DNA TYPES
Types of DNA:-
Following are the 7 main types of DNA-
1. The B-Form of DNA (B-
DNA):
Structure of B-form of DNA has been
proposed by Watson and Crick. It is
present in every cell at a very high relative
humidity (92%) and low concentration of
ions. It has antiparallel double helix,
rotating clockwise (right hand) and made
up of sugar- phosphate back bone
combined with base pairs or purine-
pyrimidine.
The base pairs are perpendicular to
longitudinal axis of the helix. The base
pairs tilt to helix by 6.3°. The B-form of
DNA is metabolically stable. The length of
a complete turn is 34 Angstroms and
number of bases pairs in a complete turn
is 10(10.4). The diameter of helix is 20
Angstroms and height of each base pair is
3.38 Angstroms. Each base pair accounts
for 36(34.6 angstrom according to 10.4
base pair per helix turn, 360/10.4 =34.6)
degree of rotation, creating a minor
groove and a major groove.
2. The A-Form of DNA (A-
DNA):
This DNA have clockwise (right
handed) rotation. The A-form of DNA
is found at 75% relative humidity in the
presence of Na+, K+ or Cs+ ions. It
contains 11 base pairs as compared to
ten base pairs of B-DNA which tilt
from the axis of helix by 20.2°. Due to
this displacement the depth of major
groove increases and that of minor
groove decreases. The A-form is
metastable and quickly turns to the D-
form. The diameter of helix is 23
Angstroms and height of each base
pair is 2.56 Angstrom. Each base pair
accounts for 32.7 degree rotation,
creating a minor groove and a major
groove.
3. The C-Form DNA (C-DNA):
This DNA have clockwise rotation. The
C-form of DNA is found at 66% relative
humidity in the presence of lithium
(Lit+) ions. As compared to A-and B-
DNA, in C-DNA the number of base
pairs per turn is 9.33. The base pairs
show pronounced negative tilt by 7.8°.
The diameter of helix is 19 Angstroms
and height of each base pair is
3.32Angstom. Each base pair accounts
for 38.6 degree rotation, creating a
minor groove and a major groove.
4. The D-Form of DNA (D-
DNA):
This DNA have anticlockwise rotation.
The D-form of DNA have very high salt
concentration. Total number of base
pairs per turn of helix is 12. This form
is also called poly (dA-dT) and poly
(dG-dC) form. There is pronounced
negative tilt of base pairs by 16.7°. The
diameter of helix is 18 Angstrom and
height of each base pair is 3.71
Angstrom. Each base pair accounts -30
degree rotation, creating a minor and a
major groove.
5. The Z-Form of DNA (Z-DNA)
or Left Handed DNA:
In 1979, Rich and coworkers at MIT
(U.S.A.) obtained Z-DNA by artificially
synthesizing d (C-G) 3 molecules in the
form of crystals. They proposed a left
handed double helix model with zig-
zag sugar-phosphate back bone
running in antiparallel direction.
Therefore, this DNA has been termed
as Z-DNA. The Z-DNA has been found
in a large number of living organisms
including mammals, protozoans and
several plant species. The diameter of
helix is 18 Angstrom and height of each
base pair is 3.7 Angstrom. The length
of complete turn of helix(pitch) is 45
Angstroms and number of base pairs
per turn is 12(6 dimer).
The repeating unit in Z-DNA is a
dinucleotide due to alternating
orientation of sugar residues, whereas
in B-DNA the repeating unit is a
mononucleotide, and sugar molecules
do not have the alternating orientation.
6. Single Stranded (ss) DNA:
Almost all the organisms contain
double stranded DNA except a few
viruses such as bacteriophage φ × 174
which consists of single stranded
circular DNA. It becomes double
stranded only at the time of
replication.
The dsDNA always remains in linear
helical form, while the ssDNA
remains in circular form; however,
it becomes double stranded only
during replication.
7. Circular and Super
Helical DNA:
Almost in all the prokaryotes and a
few viruses, the DNA is organised in
the form of closed circle. The two
ends of the double helix get
covalently sealed to form a closed
circle. Thus, a closed circle contains
two unbroken complementary
strands. Sometimes one or more
nicks or breaks may be present on
one or both strands, for example
DNA of phage PM2 (Fig. 5.7 A).
Besides some exceptions, the
covalendy closed circles are twisted
into super helix or super coils (Fig.5.7
B) and is associated with basic proteins
but not with histones found complexed
with all eukaryotic DNA.
Hindi Translation :-
डीएनए के प्रकार: -
डीएनए के 7 मुख्य प्रकार निम्नलिखित हैं-1. डीएनए का बी-फॉर्म (बी-
डीएनए के बी-रूप की संरचना वॉटसन और क्रिक द्वारा प्रस्तावित की गई। यह प्रत्येक कोशिका में बहुत उच्च सापेक्ष आर्द्रता(92%) और आयनों की कम concentration में मौजूद है । इसमें एंटीपैरल समानांतर दोहरे हेलिक्स है,
डीएनए):
यह दक्षिणावर्त (दाहिने हाथ) डीएनए है और यह शुगर फास्फेट बैकबोन तथा प्यूरीन व pyrimidine नामक क्षार से मिलकर बना है । क्षार युग्म डीएनए हेलिक्स के अनुदैधर्य अक्ष की सीध में है।हेलिक्स की तरफ क्षार युग्मो का झुकाव 6.3 ° है। बी-रूप डीएनए उचयापचय स्थिर है। इसकी लंबाई
एक पूर्ण मोड़ में 34 एंग्स्ट्रॉम और है
पूर्ण मोड़ में बेस जोड़े की संख्या
10 (10.4) है। हेलिक्स का व्यास 20 एंगस्ट्रोम है।
प्रत्येक बेस जोड़ी की ऊंचाई
3.38 एंगस्ट्रॉम हैं। प्रत्येक बेस जोड़ी
36 डिग्री ( 10.4 बेस जोड़ी प्रति हेलिक्स के लिए मोड़, 360 / 10.4 = 34.6डिग्री) के घुमाव से एक छोटी खांच और एक बड़ी खांच बनाता है।
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