Download The Structure of DNA

The Structure of DNA
James Watson and Francis Crick (1953)
•Using the work of Franklin, Wilkens, and
Chargaff they discovered the structure of
DNA.
•This discovery won them the Nobel Prize in
Medicine/Physiology (along with Wilkens) in
1962.
The Structure of DNA
Watson and Crick’s Model of DNA
•DNA has a double helix shape.
•DNA is a nucleic acid.
•A Nucleic acid is made up of a long chain of
individual nucleotides.
The Structure of DNA
The Structure of DNA
Watson and Crick’s Model of DNA
A Nucleotide has 3 parts
(1) A 5 carbon sugar (deoxyribose)
(2) A phosphate group
(3) A nitrogenous base
A Nucleotide
Adenosine Mono Phosphate (AMP)
Phosphate
HO
H+
Nucleotide
OH
P
O
Base
N
H
O
H
N
5’CH2
4’
NH2
O
1’
Sugar
3’
OH
2’
H
OH
N
N
Nucleoside
The Structure of DNA
Watson and Crick’s Model of DNA
There are 4 different nitrogenous bases in DNA
•Adenine & Guanine are called purines
because they have a double ring structure.
• Thymine & Cytosine are called pyrimidines
because they have a single ring structure.
Purines
NH2
Adenine
N
N
N
O
CH3
(DNA)
N
Guanine
NH
N
Thymine
O
NH2
Uracil
(RNA)
NH
N
O
N
N
Pyrimidines
NH
O
N
O
NH2
Cytosine
N
N
O
The Structure of DNA
Base Pairing
•Adenine (A) always bonds with Thymine (T)
and vice versa.
•Cytosine (C) always bonds with Guanine (G)
and vice versa.
•The reason only A & T and G & C can bond
together is because weak hydrogen bonds can
form between these pairs and not any others.
Base Pairing
Adenine And Cytosine
+
-
-
Base Pairing
Guanine And Thymine
+
+
Base Pairing
Adenine And Thymine
+ -
Adenine
-
+
Thymine
Base Pairing
Guanine And Cytosine
-
+
+
+
-
The Structure of DNA
Base Pairing
•The force created by the weak hydrogen bonds
between the base pairs is what holds the two
strands of the DNA molecule together.
•Due to base pairing, one strand of the DNA
molecule can be used to reconstruct the other
strand.
•Therefore, the strands are said to be
complementary.
P
HO
NH2
O
N
O
OH
N
N
CH2
H
N
O
O
CH2
O
HO
P
O
O
N
O
CH2
HO
P
O
H
O
OH
H
H2O
NH
N
NH2
N
O
O
CH2
O
H
O
H
H2O
N
O
CH2
N
O
O
O
CH2
O
P
HO
H
O
OH
HO
P
NH2
HO
P
O
H
O
HO
O
D
N
A
OH
The Structure of DNA
DNA Replication
•The process by which DNA is duplicated
before a cell divides is called DNA
replication.
•This ensures that each new cell will have a
complete set of DNA molecules.
The Structure of DNA
DNA Replication
DNA copying process
•A molecule of DNA is unzipped by enzymes
at the weak hydrogen bonds.
•Each DNA strand serves as a template for
new bases to attach to.
•This results in two DNA molecules that are
identical to each other and the original.
The Structure of DNA
RNA (ribonucleic acid)
•RNA is the nucleic acid that acts as a
messenger between DNA and the ribosomes.
•It carries out the process by which proteins
are made from amino acids.
RNA (ribonucleic acid)
There are 3 types of RNA
(1) Messenger RNA (mRNA) which differs
from DNA in three main ways.
•The 5 carbon sugar is ribose, DNA has
deoxyribose.
•It is single stranded, DNA is double stranded.
•It has the nitrogenous base uracil instead of
thymine (which DNA has).)
RNA (ribonucleic acid)
RNA (ribonucleic acid)
There are 3 types of RNA (continued)
(2) Transfer RNA (tRNA)
(3) Ribosomal RNA (rRNA)