Nucleic Acids and Protein Biosynthesis

Nucleic acids (DNA and RNA) store hereditary information and pass it on from generation to generation. They are high-molecular-weight compounds.

Their monomer is the nucleotide. It consists of a nitrogenous base, a pentose sugar and a phosphoric acid residue.

DNA is a double-stranded molecule, while RNA is single-stranded.

Protein biosynthesis

It proceeds in two stages:

• Transcription: synthesis of mRNA based on a gene in DNA, in the nucleus.
• Translation: assembly of the polypeptide chain based on mRNA, on the ribosome.

1. DNA (gene)
2. Transcription
3. mRNA
4. Translation
5. Polypeptide (protein)

One amino acid is coded by three consecutive nucleotides, that is, a triplet (codon).

Function of tRNA: to bring the amino acid to the ribosome. Its anticodon is complementary to the mRNA codon.

Key facts

• The nitrogenous bases of DNA: A (adenine), T (thymine), G (guanine), C (cytosine). In RNA, instead of thymine there is U (uracil).
• Complementarity: A=T (2 hydrogen bonds), G≡C (3 hydrogen bonds). Chargaff's rule: A=T, G=C.
• DNA contains the sugar deoxyribose, RNA contains ribose. Types of RNA: mRNA, tRNA, rRNA.
• The genetic code: triplet, universal, continuous, redundant (degenerate). Of the 64 codons, 61 code for amino acids, and 3 are stop codons (UAA, UAG, UGA).
• Start codon: AUG. ATP energy is used for protein synthesis.