Spectrum Biology - September 2016

Types of RNA

Molecules of RNA make up 50 to 10 per cent of the total weight

of the cell and have more varied biological functions than DNA.

DNA : The Supreme Heredity Commander

In most of the organisms DNA is the hereditary

material. Lets explore why?

1 Necessary Informations are Stored as

DNA in Eukaryotes

Cells store the information necessary to build

Ribonucleic Acid (RNA) and proteins as DNA, a

simple linear polymer found in all organisms.

2 Only the DNA of a Cell is Inherited

In multicellular organisms such as humans, most cells,

i.e., somatic cells, only pass their DNA to the cells

that replace them during that organism’s life time.

A specialised set of cells, i.e., germ cells are responsible

for passing DNA from one generation to the next.

A New Nucleic Acid Based

Agent for the Cure of

Immune Disorders

Stevens-Johnson Syndrome (SJS),

Toxic Epidermal Necrolysis (TEN)

and Graft Versus Host Disease

(GVHD) are distinct immune

reactions, elicited by drugs or

allogeneic antigens. They share a

pathomechanism with the

activation of cytotoxic

T-lymphocytes (CTLs). These CTLs

produce cytotoxic proteins,

cytokines, chemokines or immune

alarmins, such as granulysin (GNLY)

leading to the extensive tissue

damage and systemic inflammation

in patients with SJS/TEN or GVHD.

A new nucleic acid based agent

(CD-8 aptamer-GNLY siRNA

chimera) has been identified.

It can significantly inhibit

CTL-mediated drug hypersensitivity

as well as the alloreactivity.

RNA V/S DNA

Some RNA

is in the

nucleus

and most

is in the

cytoplasm

(ribosomes).

Possess

ribose

sugar.

Usually

consists

of a

single

strand.

Possess

uracil

instead of

thymine.

Genetic

material

of some

viruses.

Most of

the DNA

is found

in nucleus.

Contains a

deoxyribose

sugar.

Double

helical

structure.

Possess

thymine.

Genetic

material of

most of the

organisms.

Base pair

Helix of

sugar-phosphates

RNA

(Ribonuleic acid)

DNA

(Deoxyribonucleic acid)

Nucleobases

C

C

N

C N

N

H

C H

H N—C 2

H N

O

Guanine

CH

CH

N

H

C

C

N

Cytosine

O

NH 2

C

C

N

C N

N

H

CH

H C

N

Adenine

NH 2

C—CH 3

CH

N

H

C

C

H N

Thymine

O

O 2

CH

CH

N

H

C

C

H N

Uracil

O

O

Structure of tRNA; (a) Schematic drawing of a leucine tRNA from E. coli.

The three bases in the anticodon region are complementary to a codon

on the mRNA. (b) Tertiary structure of a tRNA molecule.

Part of an mRNA strand

Initiation

Capcodon

Coding

region

Noncoding region (trailer region)

Poly A

Noncoding region

(leading region)

Termination

codon

Structure of rRNA (schematic)

Unpaired

bases

Paired bases

Coiled

region

Uncoiled

region

5 ′ 3 ′

Constitutes 2-5% of total

RNA content of the cell.

It may be monocistronic

or polycistronic.

1. Messanger RNA (mRNA)

Synthesised in nucleoli

rRNA joins with proteins

to form the subunits of

ribosomes.

2. Ribosomal RNA (rRNA)


3. Transfer RNA (tRNA)
   Smallest RNA molecule.
   Synthesised in the nucleus.
   It becomes charged
   when a specific amino
   acid binds with it. Amino acid

Hydrogen

bonds

between

bases

tRNA

Amino

acid

Anticodon Anticodon

mRNA Codon

CH 3

H C—CH 3

CH 2

CH—NH 2

C==O

A O

C

G

A

U

G

C

(a)

(b)

Information is stored in the nucleus in the form of deoxyribonucleotide

sequences (DNA). Some of this DNA is transcribed in the nucleus to

form RNA, another form of information storage.

In the cytosol, messenger RNA is translated into sequences of

amino acids (polypeptides), which form proteins, another form

of information. Some proteins are synthesised on the surface

of the rough endoplasmic reticulum.

Some proteins expressed on the surface of cells act as

receptors for molecules in the extracelluar environment,

thereby capturing information about the cell’s surroundings.

Some proteins in the cytosol transmit

information from one location in the cell

to another (this is called intracellular signaling).

Some forms of information storage in cells