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Chapter 8: DNA and the Language of Life


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Title: Chapter 8: DNA and the Language of Life

Chapter 8 DNA and the Language of Life
Fredrick Griffith
  • In 1928, Fredrick Griffith was studying two forms
    or strain of a bacterial species one strain was
    fatal to mice, while the other strain was

Strain 1 Strain 2
Griffiths Experiment
  • When he injected heat-treated bacteria into mice,
    the mice remained healthy.
  • Heat kills the deadly strain of bacterium, making
    it harmless.

Strain 1 Strain 2 Heated Strain 1
Griffiths Transforming Factor
  • When he injected mice with a mixture of the
    harmless strain and the heat-treated deadly
    strain, he expected the mice to survive.
  • However, the mixture killed the mice. Some of the
    harmless bacteria had been transformed,
    becoming deadly.
  • Strain 1 Strain 2 Heated Strain 1
    Strain 2 heated Strain1

DNA or Protein?????
  • After Griffiths experiment, scientists began to
    search for the transforming factor.
  • Attention focused on two types of chemicals
    protein and DNA.
  • In 1952, biologists Alfred Hershey and Martha
    chase provided more evidence to distinguish
    between the two possibilities.
  • They conducted a series of experiments using

  • A virus is package of nucleic acid wrapped in a
    protein coat.
  • Unlike living things, viruses are not made of
  • A virus can only reproduce by infecting a living
    cell with its genetic material.
  • A virus that infects bacteria is called a
  • Phage consists of a protein coat that encloses
    the genetic
  • material. When a phage infects a bacterium, it
  • its genetic material into the bacterium, while
    its coat
  • remains outside.

Hershey and Chases Experiment
  • In the first experiment, phages with
    radioactive-labeled DNA, infected bacteria.
  • In a second experiment, phages with
    radioactive-labeled protein infected bacteria.
  • In both experiments, bacteria were separated from
    the phage coats by blending.
  • In the first experiment, most radioactivity was
    found in the infected bacteria, while in the
    second experiment most radioactivity was found in
    the phage coat.
  • These experiments demonstrated that DNA is the
    genetic material of phage and that protein does
    not transmit genetic information.

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DNA Structure
  • The heritable genetic information of an organism
    is stored in DNA.
  • The DNA molecule is made up of two strands that
    are held together by hydrogen bonds.
  • These strands are parallel to each other but run
    in opposite directions, called antiparallel.
  • Each strand of DNA consists of a large number of

DNA Structure
  • Each nucleotide is made up of a deoxyribose
    (sugar), a phosphate group and a nitrogenous
  • There are 4 kinds of nitrogenous bases, Thymine
    (T), Cytosine (C), Adenine (A), and Guanine (G).
  • Bases Thymine (T) and Cytosine (C) are
    single-ring structure called pyramidines.
  • Bases Adenine (A) and Guanine (G) are double-ring
    structures called purines.
  • In the double stranded DNA, A always pairs with T
    and C always pairs with G. This is called
    complementary base pairing.

DNA Strands
  • Nucleotides are joined to one another by covalent
    bonds that connect the sugar of one nucleotide to
    the phosphate group of the next.
  • This repeating pattern of sugar-phosphate is
    called a sugar-phosphate backbone.
  • Knowing the main components of DNA, Watson and
    Crick tried to figure out how the elements fit
  • One clue came from X-ray photographs of DNA taken
    by Rosalind Franklin suggested that the structure
    of DNA was a double helix.

DNA Replication
  • The process of copying the DNA molecule.
  • This process can be divided into 3 major parts
  • Binding of enzymes to existing DNA
  • Unwinding the double helix
  • Synthesis of a new matching strand for each
    existing strand

DNA Replication
  • 1. What do the yellow strands of DNA represent?
  • new DNA strands
  • 2. What will the two daughter DNA molecules
    consist of?
  • Each consists of one original (parent) strand
    and one new strand.

  • RNA                       DNA
  • Single-stranded                 
  • Ribose                                   
  • A, C, G and U                     
    A, C, G and T

Types of RNA
  • 1.  mRNA - messenger RNA - carries information
    from DNA to the ribosome where the protein is
  • 2.  tRNA - transfer RNA - carries amino acids to
    mRNA at the ribosome to assembly the protein
    being made. 3.  rRNA - ribosomal RNA - major
    structural component of the ribosome where
    protein synthesis occurs.

The Central Dogma
DNA is maintained by DNA replication, DNA is
transcribed into RNA, RNA is translated into
What are proteins?
  • We are protein.
  • Our hair, our nails, our skin, our blood, our
    enzymes and hormones are protein
  • indeed, our bodies contain some ten thousand to
    fifty thousand kinds of protein.

Genetic Alphabets
  • Genetic "Alphabets" - there are three alphabets
    involved in the entire process of protein
  • 1)  DNA - A, C, G and T
  • 2)  RNA - A, C, G and U
  • 3)  Protein - Twenty different amino acids
  • Triplet Code - three nucleotides code for one
    amino acid
  • 1. Codon a triplet in mRNA, pairs with triplet
    on a tRNA molecule carrying the correct amino
  • 2. Anticodon a triplet in tRNA    

The Genetic Code
  • Example 1
  • 1)  mRNA Codon AUG
  • 2)  Amino Acid Met 
  • Example 2
  •   C U A G G C A A C U U A
  • Amino Acids 2
  • Leu - Gly - Asn - Leu
  • Example 3
  • Leu Arg Arg - Lys

  • Purpose to synthesize RNA from a DNA template
  • It starts when the enzyme RNA polymerase attaches
    to a specific region of the DNA.
  • As a result, a single complementary strand of RNA
    is made.

Editing the mRNA message
  • Transcribed mRNA must first be edited before it
    can leave the nucleus for the cytoplasm.
  • The initial RNA transcripts have stretches of
    noncoding nucleotides that interrupt nucleotide
    sequences that actually code for amino acids.

  • Splicing - Introns are removed and Exons are
    joined together.
  • Intron - segment of mRNA which does NOT code for
    protein therefore, it is removed.
  • Exon - segment of mRNA which does code for
    protein therefore, it remains for expression
    in protein.

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  • It is the final step on the way from DNA to
  • It is the synthesis of proteins directed by a
    mRNA template.
  • tRNA translates the three-letter codons of mRNA
    to the amino acids that make up proteins.
  • It happens on ribosomes.

tRNA during Translation
  1. A tRNA molecule must become bound to the
    appropriate amino acid.
  2. It has to recognize the appropriate codon in the
  3. During translation, the anticodon on tRNA
    recognizes a particular codon on mRNA by using
    base-pairing rules.

  • A ribosome has two subunits
  • Small subunit binding site for mRNA
  • Large subunit consist of two binding sites for
  • P site holds the tRNA carrying the growing
    polypeptide chain.
  • A site holds the tRNA carrying the next amino
    acid to be added to the chain.

Steps of Translation
  • First, the ribosome attaches at a specific site
    on the mRNA. This site is the start codon, AUG.
  • Next, amino acids are added one by one to the
    growing chain of amino acids.
  • During translation, the ribosome moves down the
    mRNA, codon by codon, until translation is
  • This process continues until the ribosome reaches
    a stop codon UAA, UAG, or UGA.

  • A mutation is any change in the nucleotide
    sequence of DNA.
  • Mutations can change the meaning of genes.
  • Mutations can involve large regions of a
    chromosome or just a single nucleotide.
  • Mutation can be divided into 2 general categories.

1. Base Substitution
  • Base substitution replacement of one base or
    nucleotide with another.
  • It results in a change that affects the function
    of a protein.

2. Base Insertion or Deletion
  • 2. Base insertion or deletion adding or
    subtracting nucleotides.
  • Its usually more disastrous than the effects of
    base substitutions.

What Causes Mutation?
  • May occur when errors are made during DNA
  • May occur when errors happen during chromosome
    crossovers in meiosis.
  • Physical or chemical agents that cause mutations
    are called mutagens.
  • Most common physical mutagen is high-energy
    radiation. Ex X-rays
  • Mutations are the ultimate source of genetic
    diversity in the living world.

Transcription Translation
  • http//
  • Translation video-http//
  • Protein synthesis- http//

Sickle cell disease
  • Sickle cell disease is an example of a disorder
    caused by inheritance of a genetic mutation.
  • In sickle cell disease, the hemoglobin in red
    blood cells tends to bind together when oxygen
    levels are low.
  • The hemoglobin crystals deform the red blood
    cells into sickle, or crescent shapes.
  • The sickle shaped cells clog tiny blood vessels,
    dangerously blocking the normal flow of blood.

Replication of the Double Helix
  • More than a dozen enzymes are involved in DNA
  • Each incoming nucleotide pairs with its
    complementary nucleotide on the parent strand.
  • Enzymes called DNA polymerase make the covalent
    bonds between the nucleotides of the new strand.
  • DNA replication begins at specific sites called
    origins of replication. The copying proceeds