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24.1 DNA Structure and Replication. Hershey-Chase Experiments (1952) Demonstrated that DNA is the genetic material DNA stands for Deoxyribonucleic Acid. Hershey-Chase Experiments. 24.1 DNA Structure and Replication. Structure of DNA

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24 1 dna structure and replication
24.1 DNA Structure and Replication
  • Hershey-Chase Experiments (1952)
    • Demonstrated that DNA is the genetic material
    • DNA stands for Deoxyribonucleic Acid
24 1 dna structure and replication1
24.1 DNA Structure and Replication
  • Structure of DNA
    • James Watson and Francis Crick determined the structure of DNA in 1953
    • DNA is a chain of nucleotides
    • Each nucleotide is a complex of three subunits
      • Phosphoric acid (phosphate)
      • A pentose sugar (deoxyribose)
      • A nitrogen-containing base
24 1 dna structure and replication2
24.1 DNA Structure and Replication
  • Structure of DNA
    • Four Possible Bases
      • Adenine (A) - a purine
      • Guanine (G) - a purine
      • Thymine (T) - a pyrimidine
      • Cytosine (C) - a pyrimidine
    • Complimentary Base Pairing
      • Adenine (A) always pairs with Thymine (T)
      • Guanine (G) always pairs with Cytosine (C)
24 1 dna structure and replication3
24.1 DNA Structure and Replication
  • Replication of DNA
    • Semi-conservative replication
      • Each daughter DNA molecule consists of one new chain of nucleotides and one from the parent DNA molecule
    • The two daughter DNA molecules will be identical to the parent molecule
24 1 dna structure and replication4
24.1 DNA Structure and Replication
  • Replication of DNA
    • Before replication begins, the two strands of the parent molecule are hydrogen-bonded together
    • Enzyme DNA helicase unwinds and “unzips” the double-stranded DNA
    • New complementary DNA nucleotides fit into place along divided strands by complementary base pairing. These are positioned and joined by DNA polymerase
    • DNA ligase repairs any breaks in the sugar-phosphate backbone
    • The Two double helix molecules identical to each other and to the original DNA molecule
24 2 gene expression
24.2 Gene Expression
  • Gene: A segment of DNA that specifies the amino acid sequence of a polypeptide
  • DNA does not directly control protein synthesis, instead its information is transcribed into RNA
24 2 gene expression1
24.2 Gene Expression
  • RNA (ribonucleic acid)
24 2 gene expression2
24.2 Gene Expression
  • Three Classes of RNA
    • Messenger RNA (mRNA)
      • Takes a message from DNA to the ribosomes
    • Ribosomal RNA (rRNA)
      • Makes up ribosomes (along with proteins)
    • Transfer RNA (tRNA)
      • Transfers amino acids to ribosomes
24 2 gene expression3
24.2 Gene Expression
  • Gene Expression Requires Two Steps:
    • Transcription
    • Translation
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24.2 Gene Expression
  • Transcription
    • During transcription, a segment of the DNA serves as a template for the production of an RNA molecule
    • Messenger RNA (mRNA)
      • RNA polymerase binds to a promoter
      • DNA helix is opened so complementary base pairing can occur
      • RNA polymerase joins new RNA nucleotides in a sequence complementary to that on the DNA
24 2 gene expression5
24.2 Gene Expression
  • Transcription
    • Processing of mRNA
      • Primary mRNA becomes mature mRNA
      • Contains bases complementary to both intron and exon segments of DNA
        • Introns are intragene segments
        • Exons are the portion of a gene that is expressed
      • Intron sequences are removed, and a poly-A tail is added
        • Ribozyme splices exon segments together
24 2 gene expression6
24.2 Gene Expression
  • Translation
    • The Genetic Code
      • Triplet code- each 3-nucleotide unit of a mRNA molecule is called a codon
      • There are 64 different mRNA codons
        • 61 code for particular amino acids
          • Redundant code-some amino acids have numerous code words
          • Provides some protection against mutations
        • 3 are stop codons signal polypeptide termination
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24.2 Gene Expression
  • Transfer RNA
    • tRNA transports amino acids to the ribosomes
    • Single stranded nucleic acid that correlates a specific nucleotide sequence with a specific amino acid
    • Amino acid binds to one end, the opposite end has an anticodon
    • the order of mRNA codons determines the order in which tRNA brings in amino acids
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24.2 Gene Expression
  • Ribosome and Ribosomal RNA
    • Ribosome has a binding site for mRNA and for 2 tRNAs
    • Facilitate complementary base pairing
    • Ribosome moves along mRNA and new tRNAs come in and line up in order
    • This brings amino acids in line in a specific order to form a polypeptide
    • Several ribosomes may move along the same mRNA
      • Multiple copies of a polypeptide may be made
      • The entire complex is called a polyribosome
24 2 gene expression9
24.2 Gene Expression
  • Translation Requires Three Steps
    • Initiation (requires energy)
    • Elongation (requires energy)
    • Termination
24 2 gene expression10
24.2 Gene Expression
  • Genes and Gene Mutations
    • A gene mutation is a change in the sequence of bases within a gene.
    • Gene mutations can lead to malfunctioning proteins in cells.
24 2 gene expression11
24.2 Gene Expression
  • Genes and Gene Mutations
    • Causes of Mutations
      • Errors in replication
        • Rare
        • DNA polymerase “proofreads” new strands and errors are cleaved out
      • Mutagens
        • Environmental influences
        • Radiation, UV light, chemicals
        • Rate is low because DNA repair enzymes monitor and repair DNA
      • Transposons
        • “jumping genes”
        • Can move to new locations and disrupt sequences
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24.2 Gene Expression
  • Types of Mutations
    • Frameshift Mutations
      • One or more nucleotides are inserted or deleted
      • Results in a polypeptide that codes for the wrong sequence of amino acids
    • Point Mutations
      • The substitution of one nucleotide for another
        • Silent mutations
        • Nonsense mutations
        • Missense mutations
24 3 dna technology
24.3 DNA Technology
  • The Cloning of a Gene
    • Cloning: Production of many identical copies of an organism through some asexual means.
    • Gene Cloning: The production of many identical copies of a single gene
    • Two Ways to Clone a Gene:
        • Recombinant DNA
        • Polymerase Chain Reaction
24 3 dna technology1
24.3 DNA Technology
  • Using Recombinant DNA Technology
    • Restriction enzymes breaks open a plasmid vector at specific sequence of bases “sticky ends”
    • Foreign DNA that is to be inserted is also cleaved with same restriction enzyme so ends match
    • Foreign DNA is inserted into plasmid DNA and “sticky ends” pair up
    • DNA ligase seals them together
24 3 dna technology2
24.3 DNA Technology
  • Polymerase Chain Reaction
    • Amplifies a targeted DNA sequence
    • Requires DNA polymerase, a set of primers, and a supply of nucleotides
      • Primers are single stranded DNA sequences that start replication process
    • Amount of DNA doubles with each replication cycle
    • Process is now automated
24 3 dna technology3
24.3 DNA Technology
  • DNA Fingerprinting
    • Permits identification of individuals and their relatives
    • Based on differences between sequences in nucleotides between individuals
    • Detection of the number of repeating segments (called repeats) are present at specific locations in DNA
      • Different numbers in different people
      • PCR amplifies only particular portions of the DNA
      • Procedure is performed at several locations to identify repeats
24 3 dna technology4
24.3 DNA Technology
  • Biotechnology
    • Biotechnology uses natural biological systems to create a product or to achieve a goal desired by humans.
    • Transgenic organisms have a foreign gene inserted into their DNA
24 3 dna technology5
24.3 DNA Technology
  • Transgenic Bacteria
    • Medical Uses: Production of Insulin, Human Growth Hormone, Tissue Plasminogen Activator, Hepatitis B Vaccine
    • Agricultural Uses: Bacteria that protects plants from freezing, bacteria that protect plant roots from insects
    • Environmental: Bacteria that degrade oil (clean up after oil spills), bacteria that remove sulfur from coal
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24.3 DNA Technology
  • Transgenic Plants
    • Plants have been engineered to secrete a toxin that kills insects
    • Plants have been engineered to be resistant to herbicides
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24.3 DNA Technology
  • Transgenic Animals
    • Fish, cows, pigs, rabbits and sheep have been engineered to produce human growth hormone in order to increase size of the animals