An average Human cell (diploid) contains about 6.4 billion base pairs of DNA divided among 46 chromosomes. The length of each base pair is about 0.34 nm. Therefore, if the DNA molecule in a diploid cell were laid out end to end, the total length of DNA would be approximately 2 meters. Continue reading How a 2 meters long DNA is fitted into a 2 micrometers Nucleus?
Tag Archives: Nucleic Acid
Why Nature Preferred DNA over RNA?
tIt is a well-known fact that DNA acts as genetic material in most of the organisms on this planet earth. However, it is also clear that RNA also acts as genetic material, but only in some viruses (for example, Tobacco Mosaic Viruses, QB Bacteriophage, etc.).
What does it take to be a genetic material?
Genetic material should fulfill the following criteria.
- Replication: It should have the ability to replicate itself.
- Stability: It should provide stable storage for genetic information.
- Evolution: It should have the ability to evolve and change itself.
- Expression: It should be able to express the information when needed.
Now, we have the eligibility criteria for the genetic material. Let us now examine each requirement one by one and compare DNA and RNA for these functions.
Which is better at Replication?
Replication occurs when a strand acts as a template for the synthesis of new complementary strands. This is possible only when there is the presence of complementary base pairing between the two strands of nucleic acids. We already know that the complementary base pairing is present in both the nucleic acids i.e. DNA and RNA. Thus, both of them have the ability to direct their duplications.
However, DNA has an upper hand in replication, as it can replicate with very high accuracy. This is because on average there occurs only one mistake per every 109 & 1010 bases of DNA.
Which is better in Stability?
The genetic material should be stable so that genetic information can pass from one generation to another without any change during the different life stages of the organism. Now, let us see which one is more stable? DNA or RNA.
If we recall the two basic chemical differences between DNA and RNA, then we get these two differences:
1. The presence of a 2’-Hydroxyl (-OH) group on RNA.
RNA, however, is a stable molecule due to the presence of a negative charge (–ve) on the sugar-phosphate backbone. It protects RNA from attack by Hydroxyl ions (OH–) or else it would lead to Hydrolytic cleavage. But, the presence of the 2’-Hydroxyl (-OH) group makes the RNA susceptible to Base-catalyzed hydrolysis.
Moreover, A single-stranded RNA is also prone to Auto-Hydrolysis. This spontaneous cleavage reaction takes place in basic solutions where free hydroxyl ions can easily deprotonate the 2’-Hydroxyl (-OH) group of the Ribose sugar.
However, if this 2’-Hydroxyl (-OH) group is removed from the ribose sugar then the rate of such base-catalyzed hydrolysis is decreased by approximately 100 fold. Thus, the presence of the 2’-Hydroxyl (-OH) group on every nucleotide of RNA makes it labile and easily degradable.
2. The presence of Thymine at the place of Uracil in DNA.
The only structural difference between Thymine and Uracil is the presence of a methyl group in Thymine. This methyl group facilitates the repair of damaged DNA, providing an additional selective advantage.
Cytosine in DNA undergoes spontaneous deamination at a perceptible rate to form Uracil. For example, under typical cellular conditions, deamination of Cytosine to Uracil (in DNA) occurs in about every 107 Cytidine residues in 24 hours, which means 100 spontaneous events per day.
The deamination of Cytosine is potentially mutagenic because Uracil pairs with Adenine and this would lead to a decrease in G≡C base pairs and an increase in A=U base pairs in DNA of all cells. Over the time period, the Cytosine deamination could eliminate G≡C base pairs.
But, this mutation is prevented by a repair system that recognizes Uracil as foreign in DNA and removes it. Thus, the methyl group on thymine is a tag that distinguishes thymine from deaminated cytosine. But, if DNA normally contains Uracil, recognition would be more difficult.
So, we can say that the presence of Thymine in place of Uracil in DNA enhances the accuracy of genetic messages. This makes DNA is more stable than RNA.
Which is a better option for Evolution?
To act as a better genetic material, one needs to provide the scope for slow and gradual changes (i.e. evolution). Among nucleic acids, both DNA and RNA can mutate or change their sequence. But, RNA being more unstable mutate at a faster rate.
However, many pieces of evidence suggest an intimate link between rapid mutations and the process of aging and carcinogenesis. That means rapid mutations can be carcinogenic and leads to faster aging. This may be the reason for the shorter lifespan of viruses as they mutate and evolve.
However, DNA does mutate, but at a very slow rate under normal cellular conditions which do not prove to be harmful in the longer run.
Which is better in Expression?
Expression of genetic information is also a necessary criterion that should be fulfilled by genetic material. Between both nucleic acids, RNA can directly code for the synthesis of proteins, hence, can easily express the characters.
DNA, however, is dependent on RNA for the synthesis of proteins (translation). Over the period of evolution, the protein-synthesizing machinery has evolved around RNA. Thus, RNA can easily express itself in the form of proteins.
So in the above battle between DNA and RNA, DNA is proved to be victorious and can be declared as a better genetic material as it can
- Replicate with more accuracy.
- Store information with better stability.
- Undergoes slow changes and can resist rapid ones (mutations).
But, for the expression of genetic information DNA needs RNA for protein synthesis, which is then transcribed from the DNA sequence.
So from the above discussion, we can conclude that DNA is a better genetic material than RNA. Also, we can conclude that DNA is preferred for the storage of genetic information, whereas RNA is better in the transmission of genetic information 🙂. So that is all for now, meet you in my next article. Keep Reading, Keep Exploring, and Keep Sharing your Knowledge, and above all BE CURIOUS. 🙂
References:
- ‘Biochemistry Revisited: Why Is DNA (And Not RNA) A Stable Storage Form For Genetic Information?’. N.p., 2015. Web. 25 Sept. 2015.
- Wikipedia, ‘RNA Hydrolysis’. N.p., 2015. Web. 25 Sept. 2015.
Who discovered that DNA is the Genetic Material?
The discovery of DNA as the genetic material was one of the major achievements of science in the 20th century. This discovery made DNA, the chemical basis of heredity. It took almost 80 years for scientists to prove that DNA is the genetic material.
Early works:
In 1869, Johann Friedrich Miescher, a young Swiss medical student, discovered an acidic substance that he isolated from pus cells obtained from bandages used to dress humans. He found it in the form of a mixture of compounds in the nucleus of the cell and named it “Nuclein”.
The nature of Nuclein was unusual as it contained large amounts of both nitrogen and phosphorous. At that time these two elements were coexisting only in certain types of fats. The discovery of Nuclein by Friedrich Meischer was quite early. Whereas it took a very long time to discover and prove that DNA is the genetic material.
By 1926, the quest to determine the mechanism for genetic material had reached the molecular level. In addition to this, different discoveries and findings further narrowed the search for the genetic material to the chromosome levels. But, the key genetic molecule was still missing from the research and findings.
Griffith’s Experiment: Transforming Principle
In 1928, Fredrick Griffith, an English microbiologist, transformed nonpathogenic forms of Streptococcus pneumoniae into pathogenic forms by changing their physical forms. He accomplished this transformation through the following steps:
He injected mice with a mixture of heat-killed S strain (pathogenic) and live R strain (non-pathogenic) pneumococci. The mice died, and he recovered the live S strain of pneumococci from the blood of mice. That means heat-killed pathogenic strain transformed non-pathogenic live strain into pathogenic forms.- But, when he injected live R strain (non-pathogenic) and heat-killed S strain separately, the mice lived. This is because none of them were pathogenic.
He concluded that heat-killed virulent S strain transferred some “transforming principle” to the non-virulent R strain, which enabled it to become virulent. This must be due to the transfer of genetic material.
In 1931, Richard Sia and Martin Dawson performed the same experiment in vitro, showing the mice played no role in the transformation process.
Evidence for DNA as the genetic material
For a very long time after the discovery of Nuclein, it was thought that proteins carry genetic information. Whereas, the nucleic acids were thought to have only structural functions. But somehow the search for the chemical identity of the transforming principle reached its climax in 1944.
In 1944, Oswald Avery, Colin Macleod, and Maclyn McCarty provided the first solid evidence of DNA as the genetic material. They were working on the transforming principle found in Griffith’s experiment.
They supported their evidence through the following observations:
- The transformation carried out by heat-killed S strain bacteria was inhibited, when the highly purified extract of that strain was subjected to the activity of Pancreatic Deoxyribonuclease (DNases). This is because DNAase digested DNA and hence no transformation occurred.
- They also found that Protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not affect transformation. So they concluded that protein and RNA were not responsible for transformation.
Hence, they concluded that DNA is the genetic material and caused the transformation from heat-killed virulent bacteria to live non-virulent bacteria.
Hershey-Chase Experiment
The second proof that DNA is the genetic material was provided in 1952 by the experiments of Alfred Hershey and Martha Chase. They worked with bacteriophage T2, a virus that infects the bacterium Escherichia coli. The T2 phage consists of a DNA core that is surrounded by a protein coat.
They labeled the nucleic acid part of the T2 phage with radioactive phosphorus and protein coat with radioactive sulfur. After that, they infected E. coli cells with labeled T2 phage. They found out that the radioactive phosphorus (nucleic acid part) remained in the cells, whereas the radioactive sulfur (protein part) was largely lost.
This showed that proteins did not enter the E.coli from the T2 phage. Therefore, it was DNA that was transferred from the T2 phage to E. coli.
Also Read: Why Nature Preferred DNA over RNA?
So, now we know that the Hershey-Chase experiment unequivocally resolved the debate that DNA is the genetic material. Thus, at last, the journey of genetic material from Nuclein to DNA came to an end and DNA is genetic material, became a fact. So that is all for now, meet you in my next article. Keep Reading, Keep Exploring, and Keep Sharing your Knowledge, and above all BE CURIOUS. 🙂
Also Read: 11 Amazing Facts about DNA You didn’t Know.