introduction to DNA and RNA

• Nucleic acids


  An important class of biomolecules found in all cells and viruses
• Functions of nucleic acids


  • Storage and expression of genetic information
• Deoxyribonucleic acid (DNA)


  Encodes the information the cell needs to make proteins
• Ribonucleic acid (RNA)


  A related type of nucleic acid that participates in protein synthesis
• DNA has the same composition and structure in all species
• DNA


  A very efficient way of carrying complex information required to build organisms
• Gene


  A unit of DNA that contains the instructions for a gene product, commonly a protein
• From DNA to traits

  1. DNA contains instructions

  2. Instructions are read by cellular machinery

  3. Gene product (e.g. protein) is made

  4. Traits are determined by the expression of genes
• DNA structure


  • Two helical strands winding around a central axis
  • Composed of nucleotides with a sugar phosphate backbone and nitrogenous bases facing the centre of the double helix
  • Phosphate groups give DNA an overall negative charge
  • Four nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G)
• Deoxyribose


  The sugar ring in DNA, missing an oxygen atom
• Ribose


  The sugar ring in RNA, with an oxygen atom present
• DNA structure


  • A always pairs with T
  • G always pairs with C (Chargaff's rule)
  • Nitrogenous bases on opposite strands are held together by hydrogen bonds
  • A and T have two hydrogen bonds, G and C form three bonds
  • DNA has directionality, with the phosphate at the 5' end and the sugar at the 3' end
  • DNA is 'read' from 5' to 3'
• DNA structure - summary

  • Arranged as a double helix
  • 2 antiparallel strands (run in opposite directions)
  • Each strand consists of a chain of nucleotides
  • Nucleotides consist of a sugar (deoxyribose), phosphate group and a nitrogenous base
  • Nitrogenous bases on opposite strands are complementary
  • Complementary bases/two strands are held together by hydrogen bonds (easily broken)
  • Deoxyribose sugar
  • Pentose
• Organisation of DNA


  • Each DNA molecule forms a long fibre called a chromosome
  • DNA is found associated with (histone) proteins that help to package it
  • DNA and its associated proteins are called chromatin
• Structure of chromosomes


  • DNA is wound around nucleosomes like beads on a chain
  • When the nucleosome 'beads' are loose, the chromatin is 'open' and accessible
  • When the nucleosomes and DNA are tightly packed, the chromatin is 'closed'
• Histone


  Protein that the DNA wraps itself around, helping the DNA condense into chromatin
• Nucleosome


  Group of 8 histone proteins wrapped by DNA
• Chromatin


  Complex of DNA and proteins found in the nucleus
• Heterochromatin


  Tightly packed, very dense, heavily staining
• Euchromatin


  Less densely packed, rich in genes
• Chromosome terminology


  • Chromosomes have a 'center' called the centromere, where chromatids join to form a double chromosome
  • Chromatids are separated during cell division
• Telomeres


  • The section of DNA at the ends of a chromosome that prevent damage during cell division
  • As a cell divides, the telomeres continue to get shorter
  • As they shorten, more damage accumulates in the chromosome, causing aging
• Human chromosomes


  • In somatic human cells, there are 23 pairs of chromosomes
  • Each pair contains one paternal and one maternal chromosome
  • Matched pairs of chromosomes are called autosomes and are homologous
  • One pair of chromosomes is the sex chromosomes, termed X and Y - pairs can be XX (homologous) or XY (heterologous)
• Karyotypes


  • When all the chromosomes in a somatic cell are arranged by size, it is called a karyotype
  • The first 22 pairs of chromosomes are called autosomes or somatic chromosomes
  • The 23rd pair of chromosomes can be matched or unmatched, and are called sex chromosomes
• Ploidy


  • The number of pairs of chromosomes in a cell
  • In humans, the haploid number (n) is 23, so there are 23 pairs of chromosomes in a body cell
  • The diploid number for a human body cell is: (2n) = 46, i.e. 2 × 23 pairs of chromosomes
• Locus


  The position of a gene, a cluster of genes or even a single nucleotide on a chromosome
• Alleles


  Individual gene copies that can be given names like D and d depending on their characteristics
• Prokaryotic chromosomes


  • Usually single circular molecules and haploid
  • Present in the cytoplasm since prokaryotes do not have nuclei
  • Small circular DNA molecules called plasmids may float freely in the cytoplasm and carry non-essential genes
• Prokaryotic vs Eukaryotic DNA

  • Prokaryotic DNA: Small, less than 0.1pg; occurs freely in the cytoplasm; no organelle DNA; naked DNA; generally circular
  • Eukaryotic DNA: Comparatively high content, more than 1 pg; in the nucleus; has organelle DNA; nuclear DNA associated with histones; linear
• DNA vs RNA


  • DNA is coiled around histone proteins, then coiled multiple times in chromosomes to fit into the nucleus of eukaryotic cells
  • DNA is found in the nucleus, mitochondria and chloroplasts of eukaryotes
  • Prokaryotes have an unbound single large circular chromosome in the cytosol, and also contain smaller circular plasmids
  • Macroscopic view of DNA: Chromosome (telomere, centromere, chromatin, wrapped around histones forming nucleosomes)
  • Types of chromosomes: Autosome, Heterosome, Sex chromosomes, Homologous chromosomes, Double chromosomes (two chromatids), Single chromosomes