structure, function and replication

• DNA stores and transmits genetic information; it functions in the same way in all living things
• DNA

  • Informational macromolecules used to store hereditary information that determines functional and structural characteristics of organisms
  • Discovered by James Watson and Francis Crick in 1953
  • Unique molecule capable of self-replication
  • Sequence of nucleotides allows storage and passing on of information to daughter cells
  • Provides link between generations
  • Genetic code uses three bases at a time to direct protein synthesis (codon)
• Nucleic Acid

  • Long molecule made up of subunits called nucleotides
  • Components include nitrogenous base, phosphate group, and pentose sugar
  • Elements include carbon, hydrogen, oxygen, nitrogen, and phosphorus
• Types of Nucleic Acids

  • Deoxyribonucleic acid (DNA)
  • Ribonucleic acid (RNA)
• Sugar

  • Deoxyribose sugar (DNA)
  • Ribose sugar (RNA)
• Structure of DNA and RNA

  • Two polynucleotide strands intertwined in opposite directions forming a double helix (DNA)
  • Single polynucleotide chain shorter than DNA (RNA)
• Nitrogenous base pairing

  • Adenine (A) pairs with thymine (T), Guanine (G) pairs with cytosine (C) in DNA
  • Adenine (A), thymine (T), guanine (G), uracil (U) in RNA
• Nitrogenous base groups on both polynucleotide chains are matched and bound together by hydrogen bonds
• Three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA)
• Complementary base pairing is read directionally from 5'3' or 3'5'
• The nitrogenous base is attached to the sugar molecule at the first carbon atom (1') and the sugar molecule is attached to the phosphate group at the 5-carbon atom (5')
• Each DNA molecule has a sugar-phosphate backbone on the outside and nitrogenous base pairs on the inside
• What makes DNA special: helical double-stranded molecule, self-replication, complementary base pairing, held together by weak hydrogen bonds
• DNA is unbound and circular in the cytosol of prokaryotes and in the mitochondria and chloroplasts of eukaryotes
• Prokaryotic cells are made of DNA with no protein attached to it,

  chromosome in prokaryotic cell floats freely in the cytosol,

  mitochondria and chloroplasts have their own DNA separate from the nucleus
• Types of DNA

  • Nuclear DNA
  • Mitochondrial DNA
• Nuclear DNA is found in the nucleus of the cell, has 2 sets of 23 chromosomes, maternal and paternal chromosomes, DNA packed into chromatin, bounded by a nuclear envelope, double helix structure
• Mitochondrial DNA is found in mitochondria of the cells, each mitochondrion may have several copies of the single DNA molecule, maternal only chromosomes, DNA is not packed into chromatin, free of a nuclear envelope, circular structure
• Nuclear DNA can discriminate between individuals of the same maternal lineage, mitochondrial DNA cannot discriminate between individuals of the same maternal lineage
• In eukaryotes, DNA is bound to proteins (histones) in linear chromosomes found in the nucleus
• A particular gene has a particular locus on a particular chromosome
• Chromosomes are formed in eukaryotic cells by the long thread-like strands of DNA in the nucleus coiled around proteins (histones)
• Chromosomes are not normally visible as discrete units unless the cell is dividing and the long strands have condensed to form shorter, thicker rod-like structures
• Chromosomes in prokaryotic cells do not contain histones
• Information coded in the DNA determines what proteins are made within a cell and influences the structure and function of the cell
• Types of genomes

  • Prokaryotic genome
  • Eukaryotic genome
• Prokaryotic genome characteristics

  • Circular chromosome
  • No histones
  • Located in the cytoplasm
  • One per cell
  • Mostly no introns
  • Small in size
  • Condensed in nucleoid via DNA supercoiling
  • Does not have a nucleus
• Eukaryotic genome characteristics

  • Linear chromosome
  • Contains histones
  • Located in the nucleus
  • Two or more per cell
  • Have introns
  • Larger than prokaryotic genome
  • Condensed in membrane-bound nucleus via histones
  • Has nucleus
• Biological significance of complementary base-pairing

  Base pairing enable separation of polynucleotide strand so that information stored can be used for replication and transcription
• Semi-conservative DNA replication

  1. Enzyme DNA helicase unwinds the double helix by breaking the hydrogen bonds between the base pairs in a molecule

  2. Region where DNA begins replicating forms a replication fork

  3. DNA replication proceeds in the 5’ → 3’ direction, proceeding in opposite directions (bidirectional) on the same molecule

  4. DNA polymerase enzyme moves along the exposed bases of the template strand allowing complementary free DNA nucleotides to bind and form a new DNA strand

  5. Two new molecules of DNA are formed from one DNA molecule, in each molecule one strand is ‘old’ and the other is ‘new’
• Biological significance of DNA replication

  Identical copies of genetic information can be copied and transmitted to the next generation
• Enzymes involved in DNA replication include helicase, DNA polymerase, and DNA ligase