Overview of Cells

 

Overview of Cells: Prokaryotic and Eukaryotic Cells, Viruses, Viroids, Mycoplasma, and Prions

Cells are the fundamental building blocks of life. From the simplest unicellular organisms to complex multicellular beings, everything starts with a cell. However, not all biological entities fall neatly into the cellular category—some, like viruses and prions, exist at the edge of life. This article explain in details about prokaryotic and eukaryotic cells along with viruses, viroids, mycoplasma, and prions, highlighting their structure, function, and significance.

Overview of Cells

1. Types of Cells: Prokaryotic and Eukaryotic

Prokaryotic Cells

Prokaryotic cells are primitive and simple in structure. These cells lack a true nucleus and membrane-bound organelles.

Key Features:

1. Nucleoid: Contains circular DNA not enclosed in a membrane.

2. Cell Wall: Made of peptidoglycan in bacteria.

3. Plasma Membrane: Controls the entry and exit of substances.

4. Ribosomes: Smaller (70S) and free-floating.

5. Flagella or Pili: For movement and attachment.

Examples: Bacteria (e.g., Escherichia coli), Archaea (e.g., Halobacterium).

Reproduction: Primarily through binary fission.

Eukaryotic Cells

Eukaryotic cells are more complex, having a well-defined nucleus and various membrane-bound organelles.

Key Features:

1. Nucleus: Enclosed by a nuclear membrane, contains linear DNA.

2. Organelles: Includes mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and chloroplasts (in plants).

3. Cytoskeleton: Maintains cell shape and internal organization.

4. Ribosomes: Larger (80S), may be free or attached to ER.

Examples: Animals, plants, fungi, and protists.

Reproduction: Mitosis (for growth), Meiosis (for sexual reproduction).

2. Acellular and Subcellular Entities

While cells are the structural units of life, some entities like viruses and prions challenge the definition of "living."

Virus

Viruses are acellular, non-living entities that require a host cell to replicate.

Structure:

1. Genetic Material: DNA or RNA, single or double-stranded.

2. Capsid: Protein coat enclosing the genome.

3. Envelope (optional): Lipid layer derived from host.

Key Characteristics:

1. Cannot carry out metabolism on their own.

2. Exhibit host specificity.

3. Cause diseases like influenza, HIV, COVID-19.

Examples: HIV, Influenza virus, Coronavirus (SARS-CoV-2).

Viroids

Viroids are the smallest infectious pathogens, composed solely of a short strand of circular, single-stranded RNA without a protein coat.

Key Features:

1. Infect plants.

2. Do not encode proteins.

3. Cause serious agricultural damage.

Examples: Potato spindle tuber viroid (PSTVd).

Mycoplasma

Mycoplasmas are the smallest living cells known, lacking a cell wall.

Key Features:

1. Can survive without oxygen (facultative anaerobes).

2. Resistant to antibiotics like penicillin (due to absence of cell wall).

3. Highly pleomorphic (vary in shape).

4. Pathogenic in humans, animals, and plants.

Examples: Mycoplasma pneumoniae (causes atypical pneumonia).

Prions

Prions are infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of specific normal proteins.

Key Features:

1. No nucleic acid.

2. Extremely resistant to heat and enzymes.

3. Cause sponge-like degeneration of brain tissue.

Diseases Caused:

1. Creutzfeldt-Jakob disease (CJD) in humans

2. Bovine spongiform encephalopathy (BSE) or Mad Cow Disease

Comparison Table: Prokaryotes, Eukaryotes, and Acellular Entities

Feature	Prokaryotes	Eukaryotes	Virus	Viroid	Mycoplasma	Prion
Cell type	Cellular	Cellular	Acellular	Acellular	Cellular	Acellular
Nucleus	No	Yes	No	No	No	No
DNA	Circular	Linear	DNA or RNA	RNA	Circular	None
Organelles	Absent	Present	Absent	Absent	Absent	Absent
Cell wall	Peptidoglycan (in bacteria)	Cellulose (plants), Chitin (fungi)	Absent or host-derived	Absent	Absent	Absent
Reproduction	Binary fission	Mitosis/Meiosis	Host-dependent	Host-dependent	Binary fission	Induces misfolding
Living?	Yes	Yes	No	No	Yes	Debatable

Conclusion

Understanding the differences between prokaryotic and eukaryotic cells and the nature of non-cellular agents like viruses, viroids, mycoplasma, and prions is crucial in fields like microbiology, medicine, and biotechnology. While cells form the basis of life, entities like prions and viruses challenge our understanding of life itself. Recognizing these differences helps in the development of antibiotics, antiviral drugs, and treatments for prion-related diseases.

References

1. Alberts, B. et al. Molecular Biology of the Cell. 6th Ed. Garland Science.

2. Prescott, L.M., Harley, J.P., and Klein, D.A. Microbiology. McGraw-Hill.

3. Lodish, H. et al. Molecular Cell Biology. W.H. Freeman and Company.

4. NCBI: National Center for Biotechnology Information – https://www.ncbi.nlm.nih.gov/

5. WHO and CDC resources on infectious diseases

FAQs: Cells, Viruses, Viroids, Mycoplasma & Prions

1. What are the main differences between prokaryotic and eukaryotic cells?

Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells have both.

2. Are viruses considered living organisms?

No. Viruses are not considered living because they cannot reproduce or carry out metabolism independently.

3. What makes viroids different from viruses?

Viroids are smaller than viruses and consist only of circular RNA without a protein coat. They infect only plants.

4. Why are mycoplasmas resistant to antibiotics like penicillin?

Because they lack a cell wall, and penicillin targets cell wall synthesis.

5. What diseases are caused by prions?

Prions cause neurodegenerative diseases like Creutzfeldt-Jakob disease and Mad Cow Disease (BSE).

6. Do prokaryotes have DNA?

Yes, prokaryotes have circular DNA located in a region called the nucleoid.

7. What organisms have eukaryotic cells?

Animals, plants, fungi, and protists all have eukaryotic cells.

8. What is the size range of viruses?

Viruses typically range from 20 to 300 nanometers in size.

9. Can viroids infect humans?

No, viroids only infect plants.

10. Is mycoplasma the smallest living cell?

Yes, mycoplasma is the smallest known living cell capable of independent reproduction.

Quiz (10 MCQs) – Test Your Knowledge 

▶️ Click here to Play the Quiz

Worksheet: Overview of Cells and Acellular Agents

Name: _____________________  Date: ________________  Class: ___________

Section A: Match the Following

A	B
1. Prokaryotes	A. Infectious protein
2. Virus	B. No cell wall
3. Eukaryotes	C. DNA or RNA with protein coat
4. Viroid	D. Circular RNA without coat
5. Prion	E. True nucleus
6. Mycoplasma	F. No nucleus

Answers: 1 - ___, 2 - ___, 3 - ___, 4 - ___, 5 - ___, 6 - ___

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Section B: True or False

1. Prions contain DNA and RNA. ___

2. Bacteria have a peptidoglycan cell wall. ___

3. Mycoplasmas are viruses. ___

4. Eukaryotic cells are found in animals. ___

5. Viroids are made of proteins. ___

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Section C: Short Answer Questions

1. What is the major difference between prokaryotic and eukaryotic cells?

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2. How do prions cause disease?

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3. List two differences between viruses and viroids.

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4. What makes mycoplasma unique among bacteria?

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5. Why are viruses considered obligate parasites?