Nucleic Acids (DNA, RNA) – Definition, Function, Examples

Nucleic acids are the biomolecule that are formed in the nuclei of all living cells in form of nucleoprotein or chromosome. :- Definition

The nucleic acids are complex molecules having very high molecular weight. They are so named because they mainly occur in the nucleus. They also occur in the cytoplasm, but not in appreciable amount.

Nucleic acids were discovered in 1869 by a Swiss biochemist, Friedrich Miescher. Miescher called them nuclein. Later on, they were called nucleic acid due to their acidic properties.

Chemical Nature of Nucleic Acid

Nucleic acids are compounds of carbon, oxygen; hydrogen, nitrogen, and phosphorus which are variously combined to form sugars, phosphates, and nitrogen bases. The nucleic acids are in the form of long chains like proteins. They are made up of a large number of smaller units called nucleotides.

A nucleotide as shown in the figure below is formed of a molecule of pentose sugar, a nitrogen base (maybe purine or pyrimidine), and phosphate.

The nucleotides on hydrolysis (by an enzyme called phosphatase) give rise to nucleosides. Finally, a nucleoside is split into sugar and base by an enzyme called nucleosidase.

structure of nucleotide
Structure of nucleotide

It can be represented as:

Nucleic acid = Many nucleotides.

Nucleotide = Phosphate + Sugar + Nitrogen base.

Nucleoside = Nitrogen base + Sugar.

Examples of Nucleic Acid

The nucleic acids are of two types:

  • Deoxyribonucleic acid or in short DNA
  • Ribonucleic acid or in short RNA, depending on the type of sugar they possess.

Function of Nucleic Acid

Nucleic acids are the biomolecule that is formed in the nuclei of all living cells in form of nucleo-protein or chromosome.

  1. These are responsible for transmission of hereditary characters from one generation to another.
  2. Responsible for synthesis of protein needed for the growth and maintenance of our body.
  3. Investigation on the basis of DNA fingerprinting is done to catch criminals.
  4. To study biological evolution and genetics.

You can know more about Protein Synthesis from here.

Let us now know about the examples of nucleic acid that is DNA and RNA in more detail.

What is DNA?

Deoxyribonucleic acid, commonly known as DNA, is a complex molecule that contains all of the information that are necessary to build and maintain an organism.

Deoxyribonucleic acid is found in the nucleus and mainly in the chromosomes. The sugar present in it is deoxyribose.

deoxyribose
Deoxyribose, where one oxygen atom is less than ribose

Four different types of nitrogen bases (in the figure below) are found in the DNA molecule which are grouped based on their ring structure into two categories: Purine and Pyrimidine.

types or nucleotide in DNA molecule
Types or nucleotide in DNA molecule

Purine and Pyrimidine are nitrogen bases. Purine includes adenine (A) and guanine (G) bases and pyrimidine includes cytosine (C) and thymine (T) bases. These bases always occur in pairs. As a rule, one base of purine pairs with one base of pyrimidine. Phosphate is also found.

Let us know about DNA, its structure and function in detail.

Structure of DNA Molecule (Watson and Crick model)

The structure of DNA molecule was proposed by James D. Watson (Fig. 36.4) and Francis H. C. Crick in 1953 which is widely accepted. For this work, they were awarded the Nobel prize in 1962.

According to them, DNA is composed of two long chains or strands coiled around each other to form a double-helical structure. Two chains run in opposite directions. In each chain deoxyribose, sugar and phosphate are present alternately.

In two strands, the nitrogen base pairs are linked to the deoxyribose sugar. The base pairs are linked to each other by hydrogen bonds.

Diagram of DNA molecule
1. Diagrammatic representation of a small portion of DNA molecule- seen as a flexible ladder | 2. The molecular fragment assuming its shape with single twist | 3. With double twist

Adenine (A) is always linked with thymine (T) by a double hydrogen bond (A=T) and cytosine. (C) is always linked with guanine (G) by a triple hydrogen bond (C≡G).

The bases may be arranged in pairs in any sequence, for example:- (T=A) or (G≡C).

The distance between two chains i.e., diameter is about 20 Å and the distance between the base pairs is 3.4 Å. Each chain completes one spiral per ten nucleotide molecules.

The species variation is dependent upon the different arrangements of base pairs in DNA. A double-helical structure can be achieved by imagining a flexible ladder twisted twice.

The uprights of such ladder are made entirely of the phosphate and sugar of the nucleotide while the ‘rungs’ are made only of the purine and pyrimidine bases.

Double helical structure of DNA
Double Helical Structure of DNA | 1. Watson and Crick Model | 2. A portion magnified showing the arrangement of base pairs | 3. Key to symbols of the parts of a nucleotide

What is Replication of DNA?

This means that DNA duplicates itself or in other words, the DNA molecule produces a new DNA molecule that is exactly like the original in quantity and quality. The process of making an exact copy is called replication.

This is necessary to maintain the constancy in the amount and chemical nature of DNA in similar types of cells from generation to generation. This replication takes place when the maximum growth is attained by every cell and the cell becomes ready for division.

From the helical structure of DNA, it is clear that adenine can pair only with thymine and cytosine with guanine.

So the number of adenine and cytosine is equal to the number of thymine and guanine respectively. Their combination and arrangement may be in any sequence and may be repeated several times.

If the nucleotides of one strand remain fixed, naturally the order of nucleotides can be determined in another strand.

Mechanism of Replication or Duplication of DNA

As stated earlier, two strands of the double helix are connected with each other by weak hydrogen bonds. Less amount of energy is necessary to break up the bondage.

First of all, two strands become separated like a zipper by the process of uncoiling or unwinding at the point of hydrogen bonds. The nucleotides which are building blocks of DNA molecules are present numerously in the cell.

DNA Replication diagram
DNA Replication Diagram

Therefore, the proper nucleotides are picked up by the parent (original) strand to form the daughter (new) strand. Here the parent strand serves as a template for the synthesis of a new companion chain to form two molecules of DNA.

This happens when the DNA helix uncoils. In this way, gradually uncoiling continues, and simultaneously the formation of new strand proceeds.

At last, two double-stranded structures of DNA are produced, held by hydrogen bonds (i.e., one double-strand consisting of one parent strand and the other one daughter strand).

In this manner, the process of DNA replication is completed and each new nucleic acid molecule is exactly like the original one i.e., the mirror image of the other.

But the question is how two strands of DNA helix are uncoiled?

According to the modern accepted view, double strands are separated from each other by the influence of an enzyme called DNA polymerase in the presence of magnesium ions. This mechanism is known as semi-conservative replication.

Functions of DNA:

  • DNA molecules are responsible for transmitting hereditary characters from generation to generation. Therefore DNA is often called the secret of life.
  • They have got the power of duplicating themselves.
  • Provide necessary information for protein synthesis.
  • DNA is responsible for the synthesis of RNA.
  • They store all the pieces of information related to life and control almost all of the life activities of the cell directly or indirectly.

What is RNA? Ribonucleic Acid

Ribonucleic acid occurs chiefly in the cytoplasm but a little also occurs in the nucleus. The sugar present in it is ribose instead of Deoxyribose.

ribose sugar diagram
Ribose Sugar Diagram

The nitrogen bases are the same as DNA except that uracil (U) is present in place of thymine (T). The RNAs are also composed of nucleotides.

The nucleotides are arranged in a single chain. But the single-chain is folded in the middle and twisted around itself like a double helix.

In the folded region, a majority of bases are complementary and are joined by hydrogen bonds. This helps in the stability of molecules.

Types of RNA

According to their origin and function, three types of RNA are found in the cell. They are (1) ribosomal RNA (rRNA) (2) messenger RNA (mRNA) and (3) transfer RNA (t-RNA). Ribosomal and transfer RNAs comprise about 98% of all RNA.

All three forms of RNA are made on a DNA template, t-RNA and mRNA are synthesized on a DNA template of the chromosome while rRNA is derived from nucleolar DNA.

Ribosomal RNA or rRNA

It is found in the ribosomes of the cell and about 60 to 80% of the cellular RNA is found in the ribosomal particles.

The rRNA is synthesized in the nucleolus. Ribosomes are the sites for protein synthesis. The exact function of this type of RNA is not known.

The synthesis of rRNA begins at gastrulation and increases as embryonic development proceeds. It consists of a single strand twisted upon itself in some regions.

It has helical regions where most of the base pairs are complementary and joined by hydrogen bonds. rRNA is stable for at least two generations.

ribosomal RNA
Ribosomal RNA or rRNA

Messenger RNA or mRNA

This type of RNA is short-lived and is synthesized in the nucleus. It constitutes about 10% of the total RNA in the cell. It is formed from a DNA template or fragment during replication in the presence of an enzyme, called RNA polymerase.

Because the single chain of m-RNA is complementary in base sequence to one of the chains of DNA, it varies greatly in length and molecular weight.

Since most proteins contain at least a hundred amino acid residues, m-RNA must have at least 300 nucleotides on the basis of the triplet code.

m-RNA is always single-stranded and there is no base pairing. m-RNA combines with ribosomes to form polyribosomes. Synthesis of mRNA occurs during Cleavage.

The mRNA molecule is considerably smaller in size than the DNA strand. It is so named because it acts as a chemical messenger during protein synthesis between the information-carrying DNA and the ribosomes where proteins are manufactured.

Transfer RNA or tRNA

This is also called soluble RNA or acceptor RNA. Transfer RNA constitutes about 10% to 20% of the total RNA of the cell. A molecule of t-RNA is much smaller in size than the rRNA and mRNA. It is a single polynucleotide chain but the chain is folded on itself which appears to be a double helix.

Each molecule is composed of 75-80 nucleotides. t-RNA is synthesized in the nucleus on a DNA template. Synthesis of t-RNA occurs near the end of the cleavage stage.

transfer RNA
Transfer RNA or tRNA

The number of t-RNA always corresponds to the number of amino acids present inside the cell. Each t-RNA molecule has got two ends namely the carrier end and recognition end.

The part of the t-RNA molecule which becomes attached to its particular amino acid is the carrier end. The other end-bearing anticodon which recognizes the triplet nucleotide or codon of the template mRNA is called the recognition end.

The function of each t-RNA is to pick up only one specific amino acid from the cytoplasm and combines with the same and transports the amino acid to a ribosome during protein synthesis.

General Functions of RNA

All types of RNA play their important role in protein synthesis.

Know more about the function of RNA in relation to the types of RNA from the comparison table below.

Comparison of Different Types of RNA

Basis of ComparisionRibosomal RNA
(r-RNA)
Messenger RNA
(m-RNA)
Transfer RNA
(t-RNA)
1. Percentage of total RNA of the cell.60% to 80%
of total RNA
10% of
total RNA
10% to 20%
of total RNA
2. Number of Nucleotides.120-5500 nucleotidesin E. coli 900-1500
nucleotides
75-80 nucleotides
3. Molecular weight1.1 x 106500,00025,000-30,000
4. Site of SynthesisDerived from nucleolar DNASynthesized in nucleus
on DNA templates.
Synthesized in nucleus
on DNA templates.
5. Beginning of synthesisBegins at gastrulation and increases as development
proceeds.
New m-RNA synthesized
during early cleavage
Occurs at the end of cleavage stage.
6. Base relationship to DNANo obvious relationship to
DNA. r-RNA is formed from
only small section of DNA.
m-RNA shows base relationship to DNA. It is
formed from all sections
of DNA
Same as in r-RNA
7. FunctionNot exactly known. But
believed that unpaired bases may bind m-RNA and t-RNA to ribosomes.
Conveys genetic information from DNA of
chromosomes to the ribosomes where it takes
part in protein synthesis.
Adaptor for attaching amino acids to m-RNA
template.

Comparison between DNA and RNA

Similarities between DNA and RNA

  • DNA and RNA are composed of purine bases namely adenine and guanine.
  • In both, pyrimidine base namely cytosine is also present.
  • Both types of nucleic acid contain phosphate.
  • The sugar is pentose in both types of nucleic acid.

Differences between DNA and RNA

  • The sugar of DNA is deoxyribose but in RNA the sugar is ribose.
  • The pyrimidine bases of DNA are thymine and cytosine but in RNA, the bases are uracil and cytosine.
  • DNA molecule is double-stranded but RNA molecule is single-stranded.
  • DNA is present in the chromosomes but RNA is located mainly in the cytoplasm and also in the nucleolus in little amount.
  • DNA carries genetic information but RNA plays its role in the synthesis of proteins.
  • DNA molecule produces new DNA molecules by replication but RNA molecules are produced from DNA molecules.

KEYWORDS TO REMEMBER

  • Nitrogenous base: A base containing nitrogen.
  • Nucleic acids: Macromolecules containing chains of nucleotide.
  • Nucleoside: Compound formed by sugar (with five carbon atoms) and a nitrogenous base.
  • Nucleotide: Compound formed by sugar (with five carbon atoms), Phosphoric acid, and a nitrogenous base.
  • Replication: Production of exact copies of complex molecules,

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