9 9
MOLECULAR BASIS OF INHERITANCE
Figure 6.4a Nucleosome
Figure 6.4b EM picture - ‘Beads-on-String’
In some viruses the flow of information is in reverse direction, that is,
from RNA to DNA. Can you suggest a simple name to the process?
6.1.2 Packaging of DNA Helix
Taken the distance between two consecutive base pairs
as 0.34 nm (0.34×10
–9
m), if the length of DNA double
helix in a typical mammalian cell is calculated (simply
by multiplying the total number of bp with distance
between two consecutive bp, that is, 6.6 × 10
9
bp ×
0.34 × 10
-9
m/bp), it comes out to be approximately
2.2 metres. A length that is far greater than the
dimension of a typical nucleus (approximately 10
6
m).
How is such a long polymer packaged in a cell?
If the length of E. coli DNA is 1.36 mm, can you
calculate the number of base pairs in E.coli?
In prokaryotes, such as, E. coli, though they do
not have a defined nucleus, the DNA is not scattered
throughout the cell. DNA (being negatively charged)
is held with some proteins (that have positive
charges) in a region termed as ‘nucleoid’. The DNA
in nucleoid is organised in large loops held by
proteins.
In eukaryotes, this organisation is much more
complex. There is a set of positively charged, basic
proteins called histones. A protein acquires charge
depending upon the abundance of amino acids
residues with charged side chains. Histones are rich
in the basic amino acid residues lysine and arginine.
Both the amino acid residues carry positive charges
in their side chains. Histones are organised to form
a unit of eight molecules called histone octamer.
The negatively charged DNA is wrapped around the positively charged
histone octamer to form a structure called nucleosome (Figure 6.4 a). A
typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute
the repeating unit of a structure in nucleus called chromatin, thread-
like stained (coloured) bodies seen in nucleus. The nucleosomes in
chromatin are seen as ‘beads-on-string’ structure when viewed under
electron microscope (EM) (Figure 6.4 b).
Theoretically, how many such beads (nucleosomes) do you imagine
are present in a mammalian cell?
The beads-on-string structure in chromatin is packaged to form
chromatin fibers that are further coiled and condensed at metaphase stage
of cell division to form chromosomes. The packaging of chromatin at higher
level requires additional set of proteins that collectively are referred to as
2022-23
MOLECULAR BASIS OF INHERITANCE
In some viruses the flow
of information
is in reverse direction, that is,
from RNA to DNA.
Can you suggest a simple name to the process?
6.1.2 Packaging of DNA Helix
Taken the distance between two consecutive base pairs
–9
m), if the length of DNA double
ph ag
of cell division
to
form chromosomes. The packaging of chromatin at higher
level requires additional set of proteins that collectively are referred to as
202
2-2
3
99 99
Figure 6
.4
.4
.4
a
a
a
N
uc
uc
uc
le
le
le
os
os
os
om
om
om
e
a
Figure 6.4b
EM picture - ‘Beads-on-String’
helix in a typical mammalian cell is calculated (simply
by multiplying the total number of bp with distance
between two consecutive
bp
, that is, 6.6 × 10
9
bp
×
0.34 × 10
-
9
m
/
bp), it comes out to be approximately
2.2 metres. A length that is far greater than the
dimension of a typical nucleus (approximatel
y
10
6
10
10
m).
How is suc
h
a long polymer packaged in a cell?
If the length of E. coli DNA is 1.36 mm, can you
calculate the number of base pairs in E.coli?
In prokaryotes, such as,
E. coli
, though they do
not have a defined nucleus, the DNA is not scattered
throughout the cell. DNA (being negatively charged)
is held with some proteins (that have positi
ve
charges) in a region termed as ‘nucleoid’. The DNA
in nucleoid is organised in large loops held b
y
pr
oteins.
In eukaryotes, this organisation is much mor
e
complex. There is a set of positively charged, basi
c
proteins called
histones
. A protein acquires char
ge
depending upon the abundance of amino acids
residues with charged side chains. Histones are ri
ch
in the basic amino acid residues lysine and arginine.
Both the amino acid residues carry positive charges
in their side chains. Histones are organised to fo
rm
a unit of eight molecules called
histone octame
r
.
The negatively charged DNA is wrapped around the positively charged
histone octamer to form a structure called
nucleosome
(Figure 6.4 a).
A
typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute
the repeating unit of a structure in nucleus called
chromatin,
t
hr
ea
d-
like stained (coloured) bodies seen in nucleus. The nucleosomes in
chromatin are seen as ‘beads-on-string’ structure when viewed under
electron microscope (EM) (Figure 6.4 b).
Theoretically, how many such beads (nucleosomes) do you imagin
e
are present in a mammalian cell?
The beads-on-string structure in chromatin is packaged to form
chromatin fibers that
ar
e
further coiled and condensed at metaphase stage
100
BIOLOGY
Non-histone Chromosomal (NHC) proteins. In a typical nucleus, some
region of chromatin are loosely packed (and stains light) and are referred to
as euchromatin. The chromatin that is more densely packed and stains
dark are called as Heterochromatin. Euchromatin is said to be
transcriptionally active chromatin, whereas heterochromatin is inactive.
6.2 THE SEARCH FOR GENETIC MATERIAL
Even though the discovery of nuclein by Meischer and the proposition
for principles of inheritance by Mendel were almost at the same time, but
that the DNA acts as a genetic material took long to be discovered and
proven. By 1926, the quest to determine the mechanism for genetic
inheritance had reached the molecular level. Previous discoveries by
Gregor Mendel, Walter Sutton, Thomas Hunt Morgan and numerous other
scientists had narrowed the search to the chromosomes located in the
nucleus of most cells. But the question of what molecule was actually the
genetic material, had not been answered.
Transforming Principle
In 1928, Frederick Griffith, in a series of experiments with Streptococcus
pneumoniae (bacterium responsible for pneumonia), witnessed a
miraculous transformation in the bacteria. During the course of his
experiment, a living organism (bacteria) had changed in physical form.
When Streptococcus pneumoniae (pneumococcus) bacteria are grown
on a culture plate, some produce smooth shiny colonies (S) while others
produce rough colonies (R). This is because the S strain bacteria have a
mucous (polysaccharide) coat, while R strain does not. Mice infected with
the S strain (virulent) die from pneumonia infection but mice infected
with the R strain do not develop pneumonia.
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
2022-23
BIOLOGY
Non-
h
istone Chromosomal (NHC)
p
roteins
. In a typical nucleus, some
region of chromatin are loosely packed (and stains light)
and
are referred to
as
e
uc
hr
om
at
in
.
The chromatin that is more densely packed and stains
dark are called as
Heterochromatin
. Euchromatin is said to be
transcriptionally active chromatin, whereas heterochromatin is inactive.
6 2 T
HE
S
HE
EA
RC
H
FO
R
G
R
EN
ET
IC
M
A
M
TE
RI
AL
A
A
on
t
nd
c
by
r
he
e
s
a
s
wn
s
a
h
ed
at
e
202
2-2
3
110000
6.2 T
HE
S
HE
EARC
H
FOR
G
R
ENET
IC
M
A
M
M
TERIAL
AA
Even though the discovery of nuclein by Meischer and the proposition
for principles of inheritance by Mendel were almost at the same time, but
that the DNA acts as a genetic material took long to be discovered and
pr
oven.
By
1926, the
q
uest to determine the mechanism for
ge
netic
inheritance had reached the molecular level. Previous discoveries by
Gr
egor Mendel, W
alter Sutton, Thomas Hunt Mo
r
W
W
gan and numer
r
r
ou
s
ot
he
r
scientists had narrowed the search to the chromosomes located in the
nucleus of most cells. But the question of what molecule was actually the
genetic material, had not been answered.
Transforming Principle
In 1928, Frederick Griffith, in a series of ex
pe
riments with
Stre
pt
ococcus
pneumoniae
(bacterium responsible for pneumonia), witnessed a
ae
miraculous transformation in the bacteria. During the course of his
experiment, a living organism (bacteria) had changed in
ph
ysical form.
Wh
en
Streptococcus pneumoni
ae
(pneumococcus) bacteria are grown
ae
on a culture
p
late, some
p
roduce smooth shin
y
colonies (S) while others
produce rough colonies (R). This is because the S strain bacteria have a
mucous (polysaccharide) coat, while R strain does not. Mice infected with
the S strain (virulent) die from pneumonia infection but mice infected
with the R strain do not develop pneumonia.
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
101
MOLECULAR BASIS OF INHERITANCE
injected a mixture of heat-killed S and live R bacteria, the mice died.
Moreover, he recovered living S bacteria from the dead mice.
He concluded that the R strain bacteria had somehow been
transformed by the heat-killed S strain bacteria. Some ‘transforming
principle’, transferred from the heat-killed S strain, had enabled the
R strain to synthesise a smooth polysaccharide coat and become virulent.
This must be due to the transfer of the genetic material. However, the
biochemical nature of genetic material was not defined from his
experiments.
Biochemical Characterisation of Transforming Principle
Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty
(1933-44), the genetic material was thought to be a protein. They worked
to determine the biochemical nature of ‘transforming principle’ in Griffith's
experiment.
They purified biochemicals (proteins, DNA, RNA, etc.) from the
heat-killed S cells to see which ones could transform live R cells into
S cells. They discovered that DNA alone from S bacteria caused R bacteria
to become transformed.
They also discovered that protein-digesting enzymes (proteases) and
RNA-digesting enzymes (RNases) did not affect transformation, so the
transforming substance was not a protein or RNA. Digestion with DNase
did inhibit transformation, suggesting that the DNA caused the
transformation. They concluded that DNA is the hereditary material, but
not all biologists were convinced.
Can you think of any difference between DNAs and DNase?
6.2.1 The Genetic Material is DNA
The unequivocal proof that DNA is the genetic material came from the
experiments of Alfred Hershey and Martha Chase (1952). They worked
with viruses that infect bacteria called bacteriophages.
The bacteriophage attaches to the bacteria and its genetic material
then enters the bacterial cell. The bacterial cell treats the viral genetic
material as if it was its own and subsequently manufactures more virus
particles. Hershey and Chase worked to discover whether it was protein
or DNA from the viruses that entered the bacteria.
They grew some viruses on a medium that contained radioactive
phosphorus and some others on medium that contained radioactive sulfur.
Viruses grown in the presence of radioactive phosphorus contained
radioactive DNA but not radioactive protein because DNA contains
phosphorus but protein does not. Similarly, viruses grown on radioactive
sulfur contained radioactive protein but not radioactive DNA because
DNA does not contain sulfur.
2022-23
MOLECULAR BASIS OF INHERITANCE
injected a mixture of heat-killed S and live R bacteria, the mice died.
Mor
eover
, he r
ecover
ed living S bacteria fr
om the dead mice.
He
c
on
cl
ud
ed
t
ha
t
th
e
R
st
ra
in
b
ac
te
ri
a
ha
d
so
me
ho
w
be
en
transformed
by the heat-killed S strain bacteria. Some ‘transforming
pr
inci
pl
e’, transferred from the heat-killed S strain, had enabled the
R strain to
sy
nthesise a smooth
po
lysaccharide coat and become virulent.
sulfur contained radioactive protein but not radioactive DNA because
DNA does not contain sulfur
.
202
2-2
3
110011
sy poly
This must be due to the transfer of the genetic material. However
, the
biochemical nature of genetic material was not defined from his
experiments.
Biochemical Characterisation of Transformi
ng
Princ
ip
le
Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty
(1933-44), the genetic material was thought to be a protein. They worked
to determine the biochemical nature of ‘transformi
ng
p
rinc
ip
lein Griffith's
experiment
.
They purified biochemicals (proteins, DNA, RNA, etc.) from the
heat-killed S cells to see which ones could transform live R cells into
S cells. They discovered that DNA alone from S bacteria caused R bacteria
to become transforme
d.
They also discovered that protein-digesting enzymes (proteases) and
RNA-digesting enzymes (RNases) did not affect transformation, so the
transforming substance was not a protein or RNA. Digestion with DNase
did inhibit transformation, suggesting that the DNA caused the
transformation. Th
ey
concluded that DNA is the hereditar
y
material, but
not all biologists were convinced.
Can you think of any difference between DNAs and DNase?
6.2.1 The Genetic
Ma
Ma
tete
ri
ri
alal
is DNA
The un
eq
uivocal
pr
oof that DNA is the
ge
netic material came from the
ex
pe
riments of Alfred Hershe
y
and Martha Chase (1952). The
y
worked
with viruses that infect bacteria called bacteriophages.
The bacterio
ph
age attaches to the bacteria and its genetic material
then enters the bacterial cell. The bacterial cell treats the viral genetic
material as if it was its own and subs
eq
uent
ly
manufactures more virus
pa
rticles. Hersh
ey
and Chase worked to discover whether it was
p
rotein
or DNA from the viruses that entered the bacteri
a.
They grew some viruses on a medium that contained radioactive
phosphorus and some others on medium that contained radioactive sulfur
.
ur
ur
Viruses
gr
own in the
p
resence of radioactive
p
ho
sp
horus contained
radioactive DNA but not radioactive protein because DNA contains
phosphorus but protein does not. Similarly, viruses grown on radioactive
lf ta ed ad ti te but ot ad ti DNA b
102
BIOLOGY
Radioactive phages were allowed to attach to E. coli bacteria. Then, as
the infection proceeded, the viral coats were removed from the bacteria by
agitating them in a blender. The virus particles were separated from the
bacteria by spinning them in a centrifuge.
Bacteria which was infected with viruses that had radioactive DNA
were radioactive, indicating that DNA was the material that passed from
the virus to the bacteria. Bacteria that were infected with viruses that had
radioactive proteins were not radioactive. This indicates that proteins did
not enter the bacteria from the viruses. DNA is therefore the genetic
material that is passed from virus to bacteria (Figure 6.5).
Figure 6.5 The Hershey-Chase experiment
6.2.2 Properties of Genetic Material (DNA versus RNA)
From the foregoing discussion, it is clear that the debate between proteins
versus DNA as the genetic material was unequivocally resolved from
Hershey-Chase experiment. It became an established fact that it is DNA
that acts as genetic material. However, it subsequently became clear that
2022-23
BIOLOGY
Radioactive phages were allowed to attach to
E. col
i
bacteria. Then, as
i
the infection proceeded, the viral coats were removed from the bacteria by
agitating them in a blende
r
. The virus particles wer
e separated fr
om
t
he
bacteria by spinning them in a centrifuge.
Bacteria which was infected with viruses that had radioactive DNA
were radioactive, indicating that DNA was the material that passed from
the virus to the bacteria. Bacteria that were infected with viruses that had
d
c
s
om
Hershey
-
Chase experiment. It became an established fact that it is DNA
that acts as genetic material. Howev
er
, it subsequently became clear that
202
2-2
3
110022
the virus to the bacteria. Bacteria that were infected with viruses that had
radioactive proteins were not radioactive. This indicates that proteins did
not enter the bacteria from the viruses. DNA is therefore the genetic
material that is passed from virus to bacteria (Figure 6.5).
Figure 6.5
The Hershey-Chase experiment
6.2.2 Properties of Genetic Material (DNA versus RNA)
From the foregoing discussion, it is clear that the debate between proteins
versus DNA as the genetic material was unequivocally resolved from
103
MOLECULAR BASIS OF INHERITANCE
in some viruses, RNA is the genetic material (for example, Tobacco Mosaic
viruses, QB bacteriophage, etc.). Answer to some of the questions such as,
why DNA is the predominant genetic material, whereas RNA performs
dynamic functions of messenger and adapter has to be found from the
differences between chemical structures of the two nucleic acid molecules.
Can you recall the two chemical differences between DNA and RNA?
A molecule that can act as a genetic material must fulfill the following
criteria:
(i) It should be able to generate its replica (Replication).
(ii) It should be stable chemically and structurally.
(iii) It should provide the scope for slow changes (mutation) that
are required for evolution.
(iv) It should be able to express itself in the form of 'Mendelian
Characters’.
If one examines each requirement one by one, because of rule of base
pairing and complementarity, both the nucleic acids (DNA and RNA) have
the ability to direct their duplications. The other molecules in the living
system, such as proteins fail to fulfill first criteria itself.
The genetic material should be stable enough not to change with
different stages of life cycle, age or with change in physiology of the
organism. Stability as one of the properties of genetic material was very
evident in Griffith’s ‘transforming principle’ itself that heat, which killed
the bacteria, at least did not destroy some of the properties of genetic
material. This now can easily be explained in light of the DNA that the
two strands being complementary if separated by heating come together,
when appropriate conditions are provided. Further, 2
'
-OH group present
at every nucleotide in RNA is a reactive group and makes RNA labile and
easily degradable. RNA is also now known to be catalytic, hence reactive.
Therefore, DNA chemically is less reactive and structurally more stable
when compared to RNA. Therefore, among the two nucleic acids, the DNA
is a better genetic material.
In fact, the presence of thymine at the place of uracil also confers
additional stability to DNA. (Detailed discussion about this requires
understanding of the process of repair in DNA, and you will study these
processes in higher classes.)
Both DNA and RNA are able to mutate. In fact, RNA being unstable,
mutate at a faster rate. Consequently, viruses having RNA genome and
having shorter life span mutate and evolve faster.
RNA can directly code for the synthesis of proteins, hence can easily
express the characters. DNA, however, is dependent on RNA for synthesis
of proteins. The protein synthesising machinery has evolved around RNA.
The above discussion indicate that both RNA and DNA can function as
2022-23
MOLECULAR BASIS OF INHERITANCE
in
some
vi
ruses
,
RNA is the genetic material (for example, Tobacco Mosaic
viruses, QB bacteriophage
,
etc.). Answer to some of the questions such as,
why DNA is the predominant genetic material, whereas RNA performs
dy
namic functions of messenger and adapter has to be found from the
differences between chemical structures of the two nucleic acid molecules.
Can you recall the two chemical differences between DNA and RN
A?
of proteins. The protein synthesising machinery has evolved around RNA.
Th
e
ab
ov
e
di
sc
us
si
on
i
nd
ic
at
e
th
at
b
ot
h RNA
an
d DNA
ca
n
fu
nc
ti
on
a
s
202
2-2
3
110033
A molecule that can act as a genetic material must fulfill the following
criteria:
(i)
It should be able to generate its replica (Replication).
(ii)
It should be stable chemically and structurally.
(iii)
It should provide the scope for slow changes (mutation) that
are required for evolution.
(iv)
It should be able to express itself in the form of 'Mendelian
Characters’.
If one examines each requirement one by one, because of rule of base
pairing and complementarity, both the nucleic acids (DNA and RNA) have
the ability to direct their duplications. The other molecules in the living
system, such as proteins fail to fulfill first criteria itself.
The genetic material should be stable enough not to change with
different stages of life cycle, age or with change in physiology of the
organism. Stability as one of the properties of genetic material was very
evident in Griffith’s ‘transforming principle’ itself that heat, which killed
the bacteria, at least did not destroy some of the properties of genetic
material. This now can easily be explained in light of the DNA that the
two strands being complementary if separated by heating come together
,
when appr
opriate conditions ar
e pr
ovided. Furth
er
,
2
'
-OH gr
oup pr
esent
at every nucleotide in RNA is a reactive group and makes RNA labile and
easily degradable. RNA is also now known to be catalytic, hence reactive.
Therefore, DNA chemically is less reactive and structurally more stable
when compa
red to RNA.
papa
Therefore, among the two nucleic acids, the DN
A
is a better genetic material.
In fact, the presence of thymine at the place of uracil also confers
additional stability to DNA. (Detailed discussion about this requires
understanding of the process of repair in DNA, and you will study these
processes in higher classes.)
Both DNA and RNA are able to mutate. In fact, RNA being unstable,
mutate at a faster rate. Consequently, viruses having RNA genome and
having shorter life span mutate and evolve faster
.
RNA can directly code for the synthesis of proteins, hence can easily
expr
ess the characters. DNA, however
, is dependent on RNA for synthesis