DNA & RNA

DNA Structure

• Double helix (two strands that form a spiral together)
• Polynucleotide strands
• Long and tightly coiled to hold a lot of genetic information in a small space in the cell nucleus

DNA Nucleotide Structure

• Phosphate group, deoxyribose (a pentose sugar) and a nitrogen containing organic base
• Only the base in a nucleotide can vary
• Four possible bases: adenine (A), thymine (T), cytosine (C), guanine (G)

Complementary Base Pairing

• DNA polynucleotide strands join by hydrogen bonds
• A bonds to T, C bonds to G
• Two hydrogen bonds between A and T, three hydrogen bonds between C and G
• The polynucleotide strands are anti-parallel (they run in opposite directions)

RNA Structure

• Single polynucleotide strand with a sugar-phosphate backbone
• Shorter polynucleotide strand

RNA Nucleotide Structure

• Phosphate group, ribose sugar and a nitrogen containing organic base
• Only the nitrogen containing base can vary
• Four bases: adenine (A), uracil (U), cytosine (C) and guanine (G)

Complementary Base Pairing

• A bonds with U, C bonds with G

• When first observed, many scientists thought DNA was too simple to carry genetic information
• It was thought that the more chemically varied proteins carried genetic information
• Experiments that proved DNA carried the genetic code and the discovery of the double helix in DNA both happened in the same year

Shape: DNA is a twisted double helix, RNA is a single strand

Pentose sugar: DNA has a deoxyribose sugar, RNA has a ribose sugar

Bases: DNA has the bases A, T, C, G. RNA has the bases A, U, C, G

Size: DNA is long, RNA is relatively short

DNA Replication

• Occurs before cell division so each new cell has the full amount of DNA
• Semi-conservative replication: One strand of each new DNA molecule is from the original
• Genetic continuity between each cell generation: new cells inherit genes from parent cells

Stage One

• DNA helicase breaks hydrogen bonds between bases
• Helix unwinds into two separate strands

Stage Two

• Each original strand acts as a template for a new strand.
• Free-floating DNA nucleotides are attracted to complementary exposed bases

Stage Three

• DNA polymerase forms hydrogen bonds between the nucleotides

• One end of a DNA strand is 3` (3 prime) and the other is 5` (5 prime)
• DNA polymerase is complementary to the 3` end of the newly forming DNA strand
• DNA polymerase moves along the template/original strand in a 3` to 5` direction, so nucleotides are added to the new strand in a 5` to 3` direction
• Anti-parallel strands mean that a DNA polymerase working on one template strand moves in the opposite direction of the other

Evidence for Semi-conservative replication

• Originally people weren't sure whether DNA replication was semi-conservative or conservative

Meselson and Stahl's Experiment

• Used two nitrogen isotopes - heavy (15^N) and light (14^N)
• 1. Many generations of bacteria grown in nutrient broths - one with light and one with heavy. The bacteria took up the nitrogen to make nucleotides for new DNA
• 2. Sample of DNA was taken from each batch of bacteria and spun in a centrifuge. DNA from heavy nitrogen bacteria settled lower in the tube than the light nitrogen batch
• 3. The bacteria from the heavy nitrogen broth were placed in a light nitrogen broth and left for one round of DNA replication. A DNA sample was taken and spun in a centrifuge
• 4. The heavy DNA would settle at the bottom and new light DNA at the top if conservative
• 5. The new DNA settled in the middle, showing DNA molecules contained a mixture of heavy and light nitrogen due to semi-conservative replication
• Other experiments proved that semi-conservative replication is in all living things