Vitamins are organic compounds required in tiny amounts in the diet to keep the body healthy; they are not synthesised in sufficient quantity by the body and act mainly as co-factors for enzymes. A deficiency causes a specific disease. They are named by letters A, B, C, D, E and K.
Classification of vitamins. By solubility they fall into two groups. Fat-soluble vitamins (A, D, E, K) dissolve in fats and oils, are stored in the liver and adipose tissue, and can build up to toxic levels if taken in excess. Water-soluble vitamins (the B-complex and C) dissolve in water, are not stored (except B12) and so must be supplied regularly; the excess is excreted in urine. The accompanying table lists key sources and deficiency diseases.
Notable deficiencies: vitamin A → night blindness / xerophthalmia; vitamin B1 (thiamine) → beri-beri; vitamin C (ascorbic acid) → scurvy; vitamin D → rickets (children) and osteomalacia (adults); vitamin K → poor blood clotting; vitamin B12 → pernicious anaemia.
Nucleic acids are the polymers that store and transmit genetic information — DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). On complete hydrolysis each gives a pentose sugar, a nitrogenous base and phosphoric acid. The sugar is β-D-2-deoxyribose in DNA and β-D-ribose in RNA. The bases are the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) with thymine (T) in DNA or uracil (U) in RNA.
Nucleosides and nucleotides. A base joined to a sugar (at C1′) is a nucleoside. When phosphoric acid esterifies the C5′–OH of the sugar in a nucleoside, the unit is a nucleotide — the actual building block of nucleic acids. Nucleotides link through phosphodiester bonds (3′→5′) to form the sugar–phosphate backbone.
Structure of DNA (Watson–Crick double helix). DNA is two polynucleotide strands wound into a right-handed double helix. The two strands run antiparallel and are held together by hydrogen bonds between complementary bases: A pairs with T (two H-bonds) and G pairs with C (three H-bonds). Because pairing is fixed, the strands are complementary — this base-pairing is the chemical basis of accurate copying. The amount of A equals T and G equals C (Chargaff's rule).
Structure and types of RNA. RNA is usually single-stranded, contains ribose and uracil (not thymine). The three main kinds are messenger RNA (mRNA), which carries the genetic message copied from DNA; ribosomal RNA (rRNA), which forms ribosomes (the site of protein synthesis); and transfer RNA (tRNA), which brings the correct amino acid to the ribosome.
Biological functions. In replication the double helix unwinds and each strand templates a new complementary strand, so the genetic information is copied exactly before cell division. In protein synthesis, the DNA message is first transcribed into mRNA; the mRNA is then translated on the ribosome, where tRNA reads three-base codons and adds the matching amino acids, building the polypeptide. DNA thus controls heredity and directs the manufacture of every protein.
Hormones (brief). Hormones are chemical messengers secreted by endocrine glands directly into the blood to regulate metabolism and growth. They may be steroids (e.g. testosterone, estradiol), amino-acid derivatives (e.g. adrenaline, thyroxine) or peptides/proteins (e.g. insulin, which lowers blood glucose). They keep body processes in balance and work alongside the nervous system.