How does the location and structure of DNA differ between prokaryotes and eukaryotes?

Prokaryotic DNA is circular, 'naked' (no histones), and floats freely in the nucleoid region of the cytoplasm. Eukaryotic DNA is linear, associated with histone proteins, and contained within a double-membrane nucleus.

What is the difference between the ribosomes found in prokaryotic and eukaryotic cells?

Prokaryotes contain smaller 70S ribosomes throughout their cytoplasm. Eukaryotes contain larger 80S ribosomes in the cytoplasm and on the RER, though they also contain 70S ribosomes inside their mitochondria and chloroplasts.

Compare the methods of cell division used by prokaryotes and eukaryotes.

Prokaryotes divide by binary fission, a simple process of DNA replication and cytoplasmic division. Eukaryotes use mitosis or meiosis, which involves the complex assembly of spindle fibers to organize and separate linear chromosomes.

Why is it incorrect to say that eukaryotic cells only contain 80S ribosomes?

While the main cytoplasmic ribosomes are 80S, eukaryotic organelles like mitochondria and chloroplasts contain their own 70S ribosomes. This reflects their evolutionary origin from engulfed prokaryotic cells.

Is the presence of a cell wall a reliable way to distinguish a prokaryote from a eukaryote?

No, because both cell types can have cell walls. The distinction lies in the material: prokaryotes use peptidoglycan, while eukaryotes use cellulose (plants) or chitin (fungi). Animal cells lack a cell wall entirely.

True or False: All unicellular organisms are prokaryotic.

False. While all prokaryotes are unicellular, many eukaryotes (such as Amoeba or yeast) are also unicellular. The presence of a nucleus, not the number of cells, is the defining characteristic.

What is a plasmid and where is it typically found?

A plasmid is a small, circular piece of extra-chromosomal DNA that replicates independently of the main genome. It is commonly found in prokaryotic cells and often carries genes for antibiotic resistance.

Define 'ultrastructure' in the context of cell biology.

Ultrastructure refers to the fine detail of a cell's internal components that can only be seen using an electron microscope, such as the specific arrangement of organelles and membranes.

What is peptidoglycan?

Peptidoglycan is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria, forming the cell wall.

What is the functional advantage of membrane-bound organelles in eukaryotes?

Organelles allow for compartmentalization, meaning specific metabolic processes can be optimized in different environments (e.g., different pH levels) within the same cell without interfering with one another.

Prokaryotic v Eukaryotic Cells | Cambridge International Examinations AS-Level Biology

Revision Notes

AS-Level

Cambridge International Examinations

Biology

1. Cell Structure

Prokaryotic v Eukaryotic Cells

Summary

Cells are categorized into two primary types based on their internal complexity and the presence of a nucleus. Prokaryotic cells are structurally simple, lacking membrane-bound organelles and a defined nucleus, whereas eukaryotic cells possess a highly compartmentalized ultrastructure with specialized organelles that allow for complex metabolic processes and multicellularity.

1. Definition & Core Concepts

Prokaryotic cells are the most primitive form of life, characterized by the absence of a membrane-bound nucleus. Their name, derived from Greek, literally means 'before nucleus,' reflecting their evolutionary position.
Eukaryotic cells represent a more advanced level of biological organization, containing a 'true nucleus' that houses the genetic material. These cells are significantly larger and more complex than their prokaryotic counterparts.
The ultrastructure of a cell refers to its internal architecture as revealed by high-resolution microscopy. Eukaryotes have a complex ultrastructure due to extensive internal membranes, while prokaryotes have a relatively uniform internal environment.

Comparison of Prokaryotic and Eukaryotic cell structures showing the absence vs presence of a nucleus and organelles.

2. Genetic Material & Replication

In prokaryotes, the genetic material consists of a single, circular DNA molecule located in a region called the nucleoid. This DNA is 'naked,' meaning it is not associated with histone proteins.
Eukaryotic DNA is organized into multiple linear chromosomes located within the nucleus. This DNA is tightly wrapped around histone proteins, forming a complex known as chromatin.
Prokaryotes replicate through binary fission, a rapid process where the circular DNA is copied and the cell splits. Eukaryotes undergo mitosis or meiosis, involving complex spindle fibers to ensure accurate chromosome separation.

3. Organelles & Ribosomes

Eukaryotic cells contain various membrane-bound organelles, such as mitochondria, chloroplasts, the endoplasmic reticulum, and the Golgi apparatus. These structures allow different metabolic reactions to occur simultaneously in separate compartments.
Prokaryotes lack these internal membrane-bound structures. Their metabolic processes, such as respiration or photosynthesis (in some species), occur across the cell surface membrane or in the cytoplasm.
Both cell types contain ribosomes for protein synthesis, but they differ in size. Prokaryotes have smaller 70S ribosomes, while eukaryotes have larger 80S ribosomes in their cytoplasm.

4. Key Distinctions

5. Exam Strategy & Tips

Prokaryotic v Eukaryotic Cells

Summary

1. Definition & Core Concepts

Prokaryotic cells are the most primitive form of life, characterized by the absence of a membrane-bound nucleus. Their name, derived from Greek, literally means 'before nucleus,' reflecting their evolutionary position.
Eukaryotic cells represent a more advanced level of biological organization, containing a 'true nucleus' that houses the genetic material. These cells are significantly larger and more complex than their prokaryotic counterparts.
The ultrastructure of a cell refers to its internal architecture as revealed by high-resolution microscopy. Eukaryotes have a complex ultrastructure due to extensive internal membranes, while prokaryotes have a relatively uniform internal environment.

Comparison of Prokaryotic and Eukaryotic cell structures showing the absence vs presence of a nucleus and organelles.

2. Genetic Material & Replication

In prokaryotes, the genetic material consists of a single, circular DNA molecule located in a region called the nucleoid. This DNA is 'naked,' meaning it is not associated with histone proteins.
Eukaryotic DNA is organized into multiple linear chromosomes located within the nucleus. This DNA is tightly wrapped around histone proteins, forming a complex known as chromatin.
Prokaryotes replicate through binary fission, a rapid process where the circular DNA is copied and the cell splits. Eukaryotes undergo mitosis or meiosis, involving complex spindle fibers to ensure accurate chromosome separation.

3. Organelles & Ribosomes

Eukaryotic cells contain various membrane-bound organelles, such as mitochondria, chloroplasts, the endoplasmic reticulum, and the Golgi apparatus. These structures allow different metabolic reactions to occur simultaneously in separate compartments.
Prokaryotes lack these internal membrane-bound structures. Their metabolic processes, such as respiration or photosynthesis (in some species), occur across the cell surface membrane or in the cytoplasm.
Both cell types contain ribosomes for protein synthesis, but they differ in size. Prokaryotes have smaller 70S ribosomes, while eukaryotes have larger 80S ribosomes in their cytoplasm.

4. Key Distinctions

Feature	Prokaryotes	Eukaryotes
Size	Small ( $1-5 \mu m$ )	Large ( $10-100 \mu m$ )
Nucleus	Absent (Nucleoid)	Present (Nuclear Envelope)
DNA Structure	Circular, Naked	Linear, with Histones
Organelles	None membrane-bound	Many (Mitochondria, etc.)
Ribosomes	70S	80S (Cytoplasm)
Cell Wall	Peptidoglycan	Cellulose (Plants) or Chitin (Fungi)

Cell Walls: While both can have cell walls, the chemical composition is a major differentiator. Prokaryotic walls are made of peptidoglycan, whereas eukaryotic walls (if present) are made of cellulose or chitin.
Plasmids: Prokaryotes often contain small, extra-chromosomal loops of DNA called plasmids, which can carry beneficial genes like antibiotic resistance. These are rare in eukaryotes.

5. Exam Strategy & Tips

The Ribosome Trap: Always remember that while eukaryotes have 80S ribosomes in their cytoplasm, their mitochondria and chloroplasts contain 70S ribosomes. This is a common exam question regarding the endosymbiotic theory.
DNA Comparison: When asked to compare DNA, focus on three points: location (nucleoid vs nucleus), shape (circular vs linear), and protein association (naked vs histones).
Size Matters: Be prepared to convert units. Prokaryotes are typically measured in micrometers ( $\mu m$ ), while viruses (which are non-cellular) are measured in nanometers ( $nm$ ).
Common Misconception: Do not assume all eukaryotes are multicellular. Many protists are unicellular eukaryotes, but they still possess a nucleus and organelles.

Feature

Prokaryotes

Eukaryotes

Size

Small (

1-5 \mu m

)

Large (

10-100 \mu m

)

Nucleus

Absent (Nucleoid)

Present (Nuclear Envelope)

DNA Structure

Circular, Naked

Linear, with Histones

Organelles

None membrane-bound

Many (Mitochondria, etc.)

Ribosomes

70S

80S (Cytoplasm)

Cell Wall

Peptidoglycan

Cellulose (Plants) or Chitin (Fungi)