Acides nucléiques

A nucleotide consists of three components: a pentose sugar (five-carbon sugar), a nitrogenous base, and a phosphate group. The nitrogenous base attaches to carbon-1 of the sugar, while the phosphate group attaches to carbon-5. These three parts combine to form the basic building block of DNA and RNA.

DNA contains deoxyribose sugar with a hydrogen atom at carbon-2, while RNA contains ribose sugar with a hydroxyl group at carbon-2. Both share adenine, guanine, and cytosine bases, but DNA contains thymine whereas RNA contains uracil instead. These structural differences affect their stability and function in the cell.

A phosphodiester bond forms between the hydroxyl group on carbon-3 of one nucleotide's sugar and the phosphate group on carbon-5 of the next nucleotide. DNA and RNA polymerase enzymes catalyze this polymerization reaction, creating a sugar-phosphate backbone. This bonding leaves a free 5' phosphate end and a free 3' hydroxyl end on the nucleic acid strand.

Complementary base pairs follow specific pairing rules: guanine pairs with cytosine through three hydrogen bonds, while adenine pairs with thymine in DNA or uracil in RNA through two hydrogen bonds. These hydrogen bonds hold the strands together and ensure accurate replication and transcription. The specificity of base pairing is fundamental to nucleic acid function.

DNA strands are antiparallel because they run in opposite directions: the 5' end of one strand pairs with the 3' end of the complementary strand. This orientation is maintained by hydrogen bonds between nitrogenous bases and hydrophobic interactions. The antiparallel arrangement is essential for the double helix structure and for accurate DNA replication.

DNA typically exists as a double helix with two polynucleotide chains wound around each other, with the sugar-phosphate backbone on the outside and bases stacked inside. RNA is usually single-stranded but can form secondary structures through intramolecular base pairing between complementary sequences. These structural differences enable their distinct roles in storing genetic information and directing protein synthesis.

Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes. Ribosomal RNA (rRNA) is a major ribosome component that aligns mRNA and catalyzes peptide bond formation. Transfer RNA (tRNA) delivers correct amino acids during protein synthesis. MicroRNA (miRNA) regulates gene expression by interfering with mRNA messages. Each RNA type has a distinct structure and role in protein synthesis and gene regulation.