Vitamins: what they are
Vitamins are organic compounds needed in very small amounts in our food for normal growth and health. Most cannot be made by our body (vitamin D is made in the skin with sunlight; some B vitamins and K are made by gut bacteria). They are named with letters A, B, C, D, E, K; B is a group (B1, B2, B6, B12 and others).
Types of vitamins
- Fat-soluble vitamins: A, D, E, K. They dissolve in fats and oils and are stored in the liver and fatty tissue. Because they are stored, a lot of extra can build up and harm the body.
- Water-soluble vitamins: B group and C. They dissolve in water, so the extra leaves in urine and they are not stored well. We need them regularly in food. (B12 is an exception: the liver stores some.)
Sources and deficiency diseases
| Vitamin | Good sources | Lack causes |
|---|---|---|
| A | Carrots, green leaves, milk, fish liver oil | Night blindness, dry eyes (xerophthalmia), rough skin |
| B1 (thiamine) | Whole grains, yeast, pulses, milk | Beriberi (weak nerves and muscles) |
| B2 (riboflavin) | Milk, egg white, liver | Cracks at mouth corners, sore tongue |
| B6 (pyridoxine) | Yeast, cereals, egg yolk, milk | Convulsions (fits) |
| B12 (cyanocobalamin) | Meat, fish, egg, curd | Pernicious anaemia (fewer, weak red cells) |
| C (ascorbic acid) | Citrus fruits, amla, tomato, green leaves | Scurvy (bleeding gums) |
| D | Sunlight on skin, fish, egg yolk | Rickets in children, osteomalacia (soft bones) in adults |
| E | Vegetable oils, nuts, green leaves | Weak red cells, muscle weakness |
| K | Green leafy vegetables | Blood takes longer to clot |
Cooking at high heat destroys much vitamin C, and washing cut vegetables washes away water-soluble vitamins.
Nucleic acids: nucleoside and nucleotide
Nucleic acids are long polymers found in the nucleus (and other parts) of every cell. They carry the information for making proteins and pass it from parents to children. There are two kinds: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
Full hydrolysis of a nucleic acid gives three things: a pentose sugar, phosphoric acid and nitrogen bases.
- Sugar: β-D-2-deoxyribose in DNA (no O on carbon 2′), β-D-ribose in RNA.
- Bases: purines (two rings): adenine (A), guanine (G); pyrimidines (one ring): cytosine (C), thymine (T), uracil (U). DNA has A, G, C, T. RNA has A, G, C, U.
A nucleoside = base + sugar (base joined at carbon 1′ of the sugar). A nucleotide = nucleoside + phosphate (phosphate joined at carbon 5′). Nucleotides join through phosphodiester links between the 5′ carbon of one sugar and the 3′ carbon of the next, making a polynucleotide chain.
Structure of DNA: the double helix
In 1953 James Watson and Francis Crick, using X-ray pictures made by Rosalind Franklin, proposed that DNA is a double helix:
- Two polynucleotide strands twist around a common axis, like a spiral ladder.
- The sugar–phosphate backbones are on the outside; the bases point inwards.
- Bases pair by hydrogen bonds: A with T (two H-bonds), G with C (three H-bonds). A purine always pairs with a pyrimidine, so the ladder has the same width all along.
- The two strands are complementary (knowing one tells you the other) and antiparallel (one runs 5′→3′, the other 3′→5′).
Because of pairing, in any double-stranded DNA the amount of A equals T and G equals C (Chargaff's rule).
RNA and the differences between DNA and RNA
RNA is usually a single strand that may fold back on itself. There are three main types: messenger RNA (m-RNA) carries the copy of a gene, ribosomal RNA (r-RNA) forms ribosomes, and transfer RNA (t-RNA) brings amino acids to the ribosome.
| DNA | RNA | |
|---|---|---|
| Sugar | 2-deoxyribose | ribose |
| Bases | A, G, C, T | A, G, C, U |
| Strands | two (double helix) | usually one |
| Job | stores genetic information; copies itself (replication) | helps make proteins (protein synthesis) |
| Where | mainly nucleus | nucleus and cytoplasm |
Biological functions
DNA is the chemical basis of heredity: it keeps the identity of a species over generations. During cell division DNA makes an exact copy of itself (replication). The message in DNA is copied into m-RNA and then used to join amino acids in the right order to make proteins. A change in the base sequence (a mutation) can change the protein.
Hormones: types and roles
Hormones are chemical messengers. Ductless (endocrine) glands release them straight into the blood, which carries them to target organs. They keep body conditions steady and control growth and development.
Chemical types
- Steroid hormones: made from cholesterol. Testosterone, estradiol, progesterone, cortisol, aldosterone.
- Polypeptide / protein hormones: insulin, glucagon, growth hormone, endorphins.
- Amino acid derivatives: adrenaline (epinephrine), thyroxine.
Roles
- Insulin and glucagon keep blood glucose in a narrow range (insulin lowers it, glucagon raises it).
- Adrenaline prepares the body for sudden action (fight or flight).
- Thyroxine controls the rate of metabolism; too little causes hypothyroidism, and lack of iodine can cause goitre.
- Cortisol helps the body handle stress and controls some metabolism; sex hormones control development of sex organs and features.
Try it at home: pull out DNA from a banana
- Mash half a ripe banana with half a cup of warm water, 1 teaspoon of dish-wash liquid and a pinch of salt. Soap breaks the cell walls; salt helps DNA clump.
- Filter through a tea strainer into a clear glass.
- Predict first: what will appear when you slowly pour chilled rubbing alcohol (or cold surgical spirit, with an adult) down the side of the glass?
- Check: white, cloudy threads rise into the alcohol layer. That is DNA (millions of strands together). Wind it on a toothpick.
- Then open the 3D, step 6, and press "New sequence" to see how the bases pair inside each strand.
Key formulas and definitions
- Nucleoside = base + sugar · Nucleotide = base + sugar + phosphate
- Purines: A, G (2 rings) · Pyrimidines: C, T (DNA), U (RNA) (1 ring)
- DNA pairs: A=T (2 H-bonds), G≡C (3 H-bonds) → %A = %T, %G = %C, %A + %G = 50%
- Total H-bonds in a DNA stretch = 2 × (A–T pairs) + 3 × (G–C pairs)
- In double-stranded DNA: number of nucleotides = 2 × number of base pairs
- Vitamins: fat-soluble A, D, E, K · water-soluble B-group, C
Worked examples
1. A DNA sample has 30% adenine. Find the % of T, G and C.
Step 1: A pairs with T, so T = 30%. Step 2: A + T = 60%, so G + C = 40%. Step 3: G = C, so each is 20%. Answer: T 30%, G 20%, C 20%.
2. Write the complementary strand of 5′-ATGCCTA-3′.
Step 1: Swap each base: A→T, T→A, G→C, C→G. Step 2: Bases read TACGGAT. Step 3: Strands are antiparallel, so write it 3′-TACGGAT-5′. Answer: 3′-TACGGAT-5′.
3. How many hydrogen bonds hold a DNA stretch with 10 A–T pairs and 15 G–C pairs?
Step 1: A–T pairs: 10 × 2 = 20. Step 2: G–C pairs: 15 × 3 = 45. Step 3: Total = 20 + 45 = 65. Answer: 65 H-bonds.
4. A double-stranded DNA piece has 200 nucleotides, of which 60 are cytosine. How many adenines are there?
Step 1: C = G = 60, so G + C = 120. Step 2: A + T = 200 − 120 = 80. Step 3: A = T, so A = 40. Answer: 40 adenines.
5. An m-RNA is copied from the DNA template 3′-TACGGA-5′. Write the m-RNA.
Step 1: Pair each base, but RNA uses U for A's partner: T→A, A→U, C→G, G→C. Step 2: The m-RNA reads 5′-AUGCCU-3′. Answer: 5′-AUGCCU-3′.
6. Which of these DNA pieces is harder to separate by heat: one with 40% G–C or one with 60% G–C?
Step 1: G–C pairs have 3 H-bonds, A–T only 2. Step 2: More G–C means more H-bonds to break. Answer: the 60% G–C DNA needs more heat to separate.
7. A person on a diet with no oil or fat gets night blindness. Explain.
Step 1: Night blindness comes from lack of vitamin A. Step 2: Vitamin A is fat-soluble; it needs fat in food to dissolve and be absorbed. Answer: without fat, little vitamin A is absorbed, so its lack shows up.
Common mistakes
- Writing T in RNA. RNA has uracil (U) in place of thymine.
- Forgetting that the partner strand runs the other way; write 5′ and 3′ ends correctly.
- Mixing nucleoside and nucleotide. A nucleoside has no phosphate; a nucleotide has one.
- Saying vitamin C deficiency causes rickets. Rickets = lack of D; scurvy = lack of C.