Q.15 C–value paradox refers to
(A) the lack of correlation between genome size and genetic complexity of an
organism
(B) the presence of genetic sequences that propagate themselves within a genome
(C) the coexistence of multiple alleles at a genetic locus
(D) the concept that two or more genes may have the same function
C-value Paradox: Understanding the Lack of Correlation Between Genome Size and Genetic Complexity
The C-value paradox refers to the observation that genome size (C-value, or DNA content in a haploid genome) does not correlate with an organism’s genetic complexity or perceived biological sophistication. This multiple-choice question from exams like GATE Biotechnology tests core concepts in genomics, with option (A) as the correct answer. Below, each option is explained in detail for clarity.
Option Analysis
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(A) the lack of correlation between genome size and genetic complexity of an organism: Correct. The C-value paradox highlights cases like the amoeba Polychaos dubium (670 pg DNA) having a larger genome than humans (3.3 pg), despite lower complexity, due to non-coding repetitive DNA.
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(B) the presence of genetic sequences that propagate themselves within a genome: Incorrect. This describes “selfish DNA” or transposons (e.g., junk DNA hypothesis by Orgel and Crick), a proposed explanation for the paradox but not its definition.
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(C) the coexistence of multiple alleles at a genetic locus: Incorrect. This refers to genetic polymorphism or heterozygosity, unrelated to genome size variation.
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(D) the concept that two or more genes may have the same function: Incorrect. This describes genetic redundancy or gene families, not linked to the C-value paradox.
The C-value paradox, highlighting the lack of correlation between genome size and genetic complexity, challenges the assumption that more complex organisms need larger genomes. First coined by Hewson Swift in the 1950s and named by C.A. Thomas Jr. in 1971, this paradox arises because eukaryotic genomes often contain vast non-coding DNA, such as repetitive sequences and introns, inflating size without adding genes. For competitive exams like IIT JAM or GATE Biotechnology, understanding this concept is key, as seen in Q.15 from GATE BT 2023.
Causes of the C-Value Paradox
Genome size varies widely: bacteria (~1-10 Mbp), humans (3,200 Mbp), lilies (90,000 Mbp). Key factors include:
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Repetitive DNA: Tandem repeats and transposons make up 50-90% of some genomes, not contributing to complexity.
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Polyploidy and duplications: Whole-genome duplications in plants/amphibians expand DNA without proportional gene increase.
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Nucleotype effects: DNA quantity affects cell/nucleus size, independent of coding function.
Examples Across Organisms
| Organism Group | Example Genome Size (pg) | Complexity Note |
|---|---|---|
| Human | 3.3 | High (20,000 genes) |
| Amoeba (P. dubium) | 670 | Low |
| Lily | 100+ | Moderate |
| Arabidopsis | 0.16 | Plant model |
This table shows no linear genome size-genetic complexity link.
Exam Relevance for IIT JAM/GATE
In questions like GATE BT 2023 Q.15, option (A) is correct, distinguishing it from selfish DNA (B) or unrelated terms. Practice with Cot curves (DNA reassociation kinetics) to grasp repetitive DNA roles. The paradox underscores genome evolution beyond protein-coding genes.
