Nucleic Acids

Nucleic acids are the basic unit of inheritance, they constitute all the genetic material of living things

This genetic information determines how individual cells and entire organisms develop and function (by encoding for protein synthesis)

Structure of Nucleotides

  • The monomeric unit of a nucleic acid is a nucleotide, which is comprised of a 5C-sugar, a phosphate group and a nitrogenous base
  • The carbon atoms of the sugar molecule are numbered (1 - 5), the base connects to the 1'-C while the phosphate connects to the 5'-C of the sugar

Structure of a Nucleotide (Sugar, Phosphate & Base)

  • The nucleotide will contain one of four different nitrogenous bases, and when nucleotides are joined into chains, the order of these bases will determine the specific genetic information encoded by that sequence of nucleic acid
  • The four bases in DNA are:

                                Adenine                                                       Guanine                                                Thymine                                           Cytosine

  • Adenine (A) and guanine (G) are purines (double ring bases)
  • Thymine (T) and cytosine (C) are pyrimidines (single ring bases)

Biosynthesis of Nucleic Acids

  • Nucleotides a linked into a single strand via a condensation reaction
  • The phosphate group (attached to the 5'-C of the sugar) joins with the hydroxyl (OH) group attached to the 3'-C of the sugar
  • This results in a phosphodiester bond between the two nucleotides and the formation of a water molecule
  • Successive condensation reactions between nucleotides results in the formation of a long single strand

Types of Nucleic Acids

There are two main types of nucleic acids:

  • DNA:  DNA (or deoxyribonucleic acid) is found in the nucleus and carries the genetic instructions for coding proteins on sequences called genes
  • RNA:  RNA (or ribonucleic acid) functions to transfer the genetic information from the nucleus to the rest of the cell
    • mRNA (messenger RNA) is a transcript copy of a gene which encodes a specific polypeptide
    • tRNA (transfer RNA) carries the polypeptide subunits (amino acids) to the organelle responsible for protein synthesis (ribosome)
    • rRNA (ribosomal RNA) is a primary component of the ribosome and is responsible for its catalytic function

Differences Between DNA and RNA

Organisation of DNA

  • Two polynucleotide chains of DNA are held together by hydrogen bonds between complementary base pairs:
    • Adenine pairs with thymine (A=T) via two hydrogen bonds
    • Guanine pairs with cytosine (G=C) via three hydrogen bonds

                           Thymine                                         Adenine                                                               Cytosine                                           Guanine

  • In order for bases to be facing each other and thus able to pair, the two strands must run in opposite directions (i.e. they are anti-parallel)
  • As the polynucleotide chain lengthens, the atoms that make up the molecule will arrange themselves in an optimal energy configuration 
  • This position of least resistance results in the double-stranded DNA twisting to form a double helix with approximately 10 - 15 bases per twist

Structure of the Double Helix

Packaging of DNA

  • Eukaryotic chromosomes consist of DNA wrapped around histone proteins
  • This forms the basic structure of the nucleosome, which is packed together to form chromatin (in a 'beads on a string' arrangement)
  • Chromatin will supercoil and condense during prophase to form chromosomes that can be visualised under a light microscope
  • Prokaryotic DNA is not wrapped around proteins and is thus considered to be 'naked'

Arrangement of DNA into Chromosomes

Function of Nucleic Acids

  • The code carried by DNA is organised into triplets of bases (i.e. three nucleotides) called codons
  • Each codon codes for an amino acid according to a set of rules known as the genetic code
  • A sequence of codons which determines the sequence of a polypeptide is called a gene
  • As the DNA is confined to the nucleus and the ribosomes responsible for protein synthesis are in the cytoplasm, an RNA copy is made (mRNA)
    • This process by which DNA is transcribed into an RNA copy is called transcription
  • The mRNA goes to the ribosome, which synthesises a polypeptide sequence based on the order of codons in the gene
    • This process by which mRNA is translated into a polypeptide sequence is called translation

The Role of DNA and RNA in Protein Synthesis

The Genome

  • Not all DNA sequences code for proteins (i.e. not every sequence of DNA is a gene)
  • The totality of genes in a cell or organism is called the genome and the study of the way genes interact is called genomics
  • The human genome consists of ~ 20,000 - 25,000 genes