2. Learning Objective
Discuss structure of Nucleoside and Nucleotide
Discuss structure of DNA and RNA
Different Forms of DNA
Difference in DNA and RNA structures
Describe packing of DNA
3. Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA)
are nucleotide polymers, called nucleic acids, that carry
information in a form that can be passed from one
generation to the next.
DNA and RNA
4. A unit consisting of a
nitrogen base bonded to a
sugar
In each case, N-9 of a purine
or N-1 of a pyrimidine is
attached to C-1 of the sugar
Nucleosides
6. The sugar in deoxyribonucleic acid (DNA) is deoxyribose.
The C-2 atom of the sugar lacks the oxygen atom that
present in ribose (the sugar in ribonucleic acid, or RNA),
Sugars
8. A nucleotide is a nucleoside with phosphate groups linked by
an ester bond.
Site of esterification is the hydroxyl group of C-5 of the
sugar.
Nucleotides
9. Nucleotides are the monomers that are linked to form RNA
and DNA. The four nucleotide units in DNA are called
Deoxyadenylate
Deoxyguanylate
Deoxycytidylate
Deoxythymidylate or thymidylate
Nucleotides
10. The backbone is constant in DNA and
RNA the bases and sugar vary
Nucleotides
12. The base sequence is written in the 5’-to-3’ direction
The chain has a 5’ end, which is usually attached to
a phosphate, and a 3’ end, which is usually a free
hydroxyl group.
Backbone of the nucleic acid
5’ 3’
14. Watson and Crick discovered that guanine can be paired with
cytosine and adenine with thymine to form base pairs These
base pairs are held together by specific hydrogen bonds.
Base pairing in Nucleic acid
16. Bases found in nucleic acids, form
specific base pairs in such a way that
a helical structure consisting of two
strands is formed
The Double Helix Is Stabilized by
Hydrogen Bonds and Hydrophobic
Interactions between the bases
(hydrophobic effects due to base
stacking)
Structure of DNA
17. The features of the Watson-Crick model 1-
Two helical polynucleotide chains are
coiled around a common axis.
The chains run in opposite directions
form a right-handed double helix
Structure of DNA
5’
3’
3’
5’
19. The features of the Watson-Crick model
2. The sugar-phosphate backbones are
outside and, the purine and pyrimidine
bases lie inside of the helix.
Structure of DNA
20. h
e
m.
3.The bases are nearly perpendicular to t helix axis
Distance between bases are 0.34 n
The helical structure repeats every
3.4 nm, 10 bases per turn of helix.
There is a rotation of 36o / base (360o per full
turn/10 bases per turn).
The diameter of the helix is 2 nm.
Structure of DNA
21. Major and Minor groove
The major groove is wider
than the minor groove in DNA
DNA-binding proteins
interact
in the major groove and
minor groove
Structure ofDNA
Major
Minor
22. The right-handed double-helical Watson –Crick Model for
B-form DNA is the most commonly known DNA structure.
Other forms of DNA also possible.
The helical structure of DNA is depends on the
sequence as well as the environment.
Forms of DNA
24. Whether a DNA sequence will be in the A-, B-or Z-DNA
conformation depends on at least three conditions.
The first is the ionic or hydration environment, which
can facilitate conversion between different helical forms.
B-DNA found at neutral pH and physiological salt
concentrations
A-DNA is favored by low hydration (dehydration)
Z-DNA can be favored by high salt
Forms of DNA
25. The second condition is the DNA sequence:
A-DNA is favored by certain stretches of
purines (or pyrimidines)
B-DNA have no restriction of sequence
Z-DNA can be most readily formed by
alternating purine-pyrimidine steps. e.g.
GCGCGC
Forms of DNA
26. The third condition is the presence of proteins that can
bind to DNA in one helical conformation and force the DNA to
adopt a different conformation, such as proteins which bind to
B-DNA and can drive it to either A-or Z forms.
In living cells, most of the DNA is in a mixture of A and B-
DNA conformations, with a few small regions capable of
forming Z-DNA.
Forms of DNA
33. The size of the genome of E.coli (prokaryotes)
is 4.6 million base pairs (approximately 1.1
mm, if cut and stretched out).
how does this fit inside a small bacterial cell?
Packaging of DNA in Prokaryotes
34. The DNA is twisted by Supercoiling.
DNA is either under-wound (less than one turn of
the helix per 10 base pairs) or
over-wound (more than 1 turn per 10 base pairs)
from its normal relaxed state.
Some proteins involved in the supercoiling
other proteins and enzymes such as DNA gyrase
help in maintaining the supercoiled structure.
Packaging of DNA in Prokaryotes
37. Packaging of DNA in chromatin
Eukaryotes, consist
of a linear DNA
molecule.
To fit into the
nucleus long linear
DNA packed in
Chromatin with
histone proteins.
The total length of the human genome is over 3 billion
base pairs.
38. DNA is wrapped around proteins known as
histones to form structures called nucleosomes.
The histones are evolutionarily conserved proteins
that are rich in basic amino acids and form an
octamer.
The DNA (which is negatively charged because of
the
phosphate groups) is wrapped tightly around the
histone core.
Packaging of DNA in chromatin
39. Histone proteins are basic proteins, rich in Lysine and
Arginine.
They are of 5 types
i. H1
ii. H2A
iii. H2B
iv. H3
v. H4
Structure of Nucleosomes
40. Core particle is an octamer of 4 histone
proteins; H2A, H2B, H3 & H4 each present
Twice (total 8 proteins).
DNA strand of 146bp surrounds the octamer.
H1histone protein stabilizes the 2 coils around the octamer.
Linker DNA is a DNA strand of about 8 to 114 bp that joins 2
core particles or nucleosomes.
Packaging of DNA in chromatin:
41. Chromatin consists of repeating units of Nucleosomes-
appear like ‘beadsona string’.
Nucleosome form solenoid, having 6 nucleosomes per coil
Packaging of DNA in chromatin:
42. Solenoid structure compact to form the chromatin
fiber having a diameter of 30nm.
The chromatin fiber further coils with the help of
Non- histone chromosomal proteins.
Non-histone chromosomal proteins form a central
scaffold around which the solenoid forms loops. It
further coils to form chromosomes.
Non-histone chromosomal proteins are rich in
Tryptophan & Tyrosine.
Packaging of DNA in chromatin:
43.
44.
45. Eukaryotic chromosomes have two distinct regions.
1Heterochromatin is tightly packaged region,
usually contains genes that are not expressed, and
is found in the regions of the centromere and
telomeres.
2Euchromatin is less dense region, usually contains
genes that are transcribed.
Packaging of DNA
46. A chromosome consists of two
chromatids attached to each other
at the centromere
Specialized ends of
chromosomes are Telomeres.
Telomeres help the
chromosomes to maintain their
identity.
Chromosome structure:
47. Centromere divides the chromosome into two
sections, or “arms.”
The short arm of the chromosome is the “p arm.”
The long arm of the chromosome is the “q arm.”
The location of the centromere on each
chromosome gives the chromosome its
characteristic shape, and can be used to help
describe the location of specific genes.
Chromosome structure
48. In humans, each cell normally contains 23
pairs of chromosomes, for a total of 46.
Twenty-two of these pairs, called autosomes, look
the same in both males and females.
The 23rd pair, the sex chromosomes, differ
between males and females.
Females have two copies of the X chromosome
while males have one X and one Y chromosome.
Chromosome structure
49. The 22 autosomes are numbered by size.
The 23 chromosomes consist of X and Y
,are the sex
chromosomes.
The human chromosomes lined up in pairs is called a
karyotype.
50. Books
Chapters of Gene from
Harper’s Biochemistry
Lippincotts Illustrated Review:
Biochemistry
