Molecular Biology Genetic Material

MOLECULAR BIOLOGY-1
THE TERM MOLECULAR BIOLOGY WAS FIRST USED IN 1945 BY
WILLIAM ASTBURY, WHO WAS REFERRING TO THE STUDY OF THE
CHEMICAL AND PHYSICAL STRUCTURE OF BIOLOGICAL
MACROMOLECULES.
June 17 1

 At some point in the study of living cells, it is almost always necessary to examine
individual cellular reactions separately.
 This approach usually has the advantage of some simplification of a system but ignores
the fact that these reactions often interact with other cellular systems.
 Nonetheless, a great deal of information can be obtained in this way. Thus, one does not
usually study directly a phenomenon as complex as overall cell growth but in fact, looks
separately at the synthesis of DNA, proteins, and other components representing a whole
cell.
 Here, in this presentation, we shall be mainly emphasizing on DNA and RNA , their key
roles as genetic material, basic working and functions associated with them.
June 17 2

UNIT-1
GENETIC MATERIAL
MAHATHI SHARMA
MTI-15019
June 17 3

DNA
 When the abbreviation DNA is used usually, one blindly knows that it is THE key
factor that has the genetic information encoded in it which is meant to be
transferred to the next generation for ensuring the continuation of a particular
species or life itself.
 But what is not known, very often, is that why and how did we come to this
assumption that this particular component played such a vital role in the encasing of
genetic information that its loss or corruption could actually have the potential to
destroy the entire existence of an organism.
June 17 4

DNA AS GENETIC MATERIAL
Before DNA was accepted as genetic material , experiments were conducted by
scientists to find out the working of this component.
 The 3 major experiments which we shall be talking about, revolutionized our
biological thinking and paved way to an entire new study and subject called as
GENETICS.
June 17 5

GRIFFITH EXPERIMENT
Pneumonia was a serious cause of death in the wake of the post-world
war I Spanish influenza pandemic, and Frederick Griffith was studying the
possibility of creating a vaccine.
A German bacteriologist, Fred Neufeld had discovered the three
pneumococcal types (Type I,II and III) and discovered the Quellung
reaction to identify them in vitro.
Until Griffith’s experiment, bacteriologists believed that the types were
fixed and unchangeable, from one generation to another.
In the year 1928, the first experiment was conducted which gave us the
current idea that DNA is genetic material.
FREDRICK GRIFFITH
June 17 6

 In this particular experiment, Griffith used the bacterium, ‘streptococcus pneumoniae’ ,
a highly pathogenic bacteria, which causes pneumonia when injected into an organism.
 He used batches of Mice to conduct this experiment. Thus, to ensure an efficient or more
probable result ,experiments were conducted in triplets.
 There are 3 strains of this bacteria. But Griffith used 2 strains viz.,
1. The III-S strain which is capsulated, smooth and virulent.
2. The II-R strain which is non-capsulated, rough and non-virulent.
STREPTOCOCCUS PNEUMONIAE
June 17 7

 When the III-S type bacteria was injected into the mice, the mice contracted
pneumonia and died.
 When the II-R type bacteria was injected into the mice, the mice remained healthy.
 Next, the III-S type bacteria was taken into a test-tube and heat killed. Then they were
injected into the mice. The mice did not contract pneumonia.
 In the next step of the experimentation, the heat killed III-S type was mixed with the II-
R type bacteria. This solution was injected into the mice.
 What was observed was phenomenal, the mice actually contracted pneumonia.
 Thus, Griffith concluded that the III-S type bacteria had a ‘factor’ that ’Transformed’
the II-R type bacteria.
 Hence, this experiment is also called as the ’TRANSFORMING PRINCIPLE’.
June 17 8

AVERY, MACLEOD AND MCCARTY
EXPERIMENT
June 17 10

 The Avery–MacLeod–McCarty experiment was an experimental demonstration,
reported in 1944 by Oswald Avery, Collin Macleod and Maclyn McCarty, that DNA is
the substance that causes bacterial transformation, in an era where it had been
widely believed that it was proteins that served the function of carrying genetic
material.
 Interesting fact to know is that, the term protein itself was coined to indicate its
function as Primary.
 Oswald Avery, Collin Macleod and Maclyn McCarty almost 16 years later, found a
more definitive experiment analysis on DNA.
 The experiment, thus, conducted gave slight evidence that perhaps there was a factor
transferring genetic material from the parent to offspring which was supposedly DNA.
In the case of proving that DNA was the genetic material, test assays were created.
This was a different lookout as compared to the typical ’mice-conducted tests’.
June 17 11

Avery, Macleod and McCarty investigated the chemical nature of the transforming
factor - more specifically, whether the transforming factor was a protein or a nucleic
acid.
They, like Griffith, attempted to transform the II-R strain into the III-S strain by
incubating living II-R and heat-killed III-S. However, they pretreated the heat-killed III-S
with either a protease (an enzyme that degrades proteins) or with DNAase, and
enzyme that degrades DNA.
They reasoned that if the transforming factor was a protein, treatment of the heat-
killed III-S with a protease would destroy the protein and inhibit transformation and
treatment with DNAase should have no effect on the transformation process. On the
other hand, if DNA were the genetic material, the opposite would be true.
In their experiments, Avery, Macleod and McCarty found that protease did not affect
the ability of the ‘dead’ III-S to transform the II-R but DNAase did, therefore they
concluded that the Genetic material in the transformation principle was most likely
DNA.
June 17 12

HERSHEY-CHASE EXPERIMENT
June 17 14

 In 1952, the research team of Hershey and Chase published a report that concluded that
DNA is the genetic material of the bacteriophage (a virus that infects bacteria) T2 .
 They knew that when a bacteria was infected with this phage, the bacteria soon became a
machine that produced new phages. Phages have a very simple structure - they are
composed of a strand of DNA surrounded by a protein coat.
 Hershey and Chase set out to determine which component (the DNA or the protein) was
responsible for the ability of the phage to 'take over' and control the metabolic activity of
the bacteria to produce new phages. In their experiments, they used two radioactive
markers to label the proteins and the DNA of the phages.
 The proteins were labeled with 35S (a radioactive form of sulfur) and the DNA was labeled
with 32P, a radioactive form of phosphorus.
 This allowed the researchers to easily differentiate between a sample that contained
protein (thus would have 35S present) and a sample that only contained DNA (thus would
have 32P present).
June 17 16

During a phage infection, it was hypothesized that some part of the phage was injected
into the bacterium and it was this injected material that conveyed the genetic material
necessary to produce new phages.
Hershey and Chase determined that the phage injected only the DNA into the bacterium
and concluded that DNA must be the genetic material in phages.
This was done by a series of two experiments in which different sets of non-radioactive
bacteria were incubated with phages that had either their protein or their DNA labeled.
They allowed the phages to infect the bacteria for a short time, they agitated the
incubations to dislodge any loose parts of the phages. The bacteria cells were then
pelleted in a centrifuge and the location of the radioactivity (in the pellet with the
bacteria or in the supernatant).
June 17 17

They found that the radioactive DNA was always found with the bacteria cells and that the
radioactive protein was always in the supernatant.
This suggested that the DNA was injected into the bacteria but the protein coat was not. Thus
all of the information needed to produce new viruses was contained in the DNA and not the
protein.
MARTHA CHASE AND ALFRED HERSHEY
June 17 18

RNA
The transformation and blender experiments settled once and for all the question of
the chemical identity of the genetic material.
The absolute generality of the conclusion remained a question, though, because
several plant and animal viruses were known to contain single-stranded RNA and no
DNA.
The role of this particle in particular though became clear after its function in the flow
of information from gene to protein was understood.
Thus, the concept of the ‘Central dogma of the cell’ was discovered. June 17 19

RNA AS GENETIC MATERIAL
RNA is the genetic material in viruses was demonstrated in 1956 with the
experiments conducted on tobacco plant by A.Gierer and G.Schramm.
All viruses are not limited to bacterial hosts. Viruses infect and parasite plant cells, and
even some animal cells contain RNA only. In these viruses, RNA act as genetic material.
One plant virus, Tobacco mosaic virus(TMV), that contains RNA, not DNA was an
important tool for genetic Experiments.
TMV infects tobacco, causing the infected regions on leaves to become discolored and
bristled. Different strains of TMV produce clearly different inherited lesions on the
infected leaves.
 The common virus produces a green mosaic disease, but a variant Holmes rib
grass(TMV-HR), produces ring spot lesions. Moreover, the amino acid compositions
of the proteins of these two strains differ.
June 17 21

 H.Fraenkel-Conrat and B.Singer first developed the techniques for separating TMV
particles into RNA and protein. They found that virus could be broken into component
parts and they could again be reassembled or reconstituted to form functional virus.
From the two strains of TMV they were able to reconstitute viruses with the RNA
from TMV common enclosed in TMV-HR protein and TMV-HR RNA with TMV common
protein.
 When these reassembled viruses were used to infect tobacco leaves, the progeny viruses
produced were always found to be phenotypically and genotypically identical to the
parent strain from which the RNA had been obtained.
 The reassembled viruses with the TMV-common RNA and TMV-HR protein produced a
green mosaic disease characteristic of TMV-common. Recovered virus had protein
characteristic of TMV common.
 This proved that specificity of virus proteins was determined by RNA alone and that
proteins carried no genetic information. Hence RNA carries genetic information not
proteins.
 The genetic RNA is usually found to be single stranded but in some it is double
stranded as in reovirus, wound tumor virus.
June 17 22

BIBLIOGRAPHY
Molecular Biology; David Freifelder, Narosa Publishing House,2nd edition (2004).
Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by P.S.Verma and
V.K.Agarwal. S.CHAND Publications. S.CHAND & Company Ltd.
A textbook of Biology, Std.12th , Maharashtra H.S.C. Board
Advanced Molecular Biology ;R. M. Twyman, 1st Edition , (2003)
Lodish, H. et al. 1999. Molecular Cell Biology, 4th edition. New York: W. H. Freeman and
Company (Pdf).
June 17 24

CHEMICALAND
PHYSICAL NATURE OF
DNA
THE DISCOVERY OF DNA DOUBLE HELIX
IS ONE OF THE GREATEST FINDINGS OF
ALL TIME, BUT IT’S ALSO ONE OF THE
MOST CONTROVERSIAL.
Anuja Naidu
MTI- 15015
June 17 25

KEY SCIENTISTS INVOLVED
June 17 26

Rosalind Franklin
James Watson Francis Crick
Maurice Wilkins
June 17 27

DNA (Deoxyribonucleic Acid)
and RNA (Ribonucleic Acid)
DNA is a double stranded
molecule that is twisted into a
Helix (Spiraling Staircase).
DNA and RNA, the principle
genetic materials of living
oraganisms, are chemically
called nucleic acids and are
complex molecules larger than
most proteins and contain
carbon, oxygen, hydrogen,
nitrogen and phosphorus.
DNA Double Helix
June 17 28

DNA: Each strand consists
of
1) A Sugar Phosphate Backbone
June 17 29

Each strand consists of:
2) Four Base Chemicals
(Attached in Pairs)
1) A Sugar Phosphate Backbone
June 17 30

Gregor Mendel:
Introduces the concept of heredity
1865 1909 1911 1929 1944 1950
The Early Efforts
June 17 31

Wilhelm Johannsen:
Coins the term “Gene”
1865 1909 1911 1929 1944 1950
The Early Efforts
June 17 32

Thomas Hunt Morgan:
Discovers that genes are responsible for inheritance
1865 1909 1911 1929 1944 1950
The Early Efforts
June 17 33

Phoebus Levene:
Discovers that DNA is made up of nucleotides, phosphates,
sugars and 4 bases
1865 1909 1911 1929 1944 1950
The Early Efforts
June 17 34

Oswald Avery:
Shows that DNA can transform the property of cells
1865 1909 1911 1929 1944 1950
The Early Efforts
However, this idea was not universally
accepted
June 17 35

Erwin Chargaff:
Shows that: A + G = T + C = 50%
1865 1909 1911 1929 1944 1950
The Early Efforts
Chargaff’s Rule is an important equation in the
discovery of the structure of DNA
June 17 36

ERWIN CHARGAFF
 An Austrian Chemist.
 Studied Chemistry in
Vienna University and
Yale University.
 He worked at the
University of Berlin.
 A professor at the
Columbia university.
June 17 37

THEORY
 First Parity Rule
 Second Parity Rule
 GC Rule
 Cluster Rule
 Erwin Chargaff provides more evidence that DNA = genetic material
 Analysis of base composition of DNA compared between different
organisms
 Nitrogenous bases
– Adenine (A)
– Thymine (T)
– Guanine (G)
– Cytosine (C)
 Conclusions of Chargaff
• DNA composition is species specific
• The amounts of A,G,C and T are not the same between species
– Ratios of nitrogenous bases vary between species
 Fourth observation was critical to Watson and Crick as they deduced the
structure of DNA
June 17 38

In 1951 Rosalind Franklin discovers the Two Forms of DNA
through her X-ray diffraction images.
A – Dry Form B – Wet Form
TWO FORMS OF DNA- FRANKLIN'S WORK
June 17 39

What is X-Ray Crystallography ?
 An X-ray diffraction pattern is obtained after a fiber of DNA is bombarded with X-
rays (0.1-10nm).
 Some rays pass through the DNA molecule.
 Some are diffracted and emerge at a different angle.
 A fiber consists of many DNA molecules, therefore resulting in overlapping circles
of interfering diffracted waves.
 Using photographic film spots and smears are revealed giving a X-ray diffraction
pattern.
June 17 40

Watson and Crick’s Work
June 17 41

In 1951 James Watson traveled from the United States to work with
Francis Crick at Cambridge University
Watson and Crick used the “Model Building” approach
They physically built models out of wire, sheet metal, nuts and bolts to
come up with the structure of DNA
Why did they build models?
“Sometimes the fingers can grasp what the mind cannot”
(Biology the Science of Life)
Watson and Crick’s Work
June 17 42

X-Ray Crystallography
June 17 43

How Watson and Crick Solved the DNA Structure
They already knew from Franklin and Wilkins’ work
that DNA was in the form of a Double Helix
They used Chargaff’s Rule to figure out how the 4 Bases
match up in pairs
Photo 51
June 17 44

Building Model
 DNA existed and contained four bases, a
ribose sugar and phosphate. Inspired by
Pauling's successful attempts at building 3-D
models of proteins, Crick and Watson believed
this to be the correct way to proceed.
June 17 45

Bases
 John Griffith, the mathematician nephew of Fred
Griffith, calculated the attractive forces between 'like'
bases. Crick's idea was that since the bases were flat,
perhaps they could be stacked on top of one another,
and attracted that way. Griffith informed him that
adenine attracted thymine and guanine attracts
cytosine.
June 17 46

Explaination of the model
 In DNA molecule the adjacent DNA are joined in a chain by phosphodiester
bridges or bonds which link the 5' carbon of the deoxyribose of one
mononucleotide unit with the 3' carbon of deoxyribose of the next
mononucleotide unit.
 The hydrogen bonds between purines and pyrimidines are such that adenine
can bond only to thymine by two hydrogen bonds, and guanine can bond only
to cytosine by three hydrogen bonds and no other alternative is possible
between them. The specificity of the kind of hydrogen bonds that can be
formed assures that for every adenine in one chain there will be thymine in
the other.
 For every guanine in first chain there will be a cystosine in the other and so on.
Thus, the two chains are complementary to each other; that is, the sequence
of nucleotides in one chain dictates the sequence of nucleotides in the other.
The two strands run anti-parallely- that is, have opposite directions. June 17 47

Watson-Crick double helical DNA model
 One strand has phosphodiester linkage in 3'→5'
direction, while other strand phosphodiester
linkage in just reverse or 5'→3' direction. Further,
both polynucleotides strands remain separated
by 20 A° distance.
 Coiling of double helix is right handed and
complete turn occurs every 3.4 A° distance.
 The helix has two external grooves, a deep wide
one, called major groove and a shallow narrow
one, called minor groove: both these grooves are
large enough to allow protein molecules to come
in contact with the bases.
June 17 48

The Final Model
 This structure has two helical chains each coiled round the
same axis.
 Usual chemical assumptions, namely, that each chain consists
of phosphate diester groups joining ß-D-deoxyribofuranose
residues with 3',5' linkages.
 Both chains follow right- handed helices, but owing to the dyad
the sequences of the atoms in the two chains run in opposite
directions.
 An angle of 36 degrees between adjacent residues in the
same chain, so that the structure repeats after 10 residues on
each chain, that is, after 34 A. The distance of a phosphorus
atom from the fibre axis is 10 A.
 As the phosphates are on the outside, cations have easy
access to them
 The phosphates are negatively charged, and attract cations.
The phosphates, being charged, are also hydrophilic.
June 17 49

 In 1962 Watson, Crick & Wilkins won the Nobel Prize for their discovery
of the structure of DNA
The Nobel Prize
 However, there is no mention of Franklin’s key work.
June 17 50

Watson & Crick quickly published their Scientific Journal
called “Nature” on April 25th 1953
“Nature”
June 17 51

Nucleic Acids
 Nucleic acids are polymers
 Monomer---nucleotides
• Nitrogenous bases
I. Purines
II. Pyrimidines
• Sugar
I. Ribose
II. Deoxyribose
• Phosphates + nucleoside = nucleotide
}Nucleosides
June 17 52

The Sugars
 A Nucleotide consists of :
 a nitrogenous base: purine
(Adenine (A) or Guanine (G))
or pyrimidine (Cytosine (C) or
Thymine (T) (or Uracil (U)in
RNA)).
 a sugar : Deoxyribose (DNA)
or Ribose (RNA).
 a phosphate group
 A sugar and a base form a
Nucleoside.
A Nucleotide is a
phosphorylated nucleoside.
Inter-nucleotide linkages are
formed by a phosphodiester
bond between a 5'-phosphate
group and the 3'-hydroxyl
group of the next nucleotide
sugar. The nucleotide
sequence encodes the
information required for
constructing proteins.
June 17 53

The Bases
PYRIMIDINES
PURINES
June 17 54

Pyrimidines and Purines
In order to understand the structure and properties of DNA
and RNA, we need to look at their structural components.
We begin with certain heterocyclic aromatic compounds
called pyrimidines and purines.
Pyrimidine and purine are the names of the parent compounds
of two types of nitrogen-containing heterocyclic aromatic
compounds.
N
N
N
N
N
N
H
Pyrimidine Purine
June 17 55

Uracil
 It is colorless, crystalline organic compound that is involved in
the transmission of hereditary information. While Uracil can
bond with all of the other bases, it readily bonds with adenine
most often.
 It is important to know that Uracil is a component in several
enzymes as well.
 It aids in the metabolism of complex carbohydrates
June 17 56

Cytosine
 Cytosine is an important part of DNA and RNA, where it is one of the
nitrogenous bases coding the genetic information these molecules
carry. Cytosine can even be modified into different bases to carry
epigenetic information. Cytosine has other roles in the cell, too, as
the energy carrier and cofactor CTP.
Cytosine
June 17 57

 At any moment, a small but finite number of cytosines lose their amino groups to
become uracil. Imagine that during replication, a C–G base pair separates. If at that
moment the C deaminates to U, it would tend to base-pair to A instead of to G. If U
were a natural base in DNA, the DNA polymerases would just line up an adenine
across from the uracil, and there would be no way to know that the uracil was a
mistake. This would lead to a much higher level of mutation during replication.
Because uracil is an unnatural base in DNA, DNA polymerases can recognize it as a
mistake and can replace it. Thus, the incorporation of thymine into DNA, though
energetically more costly, helps ensure that the DNA is replicated faithfully.
June 17 58

Thymine
 The chemical structure of thymine contains the ring-shaped
pyrimidine molecule, similarly to each of the nucleobases.
 In the formation of DNA, thymine and adenine are always paired
together by the force of two hydrogen bonds, which creates a
stable nucleic acid structure. In a comparable fashion, guanine and
cytosine bind together in the creation of DNA. Under specific
conditions, such as exposure to ultraviolet light, thymine dimers
may also occur, although this is much less common that the
thymine-adenine pairing. In most cases, thymine is not present in
RNA structures because it is replaced by uracil.
 The scientific name of thymine, 5-methyluracil, implies that it can
be derived with methylation process of uracil at the position of the
5th carbon. Specifically in the chemical structure, this means that a
methyl- branch (-CH3) is added to the pyrimidine ring.
Thymine
June 17 59

Given that both uracil and thymine base-pair with adenine,
why does RNA contain uracil and DNA contain thymine?
 Scientists now believe that RNA was the original hereditary molecule, and
that DNA developed later. If we compare the structure of uracil and
thymine, the only difference is the presence of a methyl group at C-5 of
thymine. This group is not on the side of the molecule involved in base
pairing. Because carbon sources and energy are required to methylate a
molecule, there must be a reason for DNA developing with a base that
does the same thing as uracil but that requires more energy to produce.
The answer is that thymine helps guarantee replication fidelity. One of the
most common spontaneous mutations of bases is the natural
deamination of cytosine.
June 17 60

Adenine
 Adenine(A) is one of the four bases that make up nucleic acids. It is a purine base that complementarily
binds to Thymine (T) in DNA and Uracil (U) in RNA. This bond is formed by two hydrogen bonds, which
help stabilize the nucleic acid structures. Different structures of adenine mainly result from
tautomerization of adenine, which allows the molecule to be available in isomeric forms in chemical
equilibrium. The molecular formula of adenine is C5H5N5 .
 An adenine molecule bound to a deoxyribose, a sugar, is known as deoxyadenosine. An adenine bound
to ribose, also a sugar, is known as adenosine, a key component in Adenosine Triphosphate. When
adenosine attaches to three phosphate groups, a nucleotide, adenosine triphosphate (ATP) is formed.
Adenosine triphosphate is an important source of energy that is used in many cellular mechanisms,
primarily in the transfer of energy in chemical reactions. The phosphate of ATP can detach, resulting in
a release of energy.
 In addition to ATP, adenosine also plays a key role in other organic molecules nicotinamide adenine
dinucleotide (NAD) and flavin adenine dinucleotide (FAD), both molecules of which are
involved in metabolism.
 Also, adenine can be found in tea, vitamin B12and several other coenzymes.
June 17 61

Guanine
 Guanine is a nitrogenous base. That means that it contains plenty of nitrogen atoms (five, to be exact)
and, chemically, it's basic rather than acidic.
 Guanine's shape includes two rings, putting it in the category of purines. (All nitrogenous bases are
either purines or pyrimidines.)
 It can make three weak hydrogen bonds, allowing it to bond to its buddy cytosine.
 Guanine, like other nitrogenous bases, can be part of a nucleotide; this
means it's attached to a sugar and one or more phosphates. DNA and
RNA are both nucleic acids, made of nucleotides chained together. That
makes guanine an important part of your genetic material.
 Structure of a nucleotide, showing the sugar, the phosphate, and the
base. Guanine is one type of base.
 Nucleotide structure
 Guanine bonds to cytosine because they both share three hydrogen
bonds. When a nucleotide in one chain of DNA or RNA has guanine as
its base, the opposite chain will have cytosine in the same spot
June 17 62

DNA: Part of polynucleotide chain
Portion of polynucleotide chain of deoxyribonucleic acid (DNA). The inset shows
the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).
June 17 63

DNA structure, showing the nucleotide bases cytosine (C), thymine (T), adenine (A),
and guanine (G) linked to a backbone of alternating phosphate (P) and deoxyribose
sugar (S) groups. Two sugar-phosphate chains are paired through hydrogen bonds
between A and T and between G and C, thus forming the twin-stranded double helix
of the DNA molecule.
June 17 64

Bibliography
 https://www.britannica.com/science/DNA
 http://www.biologydiscussion.com/cell-biology/composition-and-structure-of-the-
nucleic-acids-dna-rna/3223
 Cell biology, Genetics, Molecular Biology, Evolution and Ecology by P.S. Verma and V.K.
Agarwal (S.Chand- Library issue)
 http://library.med.utah.edu/NetBiochem/pupyr/pp.htm
 https://www.researchgate.net/post/Why_is_thymine_present_in_DNA_instead_of_U
racil
June 17 65

DNA
-CHROMOSOMAL AND
EXTRACHROMOSOMAL
VYANKATESH D. ZAMBARE
MTI-15024
June 17 66

CHROMOSOMAL DNA
 DNA which is present inside the nucleus (in eukaryotes) is known as
chromosomal DNA
 Coils and supercoils to form a chromosome
 Plays a role in heredity and variations
 Important in the regulation of protein synthesis
 Generally double stranded and helical in structure
 In prokaryotes there is no specific nucleus present. So the DNA is present
directly in the cytoplasm known as nucleoid DNA
June 17 67

PROKARYOTIC CHROMOSOMAL DNA
 Prokaryotes don’t have a true chromosomal DNA.
 Prokaryotes have a basic nucleus like material named as nucleoid.
 It is also known as Genephore or Prokaryotic Chromosome.
 It is suspended in the cytoplasm itself and doesn’t have a well defined nucleus.
 It is made up of 60% of DNA with a small amount of RNA (m-RNA) and protein (transcription
factor).
 It is generally a circular and double stranded DNA.
 It has proteins that are necessary for the dynamic arrangement of the prokaryotic genome
known as nucleoid proteins or nucleoid associated proteins (NAPs) but it doesn’t have histone
proteins .
 There are about 4.6 million base pairs in the chromosome of model bacteria E.coli.
 The chromosome is very long and is formed of a single thread of long DNA.
 Nucleoid is clearly visible under the high magnification of an electron micrograph.
June 17 68

PROKARYOTIC CHROMOSOMAL DNA
Nucleoid
June 17 69

EUKARYOTIC CHROMOSOMAL DNA
 Eukaryotic Chromosomal DNA is a well defined and well organised complex
molecule
 It is a double stranded linear helix
 There are about 2,33,785 exons and 2,07,344 introns
 It is very stable in nature unlike prokaryotic DNA
 It doesn’t have specific restriction sites and antibiotic resistance sites
 Made up of DNA and histone proteins
Total No. of genes = 33,000
Active genes(Protein Coding
genes) = 20,000 – 25,000
Oncogenes = 1,200 Intronic Genes
June 17 70

EUKARYOTIC CHROMOSOMAL DNA
June 17 71

TYPES OF EUKARYOTIC DNA (POLYMORPHISM)
 Classified on the basis of following differences:
1. Helical Diameter
2. Sense of rotation
3. Number of residues (monomers) present per turn (also known as pitch)
4. Presence of Major and Minor grooves
June 17 72

SIMILARITIES BETWEEN
Z-DNA AND B-DNA
 Both are double helical
 Two polynucleotide strands of DNA are antiparallel
 Both forms exhibit G≡C pairing
June 17 73

IMPORTANT FEATURES OF DIFFERENT FORMS OF DNA
DOUBLE HELICAL STRUCTURES
Helix
Type
Conditions Base
per
turn
Rotation
per bp
Vertical Rise
per bp
Helical
Diamete
r
A 75% relative
humidity; Na+
K+,Cs- ions
11 +32.7o
(RH)
2.56 Ao 23 Ao
B 92% relative
humidity, low
ionic strength
10 +36.0o
(RH)
3.38 Ao 19 Ao
C 66% relative
humidity, Li+ ions
9.33 +38.6o
(RH)
3.32 Ao 19 Ao
Z Very lilght salt
concentration
12 -30.0o (LH) 3.71 Ao 18 Ao
June 17 75

DIFFERENCES BETWEEN Z-DNA AND B-DNA
Z-DNA
• Has left handed coiling sense
• Phosphate backbone follows zig-zag
course
• Adjacent sugar residues have opposite
orientations
• One complete helix turn is 45o long
B-DNA
• Has right handed coiling sense
• Phosphate backbone is regular
• Adjacent sugar residues have
same orientations
• One complete helix turn is 34o
long
Presence of various forms of DNA has shown that DNA is
more polymorphic that it was thought to be and it is more
flexible and can attain a variety of forms. June 17 76

COMPARISON OF CHROMOSOMAL DNAS
Eukaryotic DNA
1. Eukaryotes have a well defined
nucleus. The DNA is situated inside
the nucleus. DNA coils and
supercoils to form a solenoid fibre
which again condenses to form a
chromosome.
2. Eukaryotic DNA doesn’t have
certain restriction sites and antibiotic
resistance site.
Prokaryotic DNA
1. Prokaryotes do not have a well
defined nucleus. The chromosomal
DNA is very basic in structure and
in the form of a nucleoid (nucleus
like) material, which may be
attached to the cell membrane.
2. Prokaryotic plasmid DNA has
antibiotic resistance sites as well as
restriction sites which make them a
very important tool in
Biotechnology and Genetic
Engineering.
June 17 78

EXTRACHROMOSOMAL DNA
 DNA which is present outside the nucleus is known as extrachromosomal DNA
 May be present in the cytoplasm or in the cell organelles depending upon the type of cell
 Prokaryotes have their extrachromosomal DNA in the form of circular plasmid
 Cell organelles like mitochondria and chloroplast in eukaryotic cells also contain circular DNA which have
their individual replication mechanism
 Plasmids have a great importance in cloning techniques
Extrachromosomal DNA
Eukaryotic extrachromosomal DNA Prokaryotic extrachromosomal DNA
Mitochondrial DNA Chloroplast DNA Plasmid DNA EpisomesJune 17 80

MITOCHONDRIAL DNA
 Also known as mtDNA
 16.6 kb pairs long
 Circular in shape
Total 37 genes
13 genes code for
proteins required in
ETS
24 genes code for
other mitochondrial
proteins
2 genes produce r-RNA 22 genes produce t-RNAJune 17 81

CHLOROPLAST DNA
Also known as cpDNA, plastome or plastidome.
Present in the stroma of Chloroplast
100-200 kb pairs long
Circular
Generally codes of the proteins and enzymes requires for the dark reaction of
photosynthesis
The DNA is generally 1,20,000 – 1,70,000 base pairs long having a length of 30 – 60
micrometres
The weight is 80-130 million Daltons
June 17 83

PLASMIDS
 Circular forms of prokaryotic extrachromosomal DNA
 It is also double stranded
 Introns are absent
 Have various restriction sites and replicative sites
 Replication is autonomous
 Usually 100 kb pairs long
 May be linear in structure in some bacteria
 Some plasmids have transposons or jumping genes (Barbara
McClintock,1948)
June 17 85

EPISOMES
Definition:
Episome is a genetic material present in some bacterial cells that can replicate autonomously or can integrate to
the DNA chromosome for its replication.
Episomes:
 The term episome was proposed by Francois Jacob and Elie Wallmon
 They are the DNA molecules which are not essential. So they may or may not be present.
 If not present, not acquired by de novo synthesis
 May be acquired from other strains by infections or conjugations
 When present, may be present autonomously in the cytoplasm or may be integrated into the chromosome
 They may be lost
 Considering above criteria the elements like sex factor (F factor or fertility factor), bacteriophages and
colicinogenic factors are included in the class of episomes
(Note: Colisin is a bacteriosin (bacterial proteinaceous toxin) which inhibit the growth and metabolism of the
same type of bacteria.)
June 17 87

LINEAR DNA
 Prokaryotes:
Generally bacteria have circular DNA in the form of plasmid. But many of the gram
positive and gram negative bacteria and some spirochete bacteria are now found to have Linear
DNA ,i.e., Linear plasmid.
Structurally, there are two types of linear DNA. Linear plasmids of the spirochete bacteria
Borrelia have a covalently closed hairpin loop at each end and linear plasmids
of the Gram-positive filamentous species of the genus Streptomyces have a covalently attached
protein at each end. There is another species of gram negative bacteria named Thiobacillus versutus
which has linear DNA.
 Eukaryotes:
Eukaryotes readily have a linear DNA. The DNA structure is double helical. DNA has histone
proteins and is organized into chromosomes. Unlike eukaryotes, prokaryotes
don’t have histones. Hence, their DNA is said to be Naked.
June 17 88

DIFFERENCE BETWEEN LINEAR AND CIRCULAR DNA
Linear DNA
• Present mainly in higher organisms and some
bacteria
• Has histones
• Introns are present
• Abundant non-functional regions
• Transposons are present
• Has open ends
Circular DNA
• Present in many bacteria and some cell
organelles of higher organisms
• No histones
• Introns are absent
• Fewer non-functional regions
• No transposons
• Ends are closed
June 17 89

BIBLIOGRAPHY
 Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by P.S.Verma and V.K.Agarwal.
S.CHAND Publications. S.CHAND & Company Ltd.
 A textbook of Biology, Std.12th , Maharashtra H.S.C. Board
 www.wikipedia.com
 Advanced Molecular Biology ;R. M. Twyman, 1st Edition , (2003)
 Molecular Biology; David Freifelder, Narosa Publishing House,2nd edition (2004)
Molecular Microbiology 1993 Dec;10(5):917-22.
 Linear plasmids and chromosomes in bacteria.
 Hinnebusch J1, Tilly K.
 Laboratory of Vectors and Pathogens, National Institute of Allergy and Infectious Diseases,
 National Institutes of Health, Rocky Mountain Laboratories, Hamilton, Montana 59840.
June 17 90

DISTINGUISHING DSDNA AND SSDNA

Double stranded DNA
 DsDNA has two
phosphodiester linkages.
 Along with the
phosphodiester linkages
joining the nucleotides, the
nitrogen bases of two
adjacent nucleotides form
pairing i.e. complementary
base pairing.
Single stranded DNA
SsDNA has only one
phosphodiester strand
(phosphodiester backbone).
 The non-bridging oxygen of
phospodiester backbone joins
two adjacent nucleotides but
the nitrogen bases do not
form complementary base
pairing . June 17 92

Double stranded DNA
 DsDNA is inflexible due to
pairing of nitrogen base
pairs which forms the
double helix structure.
 Double stranded DNA
binding proteins (DBPs)
helps the DsDNA in
regulation of gene
expression.
 DBPs cause DNA
transcription, cleavage of
DNA molecules, etc.
Single stranded DNA
 SsDNA has high flexibility
and can rotate freely and
does not form a helical
structure.
 Single stranded DNA
binding proteins (SBPs)
prevent the separated
single strands from coiling.
 SBPs help in DNA
replication and
recombination.

Double stranded DNA
DsDNA on heat supplied
denaturation can separate
the two strands and further
cooling can prevent them
from renaturation. Thus
giving rise to two SsDNA
strands.
DsDNA containing genome is
found in eukaryotic cells and
some prokaryotic cells as
well as viruses.
Single stranded DNA
SsDNA can get converted
into DsDNA by the process
of renaturation immediately
after undergoing
denaturation and cooling.
Thus forming a double helix
of DNA.
SsDNA containing genome is
found in class Parvoviridae
of Viruses.

BIBLIOGRAPHY
• Wikipedia.org
• Biology-online.org
• www.nature.com/scitable
• Cell Biology, Genetics, Molecular Biology, Evolution and Ecology; Dr. P.S.
Verma and Dr. V.K. Agarwal; S.Chand Publishers.
• H.S.C Textbook.

DNA
SUPERCOILING
SANIKA SHINGWEKAR
MTI-15020
June 17 97

WONDERING WHAT IS
SUPERCOILING ????
June 17 98

SUPERCOILING IS …..
June 17 99

NUCLEUS – THE ABODE OF DNA
 Nucleus of human cell - 6 micrometer in length.
 Consists of nuclear envelope, nucleolus, nucleoplasm and chromosomes.
 During the interphase stage of mitosis, the chromosomes have to double the DNA molecules in them
and hence the chromosomal content increases.
 This causes the nuclear organelles to get disturbed and hence to maintain favorable conditions inside
the cells, the DNA contents of a chromosomes undergo structural changes.
 Thus to get adjusted in the nucleus the chromosomal DNA has to get coiled and super coiled and stay in
the condensed form .
June 17 100

NUCLEOSOMES
 The nucleosome is the fundamental
subunit of chromatin. Each
nucleosome is composed of a little
less than two turns of DNA wrapped
around a set of eight proteins called
histones, which are known as a
histone octamer. Each histone
octamer is composed of two copies
each of the histone proteins H2A,
H2B, H3, and H4.
June 17 102

PAIRING OF NUCLEOSOMES
Combined with acidic DNA the
nucleosomes make a stable
nucleoprotein called chromatin
which appears as the ‘beads-
on-string’ pattern.
June 17 103

‘BEADS-ON-STRING’ PATTERN
June 17 104

HOW DOES SUPERCOILING OCCUR ?
Adding or subtracting the number of
turns or spirals between the 10 base
pairs can cause strain.
This strain enables the DNA strand
to get twisted more.
June 17 105

If two stressed strands of DNA are joint
together into a circle then it would take the
shape of ‘Eight’.
Further the two circular DNAs in the figure
‘eight’ would twist and form numerous
helical strands.
This process of twisting and re-twisting of
DNA into miniature helical structures is
called as super coiling of DNA.
June 17 106

 The relaxed DNA gradually turning into shape ‘EIGHT’ and further goes on coiling at high
intensity.
 The phenomenon of supercoiling is based on DNA topology technique.
June 17 107

POSITIVE SUPERCOILING
If the helix is under twisted so that it becomes tighter, the edges of the
narrow grooves move closer.
Underwinding leads to positive supercoiling which is also achieved by
twisting the helix to its right side.
Supercoiling relieves the strain in the molecule but negative supercoiling
can relieve a larger amount of stress as the strands may get separated.
June 17 108

NEGATIVE SUPERCOILING
If the helix is over twisted, the edges of the narrow groove
move further apart.
Overwinding leads to negative supercoiling which is also
achieved by twisting the helix to its left side.
June 17 109

DNA TOPOLOGY
The topological components of twisting has been described in a useful way.
In case of DNA, the twisting number T is defined as the total number of
turns of the double stranded molecule. The writhe W is the number of
turns of the axis of double stranded helix in space. The linking number L is
the total number of times the two strands of double helix of a closed
molecule cross each other.
The expression can be given as
L = W + T and ∆L = ∆W + ∆T
June 17 111

DNA TOPOLOGY FOR NEGATIVE SUPERCOILING
June 17 112

DNA TOPOLOGY FOR POSITIVE SUPERCOILING
June 17 113

MAIN SUPERCOILED STRUCTURES OF DNA
Negative supercoiling takes place in
two forms : a right handed helix
called a toroid and a right handed
helix with terminal loops called as a
plectoneme.
June 17 114

CONDENSED STRUCTURES OF DNA
June 17 115

ENZYME TOPOISOMERASE
Topoisomerases are enzymes that regulate the overwinding or
underwinding of DNA. The winding problem of DNA arises due to
the intertwined nature of its double-helical structure.
During DNA replication and transcription, DNA becomes
overwound ahead of a replication fork. If left unabated, this
torsion would eventually stop the ability of RNA & DNA
polymerase involved in these processes to continue down the DNA
strand.
June 17 116

ACTION OF TOPOISOMERASE
June 17 117

TYPES OF TOPOISOMERASES
• Topoisomerases can fix the topological problem of supercoiling while
DNA transcription or replication and are separated into two types
depending on the number of strands cut in one round of action. Both
these classes of enzyme utilize a conserved tyrosine. However these
enzymes are structurally and mechanistically different.
June 17 118

TYPE I TOPOISOMERASES :
 A type I topoisomerase cuts one strand of a DNA double helix, relaxation occurs, and then the cut strand is
reannealed. Cutting one strand allows the part of the molecule on one side of the cut to rotate around the
uncut strand, thereby reducing stress from too much or too little twist in the helix. Such stress is introduced
when the DNA strand is supercoiled or uncoiled.
 Type I topoisomerases are subdivided as : type IA topoisomerases, which share many structural and
mechanistic features with the type II topoisomerases, and type IB topoisomerases, which utilize a
controlled rotary mechanism. Examples of type IA topoisomerases include topo I and topo III. In the past,
type IB topoisomerases were referred to as eukaryotic topo I, but IB topoisomerases are present in all three
domains of life.
 Recently, a type IC topoisomerase has been identified, called topo V. While it is structurally unique from
type IA and IB topoisomerases, it shares a similar mechanism with type IB topoisomerase.
June 17 119

TYPE II TOPOISOMERASES :
 A type II topoisomerase cuts both strands of one DNA double helix, passes another unbroken DNA
helix through it, and then reanneals the cut strands. These topoisomerases relaxes both negative
and positive supercoiled DNA. This class is also split into two subclasses: type IIA and type IIB
topoisomerases.
 Type IIA topoisomerase is also known as DNA gyrase.
June 17 120

IMPORTANCE OF DNA SUPERCOILING
DNA supercoiling is important because
it efficiently adjusts or packages the
genetic material into the nucleus of cell
as the length of a DNA strand could be
thousand times greater than the cell.
DNA packaging is greatly increased
during nuclear division events such as
mitosis or meiosis, where DNA must be
compacted and segregated to form
daughter cells. June 17 122

Condensins and cohesins are structural
maintenance of chromosome
(SMC) proteins that aid in the
condensation of sister chromatids and the
linkage of the centromere in sister
chromatids. These SMC proteins induce
positive supercoils.
June 17 123

Supercoiling is also required for DNA and RNA synthesis. Because DNA must be
unwound for DNA and RNA polymerase action, supercoils will result.
The region ahead of the polymerase complex will be unwound; this stress is
compensated with positive supercoils ahead of the complex.
Behind the complex, DNA is rewound and there will be compensatory negative
supercoils.
Topoisomerases such as DNA gyrase play a role in relieving some of the stress
during DNA and RNA synthesis.
June 17 124

THE PROCESS OF DNA SUPERCOILING LOOKS
LIKE …..
 Double helix of DNA.
 ‘Beads-on-string’ structure of
nucleosomes.
 Chromatin fibre i.e. network of
nucleosome and DNA.
June 17 125

Chromatin fibre.
Extended form of Chromatin fibre.
Chromosomes during Mitosis.
June 17 126

Chromosomes during Mitosis.
Chromosome in condensed form.
June 17 127

STILL CONFUSED ?????
June 17 128

LETS EXPERIENCE IT
THROUGH OUR
EYES !
June 17 129

I MEAN ……
THROUGH THE LENS !
June 17 130

BIBLIOGRAPHY
Cell Biology, Genetics, Molecular Biology, Evolution and Ecology; Dr. P.S.
Verma and Dr. V.K. Agarwal; S.Chand Publishers.
Advanced Molecular Biology, Twyman
H.S.C Textbook
Wikipedia.org
Tandem.bu.edu
Mol-bio4masters
www.boundless.com

RNA AND IT’S TYPES
PRAKHAR VYAS
MTI-13052
June 17 133

WHAT IS RNA ?
RNA is a polymer of ribonucleotides linked together by 3’-5’ phosphodiester linkage.
RNA or ribonucleic acid is used to translate instructions from DNA to make proteins in your
body.
Each RNA nucleotide consists of a nitrogenous base, a ribose sugar and a phosphate.
Each RNA molecule typically is a single strand, consisting of a relatively short chain of
nucleotides. RNA can be shaped like a single helix, a straight molecule, or may be bent or
twisted upon itself. DNA, in comparison, is double-stranded and consists of a very long
chain of nucleotides.
In RNA, the base adenine binds to uracil. In DNA, adenine binds to thymine. RNA does not
contain thymine -- uracil is an un methylated form of thymine capable of absorbing light.
Guanine binds to cytosine in both DNA and RNA.
June 17 134

RNA performs many functions in an organism, such as coding, decoding, regulating, and
expressing genes.
It is also a single stranded structure unlike DNA which is a double stranded structure,
also it is a nucleic acid.
The sugar present here is ribose and the nitrogen bases present here are Adenine –
Uracil - Cytosine and Guanine.
RNA performs many functions in an organism, such as coding, decoding, regulating, and
expressing genes.
RNA is found in both the nucleus and cytoplasm of humans cells. DNA is only found in
the cell nucleus.
RNA is the genetic material for some organisms which don't have DNA. Some viruses
contain DNA; many only contain RNA.
RNA is used in some cancer gene therapies to reduce the expression of cancer-causing
genes.
June 17 135

RNA technology is used to suppress expression of fruit ripening genes so that fruits
can remain on the vine longer, extending their season and availability for
marketing.
Friedrich Miescher discovered nucleic acids ('nuclein') in 1868. After that time,
scientists realized there were different types of nucleic acids and different types of
RNA, so there is no single person or date for the discovery of RNA. In 1939,
researchers determined RNA is responsible for protein synthesis. In 1959, Severo
Ochoa won the Nobel Prize in Medicine for discovering how RNA is synthesized.
June 17 136

Size:
 RNA molecule is much smaller in size than DNA. It consists of up to 12,000 nucleotides whereas DNA
consists of up to 4.3 million nucleotides.
Location:
 RNA found in both prokaryotic and eukaryotic cells. In eukaryotic cell RNA found in cytoplasm as well
as in nucleus. In the cytoplasm it occurs freely as well as in the ribosomes while in the nucleus it is
present in association with chromosomes. RNA also found in matrix of mitochondria and stroma of
chloroplast.
June 17 138

TYPES OF RNA
In all Prokaryotic and Eukaryotic organisms, three main classes of RNA molecule
exist :
1. Messenger RNA (m RNA)
2. Transfer RNA (t RNA)
3. Ribosomal RNA (r RNA).
The other types are –
1. Small nuclear RNA (SnRNA),
2. Micro RNA (mi RNA),
3. Small interfering RNA (Si RNA) and
4. Heterogeneous nuclear RNA (Hn RNA).
June 17 139

MESSENGER RNA (M-RNA)
When a polypeptide is required, the triplet code of it’s gene is converted into a molecule
of messenger RNA (mRNA).
This process is called transcription and is the first stage of protein synthesis.
Comprises only 5% of the RNA in the cell.
Most heterogeneous in size and base sequence.
All members in the class function as messengers carrying the information in a gene to the
protein synthesizing machinery.
mRNA transcribes the genetic code from DNA into a form that can be read and used to
make proteins. mRNA carries genetic information from the nucleus to the cytoplasm of a
cell. June 17 140

STRUCTURAL CHARACTERISTICS OF MRNA
June 17 141

The mRNA molecules are formed with the help of DNA template during the process of
transcription.
The sequence carried on m-RNA is read in the form of codons.
A codon is made up of 3 nucleotides.
The m-RNA is formed after the processing of heterogeneous nuclear RNA.
June 17 144

TRANSFER RNA (T-RNA)
Transfer RNA are the smallest among the three major species of RNA.
They have 74-95 nucleotide residue.
They are synthesized by the nuclear processing of a precursor molecule.
They transfer the amino acids from the cytoplasm to the protein synthesizing
machinery, hence the name t-RNA.
They are easily soluble and hence called soluble RNA or s-RNA.
They are also called as adapter molecules, since they act as adapters for the translation
of the sequence of nucleotides of the m-RNA into specific amino acids.
There are at least 20 species of t-RNA one corresponding to each of the amino acids
required for protein synthesis.
June 17 146

The structure of alanine transfer RNA has been revealed by Robert W. Holley and his
associates. It consists of a single polynucleotide chain of 77 subunits.
Transfer RNA is synthesised in the nucleus on a DNA template. Transfer RNA does not
show any obvious base relationship of DNA.
The main function of transfer RNA is to carry amino acids to mRNA during protein
synthesis. Each amino acid is carried by a specific tRNA.
The structure of transfer RNA molecule is conventionally represented in the form of a
clover leaf although recent evidence indicates that tRNA molecules are L-shaped.
June 17 147

There are Primary, Secondary and tertiary structures.
The Secondary structure-(Clover Leaf structure).
All t-RNA contain 5 arms or loops which are as follows :
i. Acceptor.
ii. Anticodon Arm.
iii. D H U Arm.
iv. TѰ C Arm.
v. Extra Arm.
June 17 149

1. Amino acid arm:
 It has a seven base pairs stem formed by base pairing between 5′ and 3′ ends of tRNA. At 3′ end a
sequence of 5′-CCA-3′ is added. This is called CCA arm or amino acid acceptor arm. Amino acid binds
to this arm during protein synthesis.
2. D-arm:
 Going from 5′ to 3′ direction or anticlockwise direction, next arm is D-arm. It has a 3 to 4 base pair
stem and a loop called D-loop or DHU-loop. It contains a modified base dihydrouracil.
June 17 151

3. Anticodon arm:
 Next is the arm which lies opposite to the acceptor arm. It has a five base pair stem and a loop in which there
are three adjacent nucleotides called anticodon which are complementary to the codon of mRNA.
4. An extra arm:
 Next lies an extra arm which consists of 3-21 bases. Depending upon the length, extra arms are of two types,
small extra arm with 3-5 bases and other a large arm having 13-21 bases.
5. T-arm or TψC arm:
 It has a modified base pseudouridine ψ. It has a five base pair stem with a loop.
 There are about 50 different types of modified bases in different tRNAs, but four bases are more common.
One is ribothymidine which contains thymine which is not found in RNA. Other modified bases are
pseudouridine ψ, dihyrouridine and inosine.
June 17 152

RIBOSOMAL RNA (R-RNA)
The mammalian ribosome contains two major nucleoprotein subunits- a larger one with
weight 2.8 x 106 and a smaller subunit with a molecular weight of 1.4 x 106.
The functions of the Ribosomal RNA molecules in the ribosomal particle are not fully
understood, but they are important for ribosomal assembly.
They also play a keen role in binding RNA to ribosomes and it’s translation.
rRNA component also performs peptidyl transferase activity and thus is an enzyme called
as Ribozyme.
It occurs in combination with protein as ribonucleoprotein in the minute round particles
called ribosomes which are attached to the surfaces of the intracellular membrane
system called- endoplasmic reticulum.
It constitutes about 80% of the total RNA of the cell. June 17 153

It is being synthesised on special regions of chromosomal DNA that are
concentrated in the nucleoli, small densely staining spots in the nucleus.
Ribosomal RNA molecule may be a short compact rod, a compact coil or an
extended strand.
The rRNA does not show pyrimidine equality. The rRNA strands unfold upon
heating and refold upon cooling. The rRNA has been found to be stable for at least
two generations.
June 17 154

The main difference between DNA and RNA is the sugar present in the molecules.
While the sugar present in a RNA molecule is ribose, the sugar present in a
molecule of DNA is deoxyribose. Deoxyribose is the same as ribose, except that the
former has one more OH.
DNA does not usually exist as a single molecule, but instead as a tightly-associated
pair of molecules.
 These two long strands entwine like vines, in the shape of a double helix. This
arrangement of DNA strands is called antiparallel. The asymmetric ends of DNA
strands are referred to as the 5′ (five prime) and 3′ (three prime) ends. One of the
major differences between DNA and RNA is the sugar, with 2-deoxyribose being
replaced by the alternative pentose sugar ribose in RNA.
 The four bases found in DNA are adenine (abbreviated A), cytosine (C), guanine (G)
and thymine (T). A fifth pyrimidine base, called uracil (U), usually takes the place of
thymine in RNA and differs from thymine by lacking a methyl group on its ring.
June 17 159

BIBLIOGRAPHY
12th NCERT text book.
Article by Mrs. Kanika Chabbra on mRNA and tRNA.
Xam Idea (Reference Book for 12th CBSE).
Pictures from Google images.
June 17 162

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