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

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

as 0.34 nm (0.34×10

–9

m), if the length of DNA double

ph ag

of cell division

form chromosomes. The packaging of chromatin at higher

level requires additional set of proteins that collectively are referred to as

202

2-2

99 99

Figure 6

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

, that is, 6.6 × 10

0.34 × 10

bp), it comes out to be approximately

2.2 metres. A length that is far greater than the

dimension of a typical nucleus (approximatel

–6

m).

How is suc

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

charges) in a region termed as ‘nucleoid’. The DNA

in nucleoid is organised in large loops held b

oteins.

In eukaryotes, this organisation is much mor

complex. There is a set of positively charged, basi

proteins called

histones

. A protein acquires char

depending upon the abundance of amino acids

residues with charged side chains. Histones are ri

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

a unit of eight molecules called

histone octame

The negatively charged DNA is wrapped around the positively charged

histone octamer to form a structure called

nucleosome

(Figure 6.4 a).

typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute

the repeating unit of a structure in nucleus called

chromatin,

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

are present in a mammalian cell?

The beads-on-string structure in chromatin is packaged to form

chromatin fibers that

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-

istone Chromosomal (NHC)

roteins

. In a typical nucleus, some

region of chromatin are loosely packed (and stains light)

and

are referred to

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

202

2-2

110000

6.2 T

EARC

FOR

ENET

TERIAL

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

oven.

1926, the

uest to determine the mechanism for

netic

inheritance had reached the molecular level. Previous discoveries by

egor Mendel, W

alter Sutton, Thomas Hunt Mo

gan and numer

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

riments with

Stre

ococcus

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

ysical form.

Streptococcus pneumoni

(pneumococcus) bacteria are grown

on a culture

late, some

roduce smooth shin

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.

transformed

by the heat-killed S strain bacteria. Some ‘transforming