The C-Value Paradox: Does Having A Larger Genome Make An Organisms More Complex?

The C-value paradox is the size of the genome (the amount of DNA) doesn’t correlate with how complex an organism is. In other words, the C-value paradox means that a larger genome doesn’t always lead to more complexity.

During a genetics lecture in college, someone asked, “Do humans have the most DNA?”. I thought it was an inane question. I thought wrong. Our professor chuckled, “Not by a long shot. Actually, a Japanese flower has a genome that is 50 times larger than ours”.

The whole class, which was prepared to hear a resounding “yes”, cementing human glory, stared at her in disbelief. How and why would a plant have more DNA than humans, far more evolved and complex mammals? Is there any relationship between the amount of DNA an organism has and its complexity? In other words, does size matter? 

The short answer is: yes, it does matter. But also no, it doesn’t.

What is the C-value paradox?

The C-value paradox is the size of the genome (the amount of DNA) doesn’t correlate with how complex an organism is. In other words, the C-value paradox means that a larger genome doesn’t always lead to more complexity.

One would expect that complexity requires more DNA to be generated. A larger genome, the term for the entire library of DNA in an organism, would be better equipped to support complex life. The evolution from microscopic life, like bacteria and archaea, to larger and more multiplex life, like humans, requires more information and therefore more DNA.

This is true.

Eukaryotes, organisms with a nucleus to house its genetic material (DNA), do have larger genomes compared to prokaryotes (organisms without a nucleus), such as bacteria. E.coli, every microbiologist’s best friend, has only 4 million base pairs. The fly, Drosophila Melanogaster, on the other hand, has 140 million base pairs. A fly is clearly more complex than a bacteria.

However, this logic doesn’t hold when one compares the genomes of eukaryotes.

Consider humans and the marbled lungfish. The marbled lungfish places second in the ‘largest genome’ contest, with a whopping 130 billion base pairs. Humans, for comparision, only have 4 billion base pairs worth of DNA.

Yet, humans are clearly more complex than a marbled lungfish, with our sophisticated physiological systems, and modes of communication, movement, etc. Why then does a fish have 40 times as much DNA? 

Genome Size

Comparative genome sizes. Note that there are certain amphibians, fish and plants with larger genome sizes than mammals (Photo Credit : Abizar/Wikimedia Commons)

How was C-value paradox discovered?

In 1971, C.A. Thomas Jr., a geneticist, dubbed this problem the C-value paradox. The C-value is the amount of DNA in a haploid nucleus (measured in picograms), like sperm cells. Regular, autosomal cells have two sets of DNA—one maternal and one paternal. These diploid cells have two chromosomes that work together. In that case, an autosomal human cell has 6 billion base pairs worth of DNA.

Why does the C-value paradox happen?

All DNA isn’t equal.

The entirety of the genome doesn’t hold information crucial for survival.

Genes are the stars of the genome. They are the segments of DNA that code for parts of the cellular machinery, primarily proteins, and RNA. Many scientists assumed that more DNA meant more genes which meant more protein which could do more work to make an animal complex.

But that isn’t the case. There are sections of DNA that don’t contain genes or other parts that help a gene function.

Susume Ohno, a geneticist with an impressive handlebar mustache, called it ‘junk DNA’. In eukaryotes, this supposedly useless DNA takes up a lot of space. Half the human genome is junk, while the marbled lungfish only needs 1/36th of its humongous genome.

Paris japonica Kinugasasou in Hakusan

Paris japonica, the plant with the largest genome amongst all life forms. It has approximately 149 billion base pairs, 50 times more than the human genome. (Photo Credit : alpsdake/Wikimedia Commons)

What is Junk DNA?

Junk DNA, as mentioned before, is DNA that doesn’t hold any indispensable information for the organism’s survival and is a result of mutations. Some mutations are monumental, such as whole-genome duplication mutation, where the entire genome gets doubled.

Single Chromosome Mutations

Three different types of mutations that can happen in a cell. 1 – deletion, 2 – Duplication, 3 – Inversion. In 1 and 2, the amount of DNA is altered. If this happens in a gamete, it could lead to a permanent change in genome size.  (Photo Credit: Zephyris/Wikimedia Commons)

Much of the Junk DNA doesn’t have any direct implication on the complexity of  an animal. There is evidence that junk DNA plays some functions in organisms, but to what extent and how much of junk DNA is important for complexity is still unknown.

But, the animals with the largest genomes have a lot of junk DNA that doesn’t seem to contribute to their complexity.

Tl;dr

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Eukaryotes have larger genomes than prokaryotes (bacteria), but more complex eukaryotes do not have larger genomes than less complex organisms. This is called the C-value paradox, where the C-value means the amount of DNA in a haploid cell. This is because there is part of the DNA that does not have any informational function, called junk DNA (still debatable).  Sometimes, this junk DNA litters an organism’s genome, making it much larger than it needs to be. More complex organisms achieve that level of complexity by regulating their genes better. They do this by alternative splicing, signal transduction and epigenetic mechanisms (amongst many more). This means that their genes have more functions, thus allowing for more complex expression and form.

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About the Author

Salama has a degree in Life Science and Biochemistry from St. Xavier’s College, Mumbai. She enjoys being in the water much more than being on land. She is passionate about science and wants to declutter science from its jargon to make people understand its beauty.

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