Structure of dna, types of dna forms, turners syndrome
STRUCTURE OF DNA,
TYPES OF DNA FORMS,
By Kripa Justine
• DNA as an acidic substance present in nucleus
was first identified by Friedrich Meischer in
• He named it as ‘Nuclein’.
• Due to technical limitation in isolating such a
long polymer intact, the elucidation of
structure of DNA remained elusive for a very
• James Watson and Francis Crick, based on the
X-ray diffraction data produced by Maurice
Wilkins and Rosalind Franklin, proposed a
very simple but famous Double Helix model
for the structure of DNA.
• based on the observation of Erwin Chargaff
that for a double stranded DNA, the ratios
between Adenine and Thymine and Guanine
STRUCTURE OF DNA
• DNA is a polymer containing chains of nucleotide
• Each nucleotide contains a sugar, a base, and a
• The sugar is 2′-deoxyribose which has five
carbons named 1′ , 2′, etc.
• Four types of base: adenine and guanine have two
carbon–nitrogen rings and are purines; thymine
and cytosine have a single ring and are
• Bases are attached to the 1′ carbon of the
• A sugar plus a base is termed a nucleoside.
• A nucleotide has one, two or three phosphate
groups attached to the 5′ carbon of the sugar.
• Nucleotides occur as individual molecules or
The base pairing confers a very unique
property to the polynucleotide chains.
They are said to be complementary to
each other, and therefore if the
sequence of bases in one strand is
known then the sequence in other
strand can be predicted.
If each strand from a parental DNA acts
as a template for synthesis of a new
strand, the two double stranded DNA
(daughter DNA) thus, produced would
be identical to the parental DNA
The genetic implications of the
structure of DNA became very clear.
• Nucleotide triphosphates of the four bases
are joined to form DNA polynucleotide
• Two phosphates are lost during
polymerization and the nucleotides are
joined by remaining phosphate.
• A phosphodiester bond forms between the 5′
phosphate of one nucleotide and the3′
hydroxyl of the next nucleotide.
• The polynucleotide has a free 5′ phosphate at
5′ end and a free 3′ OH at 3′ end.
• The sequence of bases encodes the genetic
• It is read 5′→3′. Polynucleotides are
extremely long. It is possible to have 4n
i. Two polynucleotide chains, where the backbone is
sugar-phosphate and bases project inside.
ii. They have anti-parallel polarity. If one chain has
polarity 5'3', the other has 3'5'.
iii. The bases in two strands are paired through hydrogen
bond forming base pairs.
iv. Adenine forms two hydrogen bonds with Thymine
from opposite strand
v. Guanine is bonded with Cytosine with three H-bonds.
The salient features of the Double-helix structure of DNA
are as follows:
vi. Always a purine comes opposite to a
pyrimidine. This generates approximately
uniform distance between the two strands of
vii. The two chains are coiled in a right-handed
fashion. The pitch of the helix is 3.4 nm and
there are roughly 10 bp in each turn. The
distance between a bp in a helix is
approximately equal to 0.34 nm.
viii.The plane of one base pair stacks over the
other in double helix. This, in addition to H-
bonds, confers stability of the helical
ix. The helix has a major groove which interacts
with proteins and a minor grove.
• In 1928, Frederick Griffith, in a series of experiments
with Streptococcus pneumoniae witnessed
transformation in the bacteria.
• During the course of his experiment, a living organism
had changed in physical form.
• When Streptococcus pneumoniae bacteria are grown on
a culture plate, some produce smooth shiny colonies (S)
others produce rough colonies (R).
• S strain bacteria have a mucous/polysaccharide coat,
while R strain does not.
• Mice infected with the S strain die from pneumonia
infection but mice infected with the R strain do not
• Griffith was able to kill bacteria by heating them.
• He observed that heat-killed S strain bacteria injected into mice did not kill them.
• When he injected a mixture of heat-killed S and live R bacteria, the mice died.
• He recovered living S bacteria from the dead mice. He concluded that the R strain
bacteria had been transformed by the heat-killed S strain bacteria.
• Some ‘transforming principle’, transferred from the heat-killed S strain enabled R
strain to synthesise a smooth polysaccharide coat and become virulent.
• This must be due to the transfer of the genetic material.
• The biochemical nature of genetic material was not defined from his experiments.
Biochemical Characterisation of
• Genetic material was thought to be a protein.
• Oswald Avery, Colin MacLeod and Maclyn McCarty,
worked to determine the biochemical nature of
‘transforming principle’ in Griffith's experiment.
• They purified biochemicals (proteins, DNA, RNA) from
heat-killed S cells to see which could transform live R
cells into S cells.
• They discovered that DNA alone from S bacteria caused
R bacteria to become transformed.
• Proteases and RNases did not affect transformation, so
transforming substance was not a protein or RNA.
• Digestion with DNase inhibited transformation
• They concluded that DNA is the hereditary material, but
not all biologists were convinced.
TYPES OF DNA
• A-DNA is one of the possible double helical structure
which DNA can adopt along with other two biologically
active helix structure(B- DNA,Z-DNA).
• It is rare.
• Right handed double helix .
• Short and fat compared to B-DNA.
• Occur only in dehydrated sample of DNA , those used
in crystallographic experiments.
• A-DNA was originally identified by X-ray diffraction
analysis of DNA fibres at 75% relative humidity.
• Deep major groove and flat minor groove.
• The bases are more tilted (to 20A°).
• The base pairs per turn is 11.
• Rise per base pair is 2.3A°.
• Described by James D. Watson &
• Commonly found in DNA.
• DNA molecule consists of 2 helical
polynucleotide chains coiled around
• 2 helices are wound in such a way so as
to produce 2 interchain spacing or
• Major/wide groove(width 12A°,depth
• Minor /narrow groove(width 6A°,depth
• These grooves provide surface with
which proteins,chemicals,drugs can
• C-DNA formed at 66% relative humidity (low)in
presence of Li+ or Mg2+.
• Right handed ,with axial rise of 3.32A° per base
• 9.33 base pairs per turn.
• Helical pitch 3.32A°×9.33°A=30.97A°.
• Base pair rotation=38.58°.
• Has diameter of 19A°,smaller than that of A-&B-
• The tilt of base is 7.8°
• One of the many possible DNA double helix structure.
• Left handed double helix structure winds to left in zig-
zag manner so they are termed as Z-DNA.
• Discovered by Rich, Nordheim &Wang in 1984.
• It has antiparallel strands as B-DNA.
• It is long and thin
• adjacent sugar have alternating orientation
• Thermodynamically relatively unstable.
Purines: bases & sugar are near & on same side
pyramidines: bases & sugar are distant ,on opposite
• Only one deep helical grooves.
• There are 12 base pairs per turn with axial rise 3.8A°,&
angle of twist 60°.
• Transition to Z-DNA facilitated by cytosine methylation
• Helix has a higher‐energy form.
• This leads to a greater distance (0.77 nm)
• segment of B‐DNA consisting of GC pairs can
be converted into Z‐DNA when the bases are
rotated 180 degrees.
• The modification of DNA by methylation of
cytosine is frequent in certain regions of the
DNA of eukaryotes.
• Z‐DNA has biological roles.
• Sequences favoring the formation of Z‐DNA
occur frequently near the promoter region,
where Z‐DNA stimulates transcription
• Condition that affects only females
• One of the X chromosomes is missing or partially
• Turner syndrome can cause a variety of medical and
developmental problems, including short height, failure
of the ovaries to develop and heart defects.
The genetic alterations of Turner syndrome may be
one of the following:
Monosomy. The complete absence of an X
chromosome generally occurs because of an error in
the father's sperm or in the mother's egg. This results
in every cell in the body having only one X
Mosaicism. An error occurs in cell division during
early stages of fetal development. This results in
some cells in the body having two complete copies of
the X chromosome. Other cells have only one copy
of the X chromosome.
X chromosome abnormalities.
Abnormal or missing parts of one of the X
chromosomes can occur.
Cells have one complete and one altered copy.
This error can occur in the sperm or egg with
Y chromosome abnormalities
In a small percentage of Turner syndrome
cases, some cells have one copy of the X
chromosome and other cells have one copy of
the X + Y chromosome material.
These individuals develop biologically as
female, but the presence of Y chromosome
material increases the risk of developing
• Turner syndrome may be suspected in
pregnancy during an ultrasound test.
• This can be confirmed by prenatal testing
- chorionic villous sampling or
amniocentesis - to obtain cells from the
unborn baby for chromosomal analysis.
• If a diagnosis is confirmed prenatally, the
baby may be under the care of a specialist
pediatrician immediately after birth.
• Diagnosis is confirmed by a blood test,
called a karyotype. This is used to
analyze the chromosomal composition of
• Wide or weblike neck
• Low-set ears
• Broad chest
• High, narrow roof of the mouth
• Arms that turn outward at the elbows
• Fingernails and toenails that are narrow and
• Swelling of the hands and feet, especially at birth
• Slightly smaller than average height at birth
• Slowed growth
• Cardiac defects
• Low hairline at the back of the head
• Receding or small lower jaw
• Short fingers and toes
• There is no cure for Turner syndrome. However, much
can be done to minimize the symptoms.
• Growth hormone, either alone or with a low dose
of androgen, will increase growth.
• Estrogen replacement therapy like birth control pill, to
promote development of secondary sexual
• Estrogens are crucial for maintaining
• good bone integrity
• cardiovascular health
• tissue health.
• Surgery for removing webbed neck.
• Women with Turner syndrome who do not have
spontaneous puberty and who are not treated with
estrogen are at high risk for osteoporosis and heart
• In females with mild signs and
symptoms of Turner syndrome, the
diagnosis is delayed until the teen or
young adult years.
• Girls and women with Turner
syndrome need ongoing medical care
from a variety of specialists.
• Regular checkups and appropriate care
can help most girls and women lead
healthy, independent lives.
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