By: Meenakshi Gupta
(M.Pharm. 2nd sem)
• Discovered by: F. Meischer in nucleus of pus cell &
called it ‘nuclein’.
• Term nucleic acid was coined by ‘altman’.
• Nucleic acids are polymers of nucleotides (=nitrogen
bases+ pentose sugars+ phosphate).
• On the basis of structure, N-Bases are broadly of two
1. Pyrimidines: consist of one pyrimidine ring.
Skeleton of ring composed of two nitrogen &
four carbon atoms. Eg. Cytosine, thymine,
2. Purines: consists of two rings i.e. one
pyrimidine ring ( 2N+4C) and one imidazole ring
(2N+3C) eg. Adenine, guanine
• N-base forms bond with first carbon of pentose sugar to form
a nucleoside. Nitrogen of third place (N3) forms bond with
sugar in case of pyrimidines while in purine, nitrogen of ninth
place (N9) forms bond with sugar.
• Phosphate form ester bond (covalent bond)
with fifth carbon of sugar to form a complete
• Discovered by- meischer
• Term was given by- zacharis
• In DNA pentose sugar is deoxyribose sugar and four
types of nitrogen bases A,T,G,C.
• Wilkins and franklin studied DNA molecule with the
help of X-ray crystallography.
• With the help of this study, watson and crick (1953)
proposed a double helix model for DNA. For this
model, watson, crick & wilkins were awarded by
nobel prize in 1962.
• According to this model, DNA is composed of
two polypeptide chains.
• Both polynucleotide chains are complimentary
and antiparallel to each other.
• In both strand of DNA direction of
phosphodiester bond is opposite ie. If the
direction of phosphodiester bond in one
strand is 3’-5’ then it is 5’-3’ in another strand.
• Both the strands are held together by
hydrogen bonds. These H-bonds are present
between nitrogen bases of both strand.
• Adenine binds to thymine by two hydrogen
bonds and cytosine binds to guanine by three
• In a double stranded DNA, amount of purine
nucleotides is equals to amount of pyrimidine
purine = pyrimidine
[A] +[G] = [T] + [C]
[A] +[G]/[T]+[C] = 1
• A+T/ G+C= Constant for a given species.
• In a DNA, if A+T> G+C= A-T Type DNA
eg., eukaryotic DNA
• In a DNA, if G+C > A+T= G-C type DNA
eg., prokaryotic DNA
• Melting point of DNA depends on G-C contents
• Hence, M.P. of prokaryotic DNA> M.P. of
Configuration of DN A
• Two strands of DNA are helically coiled like a
revolving ladder. Backbone of this ladder (reiling)
is composed of phosphates and sugars while
steps (bars) are composed of pairs of nitrogen
• Distance between two successive steps is 3.4A. In
one complete turn of DNA molecule there are
such 10 steps . So the length of complete turn is
34A. This is called helix length.
• Diameter of DNA molecule ie. Distance
between phosphates of two strands is 20A.
• Distance between sugar of two strands is
• Length of H-bonds between nitrogen bases is
2.8-3.0A. Angle between N-base & C1 carbon
of pentose is 51⁰.
• Molecular weight of DNA is 1000000 to
• In nucleus of eukaryotes the DNA is associated
with histone protein to form nucleoprotein.
Histone forms an angle of 30⁰ with DNA.
• Bond between DNA and histone is salt linkage
Types of DNA
On the basis of direction of twisting, there are two
types of DNA.
• Right handed DNA-
Clockwise twisting eg. The DNA for which watson &
crick proposed model was ‘B’ DNA.
A 28A 11 pairs 2.56A 23A
B 34A 10 pairs 3.4A 20A
C 31A 9.33 pairs 3.32A 19A
D 24.24A 8 pairs 3.03A 19A
• Left handed DNA
Anticlockwise twisting eg. Z-DNA, discovered by
Rich. Phosphate & sugar backbone is zig-zag,
units of Z-DNA are dinucleotides (purine and
pyrimidine in alternate order).
Helix length – 45.6A
Diameter – 18.4A
No. of base pairs – 12 (6 dimers)
Distance between 2 base- pairs – 3.75A
• Dehydration of DNA drives it into the A form, and this apparently protects
DNA under conditions such as the extreme desiccation of bacteria. Protein
binding can also strip solvent off of DNA and convert it to the A form, as
revealed by the structure of a rod-shaped virus.
• C-DNA also known as C form DNA. It is one of the many possible double
helical structures of DNA. This form of DNA can be observed at some
conditions such as relatively low humidity and the presence of
certain ions, such as Li+ or Mg2+.
• Z DNA is commonly believed to provide torsional strain relief
(supercoiling) while DNA transcription occurs. The potential to form a Z-
DNA structure also correlates with regions of active transcription.
• DNA absorbs U.V. rays means 2600A
• Hyperchromicity: when a double stranded
DNA molecule is denatured by heating then
denatured DNA molecule absorbs more
amount of light.
• Hypochromicity: when denatured DNA
molecule cool slowly then it becomes double
stranded and it absorbs less amount of light
• If a normal DNA molecule is placed at high temperature
(80⁰-90⁰C) then both the strands of DNA will separate
from each other due to breaking of H- bonds. It is called
• When denatured DNA molecule is placed at normal
temperature then both the strands of DNA molecule
attaches & recoiled to each other. It is called
• DNA supercoiling refers to the over- or under-winding of
a DNA strand, and is an expression of the strain on that
• Supercoiling occurs when the molecule relieves the
helical stress by twisting around itself. Overtwisting leads
to postive supercoiling, while undertwisting leads
to negative supercoiling.
• Pallindromic DNA- wilson & thomas
sequences of nucleotides same from both ends.
• DNA molecule is dextrorotatory while RNA molecule
• C-value: total amount of DNA in a haploid genome of
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