DNA REPLICATION

• One gene-one enzyme hypothesis


  Belief that each gene controlled the production of a single protein
• One gene-one protein hypothesis

  Many proteins are structural proteins, not enzymes
• One gene-one polypeptide hypothesis

  Some proteins consist of several polypeptide chains that are linked together
• As a result of the Human Genome Project, the one gene-one polypeptide hypothesis has had to be changed again
• We now know that a gene can produce more than one polypeptide depending upon how the information in the gene is read
• Differences between RNA and DNA

  • RNA contains the sugar ribose
  • RNA contains the base uracil
  • RNA is usually single stranded
  • RNA is short: one gene long at most
  • DNA contains deoxiribose
  • DNA contains thymine instead
  • DNA is usually double stranded
  • DNA is long, containing many genes
• RNA


  An intermediary (link) between DNA and a polypeptide chain
• Types of RNA


  • Messenger RNA (mRNA)
  • Ribosomal RNA (rRNA)
  • Transfer RNA (tRNA)
• Messenger RNA (mRNA)


  Carries the information that specifies a protein, each 3 bases in a row form a codon which corresponds to one amino acid
• Ribosomal RNA (rRNA)


  Combines with proteins to form a ribosome, helps to correctly align the ribosome and mRNA, and catalyzes formation of bonds between amino acids
• Transfer RNA (tRNA)


  Connector molecules that bind an mRNA codon at one end and a specific amino acid to the ribosome at the correct spot along the mRNA molecule
• Flow of information in the cell

  1. DNA replicates to form more DNA

  2. Information is transcribed into RNA

  3. Information is translated into protein
• Proteins do most of the work in the cell
• Information does not flow in the other direction
• Changes in proteins


  Do not affect the DNA in a systematic manner
• Changes in proteins can cause random changes in DNA
• Duplication


  1. Before a cell can divide, the DNA in the nucleus of the cell must be duplicated

  2. The two strands of the DNA molecule separate and new complementary strands are produced

  3. Each new DNA molecule will consist of one old strand, and a new complementary strand

  4. The gray strands in the figure are new strands in the process of being assembled
• Semiconservative replication


  In the new DNA molecule there is one old and one new strand
• Duplication needs the following participants

  • Helicase - enzymes that unwind "unzip" the DNA
  • Binding proteins - prevents the two single strands from rejoining each other
  • Primase - an enzyme that builds short complementary piece of RNA, called RNA primer attracts the DNA polymerase
  • DNA polymerase - is the enzyme that adds new DNA nucleotides that are complementary to the bases on each exposed strand
  • Ligase - an enzyme that removes each RNA primer and replaces it with correct DNA nucleotides once the new strand of DNA is in place
• Steps in DNA replication

  1. The first step is to 'unzip' the double helix structure of the DNA molecule

  2. Helicase breaks the hydrogen bonds holding the complementary bases of DNA together

  3. The separation of the two single strands creates a 'Y' shape called a replication 'fork'

  4. The leading strand is replicated continuously in the 5' to 3' direction

  5. The lagging strand is replicated discontinuously with Okazaki fragments

  6. DNA polymerase adds new complementary nucleotides

  7. DNA ligase joins the DNA segments together
• DNA replication happens at many sites, forming thousands of replication bubbles to copy the large DNA molecule faster
• Exact duplication of the DNA message is required so each new cell has the same genetic instructions
• DNA carries information to construct proteins which form structures and regulate the body's activities
• Protein synthesis


  1. Transcription - DNA message is converted into an RNA molecule

  2. Translation - RNA message is used to assemble amino acids into a protein chain
• Components of a nucleotide


  • 5-carbon sugar (deoxyribose, ribose)
  • Phosphate group
  • Nitrogenous bases (DNA: A, T, C, G; RNA: A, U, C, G)
• A=T, C=G, 1:1 ratio of nitrogenous bases
• Chromosome


  Thread-like structure, condensed state of chromatin
• Gene


  Portion or segment on DNA
• Genome


  Totality of genes