Introduction

All organisms whether they are eukaryotes or prokaryotes are complex in their structure and functions. The DNA present in all living things is broadly categorized into two groups that are functional or coding DNA and non-functional or non-coding DNA. Now let’s see what is a C-value paradox.

Well, it is the total amount of DNA present in one set of genomes that is present in a haploid genome also known as the 1C value. And this 1C value is not directly or indirectly proportional to the complexity of the organisms. Thus this 1C value is extremely variable in all the organisms (1).

In simple words, we can say that the C value is the total amount of DNA present in the genome. Their genome size is directly proportional to organism complexity. C value paradox defines that there is no direct relationship between genome size and the complexity of an organism.

What is the C value paradox?

This term is given by C. A. Thomas in 1971, when repeated sequences of DNA were discovered, explaining in every case no relationship between genome size and complexity of organisms.

The DNA content of an organism’s genome is related to the morphological complexity of eukaryotes but it is observed that it is different in higher eukaryotes.

In higher eukaryotes there is no correlation between complexity and genomic size; this is called the C value paradox. 

Genome size is the total amount of DNA contained within the copy of a single genome. It is measured in terms of picograms and base pairs.

Morphologically similar organisms appear to have different amounts of DNA in their genome.

Living organisms are classified into two categories: eukaryotes, which are complex organisms with organized nuclei, and prokaryotes, which are simpler organisms without organized nuclei.

It was commonly believed that the complexity of an organism was reflected in its DNA content, with eukaryotes having a higher percentage of DNA than prokaryotes.

However, recent findings have shown that this is not always the case, and there are many exceptions to this rule. Therefore, it is incorrect to assume that the amount of DNA in an organism is always proportional to its complexity.

In other words, we can say simpler the organism smaller the genome, complex the organism larger the genome.

The complexity of an organism can be predicted by knowing the size of the genome and the size of the genome can be predicted by the complexity of an organism.

The eukaryotic genome consists of two parts, coding DNA and non Coding DNA. Coding DNA is a protein synthesizing DNA and non-coding DNA is present in multiple copies.

The human genome consists of 1.5% of Coding DNA and 98% non Coding DNA.

Example

1. Salamanders have 40 times more DNA in comparison to humans, whereas humans are more complex organisms compared to salamanders.

2. Housefly and Drosophila both are in the same group but the housefly C value is higher than Drosophila.

Reason for C value paradox

The reason for this is the presence of repetitive DNA, which means the sequence of DNA which repeats in the genome many times.  

C value paradox for prokaryotes and eukaryotes

This is the incomplete study of genome structure. A major discovery in the understanding of genome size variation and the explanation of the C value paradox comes with the discovery that the genome of eukaryotes and some prokaryotes contain large and varying amounts of sequences that do not work as protein-coding genes but they contribute to the structure of chromosomes.

Lower Eukaryotes  and C value phenomenon

Only lower eukaryotes worked in the C value phenomenon; it means the complexity of an organism relates to genome size.

Example

Mycoplasma has more DNA bp than Nematodes.

Mycoplasma- 1 x 106 

Nematodes – 8 x 10

Higher prokaryotes

In higher eukaryotes, the complexity of an organism does not relate to genome size.

Example- Humans are the best example of highly complex eukaryotes, but they have 40 times less DNA in comparison to salamanders.

C Value enigma

C value enigma represents an updated term of the C value paradox, It was given by Dr. T. Ryan Gregory in 2001. C value enigma relates to variation in the amount of non Coding DNA found within the genomes of different eukaryotes. The variation of non-coding DNA varies from species to species.

C Value enigma explains properly the reason for the C value paradox and defines what types of non-coding DNA are found in the eukaryotic genome and its function and what proportions they are present.

C value of DNA

The term C value refers to the amount of total DNA present in the haploid nucleus and looking at the structure of their organisms. It’s defined as the linear relationship between genome size and the complexity of an organism. If the genome size is smaller the organism will be simpler and if the genomic size is large, the complexity of an organism is increased. We can say in easy terms the complexity of an organism is directly proportional to its genomic size.

C value expressed in base pairs (bp) and picograms, where 1C value 2C value expressed the Content of DNA.

Q&A

What is the c-value paradox?

There is no direct relationship between the complexity of an organism and the size of the genome. Some organisms have simpler organisms but genome size is large because in their genome repeated non Coding DNA is present in large amounts.

What is the c value paradox explained?

The C value paradox is an updated form of C value. It defines that there is no relationship between genome size and the complexity of an organism. 

Does gene duplication explain the c-value paradox?

C value paradox does not explain gene duplication. It explained the relationship between genome size and the complexity of eukaryotic and prokaryotic organisms but it fails to give a proper correlation between genome size and complexity.

Does the c-value paradox apply to prokaryotes or eukaryotes?

Yes, it applies to some prokaryotes and eukaryotes. Prokaryotes are simpler organisms but many prokaryotes have larger genome sizes than eukaryotes.

Written By: Richa Pachori 

References

1. 4th edition biochemistry Donald Voet & Judith G. Voet.

2. Lehninger, 4th edition, Principal of biochemistry, David L. Nelson & Michael.