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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


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  • 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.


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  • 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.


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  • 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.


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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).


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  • Since viruses have simple structure, they

  • wanted to know whether it was their

  • protein coat or DNA that was the genetic

  • material.


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  • 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.


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  • Then they tested the bacteria for

  • radioactivity.

  • Found radioactivity inside bacteria.


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  • 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.


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-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


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  • 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.


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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.


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pyrimidines

(single ring)

purines

(double

ring)

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


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

between one another:

-A = T (two)

-G = C (three)

** This confirms

Chargaff’s

observations.


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The Structure of DNA

  • http://www.sumanasinc.com/webcontent/anisamples/molecularbiology/DNA_structure.html


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A portion of gene has the following

sequence of nucleotides:

A

T

G

G

A

C

T

T

C

  • 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.


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Crick

Watson


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  • 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.


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-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).


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  • 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.


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Meselson-Stahl Experiment

  • DNA Replication

  • http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/meselson.html


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  • 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.


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2. In eukaryotes, there are many sites on

the DNA strand where replication takes

place.

  • At the origin of replication, a replication

  • bubble forms, where new DNA strands

  • are elongated in both directions.


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  • 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.


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-As each nucleotide is added, the last

two phosphate groups are hydrolyzed

to form pyrophosphate.


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-The exergonic hydrolysis of

pyrophosphate to two inorganic

phosphate molecules drives the

polymerization of the nucleotide to

the new strand.


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  • The sugar-phosphate

  • backbones run in

  • opposite directions.

  • One strand goes

  • from 3’  5’

  • direction. The

  • other strand goes

  • from 5’  3’

  • direction.


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

direction.


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-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.


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  • 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.


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  • Only one primer is

  • needed for the

  • leading strand.

  • A new primer is

  • needed for each

  • Okazaki fragment.


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Creates a primer.

Adds nucleotides to

the 3’ end; replaces

RNA primer.

Joins the Okazaki

fragments.

  • Helicase: Untwists DNA and separates

  • the template strands at the replication

  • fork.

  • Single-strand binding proteins: keep

  • the template strands apart during

  • replication.


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  • 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.


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  • Harmful chemicals, radioactive emissions,

  • X-rays, and ultraviolet light can change

  • nucleotides.

Also, under normal cellular

conditions, DNA can undergo spontaneous

mutations.

  • 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.


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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.


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  • 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

where

primer

previously

existed.


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  • 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.


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-Telomerases have

a short RNA fragment

that serves as a

template for a new

telomere.


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