Sherry Haynes is currently pursuing a PharmD degree and has experience in both the clinical and management sides of pharmacy.
RNA vs. DNA
Nucleic acids are huge organic molecules made of carbon, hydrogen, oxygen, nitrogen, and phosphorus. The deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are two varieties of nucleic acid. Although the DNA and RNA share many similarities, there are quite a few differences between them.
Summary of Differences Between DNA and RNA
- Pentose sugar in the nucleotide of DNA is deoxyribose whereas in the nucleotide of RNA it is ribose.
- DNA is copied via self-replication while RNA is copied by using DNA as a blueprint.
- DNA uses thymine as a nitrogen base while RNA uses uracil. The difference between thymine and uracil is that thymine has an extra methyl group on the fifth carbon.
- The adenine base in DNA pairs with thymine while the adenine base in RNA pairs with uracil.
- DNA cannot catalyze its synthesis while RNA can catalyze its synthesis.
- The secondary structure of DNA consists of mainly B-form double helix while the secondary structure of RNA consists of short regions of A-form of a double helix.
- Non Watson-Crick base pairing (where guanine pairs with uracil) is allowed in RNA but not in DNA.
- A DNA molecule in a cell can be as long as several hundred million nucleotides whereas the cellular RNAs range in length from less than one hundred to many thousands of nucleotides.
- DNA is chemically much more stable than RNA.
- The thermal stability of DNA is less compared to RNA.
- DNA is susceptible to ultraviolet damage while RNA is relatively resistant to it.
- DNA is present in the nucleus or mitochondria while RNA is present in the cytoplasm.
DNA and RNA are made of long chains of repeating nucleotides.
Each nucleotide is made of 3 parts:
A pentose sugar
A phosphate group
One of the four types of nitrogen base
To form a strand, nucleotides are linked into chains, with the phosphate and sugar groups alternating.
DNA vs. RNA: Comparison and Explanation
1. Sugars in Nucleotides
Pentose sugar in the nucleotide of DNA is deoxyribose whereas in the nucleotide of RNA it is ribose.
Both deoxyribose and ribose are five-membered ring-shaped molecules with carbon atoms and a single oxygen atom, with side groups attached to the carbons.
Ribose is different from deoxyribose in having an additional 2’ – OH group which is lacking in the latter. This basic difference accounts for one of the mains reasons why DNA is more stable than RNA.
2. Nitrogen Bases
DNA and RNA both use a different but overlapping set of bases: Adenine, thymine, guanine, uracil, and cytosine. Although the nucleotides of both RNA and DNA contain four different bases, a clear difference is that RNA uses uracil as a base whereas DNA uses thymine.
Adenine pairs with thymine (in DNA) or uracil (in RNA) and guanine pairs with cytosine. Additionally, RNA may show non-Watson and Crick pairing of bases where guanine may also pair with uracil.
The difference between thymine and uracil is that thymine has an extra methyl group on carbon-5.
3. Number of Strands
In humans generally, RNA is single-stranded whereas DNA is double-stranded. Use of double-stranded structure in the DNA minimizes the exposure of its nitrogen bases to chemical reactions and enzymatic insults. This is one of the ways DNA protects itself from mutation and DNA damage.
Additionally, the double-stranded structure of DNA allows cells to store identical genetic information in two strands with complementary sequences. Thus should damage happen to one strand of dsDNA, the complementary strand can provide the necessary genetic information to restore the damaged strand.
Nonetheless, although the double-stranded structure of DNA is more stable, the strands must be separated to generate single-stranded DNA during replication, transcription and DNA repair.
A single-stranded RNA may form an intra-stand double helix structure such as a tRNA. Double-stranded RNA exists in some viruses.
4. Chemical Stability
The extra 2’ – OH group on ribose sugar in RNA makes it more reactive than DNA.
An -OH group carries an asymmetric charge distribution. The electrons joining the oxygen and hydrogen are distributed unequally. This unequal sharing arises as a result of high electronegativity of oxygen atom; pulling the electron towards itself.
In contrast, hydrogen is weakly electronegative and exerts less of a pull on the electron. This results in both the atoms carrying partial electric charge when they are covalently bound.
The hydrogen atom carries a partial positive charge whereas the oxygen atom carries a partial negative charge. This makes the oxygen atom a nucleophile and it can chemically react with the adjacent phosphodiester bond. This is the chemical bond that links one sugar molecule to another and thus help in forming a chain.
This is why the phosphodiester bonds linking the chains of RNA are chemically unstable.
On the other hand, the C-H bond in the DNA makes it quite stable compared to RNA.
Larges grooves in RNA are more vulnerable to enzyme attack.
RNA molecules form several duplexes interspersed with singled stranded regions. The larger grooves in RNA make it more susceptible to enzyme attack. The small grooves in the DNA helix allow minimal space for enzyme attack.
The use of thymine instead of uracil confers chemical stability to the nucleotide and prevents DNA damage.
Cytosine is an unstable base which can chemically convert into uracil via a process called “deamination”. The DNA repair machinery monitors the spontaneous conversion of uracil by the natural deamination process. Any uracil if found is converted back to cytosine.
RNA does not have such a regulation to protect itself. Cytosine in RNA can also get converted and remain undetected. But it is less of a problem because RNA has a short half-life in the cells and the fact that DNA is used for the long term storage of genetic information in almost all organisms except in some viruses.
A recent study suggests another difference between DNA and RNA.
DNA appears to be using Hoogsteen bonding when there's a protein bond to a DNA site - or if there's chemical damage to any of its bases. Once the protein is released or the damage is repaired, the DNA goes back to Watson-Crick bonds.
RNA doesn't have this ability, which could explain why DNA is the blueprint of life.
5. Thermal Stability
The 2’-OH group in RNA locks the RNA duplex into a compact A-form helix. This makes the RNA thermally more stable compared to DNA’s duplex.
6. Ultraviolet Damage
The interaction of RNA or DNA with ultraviolet radiation leads to the formation of “photo-products”. The most important of these are pyrimidine dimers, formed from thymine or cytosine bases in DNA and uracil or cytosine bases in RNA. UV induces the formation of covalent linkages between consecutive bases along the nucleotide chain.
DNA and proteins are the major targets of UV-mediated cellular damage due to their UV absorption characteristics and their abundance in the cells. Thymine dimers tend to predominate because thymine has a greater absorbance.
7. Types of DNA and RNA
DNA is of two types.
- Nuclear DNA: DNA in the nucleus is responsible for the formation of RNA.
- Mitochondrial DNA: DNA in mitochondria is called non-chromosomal DNA. It makes up 1 per cent of cellular DNA.
RNA is of three types. Each type plays a role in protein synthesis.
- mRNA: Messenger RNA carries the genetic information (genetic code for the synthesis of protein) copied from the DNA into the cytoplasm.
- tRNA: Transfer RNA is responsible for decoding the genetic message in the mRNA.
- rRNA: Ribosomal RNA forms a part of the structure of the ribosome. It assembles the proteins from amino acids in the ribosome.
There are also other types of RNA such as small nuclear RNA and micro RNA.
- DNA is responsible for the storage of genetic information.
- It transmits genetic information to make other cells and new organisms.
- RNA acts as a messenger between DNA and ribosomes. It is used to transfer genetic code from nucleus to ribosome for protein synthesis.
- RNA is the hereditary material in some viruses.
- RNA is thought to have been used as the main genetic material earlier in evolution.
9. Mode of Synthesis
Transcription makes single strands of RNA from one template strand.
Replication is a process during cell division that makes two complementary strands of DNA which can base pair with each other.
10. Primary, Secondary and Tertiary Structure
The primary structure of both RNA and DNA is the sequence of the nucleotides.
Secondary structure of DNA is the extended double helix which forms between two complementary DNA strands over their full length.
Unlike DNA, most cellular RNAs exhibit a variety of conformations. Differences in the sizes and conformations of the various types of RNA permit them to carry out specific functions in a cell.
Secondary structure of RNA result from the formation of double-stranded RNA helices called RNA duplexes. There are a number of these helices separated by single-stranded regions. RNA helices are formed with the help of positively charged molecules in the environment that balance the negative charge of the RNA. This makes it easier to bring the RNA strands together.
The simplest secondary structures in single-stranded RNAs are formed by pairing of complementary bases. “Hairpins” are formed by pairing of bases within 5–10 nucleotides of each other.
RNA also forms a highly organized and complex tertiary structure. It occurs due to folding and packing of RNA helices into compact globular structures.
Organisms With DNA, RNA and Both
DNA is found in eukaryotes, prokaryotic and cellular organelles. Viruses with DNA include adenovirus, hepatitis B, papillomavirus, bacteriophage.
Viruses with RNA are ebolavirus, HIV, rotavirus, and influenza. Examples of viruses with double-stranded RNA are reoviruses, endornaviruses, and crypto viruses.
DNA or RNA: Which Came First?
RNA was the first genetic material. Most scientists believe that the RNA world existed on Earth before modern cells arose. According to this hypothesis, RNA was used to store the genetic information and catalyse the chemical reactions in primitive organisms before the evolution of DNA and proteins.
But because RNA being a catalyst was reactive and hence unstable, later in evolutionary time, DNA took over the functions of RNA as the genetic material and proteins became the catalyst and structural components of a cell.
Although there is an alternative hypothesis suggesting that the DNA or proteins evolved before RNA, today there is enough evidence to state that RNA came first.
- RNA can replicate.
- RNA can catalyse chemical reactions.
- Nucleotides alone can act as a catalyst.
- RNA can store genetic information.
How Did DNA Arise From RNA?
Today we know how DNA like any other molecules are synthesized from RNA, so it can be seen how DNA could have become a substrate for RNA.
Brian Hall, author of Evolution: Principle and Processes, explains:
“Once RNA arose, locating the two functions of information storage/replication and protein manufacture in different but linked substances would be of selective advantage.”
This book is an interesting read if you are wondering that the above facts account for the evidences for the spontaneous generation of life and want to dig deeper into the evolutionary processes.
- Rangadurai, A., Zhou, H., Merriman, D. K., Meiser, N., Liu, B., Shi, H., ... & Al-Hashimi, H. M. (2018). Why are Hoogsteen base pairs energetically disfavored in A-RNA compared to B-DNA?. Nucleic acids research, 46(20), 11099-11114.
- Mitchell, B. (2019). Cell and Molecular Biology. Scientific e-Resources.
- Elliott, D., & Ladomery, M. (2017). Molecular biology of RNA. Oxford University Press.
- Hall, B. K. (2011). Evolution: Principles and processes. Jones & Bartlett Publishers.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Sherry Haynes