Non-cellular and cellular forms

of life

“Mkhitar Gosh” University, Armenia

Medical biology courses : Lecture N 3

The living organisms are divided in

accordance with their structural organization

non-cellular cellular

viruses viroids prions prokaryotes eukaryotes

bacteria

mycoplasmas

blue-green

algae

VIRUSES

A virus (in latin means poison) is a

small infectious agent that replicates

only inside the living cells of other

organisms. Viruses can infect all types

of life forms, from animals and plants to

microorganisms, including bacteria.

Viruses are discovered by Ivanov ski

D.I. in 1892. The structure of all viruses

involves a nucleic acid and protein sheet

called capsid. In regard to the type of

genetic material contained in the viruses

there are two types of these organisms:

DNA viruses (Herpes virus) and RNA

viruses (e.g., HIV, flu virus, hepatitis B

viruses).

BACTERIOPHAGES

The viruses that affect

bacteria are called

bacteriophages, or just

phages. They are

composed of head

containing the genetic

material (DNA or RNA),

tail and tail fibers made

of protein. The

reproduction of phages

may proceed in lytic or

lysogenic cycles.

Lytic and lysogenic cycles of a

bacteriophage During lytic cycle the

phage attaches to the

host cell of bacteria and

lyses the cell wall by tail

fibers. The phage injects

its genetic material to

the host cell. The phage

uses the cell machinery

to produce own

particles, which

assemble into whole

phages.

The reproduced phages

lyse the host cell and

leave to affect new cells.

VIROIDS

Viroids present as a nucleic

acid and lack protein capsid.

The genetic material is the

short RNA (200-300

nucleotides long), which can

affect only damaged plant

cells. The only viroid of

humans described is the delta

agent that causes hepatitis D. It

is incorporated in the capsid of

hepatitis B virus (HBV) and

may affect human liver cells

only along with HBV.

COLUMNEA LATENT VIROID

PRIONS

Artificial Human

Prions Created in the Lab

Prions are infective

protein agents that do not

have any genetic material.

Prions affect brain tissue

and cause neurodegene

rative diseases (e.g. Kuru,

scrapie, “mad cow’s”

disease).

Prion diseases can be both

acquired (through infected

meat) and hereditary (gene

mutation).

PROKARYOTES

Prokaryotes are single-cell organisms measuring 0.5-5 mkm. They lack

true nucleus (pro – before, karyon – nucleus). These are bacteria,

mycoplasmas and blue-green algae. The genetic material of prokaryotes

is presented as a single circular DNA molecule called nucleoid (like a

nucleus), which is not connected to histone proteins. It is located free in

the cytoplasm, since there is no nuclear membrane. Prokaryotes have no

membranous organelles, instead they have mesosomes which are

infoldings of plasma membrane. The non-membranous organelle in

prokaryote is ribosome, which is smaller (70S) than in eukaryotes (80S).

Prokaryotes have cell wall made of muramic acid, while in eukaryotes

the cell wall is present in plant cells and is made of cellulose.

Additionally, some prokaryotes may have also a capsule. Arrangement

of microtubules in flagella and cilia of prokaryotes is “9+0”, and the

eukaryotic motility organelles are arranged as “9+2”. Division of

prokaryotes is amitosis.

THE STRUCTURE OF BACTERIA

MYCOPLASMA

Mycoplasma are a mollicute genus

of bacteria that lack a cell wall around

their cell membranes. This characte

ristic makes them naturally resistant to

many com mon antibiotics such

as penicillin or other beta-

lactam antibiotics that target cell wall

synthesis. Several species

are pathogenic in humans,

including M. pneumoniae, which is an

important cause of atypical

pneumonia and other respiratory

disorders.

EUKARYOTES

Eukaryotes can be both unicellular and multicellular

organisms. The eukaryotic cell is larger (5-100 mkm) than

prokaryotes. Eukaryotes (eu – true, karyon – nucleus) have

true nucleus: the genetic material is linear and is bound to

histone proteins organizing chromosomes; nuclear membrane

separates genetic material from cytoplasm. Membranous

organelles (e.g., endoplasmic reticulum, Golgi apparatus, etc.)

provide compartmentation of the cytoplasm (partitioning into

membranous parts), which allows simultaneous processing of

multiple metabolic reactions in various parts of the cell.

ELEMENTARY MEMRANE

PLASMA MEMBRANE

Plasma membrane consists of phospholipid bilayer with

proteins embedded in it. The phospholipids face by their

hydrophobic tails, which provide fluidity, and the

proteins have mosaic arrangement in the two layers of

phospholipids. Such a structure of plasma membrane is

known as fluid-mosaic model. Cholesterol is a

component of animal cell membrane. It provides

stability of the membrane.

Functions of plasma membrane are

following:

1. Protection and shaping.

2. Catalytic function. Some proteins in the plasma

membrane are enzymes that catalyze specific

reactions.

3. Transport /Passive transport (diffusion, facilitated

diffusion, osmosis); Active transport; Endocytosis

and exocytosis/

• 4. Signaling

• 5. Intercellular interactions (contact function).