Spectrum Biology - September 2016

DNA is ‘silenced’ in Heterochromatin

Two different mechanisms have been identified for silencing the DNA. These are as follow:

H /H N-terminal tails 34

AC

Rap 1

bound

to DNA

2. Sir3/Sir
   polymerise


3. Sir3/Sir
   attach to matrix

AC AC AC

DNA silencing

By histone modification By Rap1, Sir 3 and Sir 4 proteins

1. Histone
   deacetylase
   removes acetyl
   group from
   histone H. 3
   2. A histone methylase
   adds a methyl group
   to amino acid #
   (a lysine) on
   histone H. 3
   3. The protein HP1 binds to
   methylated histone H , triggering 3
   a change in the shape of the
   nucleosome. This induces
   neighboring nucleosomes to
   be modified in the same fashion.

Active

chromatin

H 3 HP

Ac Me Me Me Me Me Me

4. Silenced chromatin
   
   
   1. Sir3/Sir
      bind to H /H 34
   
   
   
   

RNA as Genetic Material of Viruses

Many viruses contain RNA as their genetic material. Viruses consists of

nucleic acid (DNA or RNA) surrounded by a protein coat called capsid.

The structure of a virus

Nucleic acid:

DNA or RNA, single or

double-stranded.

Protein coat:

called capsid,

made of protein

subunits called

capsomeres.

Some capsids

are icosahedral,

as here; others

are helical or

other shapes.

In addition:

a virus may have an outer

envelope a lipid bilayer

derived from the cell

membrane of the host cell.

Glycoproteins

on the envelope.

Geophis lorancai: New

Species of Earth Snake

Found in Mexico

A team of scientists has described a new

strikingly-colored species in the snake

genus Geophis from the mountains of the

Sierra Zongolica in West-central Veracruz

and the Sierra de Quimixtlán in central-East

Puebla, Mexico. Geophis is a genus of colubrid snakes in the

subfamily Xenodontinae, commonly referred to as earth

snakes. With 49 recognized species, it is one of the most

diverse genera of snakes in the Western Hemisphere.

Replication of rhinovirus (cause of common cold)

Rhinovirus

Enters host cell by endocytosic

at a receptor protein site

Epithelial cell

of nasal mucosa

Viruses escape

into tissue fluid;

may repeat

cycle of

parasitisation

of a host cell

Loss of

capsid

RNA strand

Ribosomes

Protein

forms

three

enzymes:

Translation

i.e. RNA of virus is

‘read’ at ribosome,

and protein formed

1. RNA replicase, catalyses
   replication of RNA strand
   2. Enzyme that
   catalyses formation
   of protein coats

Assembly

Nucleus

of host

cell

3. Enzyme that causes
   lysis (splitting) of host cell
   membrane (some viruses
   escape without lysis of host)

Life Cycle of an RNA Virus

Retroviruses are particular RNA

viruses. When they infect a host cell,

they bring with their proteins an

enzyme called reverse

transcriptase. The mechanism of

replication of rhinovirus (RNA virus)

is shown in the given figure.

Thus, nucleic acids work as

hereditary commanders that

transfer genetic information from

one generation to the next

generation.

When you see blood

oozing from a cut in

your finger, you might

assume that it is red

because of the iron in

it, rather as rust has a

reddish hue. But the

presence of the iron is

a coincidence. The red

colour because the

iron is bound in a ring of atoms in haemoglobin called

porphyrin and it's the shape of this structure that

produces the colour.