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Structure of DNA
1. Aman Ullah
B.Sc. Med. Lab. Technology
M. Phil. Microbiology
Certificate in Health Professional Education
Lecturer, Department of Medical Lab. Technology
Institute of Paramedical Sciences, Khyber Medical
University, Peshawar, Pakistan
2. #1. DNA Structure (an overview)
DNA has three main components
1. deoxyribose (a pentose sugar)
2. base (there are four different ones)
3. phosphate
3. #2. The Bases
They are divided into two groups
Pyrimidines and purines
Pyrimidines (made of one 6 member ring)
Thymine
Cytosine
Purines (made of a 6 member ring, fused to a
5 member ring)
Adenine
Guanine
The rings are not only made of carbon
4. #3. Nucleotide Structure
Nucleotides are formed by the condensation of a
pentose sugar, phosphate and one of the 4 bases
The following illustration represents one nucleotide
5. Nucleotides are linked together by covalent bonds
called phosphodiester linkage
#3. Nucleotide Structure
6. #4. DNA Double Helix and Hydrogen Bonding
Made of two strands of nucleotides that are joined
together by hydrogen bonding
Hydrogen bonding occurs as a result of
complimentary base pairing
Adenine and thymine pair up
Cytosine and guanine pair up
Each pair is connected through hydrogen bonding
Hydrogen bonding always occurs between one
pyrimidine and one purine
9. •Adenine always pairs
with thymine because
they form two H bonds
with each other
•Cytosine always pairs
with guanine because
they form three
hydrogen bonds with
each other
#4. DNA Double Helix and Hydrogen Bonding
10. The ‘backbones’ of DNA molecules are made of
alternating sugar and phosphates
The ‘rungs on the ladder’ are made of bases that are
hydrogen bonded to each other
#5. DNA Double Helix
11. #6. Antiparallel strands
The strands
run opposite
of each
other.
The 5’ end
always has
the
phosphate
attached.
5’ 3’
3’ 5’
12.
13. #6. When phosphodiester links
are formed . . .
A. When the covalent bonds are formed between
nucleotides the attach in the direction of 5’→3’
B. The 5’ end of one nucleotide attaches to the 3’ end
of the previous nucleotide