10.3 Vitamins, Nucleic Acids and Hormones
Vitamins we must eat, the nucleic acids that store and copy the instructions for every protein, and the hormones that carry messages around the body.
In the eighteenth century, sailors on long voyages died in large numbers from scurvy: their gums bled, old wounds reopened and they grew too weak to work. A ship's surgeon, James Lind, showed in 1747 that citrus fruit cured it. Nobody knew why for nearly two centuries, until chemists isolated the substance responsible, ascorbic acid or vitamin C. It turned out to be one of a small group of compounds that the body needs only in tiny amounts but cannot make for itself.
Two centuries later, in 1953, James Watson and Francis Crick, using X-ray photographs taken by Rosalind Franklin, worked out that DNA is a double helix whose two strands are held together by pairs of bases that fit each other exactly. That one structure explained how living things copy their genetic information and pass it on. This section covers three groups of biomolecules: the vitamins, the nucleic acids DNA and RNA, and, briefly, the hormones that act as the body's chemical messengers.
Vitamins
Vitamins are organic compounds required in small amounts in the diet for the normal growth and health of an organism, which the organism cannot make in sufficient amounts. Lack of a vitamin over a long time causes a specific deficiency disease. Vitamins are named by letters (A, B, C, D, E, K); the B vitamins are a group (B₁, B₂, B₆, B₁₂, …). By solubility they are:
- Fat-soluble vitamins: A, D, E and K. They dissolve in fats and oils, not in water, and are stored in the liver and adipose (fat-storing) tissue.
- Water-soluble vitamins: the B group and C. They are readily excreted in urine and cannot be stored (except vitamin B₁₂), so they must be supplied regularly in the diet.
| Fat-soluble vitamin | Good sources | Deficiency disease or effect |
|---|---|---|
| A | fish liver oil, carrots, butter, milk | xerophthalmia (hardening of the cornea), night blindness |
| D | sunlight on the skin, fish, egg yolk | rickets (bone deformities in children), osteomalacia (soft bones, joint pain in adults) |
| E | vegetable oils such as wheat-germ and sunflower oil | fragile red blood cells, muscular weakness |
| K | green leafy vegetables | blood takes longer to clot |
| Water-soluble vitamin | Good sources | Deficiency disease or effect |
|---|---|---|
| B₁ (thiamine) | yeast, milk, green vegetables, cereals | beri-beri (loss of appetite, poor growth) |
| B₂ (riboflavin) | milk, egg white, liver, kidney | cheilosis (cracks at the corners of the mouth and lips), digestive disorders, burning sensation of the skin |
| B₆ (pyridoxine) | yeast, milk, egg yolk, cereals, grams | convulsions |
| B₁₂ (cobalamin) | meat, fish, egg, curd | pernicious anaemia (red cells short of haemoglobin) |
| C (ascorbic acid) | citrus fruits, amla, green leafy vegetables | scurvy (bleeding gums) |
Vitamin B₁₂ is the cobalt complex met in Chapter 5 (Section 5.3); it is found almost only in animal foods, which is why strict vegetarians are advised to check their intake. Because fat-soluble vitamins are stored, taking large doses of them as supplements over a long period can be harmful, whereas excess water-soluble vitamins are mostly excreted.
Nucleic Acids: Composition
Every time a cell divides, each new cell receives an exact copy of the instructions for making all of its proteins. Those instructions are stored in nucleic acids, long chain polymers found in the nucleus (and elsewhere) of every cell. There are two kinds: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Complete hydrolysis of a nucleic acid gives three kinds of component:
- a pentose sugar: β-D-2-deoxyribose in DNA, β-D-ribose in RNA (deoxyribose has $\mathrm{H}$ in place of the $\mathrm{OH}$ on C2′);
- phosphoric acid, $\mathrm{H_{3}PO_{4}}$;
- nitrogen-containing heterocyclic bases: the purines adenine (A) and guanine (G), and the pyrimidines cytosine (C), thymine (T) and uracil (U). DNA contains A, G, C and T; RNA contains A, G, C and U.
A nucleoside is a base joined to C1′ of the pentose sugar (base $+$ sugar $-\ \mathrm{H_{2}O}$). A nucleotide is a nucleoside whose sugar carries a phosphate group, usually on C5′ (nucleoside $+$ phosphoric acid $-\ \mathrm{H_{2}O}$). Primes number the atoms of the sugar, to distinguish them from the atoms of the base.
Nucleotides are joined into a chain by phosphodiester linkages between C5′ of one sugar and C3′ of the next: the phosphate is an ester of both. A nucleic acid is a polynucleotide with a sugar–phosphate backbone and one base hanging from each sugar; the chain has a free 5′ end and a free 3′ end, and its sequence is written from 5′ to 3′.
The DNA Double Helix and Base Pairing
In 1950 Erwin Chargaff found that, in DNA from any organism, the amount of adenine always equals the amount of thymine, and the amount of guanine equals that of cytosine, although the ratio of A + T to G + C varies from species to species. Watson and Crick explained why.
- DNA consists of two polynucleotide strands coiled round a common axis in a right-handed double helix. The sugar–phosphate backbones are on the outside and the bases point inwards. The strands run in opposite directions (one 5′→3′, the other 3′→5′).
- The strands are held together by hydrogen bonds between pairs of bases, one from each strand. A purine always pairs with a pyrimidine: adenine with thymine (two hydrogen bonds) and guanine with cytosine (three hydrogen bonds).
- The two strands are therefore complementary: the sequence of one fixes the sequence of the other. Hence in double-stranded DNA $n(\mathrm{A})=n(\mathrm{T})$ and $n(\mathrm{G})=n(\mathrm{C})$ (Chargaff's rules), and purines $=$ pyrimidines.
RNA, the Functions of Nucleic Acids, and Hormones
RNA molecules are usually single strands, which may fold back on themselves. Their sugar is ribose and uracil takes the place of thymine. Cells make three main types.
- Messenger RNA (m-RNA) carries a copy of the instructions of a gene from DNA to the sites of protein synthesis; it is made on a DNA template (transcription).
- Ribosomal RNA (r-RNA) forms, with proteins, the ribosomes where proteins are assembled.
- Transfer RNA (t-RNA) carries particular amino acids to the ribosome and matches them to the message on m-RNA.
Functions. DNA is the chemical basis of heredity: it stores the genetic information that is passed from parents to offspring, and its unique sequence in each organism makes it possible to identify individuals. Before a cell divides, its DNA replicates: the strands separate and each serves as a template for a new complementary strand, so each daughter cell receives an identical copy. Proteins are made by the cell from the message in DNA, through RNA: DNA → m-RNA (transcription) → protein (translation, with r-RNA and t-RNA).
| Feature | DNA | RNA |
|---|---|---|
| sugar | β-D-2-deoxyribose | β-D-ribose |
| bases | A, G, C, T | A, G, C, U |
| strands | two (double helix) | usually one |
| Chargaff's rules | A = T, G = C | do not apply |
| main role | stores genetic information, replicates | protein synthesis |
Hormones are the body's chemical messengers. They are made by endocrine glands, released into the blood, and carried to the organs (targets) where they act, in very small amounts. Some are steroids, some are polypeptides and some are derived from amino acids; they are needed to keep the body's activities in balance (homeostasis). Only an elementary idea of them is needed here; their structures are not required.
- Types. Steroids (oestrogens and androgens, the sex hormones); polypeptides (insulin, glucagon, endorphins); amino acid derivatives (adrenaline, also called epinephrine; noradrenaline or norepinephrine; thyroxine).
- Blood glucose. Insulin (released when blood glucose rises) and glucagon (released when it falls), both from the pancreas, together keep blood glucose within a narrow range.
- Thyroxine, from the thyroid gland, which contains iodine, controls the rate of metabolism: too little causes hypothyroidism (lethargy, weight gain); too much, hyperthyroidism. A lack of iodine in the diet enlarges the thyroid (goitre), which is why salt is iodised.
- Adrenaline and noradrenaline prepare the body for sudden stress ("fight or flight"). The adrenal cortex releases steroid hormones that control carbohydrate metabolism and inflammation (glucocorticoids) and the excretion of sodium and potassium (mineralocorticoids); a failure of the adrenal cortex causes Addison's disease.
- Sex hormones (testosterone, oestradiol, progesterone) control the development of sexual characteristics and reproduction; growth hormones control growth.
- Complement: replace A↔T and G↔C, and remember the new strand runs the other way (5′→3′ becomes 3′→5′).
- Hydrogen bonds: $2\times$(number of A–T pairs) $+\ 3\times$(number of G–C pairs).
- Composition of double-stranded DNA: A $=$ T, G $=$ C, so knowing one percentage gives all four.
- mRNA: complementary to the template strand, with U in place of T; it has the same sequence as the other (coding) strand.
1. Nucleoside vs nucleotide. Nucleoside $=$ base $+$ sugar; nucleotide $=$ nucleoside $+$ phosphate ("t" for phosphate).
2. Uracil in DNA. No: DNA has thymine; RNA has uracil.
3. Chargaff's rules for RNA. They hold only for double-stranded DNA.
4. Counting hydrogen bonds. A–T has two, G–C three; G–C-rich DNA is held together more strongly.
5. Fat-soluble vs water-soluble vitamins. A, D, E, K are fat-soluble; B and C are water-soluble.
In JEE Main and NEET 2026 (biological functions of nucleic acids; NEET Biology names the genetic code); not in the CBSE Chemistry syllabus or JEE Advanced.
The message in m-RNA is read in groups of three bases, called codons, each standing for one amino acid (or a "stop" signal). With four bases there are $4^{3}=64$ possible codons, enough for 20 amino acids; pairs of bases would give only $4^{2}=16$, too few. Most amino acids therefore have more than one codon; AUG codes for methionine and also marks the start of a protein. A gene of 900 coding bases (excluding the stop codon) thus codes for a chain of $900/3=300$ amino acids.
Exercises
- 1 What are vitamins? Why are they needed only in small amounts?
- 2 Name the fat-soluble vitamins. Where are they stored in the body?
- 3 Why must water-soluble vitamins be supplied regularly in the diet? Which one is an exception?
- 4 Name the deficiency diseases of vitamins A, B₁, B₆ and D.
- 5 Name the vitamin, and one good source of it, whose deficiency causes (a) bleeding gums, (b) increased blood-clotting time, (c) cheilosis.
- 6 Which vitamin can the skin make with the help of sunlight? Why do children who rarely go outdoors risk rickets?
- 7 Why is it more dangerous to take very large doses of vitamin A than of vitamin C?
- 1 Name the three kinds of product obtained by complete hydrolysis of DNA.
- 2 How does 2-deoxyribose differ from ribose?
- 3 Name the purine and the pyrimidine bases found in DNA, and the base in RNA that replaces thymine.
- 4 Define nucleoside and nucleotide. Through which carbon atoms of the sugar are the base and the phosphate attached?
- 5 What is a phosphodiester linkage? Which carbon atoms does it join?
- 6 Find the molar mass of adenosine (adenine + ribose).
- 7 Find the molar mass of guanosine 5′-monophosphate (GMP).
- 8 Find the molar mass of deoxycytidine and of uridine 5′-monophosphate.
- 1 Describe the double-helix structure of DNA in three sentences.
- 2 Write the complementary strand of 5′-GGATCCTA-3′, showing its 5′ and 3′ ends.
- 3 How many hydrogen bonds hold together the double-stranded segment in part 2?
- 4 A double-stranded DNA contains $18\%$ adenine. Find the percentage of each other base.
- 5 A DNA segment of $1000$ base pairs contains $40\%$ G + C. How many hydrogen bonds hold its strands together?
- 6 Why does a purine always pair with a pyrimidine?
- 7 A segment rich in G–C pairs needs a higher temperature to separate its strands than one rich in A–T pairs. Explain.
- 8 A DNA sample has $n(\mathrm{G})=n(\mathrm{C})$ but $n(\mathrm{A})\ne n(\mathrm{T})$. Is it likely to be double-stranded? Explain.
- 1 Name the three types of RNA and give the function of each.
- 2 What is meant by replication of DNA? Why does it give two identical molecules?
- 3 Write the m-RNA that has the same sequence as the DNA coding strand 5′-ATGTTCGAA-3′.
- 4 Give two biological functions of nucleic acids.
- 5 What are hormones? Name the three structural classes with one example each.
- 6 Name the hormone deficient in (a) hypothyroidism, (b) a person whose blood glucose stays too high because the pancreas fails to respond.
- 7 What is the role of adrenaline in the body?
E1. Board · 2 marks Differentiate between a nucleoside and a nucleotide, giving one example of each.
E2. NEET Which pair of vitamin and deficiency disease is correctly matched? (A) vitamin B₁₂ – rickets (B) vitamin K – scurvy (C) vitamin A – night blindness (D) vitamin C – beri-beri
E3. JEE Main Numerical value. One strand of a DNA segment is 5′-AGCTTGCAGC-3′. Find the total number of hydrogen bonds between the two strands of this segment.
E4. JEE Main NEET A double-stranded DNA contains $17\%$ thymine. The percentage of guanine in it is (A) $17\%$ (B) $33\%$ (C) $34\%$ (D) $66\%$
E5. Board · 3 marks Give three differences between DNA and RNA, and name the three types of RNA.
E6. Board · Case study In many countries, children who spend little time outdoors and drink little milk are found to have low levels of vitamin D, and some develop soft, bent bones. Doctors also warn that taking high-dose vitamin A or vitamin D tablets for months without advice can be harmful, whereas an extra orange a day cannot cause vitamin C poisoning. (a) Name the disease described and say how sunlight helps prevent it. (2 marks) (b) Why can excess vitamin A or D be harmful? (1 mark) (c) Why is excess vitamin C not dangerous in the same way? (1 mark)
Research question. How much DNA is packed into a single human cell, and how long would it be if it were stretched out?
Why it matters. Genome projects, forensic DNA profiling, cancer diagnostics and newborn screening all begin by extracting DNA from a few cells. Laboratories in every country need to know how much DNA to expect from a cheek swab or a drop of blood, and engineers designing DNA sequencers need to know how long the molecules are.
What to do.
- From the nucleotide builder (Interactive 1), find the molar masses of dAMP, dTMP, dGMP and dCMP. In a chain each nucleotide loses one $\mathrm{H_{2}O}$; find the average mass of one base pair in a DNA chain.
- (Illustrative figures, constructed for this exercise.) Take a human cell nucleus to contain $6.4\times10^{9}$ base pairs. Using $N_{A}=6.022\times10^{23}\ \mathrm{mol^{-1}}$, find the mass of DNA in one cell, in picograms ($1\ \mathrm{pg}=10^{-12}\ \mathrm{g}$).
- Successive base pairs in the double helix are about $0.34\ \mathrm{nm}$ apart. Find the total length of the DNA in one cell.
- A forensic sample yields $1.0\ \mathrm{ng}$ of DNA. Roughly how many cells did it come from?
Tools: A calculator; Interactive 1 and 2; a reliable source on DNA extraction or forensic profiling.
Hand in: Your calculations, and one paragraph on how a cell packs metres of DNA into a nucleus a few micrometres across.
Going further. Find out what PCR (the polymerase chain reaction) does, and why it relies on the strands of DNA separating on heating and pairing again on cooling.
🧪 Self-check quiz
Q1. The sugar present in DNA is
Q2. Which base is found in RNA but not in DNA?
Q3. The number of hydrogen bonds between guanine and cytosine in DNA is
Q4. Scurvy is caused by a deficiency of
Q5. A nucleotide consists of