DNA Structure and Replication - PowerPoint PPT Presentation


PPT – DNA Structure and Replication PowerPoint presentation | free to download - id: 1fcf0-NzE3M


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation

DNA Structure and Replication


DNA and RNA were first described by Friedrich Miescher in 1869. ... DNA was known to be a linear polymer of just 4nucleotides, and it was thought to ... – PowerPoint PPT presentation

Number of Views:709
Avg rating:3.0/5.0
Slides: 16
Provided by: Mitric
Learn more at: http://www.bios.niu.edu


Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: DNA Structure and Replication

DNA Structure and Replication
DNA is the Genetic Material
  • DNA and RNA were first described by Friedrich
    Miescher in 1869. He isolated a
    phosphorus-containing material from the nuclei
    of cells found in pus from discarded surgical
    bandages, and called in nuclein. He later
    found the same material in salmon sperm. Later
    it was recognized that DNA and RNA are slightly
    different in structure.
  • In pre-digital days it seemed impossible for
    complex phenotypic traits to be coded in a simple
    linear fashion. DNA was known to be a linear
    polymer of just 4nucleotides, and it was thought
    to be just a scaffold for the actual genes.
    Proteins were considered the most likely genetic
    material, or perhaps some undiscovered substance.
  • Definitive proof of DNAs central role came in
    the 1950s, but important experiments were done

Transformation Experiments
  • In 1928, F. Griffith did a series of experiments
    on mice infected with Streptococcus bacteria.
    These experiments were followed up Avery,
    MacLeod, and McCarty in 1943, who demonstrated
    that at least in this case, DNA was the
    hereditary material, and not proteins.
  • Griffith had 2 strains of Streptococcus. The S
    strain had a polysaccharide coat around each
    cell, causing the colonies to have a smooth,
    glossy appearance. The R strain lacked the coat,
    and its colonies had a rough appearance. More
    importantly, the S strain was virulent when
    injected into mice, they developed pneumonia and
    died. The R strain was avirulent it did not
    kill the mice upon injection.
  • When S cells were killed by heat, injecting them
    had no effect on the mice. Heat killed R cells
    also had no effect.
  • The surprising result when live R cells were
    mixed with heat-killed S cells and injected, the
    mice developed an infection and died. When
    bacteria were isolated from the dead mice, they
    were found to be S type.
  • Conclusion something from the dead S cells had
    transformed the live R cells into S.

More Transformation
  • Avery, MacLeod, and MacCarty carried this result
    further by fractionating the dead S cells. They
    mixed various components (such as lipids,
    polysaccharides, protein, nucleic acids) of the S
    cells with live R cells, to determine which
    component caused the transformation. They found
    the DNA by itself caused the transformation, and
    no other component had any effect.
  • This demonstrated that DNA was the hereditary
    material, but their results were not considered
    to be generally applicable to inheritance.

Hershey-Chase Experiment
  • Another experiment, performed by Hershey and
    Chase in 1952, demonstrated the essential role of
    DNA using bacteriophage. At the time, a group of
    physicists who had previously worked on nuclear
    energy were moving into biology. They started
    work on the genetics of bacteria and
    bacteriophage that grew into modern molecular
  • An important element of the Hershey-Chase
    experiment is that DNA contains phosphorus but
    not sulfur, while protein contains sulfur but not
    phosphorus. Thus it is possible to label the two
    types of molecule independently, with radioactive
    32P and 35S.
  • Phage contain only DNA and protein, and no other
    types of molecule.
  • They used phage T2, infecting E. coli.
  • Hershey and Chase showed that 32P-labelled DNA
    entered the bacterial cells when the phage
    infected them, and that the new generation of
    phage contained a significant amount of that
    labelled DNA.
  • In contrast, the 35S-labelled protein stayed
    outside the cells during an infection, and none
    of it ended up in the new phage.
  • This implies that DNA is necessary for phage

Structure of DNA
  • Once the importance of DNA was recognized, it was
    necessary to deduce how the DNA molecule is
    structured. A race between various lab groups
    ensued, and in 1953 James Watson and Francis
    Crick published a model of DNA structure. Their
    work was based on X-ray crystallography data
    provided by Maurice Wilkins and Rosalind
  • DNA consists of two anti-parallel chains twisted
    into a helix. The nitrogenous bases are paired
    in the center of the molecule, and the
    phosphate-sugar backbones are on the outside.
  • Although DNA is the genetic material of all
    living cells, some viruses use RNA as their
    genetic material.

Nucleotide Structure
  • DNA and RNA are macromolecules composed of
    subunits called nucleotides.
  • Each nucleotide of DNA or RNA has 3 parts a
    nitrogenous base, a sugar, and a phosphate group.
  • The phosphate group, PO4, links two sugar
    molecules in the backbone. Each phosphate
    carries a -1 charge. This causes DNA to have an
    overall negative charge.
  • The sugar is ribose in the case of RNA and
    deoxyribose in the case of DNA. has 5 carbons,
    numbered 1 through 5.
  • the nitrogenous base is attached to the 1 carbon
  • the 2 carbon has a free -OH group in the case of
    RNA, but a -H group in the case of DNA. The lack
    of the oxygen atom makes DNA far less reactive
    than RNA.
  • the 3 carbon has an -OH group on it that links
    to the phosphate group on the next base. The
    end of the DNA molecule is a free 3 OH group.
  • the 5 carbon is attached to the phosphate group.

More Nucleotide Structure
  • There are 4 possible DNA bases adenine (A),
    guanine (G), cytosine (C), and thymine (T).
  • Adenine and guanine are purines they consist of
    two linked rings of mixed nitrogen and carbon
  • Thymine and cytosine are pyrimidines, which
    consist of a single ring. In RNA, thymine is
    replaced by uracil (U), which looks like thymine
    except for a single methyl group.
  • Each strand of DNA pairs with a complementary DNA
    strand. This pairing happens because each A is
    paired with a T, and each G is paired with a C.
    Thus, the information on one DNA strand easily
    allows the other strand to be deduced. The
    amount of A in DNA always equals the amount of T,
    and the amount of G always equals the amount of
    C. This is not true in RNA, which is usually
  • Pairing is caused by hydrogen bonds, weak links
    between oxygen and nitrogen atoms where one of
    them has a hydrogen attached.
  • A-T pairs have 2 hydrogen bonds, while G-C pairs
    have 3 hydrogen bonds. G-C pairs are stronger,
    and they are more frequent in high temperature

Paired Nucleotides
Semi-conservative Replication
  • Watson and Crick recognized that the double
    stranded DNA molecule could replicate by
    unwinding, then synthesizing a new strand for
    each of the old stands.
  • This mode of replication is called
    semi-conservative. It means that after one
    DNA molecule has replicated to become 2 DNA
    molecules, each new molecule consists of one old
    strand (from the original molecule) and one new
  • The information from each old strand can be used
    to create the new strands, since A always pairs
    with T, and G always pairs with C.
  • DNA replication starts at specific locations
    origins of replication, and proceeds in both

Replication Components
  • The raw materials of DNA synthesis are
    nucleoside triphosphates, often written as
  • dNTPs have a chain of 3 phosphate groups attached
    to the 5 carbon of the deoxyribose sugar. Just
    as with ATP, the bonds between the phosphates are
    high energy bonds, and releasing them produces
    the energy needed to drive the synthesis of DNA.
  • Each new nucleotide is added to a growing DNA
    chain by removing the outer 2 phosphates and
    attaching the remaining phosphate to the 3 OH
    group of the previous nucleotide.
  • The DNA chain is said to grow from 5 to 3,
    which means that the first DNA base has a free 5
    end, with attached phosphates. The last
    nucleotide has a free 3 OH group on it. All
    other bases have their 5 carbons attached to a
    phosphate, which is attached to the 3 OH group
    of the previous nucleotide.
  • DNA polymerase is the main enzyme used to
    replicate DNA. However, DNA polymerase is only
    one enzyme in the replication complex. Several
    other enzymes are needed to cause replication to

Continuous and Discontinuous Synthesis
  • DNA can only be synthesized from 5 to 3, by
    adding new nucleotides to the 3 end.
  • This is a problem, because both strands must be
    synthesized at the replication fork, and one
    strand will necessarily be synthesized in the
    opposite direction from the movement of the
    replication fork.
  • In reality, one strand is synthesized
    continuously, in the same direction that the
    replication form is moving. This is called the
    leading strand.
  • The other strand is synthesized in short,
    discontinuous pieces, that are then attached
    together to form the final DNA strand. This is
    the lagging strand. Each fragment of the
    lagging strand is called an Okazaki fragment,
    and they are synthesized in the opposite
    direction that the replication fork moves.

Discontinuous Synthesis
  • Another peculiarity of DNA synthesis is that DNA
    polymerase must attach new bases to the 3 end of
    a pre-existing nucleic acid chain. All DNA
    synthesis starts at a short double-stranded
  • In the cell, short pieces of RNA, called
    primers are paired with the DNA bases to create
    to the short double stranded regions that DNA
    synthesis builds.
  • The RNA primers are synthesized by an enzyme
    called primase, and they are removed by DNA
    polymerase during the synthesis of the next
    Okazaki fragment.
  • Joining of the Okazaki fragments is done by the
    enzyme DNA ligase.

Replication Miscellany
  • Errors occur in DNA replication fairly
    frequently the wrong base gets inserted due to
    the peculiarities of nucleotide chemistry.
    However, DNA polymerase has a built-in editing
    function that removes most of the incorrect
  • Enzymes that replicate the RNA genomes of some
    viruses do not have editing functions. Thus,
    mutation rates in RNA viruses are 100-1000 times
    higher than in DNA viruses. This rapid mutation
    rate makes it easy for RNA viruses to evade the
    immune system.
  • DNA in cells is supercoiled, twisted into tight
    knots because the helix is given a few extra
    twists beyond what it needs to maintain its
    shape. Supercoiling has the advantage of causing
    the DNA to be more compact inside the cell, but
    it must be created and maintained by various
    enzymes that wind the DNA, break and rejoin DNA
    strands so they can pass through one another in
    the winding process, and stabilize single
    stranded molecules.
About PowerShow.com