Prokaryotes and Eukaryotes

Md. Atick Chowdhury Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100, Bangladesh.

E-mail: atickturja@gmail.com

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CONTENTS

Topics Pages

01. Introduction 03

02. Prokaryotes 04

03. Prokaryotic Cell 05

04. Eukaryotes 05

05. Eukaryotic Cell 07

06. Comparison between Prokaryotic and Eukaryotic Cell 08

07. Conclusion 09

08. References 09

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01. Introduction:

The smallest unit of life is cell. All living beings except the viruses are composed of one or many

cells. On the cytological basis, the cellular organisms fundamentally may have either

prokaryotic cells or eukaryotic cells.

The cell can be defined as a basic functional unit of life. The term cell is derived from the Latin

word and was first observed by a scientist named Robert Hook in the year 1665. All living

organisms are composed of one or many cells to perform their individual functions. A cell is a

smallest unit of a life, which is able to control and perform several functions in all living

organisms. All unicellular organisms (single cell) including bacteria and archae, which are

composed of a single cell, are examples of prokaryotic cells. All multicellular organisms (many

or more than one cell) including humans, which are composed of complex or many cells, are

examples of eukaryotic cells. Both prokaryotic cells and eukaryotic cells have cytoplasm, cell

membrane and genetic material in common.

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02. Prokaryotes: Greek word ‘pro’ means before and ‘karyon’ means nut or kernel, referring to the cell nucleus.

Prokaryotes are organisms without a cell nucleus, or any other membrane bound organelles.

Most prokaryotes are unicellular, but some prokaryotes are multicellular.

Examples: Blue-green algae (cyanobacteria), bacteria, archaea etc.

Figure: Micrscopic view of Prokaryote (Bacteria)

Bacteria (E.coli) Bacterial Colony

Cyanobacteria Archaea

Figure: Some Prokaryotes

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03. Prokaryotic Cell: The cell which are composed of primitively organized cytoplasm and nucleus; the nuclear

substances (i.e. DNA and RNA) do not remain isolated or separated from the cytoplasm which

lacks in mitochondria, endoplasmic reticulum, Golgi complex, lysosomes, centrioles etc., by any

nuclear membrane, are called prokaryotic cells.

The simplest types of cells are prokaryotic cells. Bacteria, the oldest form of life on Earth, are

prokaryotes. The DNA of prokaryotic cells is not contained within a nucleus. Instead,

prokaryotic chromosomes are found in a localized area of the cell called the nucleoid region.

Examples: The prokaryotic cells in blue-green algae and bacteria.

04. Eukaryotes:

Greek word ‘eu’ means good or true and ‘karyon’ means nut or kernel reffering to the cell

nucleus.

Eukaryotes are organisms whose cells are organized into complex structures by internal

membranes and cytoskeleton. The most characteristic membrane bound structure is the nucleus.

This feature gives them the name.

Examples: Animals, plants, fungi and protists etc.

Figure: Typical Prokaryotic Cell

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Figure: Microscopic View of Eukaryote (Animal cell)

Tenualosa ilisha

Homo sapiens

Macaca fascicularis

Oryza sativa

Nymphaea rubra

Ailuropoda melanoleuca

Figure: Some Eukaryotes

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05. Eukaryotic Cell: According to Encyclopædia Britannica, “Eukaryote, any cell or organism that possesses a

clearly defined nucleus. The eukaryotic cell has a nuclear membrane that surrounds the nucleus,

in which the well-defined chromosomes (bodies containing the hereditary material) are located.

Eukaryotic cells also contain organelles, including mitochondria (cellular energy exchangers), a

Golgi apparatus (secretory device), an endoplasmic reticulum (a canal-like system of membranes

within the cell), and lysosomes (digestive apparatus within many cell types).”

From Wikipedia, the free encyclopedia, “A eukaryote is any organism whose cells contain

nucleus and other organelles enclosed within membranes.”

The eukaryotic cells have a true nucleus, i.e. the nuclear substances such as DNA containing

stainable and thread like chromosomes, RNA rich nucleolus and nucleoplasm, all remain well

separated from the cytoplasm by two perforated lipo-proteinous nuclear membrane.

A typical eukaryotic cell is composed of an outer plasma membrane, cytoplasm and nucleus.

Examples: Eukaryotic cells occur in most plants (from algae to angiosperms) and animals (from

protozoa to mammals).

Figure: Highly Schematic Diagram of a Composite Eukaryotic Cell, Half Plant and Half Animal

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06. Comparison between Prokaryotic and Eukaryotic Cell:

The distinction between prokaryotes and eukaryotes is considered to be the most important

distinction among groups of organisms. Eukaryotic cells contain membrane-bound organelles,

such as the nucleus, while prokaryotic cells do not. Differences in cellular structure of

prokaryotes and eukaryotes include the presence of mitochondria and chloroplasts, the cell wall,

and the structure of chromosomal DNA. Prokaryotes were the only form of life on Earth for

millions of years until more complicated eukaryotic cells came into being through the process of

evolution.

Table: Comparison between Prokaryotic and Eukaryotic Cell

Criteria Prokaryotic Cell Eukaryotic Cell

01. Nucleus Absent Present

02. Number of

chromosomes

One, but not true chromosome:

Plasmids

More than one

03. Cell Type Usually unicellular (some

cyanobacteria may be multicellular)

Usually multicellular

04. True Membrane

bound Nucleus

Absent Present

05. Genetic

Recombination

Partial, undirectional transfers DNA Meiosis and fusion of gametes

06. Lysosomes and

peroxisomes

Absent Present

07. Microtubules Absent or rare Present

08. Endoplasmic

reticulum

Absent Present

09. Mitochondria Absent Present

10. Cytoskeleton May be absent Present

11. DNA wrapping

on proteins

Multiple proteins act together to fold

and condense prokaryotic DNA.

Folded DNA is then organized into a

variety of conformations that are

supercoiled and wound around

tetramers of the HU protein.

Eukaryotes wrap their DNA around

proteins called histones.

12. Ribosomes Smaller Larger

13. Vesicles Present Present

14. Golgi apparatus Absent Present

15. Chloroplasts Absent; chlorophyll scattered in the

cytoplasm

Present (in plants)

16. Flagella Submicroscopic in size, composed of

only one fiber

Microscopic in size; membrane bound;

usually arranged as nine doublets

surrounding two singlets

17. Permeability of

Nuclear Membrane

Not present Selective

18. Plasma membrane

with steroid

Usually no Yes

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19. Cell wall Usually chemically complexed Only in plant cells and fungi

(chemically simpler)

20. Vacuoles Present Present

21. Cell size 1-10μm 10-100μm

22. Example Bacteria and Archaea Animals and Plants

07. Conclusion:

For cellular study in genetics the knowledge about prokaryotic and eukaryotic cell must be

studied clearly. Microorganisms and all other living organisms are classified as prokaryotes or

eukaryotes. Prokaryotes and eukaryotes are distinguished on the basis of their cellular

characteristics. For example, prokaryotic cells lack a nucleus and other membrane‐bound

structures known as organelles, while eukaryotic cells have both a nucleus and organelles.

Prokaryotic and eukaryotic cells are similar in several ways. Both types of cells are enclosed by

cell membranes (plasma membranes), and both use DNA for their genetic information.

Prokaryotes include several kinds of microorganisms, such as bacteria and cyanobacteria.

Eukaryotes include such microorganisms as fungi, protozoa, and simple algae. Viruses are

considered neither prokaryotes nor eukaryotes because they lack the characteristics of living

things, except the ability to replicate (which they accomplish only in living cells).

08. References: i. Genetics, P.S. Verma & V.K. Agarwal

ii. Carp Genetic Resources, M.G. Hussain and M.A. Mazid, WorldFish Center

iii. Encyclopædia Britannica iv. Wikipedia, the free encyclopedia