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Ch. 16 DNA: The Genetic Material Intro In 1953, James Watson and Francis Crick presented their model of DNA to the world. Nucleic Acids (DNA: Deoxyribonucleic acid, RNA: Ribonucleic acid) have a unique ability to replicate itself. DNA’s ability to replicate itself precisely is

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Ch. 16

DNA: The Genetic Material



  • In 1953, James Watson and Francis Crick
  • presented their model of DNA to the world.
  • Nucleic Acids (DNA: Deoxyribonucleic acid,
  • RNA: Ribonucleic acid) have a unique ability
  • to replicate itself.
  • DNA’s ability to replicate itself precisely is

important for its transmission from one

generation to the next.

  • The search for genetic material led to the
  • discovery of DNA and its structure.
  • Before the 1940’s it was thought that
  • proteins were the genetic material.

The genetic role of DNA was first researched

  • by Frederick Griffith in 1928.
  • Studied Streptococcuspneumoniae, a
  • bacterium that causes pneumonia in
  • mammals.
  • He discovered one strain that was
  • nonvirulent (harmless) - R strain.
  • Another strain was virulent (causes
  • pneumonia) – S strain.
  • His experiment:
  • Mixed heat-killed S strain with
  • live R strain and injected it into mice.

The mouse died and Griffith took a

  • blood sample.
  • He found that some of the R strain
  • had changed into the S strain. He
  • called this transformation. Some
  • chemical component had changed the
  • R strain into S strain.

After Griffith’s experiment, researchers tried

to discover this transforming material.

  • Finally in 1944, Oswald Avery, Maclyn
  • McCarty and Colin MacLeod announced that
  • the transforming substance was DNA.
  • They took various chemicals from the
  • heat-killed pathogenic bacteria and tried
  • to transform nonharmless bacteria with
  • them. Only DNA worked.
  • In 1952, Alfred Hershey and Martha Chase
  • showed that DNA was the genetic material
  • of the phage T2 (a virus that infects
  • e. coli bacteria).

They studied bacteriophages – viruses

  • that infect bacteria. They knew that
  • viruses need bacteria in order to
  • replicate.
  • Since viruses have simple structure, they
  • wanted to know whether it was their
  • protein coat or DNA that was the genetic
  • material.

Their experiment:

  • They had two batches of viruses:
  • -Viruses with radioactive sulfur (S-35)
  • labeling their protein coat.
  • -Viruses with radioactive phosphorus
  • (P-32) labeling their DNA.
  • They allowed for the two batches to
  • infect bacteria. After infection, they
  • put the virus/bacteria mixture in a
  • blender so that the viral parts outside
  • of the bacteria could be separated.

They centrifuged the mixture so that

  • the bacteria form a pellet at the
  • bottom of the tube.
  • Then they tested the bacteria for
  • radioactivity.
  • Found radioactivity inside bacteria.

They concluded that the injected DNA,

  • radioactively labeled was the genetic
  • material.
  • In 1947, Erwin Chargaff had developed a
  • series of rules based on a survey of DNA
  • composition in organisms.
  • He already knew that DNA was a polymer
  • of nucleotides consisting of a nitrogenous
  • base, deoxyribose, and a phosphate
  • group.
  • The bases could be adenine (A), thymine
  • (T), guanine (G), or cytosine (C).
  • Chargaff noticed that the DNA
  • composition varied from species to
  • species.

He found that the bases were present in

  • all species in very regular ratios:

-The number of Adenine = Thymine

-The number of Cytosine = Guanine

  • Watson and Crick: By the 1950’s it was now
  • accepted that DNA was the genetic material.
  • The race was on to discover its structure.

-Linus Pauling

-Maurice Wilkins and Rosalind Franklin


Wilkins and Franklin used X-Ray crystallo-

  • graphy to study the structure of DNA.
  • From their picture
  • of DNA, Watson
  • and Crick were able
  • to see its helical
  • structure.
  • Double-Helix model of DNA proposed by
  • Watson and Crick:
  • DNA is made up of nucleotides.

2. DNA looks like a ladder. It has two

strands, each strand with the sugar-

phosphate chains on the outside and the

nitrogenous bases on the inside.

  • The nitrogenous bases paired up, forming
  • the rungs of the ladder.
  • The ladder is then twisted, forming a coil.

The nitrogenous bases are paired up very

  • specifically:
  • A – T
  • G - C


(single ring)




-Only a pyrimidine-purine pairing would produce the 2-nm diameter indicated by the X-ray data.


-The A & T, C & G form hydrogen bonds

between one another:

-A = T (two)

-G = C (three)

** This confirms




The Structure of DNA


The sequence of nucleotides on one DNA

  • strand can vary in numerous ways. Each
  • gene has a specific sequence of
  • nucleotides.

A portion of gene has the following

sequence of nucleotides:










  • T
  • A
  • C
  • C
  • T
  • G
  • A
  • A
  • G

-Watson and Crick presented

their DNA model in 1953.

-They, along with Maurice

Wilkins won the Noble Prize in

Medicine in 1962.





DNA Replication:

  • After Watson and Crick presented their DNA
  • model, they wrote about how DNA replicates.
  • They said that the two strands of DNA
  • are complimentary to one another.
  • When they are separated, they can act
  • as templates for synthesizing a new
  • strand of DNA.

Watson and Crick’s model of replication

  • was called “Semiconservative replication.”

-This means that when two strands

of DNA are made, each one will have

a new strand and an old one. The old

strands will act as “templates” to the

new complimentary strand.

  • Experiments done in the late 1950s by
  • Matthew Meselson and Franklin Stahl
  • supported the semiconservative model.

1. In their experiments, they labeled the

nucleotides of the old strands with a

heavy isotope of nitrogen (15N) while any

new nucleotides would be indicated by a

lighter isotope (14N).


After they labeled the DNA and let it

  • replicate, they found that each DNA
  • molecule had one strand labeled with
  • N-15 and the other with N-14.
  • They then allowed for the DNA to
  • replicate once more and they found that
  • the only strands with N-15 were the
  • original two strands of DNA.

Meselson-Stahl Experiment

  • DNA Replication

More than a dozen enzymes and proteins

  • carry out DNA replication:
  • E. coli can replicate its DNA in less than
  • an hour.
  • Human cells can replicate its 6 billion
  • base pairs in only a few hours.
  • Replication is highly accurate; only

one error per billion nucleotides.

  • DNA replication starts at the origins of
  • replication.
  • In bacteria, there are very specific
  • nucleotide sequences that enzymes
  • recognize as sites where replication
  • begins.

2. In eukaryotes, there are many sites on

the DNA strand where replication takes


  • At the origin of replication, a replication
  • bubble forms, where new DNA strands
  • are elongated in both directions.

DNA Polymerase is the enzyme that

  • elongates the new DNA at a replication
  • fork.
  • The rate of elongation is about 500
  • nucleotides per second in bacteria and
  • 50 per second in human cells.

-The nucleotides that are attached to the

newly formed strands are called

nucleoside triphosphates.

Each has a nitrogen base, deoxyribose,

and a triphosphate tail.


-As each nucleotide is added, the last

two phosphate groups are hydrolyzed

to form pyrophosphate.


-The exergonic hydrolysis of

pyrophosphate to two inorganic

phosphate molecules drives the

polymerization of the nucleotide to

the new strand.


The strands in the double helix are

  • antiparallel.
  • The sugar-phosphate
  • backbones run in
  • opposite directions.
  • One strand goes
  • from 3’  5’
  • direction. The
  • other strand goes
  • from 5’  3’
  • direction.

DNA polymerases can only add

  • nucleotides to the free 3’ end of a
  • growing DNA strand.

-DNA can only replicate in the 5’  3’



-Leading strand

replicates from 5’ 3’.

-Lagging strand

replicates from 5’ 3’,

but by forming

Okazaki fragments.

-The Okazaki fragments

(100-200 nucleotides),

are then joined by

DNA ligase.


DNA replication starts with a primer (a

  • short fragment of RNA).
  • A primer is
  • created by an
  • enzyme called
  • primase.
  • Once the primer
  • is made, DNA
  • polymerase can
  • start adding
  • nucleotides at the
  • 3’ end.
  • The primer is
  • then converted
  • into deoxyribo-
  • nucleotides.

Only one primer is

  • needed for the
  • leading strand.
  • A new primer is
  • needed for each
  • Okazaki fragment.

Enzymes involved in DNA replication:

  • 1. Primase:
  • 2. DNA Polymerase:
  • 3. DNA Ligase:

Creates a primer.

Adds nucleotides to

the 3’ end; replaces

RNA primer.

Joins the Okazaki


  • Helicase: Untwists DNA and separates
  • the template strands at the replication
  • fork.
  • Single-strand binding proteins: keep
  • the template strands apart during
  • replication.

Enzymes proofread DNA during replication

  • and repair existing damaged DNA.
  • Mistakes during DNA synthesis can
  • occur at a rate of one error per 10,000
  • base pairs.
  • DNA Polymerase proofreads the new
  • DNA strand. If there is a mistake, DNA
  • polymerase removes the incorrect
  • nucleotide and resumes synthesis.
  • After proofreading, the error rate is
  • one per billion nucleotides.

Harmful chemicals, radioactive emissions,

  • X-rays, and ultraviolet light can change
  • nucleotides.

Also, under normal cellular

conditions, DNA can undergo spontaneous


  • There are over 130 enzymes that help
  • repair damaged and mutated DNA.
  • Defects in enzymes that help repair
  • mismatched nucleotides are associated
  • with colon cancer.
  • Nucleases are enzymes that excise
  • (cut out) damaged nucleotides. After
  • they are cut out, the gap is filled in
  • with the correct nucleotide via DNA
  • polymerase and ligase.

Example: The inherited disorder called

Xeroderma Pigmentosum causes an

individual to be very sensitive to sunlight.

UV light can cause two adjacent Thymine

nucleotides to form a dimer. The dimer

buckles the DNA strand and interferes with

DNA replication.

 Causes skin cancer.


The End-Replication problem: When

  • eukaryotic DNA replicates, a gap is left at
  • the 5’ end of each new strand because DNA
  • polymerase can only add nucleotides at the
  • 3’ end.

Gap formed






To help this problem, eukaryotic DNA

  • have telomeres at their ends. Telomeres
  • are not genes but the sequence
  • repeated between 100 to 1,000 times.
  • The telomeres prevent any important
  • genes from being deleted over time due
  • DNA shortening with repeated replication.
  • The enzyme, telomerase, catalyzes the
  • lengthening of telomeres.

-Telomerases have

a short RNA fragment

that serves as a

template for a new