DNA (2.6 SL)
IB Diploma Biology
Essential Idea: The structure of
DNAallows efficient storage of
genetic information.
2.6.1 The nucleic acids DNA are polymers of nucleotides.
Starter activity:
Observe figure 1.1 and list some
traits of DNA.
like:
1. ……..
2. ……..
3. ……..
4. ……..
Figure 1.1
2.6.1 The nucleic acids DNA are polymers of nucleotides.
ANucleotide: Asingle unit of a Nucleic Acid polymer
Nucleic acids are very large
molecules that are constructed
by linking together nucleotides
to form a polymer.
There are two types of Nucleic Acids: DNAand RNA.
2.6.1 The nucleic acids DNA and RNA are polymers of nucleotides.
ANucleotide: Asingle unit of a Nucleic Acid polymer
•
•
Acidic
Negatively charged
•
•
Covalent bond
•
•
•
Five carbon atoms = a pentose sugar
If the sugar is Deoxyribose the polymer is
Deoxyribose Nucleic Acid (DNA)
If the sugar Ribose the polymer is Ribose
Nucleic Acid (RNA)
Covalent bond
Contains nitrogen
Has one or two rings in it’s
structure
2.6 The nucleic acids DNA and RNA are polymers of nucleotides.
There are four nitrogen bases in DNA:
Adenine (A)
Guanine (G)
Thymine (T)
• Adenine & Guanine are two-ringed bases called Purines
• Thymine & Cytosine are one-ringed based called Pyrimidines
RNAshares the same bases except that Uracil (U) replaces Thymine
NOTE: When talking about bases always use
the full name on the first instance
Cytosine (C)
2.6 complementary base pairs.
2.6 complementary base pairs.
complementary base pairing
• A only pairs with T sharing two hydrogen
bonds; G only pairs with C sharing three
hydrogen bonds
• A purine must base-pair with a pyrimidine
2.6. The nucleic acids DNA is polymers of nucleotides.
2.6.3 DNA is a double helix made of two antiparallel strands of nucleotides linked
by hydrogen bonding between complementary base pairs.
antiparallel strands of polynu leoti es
5' (5-prime)
the end nearest
carbon number 5
A purine must base-pair with
a pyrimidine
3' (3-prlme)
covalent bonds link
carbon 3 and 5
Hydrogen bond also
hold the structure
of the double helix
3' (3-prime)
the end nearest
carbon number 3
1
5' (5-prime)
2.6 DNA differs from RNA in the number of strands present, the base
composition and the type of pentose.
DNA
RNA
Number of
Strands
Two anti-parallel,
complementary
strands form a double
helix
Single stranded, and
often, but not always,
linear in shape
Nitrogen Bases
Adenine
(A)
Guanine
(G)
Thymine
(T)
Cytosine
(C)
Adenine
(A)
Guanine
(G) Uracil
(U)
Cytosine
(C)
Type of Pentose
Sugar
2.6.2 DNA differs from RNA in the number of strands present, the base
composition and the type of pentose.
AUCG
Cytosine
Cytosine
Guanine
Guanine
0
..
Adenine
Adenine
suoar
Pho
PMtO 8
bon
c.
It -
Thymine
Uracil
0
I
..
DNA
Dooxyrlbonuc:l c; Ac;ld
RNA
Rlbonuclo
Acid
2.6.5 Drawing simple diagrams of the structure of single nucleotides of DNA and RNA, using
circles, pentagons and rectangles to represent phosphates, pentoses and bases.
DNA:
RNA:
The human genome project which has decoded the case sequence
for the whole 6 feet of the human genome requires a data
warehouse (pictured) to store the information electronically.
Scientists have programmed nearly 500,000
DVD’s worth of data into 1 gram of DNA!
2.6.4 Crick and Watson’s elucidation of the structure of DNA using model making.
“We have discovered the secret of life!”
– Francis Crick (An English pub, 1953)
In early 1953, Linus Pauling, an American chemist proposed a
model for DNAwith phosphate groups in the core of the
molecule and the nitrogen bases facing outward…
After this was disproved, three major groups, including
Pauling’s Cal Tech group, James Watson and Francis Crick at
Cambridge, and Maurice Wilkins and Rosalind Franklin at the
University of London, were competing to elucidate the
correct structure of the molecule…
Whilst others worked using an experimental basis Watson and
Crick used stick-and-ball models to test their ideas on the
possible structure of DNA. Building models allowed them to
visualize the molecule and to quickly see how well it fitted the
available evidence.
Watson and Crick ultimately won the race, publishing their
model of DNAin a 900 word paper later in 1953
http:// www.hhmi.org/ biointeractive/watson-constructing-base-pair-models
2.6.4 Crick and Watson’s elucidation of the structure of DNA using model making.
It was not all easy going however. Their first model, a
triple helix, was rejected for several reasons:
•
•
The ratio of Adenine to Thymine was not 1:1 (as discovered
by Erwin Chargaff)
It required too much magnesium (identified by Franklin)
From their setbacks they realized:
•
•
•
DNAmust be a double helix.
The relationship between the bases and base pairing
The strands must be anti-parallel to allow base pairing to
happen
Because of the visual nature of their work the second
and the correct model quickly suggested:
•
•
Possible mechanisms for replication
Information was encoded in triplets of bases
Watson and Crick gained Nobel prizes for their discovery.
It should be remembered that their success was based
on the evidence they gained from the work of others. In
particular the work of Rosalind Franklin and Maurice
Wilkins, who were using X-ray diffraction was critical to
their success.
THE END...