the G-value paradox.