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Chemistry · Biomolecules

Biological Functions of Nucleic Acids

Nucleic acids are not merely structural curiosities of the cell nucleus; they are the molecules that carry, copy and express the instructions of life. NCERT Class XII Chemistry §10.5.3 ("Biological Functions of Nucleic Acids") and NIOS Chapter 29 §29.4.2 condense this story into two themes — DNA as the chemical basis of heredity, and the synthesis of proteins by RNA. For NEET, this subtopic is reliably mined for assertion–reason and statement-matching questions on replication, base pairing and the division of labour among mRNA, tRNA and rRNA.

The two functions, in one sentence

The particles in the nucleus of a cell that are responsible for heredity are the chromosomes, which are made up of proteins and nucleic acids. NCERT distils everything that follows into two clauses: DNA is the chemical basis of heredity and holds the coded message for proteins to be synthesised in the cell, while RNA actually carries out the synthesis. Hold these two clauses in mind — every fact in this subtopic is a footnote to one of them.

Nucleic acids are long-chain polymers of nucleotides (polynucleotides). Each nucleotide combines a nitrogenous base, a pentose sugar and a phosphate. The sugar is $\beta$-D-2-deoxyribose in DNA and $\beta$-D-ribose in RNA. DNA carries the bases adenine (A), guanine (G), cytosine (C) and thymine (T); RNA replaces thymine with uracil (U). These structural facts, covered in the sibling note on nucleic-acid structure, are the scaffolding on which the biological functions rest.

FunctionMolecule responsibleOne-line description
Storage of heredityDNAReserve of genetic information; identical strands passed to daughter cells.
Self-duplicationDNAReplication during cell division yields two identical molecules.
Carrying the message outmRNATemplate copied from DNA; leaves nucleus for cytoplasm.
Delivering amino acidstRNABrings amino acids to mRNA where peptide bonds form.
Building proteinsrRNAOne of three RNA types that carry out protein synthesis.

DNA as the chemical basis of heredity

NCERT states the central claim plainly: DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. The reason is mechanical: a DNA molecule is capable of self-duplication during cell division, and the identical DNA strands so produced are transferred to the daughter cells. Because each daughter cell receives a faithful copy, the characters of the parent are transmitted to the offspring — the chemical embodiment of heredity.

This is also why a sequence of bases on DNA is unique to an individual and stable: it is the same in every cell and cannot be altered by any known treatment. That property underlies DNA fingerprinting, used in forensic identification, determining paternity, and identifying bodies after accidents by comparing DNA with that of parents or children. The constancy of the base sequence is the practical face of "chemical basis of heredity".

"The complementary pairing of nucleotide bases explains how identical copies of parental DNA pass on to two daughter cells." — NCERT, on the Watson–Crick double helix.

Complementary base pairing

The double helix proposed by James Watson and Francis Crick resembles a gently twisted ladder: the rails are alternating units of phosphate and the sugar deoxyribose, and the rungs are each a pair of purine/pyrimidine bases. The two strands are wound about each other and held together by hydrogen bonds between specific pairs of bases. The pairing is not random — it is complementary, and the rule is fixed:

In DNAPairs withType
Adenine (A)Thymine (T)Purine–Pyrimidine, via H-bonds
Cytosine (C)Guanine (G)Pyrimidine–Purine, via H-bonds
Adenine (A) — in RNAUracil (U)Thymine replaced by uracil in RNA

Because A always faces T and C always faces G, the two strands carry the same information in mirror form — knowing one strand fixes the other completely. This single fact does double duty: it explains both how DNA is faithfully copied and how a true template for RNA can be made.

Figure 1 · Complementary base pairing 5′ 3′ 3′ 5′ A T C G T A G C A T Dashed lines = hydrogen bonds. A–T and C–G only; strands are complementary.
NEET Trap

A pairs with T, not with U — in DNA

Examiners swap thymine and uracil to test whether you remember which polymer you are in. In DNA the pairs are A–T and C–G. Uracil replaces thymine only in RNA, where adenine then pairs with uracil. Never write "A–U" for DNA or "A–T" for RNA.

Rule: DNA → A·T, C·G. RNA → A·U, C·G.

Replication: semiconservative self-duplication

Replication is the chemical process by which DNA copies itself. NIOS §29.4.2 describes the mechanism in three movements. First, the process starts with the unwinding of the two chains of the parent DNA. Second, as the two strands separate, each strand serves as a master copy for the construction of a new partner — done by bringing the appropriate nucleotides into place and linking them together. Third, because the bases must be paired in the specific manner (adenine to thymine, guanine to cytosine), each newly built strand is complementary to the old one, not identical to it.

When replication is completed there are two DNA molecules, each identical to the original. Crucially, each new molecule is a double helix that has one old strand and one new strand. This is exactly what "semiconservative" means: half of each daughter molecule is conserved from the parent. The old strands are then transmitted to the daughter cells.

Figure 2 · Semiconservative replication fork Parent DNA new old old new Daughter 1 Daughter 2 Each daughter helix keeps one old (dark) strand + one new (teal) strand → semiconservative.
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Build the foundation first

Replication and base pairing only make sense once you know the sugar–phosphate backbone and the four bases. Review Nucleic Acids — DNA & RNA for the structural groundwork.

Transcription and the central flow

The second great function is protein synthesis. NCERT is careful about the division of labour: the proteins are synthesised by various RNA molecules in the cell, but the message for the synthesis of a particular protein is present in DNA. The transfer of that message begins with transcription. In NIOS's words, "the information from DNA is transmitted to another nucleic acid called messenger RNA, which leaves the nucleus and goes to the cytoplasm of the cell." DNA does not leave the nucleus; mRNA is its mobile messenger.

The same complementary base-pairing rule that governs replication governs this copying step — the mRNA sequence is read off the DNA template. The overall flow of information, as the chemistry text presents it, can be written compactly:

Figure 3 · Information flow DNA mRNA Protein transcription translation (coded message) (mRNA + tRNA + rRNA)

Translation: mRNA, tRNA and rRNA

Once the messenger RNA reaches the cytoplasm, it acts as a template for the incorporation of amino acids in the proper sequence in the protein. The amino acids do not arrive on their own: they are brought to the messenger RNA by transfer RNA (tRNA), and at the template they are joined by peptide bonds. NCERT names three types of RNA — messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA) — and states that these three "actually carry out the protein synthesis in the cell."

The peptide bond that links the amino acids is the same amide linkage you met in the proteins note; here it is being formed in the order dictated by the mRNA template. So the chemistry of protein structure and the chemistry of nucleic-acid function meet at exactly this step.

RNA typeAbbreviationRole in protein synthesis
Messenger RNAmRNACarries the coded message from DNA out of the nucleus; acts as the template for amino-acid sequence.
Transfer RNAtRNABrings the amino acids to the mRNA, where peptide bonds form.
Ribosomal RNArRNAOne of the three RNA types that together carry out the synthesis of protein in the cell.
NEET Trap

DNA stores; RNA builds

A favourite statement-pair asks whether DNA or RNA synthesises proteins. The NCERT-correct framing: DNA contains the coded message for protein synthesis, whereas RNA actually carries out the synthesis. Saying "DNA synthesises proteins directly" is the trap — the message is in DNA but the work is done by the RNA molecules.

Rule: DNA = blueprint (message); RNA = workforce (synthesis).

The genetic code

What exactly is the "message" in DNA? It is a specific sequence of bases. As NIOS puts it, "the specific sequence of bases in DNA represents coded information for the manufacture of specific proteins." That coded relationship between base sequence and the amino-acid sequence of a protein is the genetic code. The four-letter alphabet of bases is translated, through mRNA, into the twenty-letter alphabet of amino acids.

NCERT marks the chemical significance of this with a Nobel citation: Har Gobind Khorana shared the 1968 Nobel Prize for Medicine and Physiology with Marshall Nirenberg and Robert Holley "for cracking the genetic code." For NEET chemistry, the examinable points are conceptual — that the code lives in the base sequence and that Khorana is the name associated with it — rather than the codon tables of biology.

Worked Example

A single DNA strand reads 3′–T A C G G A–5′. Write the base sequence of the complementary DNA strand and of the mRNA transcribed from this template.

Complementary DNA strand: apply A·T and C·G → 5′–A T G C C T–3′.

mRNA from the template (T→U): read the template 3′–TACGGA–5′ with A·U, C·G → 5′–A U G C C U–3′. Note the uracil in place of thymine — the single chemical signature of RNA.

DNA vs RNA: roles compared

Because NEET so often tests the two molecules side by side, it is worth tabulating their functional contrast as the chemistry text frames it. Structure (single vs double strand, sugar, fourth base) feeds directly into function (storage vs synthesis).

PropertyDNARNA
Pentose sugar$\beta$-D-2-deoxyribose$\beta$-D-ribose
StrandsDouble helixSingle strand (may fold back on itself)
Fourth baseThymine (T)Uracil (U)
Primary roleChemical basis of heredity; reserve of genetic informationCarries out protein synthesis
Self-duplicationYes (semiconservative replication)Not the storage molecule; transcribed from DNA
Sub-typesOne (the genome)Three: mRNA, tRNA, rRNA
Quick Recap

Biological Functions of Nucleic Acids — in one pass

  • DNA is the chemical basis of heredity and the reserve of genetic information; it maintains species identity over millions of years.
  • DNA self-duplicates during cell division by semiconservative replication: strands unwind, each templates a complementary partner, giving two helices each with one old + one new strand.
  • Base pairing is fixed: A–T and C–G via hydrogen bonds in DNA; A–U in RNA.
  • The message for a protein lives in DNA's base sequence (the genetic code; Khorana, Nobel 1968).
  • Protein synthesis: mRNA carries the message out of the nucleus and templates the amino-acid order; tRNA delivers amino acids; the three RNAs (mRNA, tRNA, rRNA) carry out synthesis. Amino acids join by peptide bonds.
  • Mantra: DNA stores and copies; RNA builds.

NEET PYQ Snapshot — Biological Functions of Nucleic Acids

Real NEET items from the Biomolecules bank that touch nucleic-acid identity and function; concept cards fill the rest.

NEET 2016 · Q.38

The correct statement regarding RNA and DNA, respectively, is:

  1. The sugar component in RNA is ribose and the sugar component in DNA is 2-deoxyribose
  2. The sugar component in RNA is arabinose and the sugar component in DNA is ribose
  3. The sugar component in RNA is 2′-deoxyribose and the sugar component in DNA is arabinose
  4. The sugar component in RNA is arabinose and the sugar component in DNA is 2′-deoxyribose
Answer: (1)

RNA contains ribose; DNA contains 2′-deoxyribose. This sugar difference underlies the functional split: ribose-based RNA does the synthesis work, deoxyribose-based DNA is the stable heredity store.

NEET 2023 · Q.71

Statement-I: A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside. Statement-II: When nucleoside is linked to phosphorous acid at 5′-position of sugar moiety, we get nucleotide. Choose the correct answer.

  1. Statement-I is false but Statement-II is true
  2. Both Statement-I and Statement-II are true
  3. Both Statement-I and Statement-II are false
  4. Statement-I is true but Statement-II is false
Answer: (4)

Statement-I is correct. Statement-II is false: a nucleotide is formed when a nucleoside is linked to phosphoric acid (not phosphorous acid) at the 5′-position. Nucleotides are the monomers whose polymer — the polynucleotide — performs every function above.

Concept

Which RNA leaves the nucleus to act as the template for the sequence of amino acids in a protein?

Answer: Messenger RNA (mRNA)

DNA holds the coded message but stays in the nucleus; mRNA is transcribed from it and travels to the cytoplasm to template synthesis. tRNA then ferries the amino acids in.

Concept

Why is DNA replication described as semiconservative?

Answer: Each daughter helix retains one parental strand + one new strand

The two parent strands unwind and each templates a complementary new strand. Half of every daughter molecule is conserved from the parent — hence "semiconservative".

FAQs — Biological Functions of Nucleic Acids

The questions NEET aspirants ask most about how DNA and RNA do their jobs.

Why is DNA called the chemical basis of heredity?
DNA is the reserve of genetic information and is exclusively responsible for maintaining the identity of different species of organisms over millions of years. A DNA molecule is capable of self-duplication during cell division, and identical DNA strands are transferred to daughter cells, so the characters of parents are transmitted to offspring.
What does it mean that DNA replication is semiconservative?
During replication the two chains of the parent DNA unwind; each separated strand serves as a master copy on which a new complementary strand is built. When replication is complete there are two DNA molecules, each a double helix containing one old strand and one new strand. Because half of each daughter molecule is conserved from the parent, the process is called semiconservative.
Which base pairs with which during base pairing in DNA?
Pairing is specific and complementary: adenine forms hydrogen bonds with thymine (A–T) and cytosine forms hydrogen bonds with guanine (C–G). In RNA, thymine is replaced by uracil, so adenine pairs with uracil (A–U).
What are the roles of mRNA, tRNA and rRNA in protein synthesis?
The coded message for a specific protein is present in DNA. The information is transmitted to messenger RNA (mRNA), which leaves the nucleus and acts as a template for incorporating amino acids in the proper sequence. Transfer RNA (tRNA) brings the amino acids to the mRNA, where peptide bonds form. Ribosomal RNA (rRNA) is one of the three RNA types that actually carry out protein synthesis in the cell.
What is the genetic code in the NCERT chemistry context?
The specific sequence of bases in DNA represents coded information for the manufacture of specific proteins. This coded message is the genetic code. Har Gobind Khorana shared the 1968 Nobel Prize in Medicine and Physiology with Marshall Nirenberg and Robert Holley for cracking the genetic code.
How does protein synthesis link DNA, RNA and amino acids?
Proteins are synthesised by various RNA molecules, but the message for a particular protein is present in DNA. In short, DNA contains the coded message for protein synthesis whereas RNA actually carries out the synthesis: mRNA carries the message out of the nucleus, tRNA delivers the amino acids, and the amino acids are joined by peptide bonds in the sequence dictated by the mRNA template.
Related Topics
Biomolecules — Chapter Hub All NEET biomolecules subtopics in one place. Nucleic Acids — DNA & RNA Bases, sugars, nucleosides, nucleotides and the double helix. Protein Structure Levels Primary to quaternary — the proteins that synthesis builds. Amino Acids The monomers tRNA delivers and peptide bonds join. Protein Denaturation What happens when a synthesised protein loses its shape. Enzyme Mechanism Biocatalysts — the working face of many proteins.