The document provides an overview of DNA structure. It discusses that DNA is composed of deoxyribose, phosphate groups, and four nitrogenous bases. The bases, adenine, guanine, cytosine, and thymine, form pairs through hydrogen bonding between complementary bases. Adenine pairs with thymine, and cytosine pairs with guanine. Nucleotides are formed from a pentose sugar, phosphate group, and one of the four bases. DNA takes the form of a double helix with the backbones made of alternating sugars and phosphates, and the bases forming rungs between the strands via hydrogen bonding of complementary base pairs. The two strands of DNA run in opposite directions, with the 5' end having

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

7. Complimentary base pairing of pyrimidines and
purines
#4. DNA Double Helix and Hydrogen Bonding

8. #4. DNA Double Helix and Hydrogen Bonding

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’

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