Molecular Genetics Part 1

• Main Components of DNA

  • Deoxyribose sugar
  • Phosphate group
  • Nitrogenous base
• Monomers


  Called nucleotides
• Nucleotides are charged
• Nucleotides are composed of a deoxyribose sugar, a phosphate group, and a nitrogenous base
• Structure of Nucleotides


  • Deoxyribose sugar (5-carbon)
  • Phosphate group attached to the 5' carbon
  • Nitrogenous base attached to the 1' carbon
• The carbons in the deoxyribose sugar are numbered starting from the phosphate group
• The first carbon is called the 1' carbon
• The nitrogenous base is attached to the 1' carbon
• The phosphate group is attached to the 5' carbon
• Purines (A and G) are always bonded to pyrimidines (C and T) in DNA
• This is called complementary base pairing
• Complementary bases

  Attached together by hydrogen bonds
• Hydrogen bonds

  • Relatively strong
  • This is why DNA is very stable
• Hydrogen bonds


  • A and T use
  • G and C use
• DNA helix

  • Right-handed
  • Turns every 10 nucleotides
  • Two strands run antiparallel - one in the 5'-3' direction, the other in the 3'-5' direction
• 3' end terminates with a hydroxyl group and 5' with a phosphate group
• Example DNA sequence


  • 5'-ATGCCGTTA-3'
  • 3'-TACGGCAAT-5'
• DNA replication - Initiation

  1. DNA is opened at the replication origin

  2. Helicase unwinds the DNA by breaking hydrogen bonds

  3. Replication goes in both directions from the origin
• DNA replication - Elongation


  1. Need a RNA primer to act as a starting point for adding nucleotides

  2. DNA Polymerase III can only add nucleotides to the 3' end

  3. Leading strand is continuous, lagging strand is discontinuous (Okazaki fragments)

  4. RNA primers must be removed and gaps filled by DNA
• DNA replication - Termination

  1. Daughter molecules re-wind

  2. A gap is left at the 5' end due to the directionality of DNA synthesis
• Telomeres


  • Highly repetitive DNA at the 5' end of a chromosome
  • Telomerase enzyme adds the repeats
  • Loss of telomeres is linked to aging
• There are 3 billion nucleotides per cell
• DNA replication enzymes make mistakes at a rate of about 1 per every 100,000 nucleotides
• In humans, with our 6 billion base pairs in each diploid cell, that would amount to about 120,000 mistakes every time a cell divides
• Main mechanisms for DNA repair

  • Proofreading
  • Mismatch Repair
• Proofreading


  1. Polymerases I and II act as "proofreaders" and recognize structural imperfections between improperly paired nucleotides

  2. When a mistake is found, the enzyme "exonuclease" backtracks past the mis-paired nucleotide and excises it

  3. Exonuclease then replaces the missing nucleotides
• Proofreading fixes about 99% of these types of errors, but that's still not good enough for normal cell functioning
• It is important to catch mistakes right away otherwise it will be copied to all other strands in the future
• Mismatch Repair


  1. If an error is missed in the replication process, mismatch repair will likely catch it

  2. It works in the same manner as proofreading
• If a mistake in DNA is not repaired


  It can mess up DNA structure, genes that code for proteins, and their function (worst case)
• DNA


  Transmits information within the cell
• Proteins


  A bunch of amino acids joined together (peptides and polypeptides)
• RNA


  Ribonucleic acid, single-stranded, with ribose sugar and uracil instead of thymine
• Transcription of DNA


  1. DNA RNA PROTEIN (central dogma of Molecular Biology)

  2. RNA is used to get the message out of the nucleus and into the cytoplasm where proteins are made
• There are 20 amino acids
• There are 64 codons (combination of 3 nucleic acids)
• From DNA to RNA (Transcription)

  1. Initiation (start signal located)

  2. Elongation (copying of DNA to RNA)

  3. Termination (stop signal located)

  4. Processing (final changes made)
• Initiation


  • Each gene of double stranded DNA has a coding and a non-coding (template) strand
  • The coding strand has the same sequence as the mRNA
  • The start of a gene is indicated by a nucleotide sequence (promoter sequence) where RNA polymerase can bind
• Promoter sequence


  • Has two specific regions: one rich in T and A nucleotides (TATA box), and an upstream element
• Elongation

  1. RNA polymerase opens the DNA one section at a time

  2. RNA is built in the 3' direction using the template strand

  3. Nucleotides are added to the 5' OH group