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IB Biology HL topic 7
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IB biology notes for higher level topic 7
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☒•¥÷ ¥:☒.• E.÷I• ¥↳• • Nucleic Acids By A rathi Anand
0488
TO PICS 7 I 7 2,7 3
.
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,DNA is 2M long
topic 7
Essential Idea : The structure of DNA is ideally suited to its function .
Skill :
Analysis of results of the
Hershey and chase experiment providing
evidence that DNA is the genetic material .
In the mid twentieth century , scientists were still unsure as to whether
-
DNA or protein was the genetic material of the cell .
•
Known that some viruses consisted solely of DNA and a protein coat
and could transfer their genetic material into hosts .
→ 1952 , Alfred and Martha Chase conducted
Hershey a series of
experiments to prove that DNA was the
genetic material .
•
viruses ( T2 bacteriophage) were grown in one of two
isotopic
mediums in order to radioactively
viral component
label a
specific .
viruses grown in radioactive sulfur ( 3 s s) had radiolabelled
proteins
•
( sulfur is present in proteins but not DNA ) 32 P )
.
•
viruses grown in radioactive phosphorus ( had radiolabeled
DNA (
phosphorus is present in DNA but not proteins ) .
The viruses were then allowed to infect a bacterium (E -
coli ) and the
the virus and bacteria were separated via
centrifugation .
• The larger bacteria formed a solid pallet while the smaller
viruses remained in the supernatant .
bacterial pallet was found to be radioactive when infected
The
32
by
the p viruses ( DNA ) but not the 35 S
-
viruses ( proteins ) -
.
• This demonstrated that DNA , NOT protein , was the genetic
material because DNA was transferred to the bacteria .
- -
protein part not in
of phage phage Ghosts
Application Rosalind Franklin 's
: and Maurice Wilkins ' investigation of DN
ray diffraction
structure by X -
.
Rosalind Franklin and Maurice Wilkins used a method of X -
ray
diffraction to investigate the structure of DNA .
, DNA was
purified and then fibres were stretched thin glass tube
•
in a
(to make most of the strands parallel )
•
The DNA was targeted by a X -
ray beam , which was diffracted
when it contacted an atom .
The scattering pattern ray was recorded
of the X film and
• -
on a
used to elucidate details of molecular structure .
Summary of the Process of X-Ray Crystallography
From the
scattering pattern
produced by a DNA molecule ,
certain inferences could be
made about its structure .
•
COMPOSITION :
DNA is a
double stranded molecule
• ORIENTATION : Nitrogenous
bases are closely packed together on the inside and phosphates
form an outer backbone .
• SHAPE ! The DNA molecule twists at regular intervals (every 34
Ang storm ) to form a helix ( two strands = double helix .
Franklin 's data shared by Wilkins with James Watson ( without
was
Franklin 's permission ) who , with the help of Francis Crick , used
the information to create a molecular model of the basic structu
of DNA .
→ In 1962 , Watson , Crick and Wilkins ( but not Franklin ) were awarde
Nobel prize .
understanding : DNA structure
suggested a mechanism for DNA
replication .
FrankKin 's X ray diffraction experiments demonstrated that the DNA
-
helix is both tightly packed and regular in structure .
•
Phosphates ( and sugars ) form the outer backbone and nitrogenous
within the interior
bases are packaged .
Chargaff demonstrated that DNA is composed of an equal numbe
of purines ( At a) and (CtT) pyrimidines .
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