Summary
Nucleic acids: Structures
- Nucleic acids are polymers of nucleotides. Each nucleotide has three distinct part: a base (either a purine, Adenine or Guanine, or a pyrimidine, Cytosine or Thymine or Uracil), a sugar moiety (a pentose, either a ribose (RNA), or a deoxyribose (DNA)) and a phosphate group.
- Polynucleotide chains are long, linear polymers of nucleotides, also referred to as "strands"
- DNA (deoxyribo nucleic acid) is usually a double stranded helix, with two strands running in opposite directions and maintainded together by hydrogen bonds between bases. Some viruses consist of single stranded DNA.
- Bases pair by hydrogen bonds. The main bases pair involve one purine and one pyrimidine: A-T and G-C in DNA, A-U and G-C in RNA.
- Structurally, there are three main conformation of DNA helices: A, B, and Z.
- DNA molecules may have topological constraints, such as supercoiling in circular DNA.
- Only one DNA strand is used for RNA synthesis: the "template" strand, which is complementary to the coding strand.
- The sequence of the mRNA is the sequence of the DNA coding strand, with T replaced by U.
- Three types of RNA are involved in protein synthesis: messenger RNA (mRNA, the carrier of genetic information), transfer RNA (tRNA, that brings the correct amino acid during protein synthesis), and ribosomal RNA (rRNA, a major constituent of the ribosome, where protein synthesis occurs).
- The message carried by mRNA is "read" as a collection of words of 3 letters, or codons. There are 64 possible codons, that code for 20 possible amino acids. AUG is the initiation codon, that also codes for Methionine. UAA, UAG and UGA are stop codons. There is a redundancy in the genetic code, usually related to the third base in the codon.
- RNA bases can be free, involved in base pairs, base triplets, or even base quadruplets.
- RNA contains single stranded regions, hairpin loops, bulges, and internal loops (the secondary structures).
- RNA secondary structures can interact to form pseudoknots, kissing hairpins or hairpin-bulge complexes.
- THe wobble hypothesis related to the degeneracy of the genetic code is based on the presence in some tRNA of Inosine at the 5'end of the anticodon