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Introduction

Microorganisms were originally divided into plant and animal kingdoms. Because this was unsatisfactory, they were then classified under a third kingdom, protista. Based on cellular organizations and biochemistry they are further classified into :

  1. Prokaryotes :  Bacteria & Blue Green Algae
  2. Eukaryotes    :  Other Algae, Fungi, Slime moulds, Protozoa

Comparison of medically important organisms

Characteristic

Viruses

Bacteria

Fungi

Protozoa/ Helminths

Cells

 

No

Yes

Yes

Yes

Approx diam. (μm)

 

02-0.2

1-5

3-10(yeast)

15-25 (trophozoite)

Nucleic acid

DNA or RNA

 

DNA & RNA

DNA & RNA

DNA & RNA

Type of nucleus

None

 

Prokaryotic

Eukaryotic

Eukaryotic

Ribosomes

Absent

 

70s (50s+30s)

80s (60s+40s)

80s (60s+40s)

Mitochondria

Absent

 

Absent

Present

Present

Outer surface

Capsid

 

Rigid wall peptidoglycan

 

Rigid wall chitin

Flexible Membrane

Motility

None

 

Some

None

Most

Replication

--

Binary fission

Budding mitosis

Mitosis

 

Difference between Prokaryote and Eukaryote
 

Characteristic

Prokaryote

Eukaryote

Genetic material

 

Nuclear membrane

Absent

Present

Nucleolus

Absent

Present

Histones

Absent

Present

Chromosome

One (circular)

Multiple (linear)

Replication

Binary fission

Mitosis/meiosis

Extra chromosomal DNA

Plasmids

Mitochondria

Cytoplasm

 

Cytoplasmic streaming

Absent

Present

Pinocytosis

Absent

Present

Mitochondria

Absent

Present

Lysosomes

Absent

Present

Golgi apparatus

Absent

Present

E.R.

Absent

Present

Ribosomes

70s, 50s, 30s

80s, 60s, 40s

Chemical composition of cell envelope

 

Sterols

Absent*

Present

Muramic acid

Present

Absent

* Present in Mycoplasma

 





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