Difference between DNA and RNA, Practice Problems and FAQs

Have you watched any of the ‘Jurassic Park’ movies? The movies revolve around an amusement park which houses dinosaurs that had become extinct millions of years ago but were recreated using scientific technology by a bunch of brilliant scientists. All of that could be possible because the scientists could extract a piece of dinosaur DNA from a fossilised mosquito which might have once sucked on dinosaur blood. Although the premise of the movie is very far-fetched, it correctly portrays the significance of one of the most abundant and most important biomolecules on this planet, that is DNA. Do you know what this DNA is?

DNA or deoxyribonucleic acid is our genetic material. In fact, it serves as the genetic material for all living beings except for a few viruses. All the characters that we possess, the ones that we share with our family and also the ones that make us unique amongst 7.9 billion human beings, are determined by a unique sequence of molecules in the DNA which is referred to as the genetic code.

DNA is a polynucleotide molecule that combines with proteins to form the chromosomes that help to transfer genetic information from one generation to the other. But did you know that extremely primitive and simple organisms such as certain viruses do not have DNA as their genetic material? They have a simpler but fairly unstable molecule known as RNA or ribonucleic acid as their genetic material. Although RNA is not the genetic material for most living organisms, it is still required by the cells to translate the genetic code carried by the DNA into proteins which influence the characteristics of the cell. So how are DNA and RNA different from each other?

Let’s discuss some main differences between these two molecules in this article.

Table of contents

• Nucleic acids
• DNA vs RNA
• Significance of DNA
• Significance of RNA
• Practice Problems
• FAQs

Nucleic acids

Nucleic acids are described as the polymeric compounds made up of long chains of nucleotides, which serve as the monomers. Thus, they are also known as polynucleotides. They are involved in the inheritance and transmission of genetic information from one generation to another. Each nucleotide consists of three components: a nitrogen-containing ring structure known as a nitrogenous base, a five-carbon sugar, and one phosphate group.

                                             Fig: Nucleotide

Types of nucleic acids

There are mainly two types of nucleic acids:

• Deoxyribonucleic acids (DNA)
• Ribonucleic acids (RNA)

Except for some viruses, which have RNA as their genetic material, all living organisms have DNA as their genetic material.

                                                        Fig: Types of nucleic acid

DNA

The structure of B DNA was given by Watson and Crick. It is considered the most stable form of DNA. It has a double-helix structure, meaning two polynucleotide chains are wrapped around one another in a helical pattern.

                 Fig: Double helix structure by Watson and Crick

Each polynucleotide chain is made up of repeating units of deoxyribonucleotides each of which in turn are made up of a deoxyribose sugar having a nitrogenous base (adenine, guanine, cytosine or thymine) attached to its 1^st carbon molecule and a phosphate group attached to its 5^th carbon molecule. The deoxyribonucleotides in a chain are connected by phosphodiester linkages between their 3'OH groups and the 5' phosphate groups of the nucleotides next to them.

                               Fig: Phosphodiester bond between nucleotides

The polynucleotide chain has a sugar residue at one end with a free 5' carbon joined to an unlinked phosphate group. This end is known as the 5’ end. A free 3' carbon is present at the other end of a sugar residue that is connected directly to an unlinked -OH group. This end is known as the 3’ end.

                                            Fig: 5’ and 3’ ends of DNA double helix

As the two polynucleotide chains are orientated in opposite directions to one another, with the 3' end of one polynucleotide chain being opposite the 5' end of the other, the two DNA strands are said to be antiparallel.

                          Fig: Antiparallel polynucleotide chains of DNA

Two strands of the polynucleotide are joined together by hydrogen bonds that are present between the nitrogenous bases. A purine is always paired with a pyrimidine that is complementary to it. As a result, the two strands of the helix are separated by a nearly constant distance, and the strand's diameter remains constant at 20 Å. Adenine pairs with thymine with two hydrogen bonds and cytosine pairs with guanine using three hydrogen bonds.

                        Fig: Hydrogen bonds with different nitrogenous bases

The DNA backbone is composed of pentose sugar, phosphate, and the nitrogenous bases are projected inside to form a staircase-like structure. In a double helix, each helical turn has an ascent of 36 degrees. One complete turn of the helical strand involves ten base pairs. Every base pair is separated by 3.4Å, or 0.34nm. As a result, the length of one full turn, or the pitch of the DNA helix, is 3.4 nm, or 34Å.

                             Fig: Structure of DNA helix

DNA is categorised into five major types:

• A-DNA
• B-DNA
• C-DNA
• D-DNA
• Z-DNA

                                                     Fig: Types of DNA

A-DNA

It has the widest helical diameter when compared to other DNA forms and is a right-handed double helix identical to the B-DNA form. It exists at a relative humidity of 75%. It exists as a right-handed helix that contains 11 nucleotide pairs per turn and has a diameter of 2.3 nm and a pitch of 3.4 nm.

B-DNA

The majority of DNA has the B-form, which is the most prevalent form at neutral pH and physiological salt concentrations. There are 10 base pairs present per round of the helix axis. The helical diameter of B-DNA is 20Å or 2 nm. Watson-Crick’s double helix model is referred to as a B-form of DNA.

C-DNA

It is seen with a few ionic species, such as lithium(Li⁺), and with a relative humidity of 66%. For each turn, there are 9.33 base pairs. The helix has a diameter of around 19Å and a vertical rise of 3.32Å for each base pair in the right-handed helix.

D-DNA

It is a rare and extreme version of DNA. With an axial rise of approximately 3.03Å, the 8 base pairs are inclined negatively from the helix axis.

Z-DNA

The Z-DNA is a left handed helix. The sugar-phosphate linkage occurs in several forms, which results in a zigzag pattern. It is present where there is an abundance of salt concentration. The Z-DNA helix has a diameter of 1.8nm, pitch of 4.5nm and has 12 base pairs per turn.

RNA

RNA is described as a single-stranded structure that is composed of a single polynucleotide chain. This single polynucleotide chain is often folded to form helices. The ribonucleotides in RNA are joined by 3’-5’ phosphodiester bonds. A ribose sugar with a nitrogenous base (adenine, guanine, cytosine, or uracil) attached to its 1^st carbon and a phosphate group attached to its 5^th carbon are the components of a ribonucleotide monomer.

The RNA molecule is categorised into four types on the basis of their functions:

• messenger RNA or mRNA
• transfer RNA or tRNA
• ribosomal RNA or rRNA
• Small nuclear RNA or snRNA

These RNA forms are transcribed from various parts of the DNA strand, and their sequences are complementary to the DNA strand from which they are synthesised. However, in RNA, uracil takes the place of thymine. The DNA's 3'–5' strand, which serves as a template for RNA production, is known as the template strand.

mRNA

mRNA aids in the movement of chromosomal DNA's genetic information from the nucleus to the cytoplasm, where it serves as a template for the synthesis of proteins. It has a DNA sequence that is complementary to the template strand of the DNA from which it is synthesised and is linear in form. mRNA makes up about 5% of the cell's total RNA.

                                                             Fig: mRNA

rRNA

Out of the total RNA content of a cell, rRNA forms 80% composition and originates from nucleolar DNA. It is located in the ribosomal subunits in close proximity to proteins and is strongly coiled. Ribosomal subunit production is assisted by rRNA.

tRNA

tRNA constitutes 15% of the total RNA content of a cell. These are the tiniest RNA molecules and are found in 60 different varieties. Each type of tRNA binds to a specific type of amino acid at its 3’ end. The amino acid is transported from the cytoplasm to the mRNA for protein synthesis. The CCA sequence at the 3'end serves as the location where amino acids are attached. On the other hand, the 5’ end of tRNA terminates with a guanine base. Despite being single-stranded, tRNA has a clover leaf shape due to its numerous complementary sequences and pairing regions. One molecule of tRNA has four different regions:

• Amino acid recognition site or dihydroxy uracil loop (DHU)
• Amino acid binding site at 3’ end
• Ribosome recognition site or TψC loop where T is ribothymidine, ψ is pseudouridine and C is ribocytidine.
• Codon recognition site or anticodon loop

                             Fig: Structure of tRNA

snRNA

This type of RNA is used to form complexes with the help of proteins that are used in RNA processing in eukaryotes.

DNA vs RNA

The differences between DNA and RNA are given below:

Significance of DNA

DNA molecules are macromolecules and are very long strands of nucleotides. Without being tightly packed, these long and narrow structures cannot fit into a cell. Therefore, DNA strands are tightly wrapped around proteins to form chromosomes. Chromosomes are inherited from the parents during reproduction. In sexually reproducing organisms, 50% of the chromosomes are inherited from the male parent and the other 50% are inherited from the female parent. As these chromosomes contain DNA, they help in transfer of genetic information from one generation to another.

The specific sequence of nitrogenous base pairs of the nucleotides in the DNA forms the genetic code. A specific segment of DNA having a specific nucleotide sequence that codes for a specific protein forms a gene which is the unit of heredity. The genetic code carried by the DNA determines every characteristic of our body by expressing it in the form of proteins. It controls our metabolic rate, growth rate, eye colour, hair colour, etc.

During gamete formation and gamete fusion, there is reshuffling of the DNA in the chromosomes, which leads to inheritance of genetic information with slight variations. Variations bring about individuality and increased adaptability in every organism.

The nucleotide sequence of DNA is susceptible to sudden inheritable changes called mutations which is the prime source of variation.

Significance of RNA

RNA is primarily responsible for decoding the genetic information present on DNA and converting this information into proteins. This occurs in ribosomes and is an essential process because proteins play a critical role in the functioning of living organisms. Moreover, proteins act as a catalyst in various biochemical reactions occurring in the living system. These biochemical reactions occur inside the cells because they maintain the structure and regulate the rate of internal functioning. These processes include photosynthesis in plants and respiration and other processes in animals. In general, the production of protein is essential for life forms to function properly.

Practice Problems

1. From the given options, based on which property DNA and RNA can be differentiated?

1. Nitrogen bases and sugars
2. Nitrogen bases and phosphate groups
3. Number of C-atoms in sugars
4. Sugar and phosphate groups

Solution: Nucleic acids are described as the polymers made up of long chains of nucleotide monomers, and therefore, they are also known as polynucleotides. Each nucleotide consists of three components: a nitrogen-containing ring structure known as a nitrogenous base, a five-carbon pentose sugar, and one phosphate group. The nitrogenous bases found in DNA are adenine, guanine, cytosine and thymine. In RNA, thymine is replaced by uracil while the other three nitrogenous bases remain the same. Also, the pentose sugar present in DNA is the deoxyribose sugar whereas in RNA it is ribose sugar. Hence, the correct option is a.

2. RNA is synthesised from DNA in which of the following parts of the cell?

1. Cytoplasm
2. Nucleus
3. Chromosomes
4. All of the above

Solution: The process of synthesis of RNA from a DNA template is called transcription and this process takes place in the nucleus. Hence, the correct option is b.

3. Purines that are present in both DNA and RNA are

1. cytosine and thymine
2. adenine and thymine
3. adenine and guanine
4. guanine and cytosine

Solution: DNA and RNA are the polymers of nucleotides. A nucleotide is composed of three components namely, a pentose sugar, a nitrogenous base and a phosphate group. Nitrogenous bases are of two types in DNA and RNA and these are purine and pyrimidines. The purines are adenine and guanine which are present in both DNA and RNA. The pyrimidines found in DNA are cytosine and thymine and that in RNA are cytosine and uracil. Hence, the correct option is c.

4. How many base pairs are present in one complete turn of the DNA structure explained by Watson and Crick?

1. 11
2. 10
3. 12
4. 9

Answer: The structure of B DNA was given by Watson and Crick. It is considered the most stable form of DNA. It has a double-helix structure, meaning two polynucleotide chains are wrapped around one another in a helical pattern. The two strands of the helix are separated by a nearly constant distance, and the strand's diameter remains constant at 20 Å. The DNA backbone is composed of pentose sugar, phosphate, and the nitrogenous bases are projected inside to form a staircase-like structure. One complete turn of the helical strand involves ten base pairs. Every base pair is separated by 3.4Å, or 0.34nm. As a result, the length of one full turn, or the pitch of the DNA helix, is 3.4 nm, or 34Å. Thus, the correct answer is option b.

FAQs

1. Why DNA is better genetic material as compared to RNA?

Answer: Compared to RNA, DNA is more physically stable and less chemically reactive. Therefore, DNA is considered a better genetic material.

2. Which type of RNA is short-lived?

Answer: mRNA is short-lived RNA. This is because mRNA is a temporary structure that is created during protein synthesis and exits the nucleus just to transport the genetic message from the nucleus to the cytoplasm. It quickly degrades after being translated into proteins.

3. How much percent of DNA is similar in all humans?

Answer: Approximately 99.9% of DNA is similar in all humans. This means that all humans are 99% genetically identical. Only 0.1% of DNA is different which makes an individual.

4. Why is the structure of DNA twisted in shape?

Answer: The nucleus contains a densely packed double-helical structure called DNA. It is composed of phosphate molecules, nitrogenous bases, and deoxyribose sugars. Due to the interaction between the water and DNA molecules, the form is twisted. The packing of DNA into chromatin fibres is made easier by the twisted structure. Protein synthesis and DNA replication is also made easier by it.

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