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DNA Replication and Protein Synthesis

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  • 1. Processes of DNA Replication and Protein Synthesis
  • 2. DNA Replication
  • 3. Replication
    • Makes duplicate copies of DNA in the process of replication
    • DNA replication involves separating the DNA molecule into 2 strands.
    • Each strand than serves as a pattern, or template, to make a new DNA molecule.
    • The result is 2 identical double-stranded molecules of DNA
  • 4. Replication (con’t)
    • Because each of these double-stranded molecules of DNA consists of a single strand of old DNA (the template) and a single strand of new, replicated DNA (the complement), this is called semi-conservative replication.
  • 5. Steps
    • Helicase unwinds DNA, creating a replication fork. Proteins bind to the single strand to keep the DNA from recombining.
    • RNA primase starts replication at special base sequences by adding RNA primers.
    • DNA polymerase attaches to the RNA primers and begins elongation. (Elongation = the adding of DNA nucleotides to the complement strand)
  • 6. Steps (con’t)
    • The leading complementary strand is assembled continuously as the DNA uncoils.
    • The lagging complementary strand is assembled in short pieces called Okazaki fragments. The pieces are joined together by DNA ligase.
    • The RNA primers are replaced by DNA nucleotides.
  • 7. Protein Synthesis
  • 8. Protein Synthesis
    • Proteins can function as:
      • Enzymes
      • Repairing agents for the cell
    • Proteins determine the shape and function of organisms.
    • Each gene is responsible for one protein. So, the message of the DNA code is information for building proteins.
  • 9. Protein Synthesis (con’t)
    • How do genes code for proteins? THE GENETIC CODE!!!
    • Genetic code: sequence of nitrogen bases along one of the two strands that codes for the synthesis of proteins.
    • The nitrogen bases set up in sets of 3 that are known as codons.
  • 10. Protein Synthesis (con’t)
    • Because the sequence of 3 nitrogen bases forms the code for an amino acid, the DNA code is often called the triplet code.
    • 64 combinations are possible when a sequence of 3 bases is used. So, 64 different codons are in the genetic code.
    • 61 of the codons code for amino acids, and the other 3 code for protein synthesis to start or stop.
  • 11. Protein Synthesis (con’t)
    • More than one codon can code for an amino acid; however, for any one codon, there can only be one amino acid.
    • The genetic code is universal. Universal means that the codons represent the same amino acids in all organisms.
    • Protein synthesis: process that describes how enzymes and other proteins are made from DNA.
  • 12. Protein Synthesis (con’t)
    • Steps of Protein Synthesis:
      • Transcription
      • RNA processing
      • Translation
    • There are 3 types of RNA:
      • mRNA (messenger): provides the instructions for assembling amino acids into a polypeptide.
      • tRNA (transfer): delivers amino acids to a ribosome for their addition into a growing polypeptide chain
      • rRNA (ribosomal): combines with proteins to form ribosomes
  • 13. Transcription: From DNA to RNA RNA is transcribed from a DNA template by a process that resembles DNA replication.
  • 14. Steps of Transcription:
    • Initiation
      • The “start transcribing” signal is a nucleotide sequence called the promoter, located in the DNA at the beginning of the gene. Promoter is a specific place where RNA polymerase attaches.
      • RNA polymerase attaches to the promoter
      • Start of RNA synthesis
  • 15. Steps of Transcription (con’t)
    • RNA Elongation
      • During the 2 nd phase of transcription, elongation, the RNA grows longer.
      • As RNA synthesis continues, the RNA peels away from DNA and allows the DNA strand to rejoin.
  • 16.
    • Termination
      • The 3 rd and final phase, RNA polymerase reaches the terminator sequence of bases in DNA.
      • This sequence signals the end of the gene and the polymerase molecules detaches from the RNA molecule and the gene.
    Steps of Transcription (con’t)
  • 17. RNA Processing The RNA copy that is formed is called mRNA. The mRNA carries the information for making the protein chain to the ribosomes, where it goes through RNA processing.
  • 18. Translation Conversion between different languages – from nucleic acid to protein. Occurs on the ribosomes and uses tRNA. End product is a very long protein chain.
  • 19. tRNA
    • Composed of about 75 nucleotides
    • Each tRNA recognizes only one amino acid.
    • The amino acid becomes bonded to the top of the tRNA.
    • Located on the bottom of the tRNA are 3 nitrogenous bases, called an anticodon, which pairs with mRNA codons during translation.
  • 20. Steps of Translation
    • Initiation
      • 1 st phase brings together:
        • mRNA
        • 1 st amino acid with its attached tRNA, and
        • The 2 subunits of a ribosome
  • 21. Steps of Translation (con’t)
    • Elongation
      • Amino acids are added one by one in a 3-step process
        • Step 1
          • Codon recognition
        • Step 2
          • Peptide bond formation
        • Step 3
          • Translocation
  • 22.
    • Termination
      • Elongation continues until a stop codon (UAA, UAG, or UGA) is reached.
      • Polypeptide is released.
      • Ribosome splits into subunits and falls off.
    Steps of Translation (con’t)