History of DNA. introduction of DNA with short history and findings. different types of DNA with structures variations. A -DNA, B- DNA, C- DNA E- DNA D- DNA And Z DNA Detail information of these DNA with their comparison tables, different types of unusual DNA and sequences. Functions of DNA with their explanations . Nucleic acid chemical basis : Denaturation and annealing of DNA with factors for that. New DNA.
3. TIMELINE
1869
Friedrich Miescher : identified the “nuclein” by isolating a
molecule from a cell nucleus that would later become known as DNA
1881
Albrecht Kossel, who is credited with naming DNA, identified nuclein as a nucleic
acid. He also isolated those five nitrogen bases : adenine (A), cytosine (C), guanine (G), thymine
(T) and uracil (U)
1950
Avery, McCleod & McCarty- Transforming principle is DNA. i.e
carrier of genetic information.
1944
Roslind Franklin’s work in X-ray crystallography, started taking X-ray
diffraction photographs of DNA. Images showed the helical form, which was
confirmed by Watson and Crick nearly two years later.
Erwin Chargaff :Chargaff’s Rules, proved that guanine = cytosine , as well as
adenine= thymine . Sum of Purine is equal to sum of Pyrimidine
1951
1953 J Watson and F Crick :published on DNA’s double helix structure
that twists to form the ladder-like structure
4. DNA is a polymer of deoxyribonucleoside
monophosphates covalently linked by 3′→5′–
phosphodiester bonds
Found in chromosomes, mitochondria and
chloroplasts
Carries the genetic information
7. DNA double helical structure coils round Histones.
DNA bound to histones forms NUCLEOSOMES
(10nm FIBRES)
Nucleosomes contain 146 nucleotides
8. 1. Genetic Information (Genetic Blue Print):
DNA is the genetic material which carries all the hereditary
information.
The genetic information is coded in the arrangement of its nitrogen bases.
2. Replication:
DNA has unique property of replication or production of carbon
copies (Autocatalytic function).
This is essential for transfer of genetic information from one cell to its daughters
and from one generation to the next.
3. Mutations:
Changes in sequence of nitrogen bases due to addition, deletion or
wrong replication give rise to mutations.
Mutations are the fountain head of all variations and evolution.
4. Transcription:
DNA gives rise to RNAs through the process of transcription. It is
heterocatalytic activity of DNA.
9.
5. Cellular Metabolism:
It controls the metabolic reactions of the cells
through the help of specific RNAs, synthesis of specific proteins,
enzymes and hormones.
6. Differentiation:
Due to differential functioning of some specific regions of DNA or
genes,
different parts of the organisms get differentiated in shape, size
and functions.
7. Development:
DNA controls development of an organism through working of an
internal genetic clock
with or without the help of extrinsic information.
8. DNA Finger Printing:
Hypervariable microsatellite DNA sequences of each individual are
distinct.
They are used in identification of individuals and deciphering their
relationships.
The mechanism is called DNA finger printing.
9. Gene Therapy:
Defective heredity can be rectified
by incorporating correct genes in place of defective ones.
11. DNA STRUCTURE
Most important clue to the structure of DNA came
From the work of Erwin Chargaff and his colleagues in the late 1940s.
The data, collected from DNAs of different species, led Chargaff to
following conclusions:
The base composition of DNA generally varies from one species to another.
DNA specimens isolated from different tissues of the same species have the
same base composition.
The base composition of DNA in a given species does not change with an
organism’s age , nutritional state, or changing environment.
In all cellular DNAs, the number of adenosine residues is equal to the number
of thymidine residues (that is, A=T), and the number of guanosine residues is
equal to the number of cytidine residues (G=C). Thus it follows that the sum
of the purine residues equals the sum of the pyrimidine residues; that is,
A+G = T+ C.
11
12. DNA STRUCTURE
Double helix
In 1953 Watson and Crick
postulated a three dimensional
model of DNA structure
which consists of two helical DNA
chains
wound around the same axis to form
a right handed double helix
Width of double helix is 20 Ao
(2nm).
Each turn of helix is 34Ao(3.4nm) with
ten pairs of nucleotides;
Each pair placed at distance of 3.4Ao.
12
13. DNA STRUCTURE
Double helix
Each strand has :
The hydrophilic backbones of alternating
deoxyribose and phosphate groups on the
outside of the double helix
facing the surrounding water
The purine and pyrimidine bases of both
strands stacked inside the double helix,
with their hydrophobic & nearly planar ring
structures very close together &
perpendicular to the long axis
13
14. BASE PAIRING
oPurines - adenine (A)
and guanine (G)
oPyrimidines - cytosine
(C) and thymine (T)
oThe two strands are
complementary
oDNA double helix is held
together by 2 forces
H-bonding between
complementary base pairs
van der Waals
interactions between
stacked bases
14
15. Major groove :
Sequence-specific interactions
provide access for the binding of regulatory proteins
Maintained by hydrogen bonds, ionic interactions and van der Waals
forces.
Minor groove
Offers the highest sequence specificity binding between a small molecule and DNA
To disrupt the specific gene expression
Certain anticancer drugs, such adactinomycin (actinomycin D),
exert their cytotoxic effect by intercalating into the narrow groove of
the DNA double helix, thus interfering with DNA synthesis
• Compared to minor groove binding,
interactions between small molecules
and the DNA major groove have not been
extensively explored.
16. DIFFERENT 3-DIMENSIONAL FORMS OF DNA
DNA is a remarkably flexible.
Considerable rotation is possible around several types of bonds
Thermal fluctuation can produce bending, stretching and unpairing (melting)
of the strands.
The structural variations reflect 3 things:
Different possible conformations of deoxyribose
Rotation about the contiguous bonds that make up the phospho-
deoxyribose backbone
Free rotation about C-1’-N glycosyl bond
16
17. VARIOUS TYPES OF DNA
At least 6 different forms of DNA (A-E, Z) have been found
out.
Amongst these, A,B,Z are predominant.
The Watson-Crick structure is also referred to as B form DNA,
or B DNA.
Most stable structure for a random-sequence DNA molecule
under physiological conditions
Therefore the standard point of reference in any study of the
properties of DNA.
17
18. A-DNA
o X-ray diffraction studies of less-hydrated DNA fibers revealed a A
form DNA
o When the relative humidity of B-form DNA falls to less than 75%,
o the B-form undergoes a reversible transition into the A-form of DNA
o It is also a right-handed helix
o Helix is wider ,
o Number of base pairs per helical turn is 11
o The plane of the base pairs is tilted about 20° with respect to the
helix axis.
18
• In A-DNA, C-3’ lies out of
the plane (a conformation
referred to as C-3’endo)
formed by the
other four atoms of the ring
19. Z-DNA
o Z-DNA is a left handed helix containing 12 base pairs per turn.
oTo form the left-handed helix in Z-DNA
o the purine residues flip to the synconformation, alternating with pyrimidines
in the anti conformation.
owhen the sequence of nucleotides consists of alternating
purine/pyrimidine stretches- form, Z-DNA
o is also favored at high ionic concentrations
oDNA with a zigzag configuration along the sugar phosphate backbone.
o hence named Z-DNA
oThe major groove is barely apparent in Z-DNA, and the minor groove
is narrow and deep.
o The Z-DNA tracts may play a role (as yet undefined) in regulating the
expression of some genes or in genetic recombination.
19
20. Blue: sugar phosphate
backbone
Yellow: for pyrimidines
(thymine and cytosine)
brown for
purines (adenine and
guanine)
23. Observed at some conditions such as
relatively low humidity and the presence
of certain ions, such as Li+ or Mg2+
Not very stable and Not very common.
Right handed helice
Has 9 base pairs per turn of spiral
Has diameter of 19A°,smaller than that
of A-&B- DNA.
The tilt of base is 7.8°
24. Extremely rare variant with only 8base pairs per helical
turn .
This forms of DNA found in some DNA molecules devoid
of guanine.
Axial rise of 3.03A°per base pairs .
Tilt of 16.7° from axis of helix
25. E-DNA - extended &eccentric double helix
Cytosine methylation of or bromination of DNA
sequence d(GGCGCC)2
E-DNA has a long helical axis rise and base
perpendicular to the helical axis.
Deep major groove and shallow minor groove.
E-DNA allowed to crystallize for a period time
longer, the methylated sequence forms
standard A-DNA.
26. E-DNA is the intermediate in the transition to A-
DNA.
E-DNA is the intermediate in the
crystallographic pathway from B-DNA to A-DNA
27.
28. Mitochondria also have a small amount of their own DNA.
mitochondrial DNA
In humans, mitochondrial DNA spans about 16,569 DNA
building blocks (base pairs)
Mitochondrial DNA contains 37 genes,
Thirteen of these genes
Provide instructions for making enzymes involved in oxidative
phosphorylation.
The remaining genes
Provide instructions for making molecules called transfer RNA
(tRNA) and ribosomal RNA (rRNA), which are chemical cousins of
DNA.
29. Mitochondrial DNA Nuclear DNA
Location Mitochondria Cell Nucleus
Copies per somatic cell 100-1,000 2
Structure Circular and closed Linear and open ended
Membrane enclosure
Not enveloped by a
membrane
Enclosed by a nuclear
membrane
Genome size
1 chromosome with 16,569
base pairs
46 chromosomes with 3.3
billion base pairs
Number of genes 37 genes 20,000-25,000 genes
Method of inheritance Maternal Maternal and Paternal
Method of translation
Some codons do not follow
universal codon pattern
Follows universal codon
pattern
Method of transcription Polycistronic Monocistronic
30. SATELLITE DNA
Originally identified as a subfraction of DNA with a
buoyant density slightly lower than that of genomic DNA
because of its higher content of AT base pairs.
Consists of clusters of short, species-specific, nearly
identical sequences that are tandemly repeated hundreds
of thousands of times.
These clusters lack protein-coding genes
Found principally near the centromeres of chromosomes.
Two types
Microsatellite sequences
1-6 bp repeat units flanked by conserved sequences
Minisatellite sequences
11-60 bp flanked by conserved restriction sites.
Used for DNA matching or finger printing as first found
out by Jeffreys et al (1985).
31. Viral DNA (and in some, RNA)
Some circular, some linear
Some double stranded, some single stranded
Very small amount, packed very tightly
Small size is an advantage
Viruses use host cell enzymes, need few genes
Bacterial DNA
Usually single copy of double stranded
Usually circular
Eukaryotic DNA
Linear, in several pieces
32. UNUSUAL SEQUENCES /
STRUCTURES OF DNA
Important for molecular recognition of DNA by proteins
and enzymes
Bent DNA:
32
Bends occur in the DNA helix
wherever four or more adenosine
residues appear sequentially in one
strand.
Cause a collapse of the helix into the
minor groove.
Six adenosines in a row produce a bend
of about 18°.
Drugs, photochemical damage,
mispairing of bases can also produce
33. DNA duplex possesses areas where sequence of nucleotides is the
same but opposite in the two strands.
These sequences are recognized by restriction endonucleases and
are used in genetic engineering.
It is similar to palindrome words having same words in both
forward and backward direction, e.g., NITIN, MALAYALAM
When the inverted repeat occurs within each individual strand of
the DNA, the sequence is called a mirror repeat.
34. Palindromic DNA sequences can form alternative
structures with intrastrand base pairing.
(a) When only a single DNA (or RNA) strand is involved, the
structure is called a hairpin.
(b) When both strands of a duplex DNA are involved, it is
called a cruciform.
35. TRIPLEX DNA/ TRIPLE
STRANDED DNA
Due to additional H-bond between bases
particularly with functional groups arrayed
in the major groove.
The bonds are called as Hoogsteen H-bonds
The non-Watson-Crick pairing is called
Hoogsteen pairing, after Karst Hoogsteen,
who in 1963 first recognized the potential for
these unusual pairings
Triple helical structure is less stable than
double helix - increased electrostatic
repulsion.
36. G-TETRAPLEX / FOUR STRANDED DNA
High content of Guanine – form tetrameric structure called G-quartets.
These structures are planar & are connected by Hoogsteen hydrogen
bonds.
Eukaryotic chromosomes - Telomeres are rich in guanine - forms G-
tetraplexes.
Ref: Lehninger Principles of Biochemistry
37.
38. Type of deoxyribonucleic acid, discovered in cell
nuclei in 2018,
that is found in the nuclei of human cells.
I-motifs are four-stranded quadruplex structures
formed by cytosine-rich DNA.
C-rich DNA regions are common in gene regulation
portions of the genome.
(Zeraati et al., Nat Chem, 2018)
39. Postulated to play a role in gene regulation and
expression in the cell
I-motifs have potential applications in nano-technology
and nano-medicine,
because size is more than 1nm and less than 100nm due to
their unique pH sensitivity
Have been used as biosensors, nanomachines, and
molecular switches.
40. Nucleic Acids Research, Volume 30, Issue 21, 1 November 2002, Pages 4618–4625, https://doi.org/10.1093/nar/gkf597
The content of this slide may be subject to copyright: please see the slide notes for details.
FIGURE 6. SOME POSSIBLE MODELS OF DUPLEX, I‐MOTIF AND G‐QUADRUPLEX
ASSOCIATION FOR THE HUMAN TELOMERE ...
41. NUCLEIC ACID CHEMISTRY
The chemical transformations that do occur are generally
very low in the absence of an enzyme catalyst.
Understanding the nucleic acid chemistry gives us:
Powerful array of technologies that have applications in molecular
biology, medicine & forensic medicine.
DENATURATION
Disruption of the hydrogen bonds between paired bases and
of base stacking
causes unwinding of the double helix to form two single
strands,
completely separate from each other along the entire length or
part of length 41
42. DENATURATION
Causes:
Extreme pH,
Temperature above 80 ̊C,
Chemicals such as formamide and urea
Hyperchromic effect
Denaturation of a double stranded nucleic acid produces an increase in
absorption of UV light
Relative to that of a solution with the same concentration of paired
complementary nucleic acid strands
The transition from double stranded DNA to the single stranded
denatured form can be detected by monitoring UV absorption at
260nm
42
43. DENATURATION
43
Each species of DNA has a characteristic
denaturation temperature or melting poin
Melting temperatures (Tm) of DNA
molecules with different nucleotide
compositions
44. ANNEALING
Process by which separated complementary strands
form a double helix
Occurs when the temperature and pH is returned to the
range in which most organisms live
Rapid one step process
If double helical segment of a dozen or more residues still
unites the two strands
44
45. ANNEALING
If the two strands are completely separated, renaturation
occurs in two steps
First step:
Slow, the two strands find each other by random collisons
Form a short segment of complementary double helix
Second step:
Faster, the remaining unpaired bases successively come into
register as base pair
The two strands unite to form double helix
45
47. ORGANIZATION OF DNA
A segment of DNA molecule that contains the
information required for the synthesis of a functional
biological product (RNA or protein) is called gene
A cell has many thousands of genes and so the DNA
molecules are very large
Large DNA molecules must be packaged in such a way
that they can fit inside the cell and still be functional
47
48. ORGANIZATION OF DNA
Nuclear DNA in eukaryotes is found in chromatin associated
with histones and non histone proteins
48
49. ORGANIZATION OF DNA
DNA (negatively charged) loops twice around the histone octamer to
form nucleosomes (series of nucleosomes: beads on a string)
Histone H1 is associated with the linker DNA found between
nucleosomes to help package them into a solenoid like structure
Further condensation occurs to eventually form the chromosome.
49