MICROBIOLOGY

1. STRUCTURE OF DNA
NAME – SAMADRITA BANIK
ST. GEORGE COLLEGE OF MANAGEMENT
AND SCIENCE
M.Sc MICROBIOLOGY
2ND SEMESTER

2. INTRODUCTION
DNA is made up of molecules called
nucleotides. Each nucleotide contains a
phosphate group, a sugar group and a
nitrogen base. The four types of nitrogen
bases are adenine (A), thymine (T),
guanine (G) and cytosine (C). The order
of these bases is what determines DNA's
instructions, or genetic code.

3. DNA

4. DNA STRUCTURE
DNA is a nucleic acid, one of the four major groups of biological macromolecules.
• Nucleotides
All nucleic acids are made up of nucleotides. In DNA, each nucleotide is made up of three
parts: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.
DNA uses four kinds of nitrogenous bases: adenine (A), guanine (G) cytosine (C), and
thymine (T).
RNA nucleotides may also contain adenine, guanine and cytosine bases, but instead of
thymine they have another base called uracil (U).

5. • Chargaff's rules
In the 1950s, a biochemist named Erwin Chargaff discovered that
the amounts of the nitrogenous bases (A, T, C, and G) were not
found in equal quantities. However, the amount of A always
equalled the amount of T, and the amount of C always equalled the
amount of G.
These findings turned out to be crucial to uncovering the model of
the DNA double helix.

6. • Double helix
The discovery of the double helix structure of DNA was made thanks to a number of scientists
in the 1950s.
DNA molecules have an antiparallel structure - that is, the two strands of the helix run in
opposite directions of one another. Each strand has a 5' end and a 3' end.
Solving the structure of DNA was one of the great scientific achievements of the century.
Knowing the structure of DNA unlocked the door to understanding many aspects of DNA's
function, such as how it is copied and how the information it carries can be used to produce
proteins.

8. DNA replication
DNA replication is semi-conservative. This
means that each of the two strands in double-
stranded DNA acts as a template to produce two
new strands.
Replication relies on complementary base
pairing, that is the principle explained by
Chargaff's rules: adenine (A) always bonds with
thymine (T) and cytosine (C) always bonds with
guanine (G).

10. THE REPLICATION PROCESS
DNA replication occurs through the help of several
enzymes. These enzymes "unzip" DNA molecules by
breaking the hydrogen bonds that hold the two
strands together.
Each strand then serves as a template for a new
complementary strand to be created.
Complementary bases attach to one another A-T
and C.
The primary enzyme involved in this is DNA
polymerase which joins nucleotides to synthesize the
new complementary strand. DNA polymerase also
proofreads each new DNA strand to make sure that
there are no errors.

11. Leading and lagging strands
DNA is made differently on the two strands at a replication fork.
One new strand, the leading strand, runs 5' to 3' towards the fork and is made continuously.
The other, the lagging strand, runs 5' to 3' away from the fork and is made in small pieces
called Okazaki fragments.

12. Common mistakes and misconceptions
• DNA replication is not the same as cell division. Replication occurs before
cell division, during the S phase of the cell cycle. However, replication
only concerns the production of new DNA strands, not of new cells.
• Some people think that in the leading strand, DNA is synthesized in the 5’
to 3’ direction, while in lagging strand, DNA is synthesized in the 3’ to 5’
direction. This is not the case. DNA polymerase only synthesizes DNA in
the 5’ to 3’ direction only. The difference between the leading and lagging
strands is that the leading strand is formed towards replication fork, while
the lagging strand is formed away from replication fork.

13. CONCLUSION
Each strand of a DNA molecule is composed of a long chain of monomer nucleotides.
The nucleotides of DNA consist of a deoxyribose sugar molecule to which is attached a
phosphate group and one of four nitrogenous bases: two purines (adenine and guanine)
and two pyrimidines (cytosine and thymine). The nucleotides are joined together by
covalent bonds between the phosphate of one nucleotide and the sugar of the next,
forming a phosphate-sugar backbone from which the nitrogenous bases protrude. One
strand is held to another by hydrogen bonds between the bases; the sequencing of this
bonding is specific—i.e., adenine bonds only with thymine, and cytosine only with
guanine.