3.2.1 - Structure of prokaryotic cells and of viruses

Question 1

What are the distinguishing features of prokaryotic cells?

Answer 1

● Cytoplasm lacking membrane-bound organelles
● So genetic material not enclosed in a nucleus

Question 2

Examples of prokaryotic organisms

Answer 2

bacteria and archaea (always unicellular)

Question 3

Describe the general structure of prokaryotic cells

Answer 3

Always present:
- Cell-surface membrane
- Cell wall
> contains murein - a glycoprotein
- Cytoplasm
- Small ribosomes
- Circular DNA
> free in cytoplasm
> not assoicated with histones/proteins

Sometimes present:
- Capsule
- Plasmids (small rings of DNA)
- Flagella

Question 4

Compare and contrast the structure of eukaryotic and prokaryotic cells

Answer 4

• Eukaryotic cells have membrane-bound organelles, whereas prokaryotic cells don’t have any membrane-bound organelles
• Eukaryotic cells have a nucleus containing DNA, whereas prokaryotic cells don’t have a nucelus and instead have their DNA free in their cytoplasm
• Eukaryotic cells have DNA that is long and linear and associates with histones/proteins, whereas prokaryotic cells have DNA that is short and circular and not associated with histones/proteins
• Eukaryotic cells have larger (80S) ribosomes whereas prokaryotic cells have smaller (70S) ribosomes
• Eukaryotic cells only have cell walls if they are plants, algae and fungi and these cell walls contain cellulose or chitin, whereas prokaryotic cells all have cell walls and these contain murein (a glycoprotein)
• Eukaryotic cells never have a capsule/plasmids, whereas prokaryotic cells sometimes have a capsule/plasmids
• Eukaryotic cells have a larger overall size compared to prokaryotic cells which have a much smaller overall size.
• Eukaryotic DNA contains introns, Prokaryotic DNA does not.

Question 5

Explain why viruses are describes as acellular and non-living

Answer 5

● Acellular - not made of cells, no cell membrane / cytoplasm / organelles
● Non-living - have no metabolism, cannot independently move / respire / replicate / excrete

Question 6

Describe the general structure of a virus particle

Answer 6

1. Nucleic acids surrounded by a capsid
   (protein coat)
2. Attachment proteins allow attachment
   to specific host cells
3. No cytoplasm, ribosomes, cell wall,
   cell-surface membrane etc.
4. Some also surrounded by a lipid
   envelope eg. HIV

Question 7

What are similarities between DNA in eukaryotic cells with the DNA in prokaryotic cells.

Answer 7

• Nucleotide structure is identical
• Nucleotides joined by phosphodiester bond
• DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes