NCERT Solutions · Class 12 Chemistry Biomolecules

A nucleic acid is the polymer that stores and reads out hereditary information — a long chain built from repeating nucleotide units, not a single molecule with one fixed formula.Every nucleotide, the repeating unit, is itself built from three pieces joined together: The base attaches to the sugar's $\displaystyle 1$' carbon by an N-glycosidic bond to give a base + sugar unit called a nucleoside (for example adenine + ribose = adenosine). Adding the phosphate at the $\displaystyle 5$'-OH of that nucleoside gives the nucleotide (for example adenosine $\displaystyle 5$'-monophosphate). Nucleotides then link into a chain: the $\displaystyle 5$'-phosphate of one nucleotide's sugar forms a phosphodiester bond to the $\displaystyle 3$'-OH of the next sugar, and this repeats down the chain — sugar-phosphate-sugar-phosphate — with one base hanging off each sugar. That sugar-phosphate backbone with bases attached is the polynucleotide, and a very long polynucleotide is the nucleic acid.Two nucleic acids exist, distinguished by the sugar and by which pyrimidine they carry: Their two important functions:1. Storage and transmission of hereditary information. The specific sequence of the four bases along a DNA strand is the genetic code for an organism, and DNA replication — each strand acting as a template for a new complementary strand — copies that sequence exactly so it passes from parent cell to daughter cell, and from one generation to the next.2. Protein synthesis. Genetic information is not read directly off DNA at the ribosome; it is first transcribed into messenger RNA (mRNA), which carries the coded message to the ribosome, while transfer RNA (tRNA) reads that code three bases (a codon) at a time and delivers the matching amino acid, and ribosomal RNA (rRNA) forms the structural and catalytic core of the ribosome that links the amino acids into a polypeptide. Through this DNA to mRNA to protein flow, nucleic acids control which proteins — and therefore which enzymes and traits — a cell makes.**Answer: Nucleic acids are polynucleotides — long chains of nucleotide units (nitrogenous base + pentose sugar + phosphate group) joined by $\displaystyle 3$'-$\displaystyle 5$' phosphodiester bonds — occurring as DNA (sugar = deoxyribose; bases A, G, C, T; double helix) and RNA (sugar = ribose; bases A, G, C, U; single strand). Their two chief functions are (i) storage and transmission of hereditary information from one generation to the next via DNA replication, and (ii) protein synthesis, in which mRNA, tRNA, and rRNA together translate that stored information into specific polypeptides.

A nucleoside is only the sugar joined to the base; a nucleotide is the sugar joined to the base AND to a phosphate group. The phosphate group is what turns one into the other, and it is also what lets nucleotides link up into DNA and RNA.Start with the two pieces that both structures share:A pentose sugar — either \(\displaystyle \beta\text{-D-ribose} \) (in RNA) or \(\displaystyle \beta\text{-D-2-deoxyribose} \) (in DNA, missing the \(\displaystyle -\text{OH} \) at carbon $\displaystyle 2$).A nitrogenous base attached at carbon $\displaystyle 1$ of that sugar — one of the purines adenine or guanine, or one of the pyrimidines cytosine, thymine or uracil.Nucleoside — base + sugar, nothing else. The base is attached to carbon $\displaystyle 1$' of the sugar through a \(\displaystyle \beta\text{-N-glycosidic} \) bond (an \(\displaystyle \text{N-C} \) bond joining a ring nitrogen of the base to \(\displaystyle C1' \) of the sugar). No phosphate is present. For example, adenine joined to ribose this way is the nucleoside adenosine; cytosine joined to deoxyribose is the nucleoside deoxycytidine. In condensed form, a nucleoside is written simply as (base)–(sugar), e.g. adenine–ribose.Nucleotide — base + sugar + phosphate. A phosphate group is esterified onto the sugar, most commonly at the \(\displaystyle 5' \)-\(\displaystyle \text{OH} \) of the sugar, through a phosphoester bond (an \(\displaystyle \text{O-P} \) bond between the sugar's \(\displaystyle 5' \)-oxygen and the phosphorus of \(\displaystyle \text{PO}_4^{3-} \)). So a nucleotide is (base)–(sugar)–(phosphate), e.g. adenine–ribose–phosphate is adenosine $\displaystyle 5$'-monophosphate (AMP), and adenine–deoxyribose–phosphate is deoxyadenosine $\displaystyle 5$'-monophosphate (dAMP).Why this distinction matters structurally: the nucleotide's phosphate group carries a second reactive \(\displaystyle \text{-OH} \) that can esterify onto the \(\displaystyle 3' \)-\(\displaystyle \text{OH} \) of the sugar of the next nucleotide. This is exactly the phosphodiester linkage that repeats down the backbone of DNA and RNA — so nucleotides, not nucleosides, are the actual building blocks of the polynucleotide chain. A nucleoside, lacking the phosphate, has no way to form that second ester bond and cannot be strung into a chain.Summary in one line: nucleoside \(\displaystyle = \) base \(\displaystyle + \) sugar (joined by an N-glycosidic bond); nucleotide \(\displaystyle = \) base \(\displaystyle + \) sugar \(\displaystyle + \) phosphate (the sugar–phosphate joined by a phosphoester bond), and it is this added phosphate that lets nucleotides polymerise into nucleic acids.Answer: A nucleoside is a base linked to a pentose sugar (by an N-glycosidic bond) with no phosphate — e.g. adenosine (adenine + ribose). A nucleotide is a base linked to a pentose sugar which is in turn esterified to a phosphate group (e.g. AMP, adenine + ribose + phosphate at \(\displaystyle C5' \)) — the extra phosphate is what allows nucleotides to join into the phosphodiester-linked chain of DNA/RNA, which nucleosides alone cannot do.

Complementary means each base on one strand fixes exactly one particular base opposite it on the other strand — the two strands end up carrying different, paired sequences, not the same sequence.DNA is built from two long polynucleotide chains that wind around a common axis to form a double helix. Each nucleotide in a chain is made of a nitrogenous base — adenine (A) or guanine (G), which are purines with two fused rings, or cytosine (C) or thymine (T), which are pyrimidines with a single ring — joined to a deoxyribose sugar, which is joined to a phosphate group. The sugar-phosphate units link up to form the backbone of each strand, and the bases point inward from the backbone, toward the bases of the other strand.The two strands are held together by hydrogen bonds between the bases facing each other at every rung of this ladder-like structure, and this bonding follows one fixed rule, not a random pairing: This rule is forced by geometry. A purine (A or G) is a two-ring base and is large; a pyrimidine (T or C) is a one-ring base and is small. Only a purine-pyrimidine pair spans the fixed, uniform width of the double helix and lines up the correct hydrogen-bond donor and acceptor atoms at the right distance to form $\displaystyle 2$ (A-T) or $\displaystyle 3$ (G-C) hydrogen bonds. A purine opposite a purine (A opposite G, say) would be too wide to fit inside the helix; a pyrimidine opposite a pyrimidine (C opposite T) would leave the two bases too far apart to hydrogen-bond at all. So whatever base sits at a given rung on one strand, only one particular base can chemically and geometrically complete that rung on the other strand.Because of this rule, the base sequence of one strand completely determines the base sequence of the other, position by position: wherever strand $\displaystyle 1$ has A, strand $\displaystyle 2$ must have T at that position; wherever strand $\displaystyle 1$ has G, strand $\displaystyle 2$ must have C; and vice versa. For example, if one strand, read $\displaystyle 5$' to $\displaystyle 3$', is$\displaystyle 5$'-A-T-G-C-A-$\displaystyle 3$'then the other strand, running antiparallel to it, must read$\displaystyle 3$'-T-A-C-G-T-$\displaystyle 5$'Strand $\displaystyle 2$ is not a copy of strand $\displaystyle 1$ — it does not repeat A opposite A or T opposite T — it carries the base that pairs with (complements) each base of strand 1. The two strands are also antiparallel: one runs $\displaystyle 5$'→$\displaystyle 3$' while the other runs $\displaystyle 3$'→$\displaystyle 5$' along the same axis, which is another respect in which they are not identical copies of each other.This is exactly why the two strands are called complementary and not identical: their base sequences differ from each other, yet each sequence is completely predictable from the other because of the fixed A-T and G-C pairing rule. This complementarity is also what makes faithful DNA replication possible — each old strand serves as a template on which a new partner strand is built, one base at a time, according to the same A-T / G-C rule, so that the new double helix reproduces the original genetic information exactly.**Answer: The two strands of DNA are complementary, not identical, because base pairing between them follows one fixed geometric rule — adenine pairs only with thymine (A=T, $\displaystyle 2$ hydrogen bonds) and guanine pairs only with cytosine (G≡C, $\displaystyle 3$ hydrogen bonds), since only a purine-pyrimidine pair fits the constant width of the double helix. This means the base sequence of one strand is always exactly predictable from the other (complementary), but is a different sequence from it, not a copy (not identical); the strands are also antiparallel, running $\displaystyle 5$'→$\displaystyle 3$' and $\displaystyle 3$'→$\displaystyle 5$' in opposite directions.

The single biggest difference is one hydroxyl group on the sugar — everything else about DNA vs RNA follows from what that group does and does not allow.Structural differences1. The sugar in the backbone. DNA's repeating sugar is $\displaystyle 2$-deoxyribose, \(\displaystyle \text{C}_5\text{H}_{10}\text{O}_4 \) — a five-carbon sugar with no -OH on carbon $\displaystyle 2$'. RNA's sugar is ribose, \(\displaystyle \text{C}_5\text{H}_{10}\text{O}_5 \) — the same skeleton but with an -OH at C-$\displaystyle 2$'. That extra -OH is the reason RNA is chemically fragile: the $\displaystyle 2$'-oxygen can attack the adjacent phosphodiester bond and cleave the chain, so RNA hydrolyses easily. DNA, lacking that oxygen, cannot do this and is chemically far more stable — which is exactly why the cell trusts DNA, not RNA, to hold its genetic information for a lifetime.2. The base set. Both nucleic acids use adenine (A), guanine (G) and cytosine (C), but the fourth base differs: DNA uses thymine (T, $\displaystyle 5$-methyluracil), RNA uses uracil (U, the same ring without the $\displaystyle 5$-methyl group). So DNA's four bases are A, G, C, T; RNA's are A, G, C, U.3. Number of strands. DNA is double-stranded: two polynucleotide chains run antiparallel to each other and coil into a double helix, held together by hydrogen bonds between bases on opposite strands — A pairs with T through two H-bonds, G pairs with C through three H-bonds, so the two strands are complementary and either one can be used to reconstruct the other. RNA is ordinarily single-stranded; it can fold back on parts of itself (as in the cloverleaf shape of tRNA) but does not have a second, separate complementary strand running its whole length.4. Size. A DNA molecule (a chromosome) is enormous — its molecular mass runs into billions of daltons — because it must encode an organism's entire genome in one continuous molecule. RNA molecules are much shorter, since each is transcribed to do one job: one mRNA carries the message of one gene (or a few), one tRNA is about $\displaystyle 75$–$\displaystyle 90$ nucleotides, one rRNA matches the size of the ribosomal subunit it belongs to.5. Location in the cell. DNA is confined almost entirely to the nucleus (with a small amount in mitochondria and chloroplasts). RNA is transcribed in the nucleus but does its actual work mostly outside it, in the cytoplasm, at the ribosome.Functional differences1. DNA stores hereditary information; RNA acts on it. DNA is the genetic material: it carries the coded instructions for building and running the organism, and this information must be passed on essentially unchanged from one generation of cells to the next.2. DNA can duplicate itself; RNA is made fresh from DNA whenever needed. DNA replicates — each strand is used as a template to build a new complementary strand, so one DNA molecule becomes two identical ones before a cell divides. RNA is transcribed anew from the DNA template only when a particular gene needs to be expressed; RNA does not routinely copy itself.3. RNA carries out protein synthesis, using three different molecules for three different jobs. Messenger RNA (mRNA) is transcribed from a gene and carries its coded message out of the nucleus to the ribosome. Ribosomal RNA (rRNA) is a structural and catalytic component of the ribosome itself, the site where the message is read. Transfer RNA (tRNA) reads each three-base codon on the mRNA and delivers the matching amino acid, linking the genetic code to the growing protein chain. DNA performs none of these three jobs directly — it only supplies the template from which RNA is transcribed.4. An exception worth naming, so the rule is not overstated: in retroviruses (e.g. HIV), RNA itself is the genetic material and is copied by reverse transcriptase — so "DNA stores, RNA executes" describes the normal cell, not every biological system.Answer: DNA has $\displaystyle 2$-deoxyribose sugar, the bases A, G, C, T, and is a stable double helix of two complementary, hydrogen-bonded strands, confined mainly to the nucleus, with an enormous molecular mass; functionally, it stores hereditary information and replicates itself. RNA has ribose sugar ($\displaystyle 2$'-OH present), the bases A, G, C, U, is usually single-stranded, chemically less stable, of much smaller molecular mass, and found in both nucleus and cytoplasm; functionally, it is synthesized from DNA as needed and carries out protein synthesis through its three forms — mRNA (carries the message), tRNA (delivers amino acids), and rRNA (builds the ribosome).

A cell needs three different RNAs because protein synthesis has three separate jobs — carrying the code, reading the code, and doing the assembly — and one molecule cannot do all three.Deoxyribonucleic acid (DNA) stores the genetic information, but DNA itself never leaves the nucleus to make a protein. That job is carried out by ribonucleic acid (RNA), and the cell uses three distinct kinds of RNA, each shaped for a different step of translation.1. Messenger RNA (mRNA)mRNA is transcribed from a gene on the DNA template strand and carries that gene's coded information, as a sequence of codons (groups of three bases), out of the nucleus to the ribosome. It is a long, single-stranded, largely unbranched molecule — essentially a disposable working copy of one gene's instructions. Because it only carries the message and does not fold into a stable, reusable shape, it is also the shortest-lived of the three RNAs, being degraded soon after it has been read.2. Transfer RNA (tRNA)tRNA is the smallest of the three RNAs, roughly $\displaystyle 75$–$\displaystyle 90$ nucleotides long. Its single strand folds back on itself and base-pairs internally to give a cloverleaf shape in two dimensions, which then folds further into an L-shaped three-dimensional structure. This shape gives it two functional ends: During translation, the anticodon on the tRNA base-pairs with the complementary codon on the mRNA, which is how the correct amino acid is delivered to the correct position specified by the code. Because there are (with some codon degeneracy) different tRNAs for different amino acids, tRNA is the molecule that physically "translates" a base sequence into an amino-acid sequence.3. Ribosomal RNA (rRNA)rRNA is the RNA that, together with proteins, makes up the ribosome itself — the structure on which mRNA is read and the polypeptide chain is built. It is the most abundant of the three types of RNA in the cell, since every ribosome (and a cell contains very many ribosomes) is built largely of rRNA. Unlike mRNA and tRNA, rRNA is not a messenger or an adaptor; it forms the physical framework and, in part, the catalytic machinery of the ribosome that carries out peptide-bond formation.How the three work together: DNA is transcribed into mRNA, which travels to a ribosome built of rRNA and protein; there, tRNA molecules — each charged with one specific amino acid — bring their amino acids to the ribosome and pair their anticodon with the matching mRNA codon, so the amino acids are joined in the order the mRNA specifies.Answer: The cell contains three types of RNA — messenger RNA (mRNA), which carries the genetic code from DNA to the ribosome as a sequence of codons; transfer RNA (tRNA), a small cloverleaf-shaped molecule that reads each codon via its anticodon and delivers the corresponding amino acid; and ribosomal RNA (rRNA), the most abundant RNA, which forms the structural and functional core of the ribosome on which the polypeptide chain is assembled.