2. Prokaryotic Cells And Viruses

Question 1

What type of organelles do prokaryotes not contain?

Answer 1

Membrane bound organelles.

Question 2

What is the difference in size between prokaryotic and eukaryotic cells?

Answer 2

Prokaryotic cells are extremely small, less than 2um in diameter. Eukaryotic cells can be up to 50 times bigger, 0.1mm.

Question 3

What is the flagellum?

Answer 3

A long, hair like structure that rotates to make the prokaryotic cell move. Not all prokaryotes have a one, and some have more than one.

Question 4

Describe the function of the cell surface membrane in prokaryotes.

Answer 4

It is mainly made up of lipids and proteins and controls the movement of substances into and out of the cell.

Question 5

Describe the cell wall in prokaryotes.

Answer 5

The cell wall supports the cell and stops it from changing shape. It’s made up of a polymer called murein, a glycoprotein.

Question 6

How are ribosomes different in prokaryotes to eukaryotes?

Answer 6

They are smaller.

Question 7

Describe the capsule that surrounds prokaryotic cells.

Answer 7

Some prokaryotes e.g. bacteria have this capsule, made up of secreted slime.
It helps to protect the bacteria from attack by cells of the immune system.

Question 8

What form does DNA take in a prokaryotic cell?

Answer 8

Prokaryotes don’t have a nucleus.
Instead the DNA floats free in the cytoplasm. It’s circular, present as one long coiled up strand and isn’t attached to any histone proteins.

Question 9

What are plasmids?

Answer 9

They are small loops of DNA that aren’t part of the main DNA molecule. They contain genes for things like antibiotic resistance, and can be passed between prokaryotes. Plasmids are not always present in prokaryotic cells. Some prokaryotic cells have several.

Question 10

What is the process called by which prokaryotic cells carry out replication?

Answer 10

Binary fission.

Question 11

Describe the steps of binary fission.

Answer 11

1. The circular DNA replicates once and the plasmids replicate many times.
2. The cell gets bigger and the DNA loops move to opposite poles (ends) of the cell.
3. The cytoplasm begins to divide and new cell walls begin to form.
4. The cytoplasm divides and two daughter cells are produced. Each has one copy of the circular DNA, but can have a variable number of copies of the plasmids.

Question 12

What is meant by viruses are ‘acellular’?

Answer 12

Viruses are not cells, nor are they alive. They’re just nucleic acids surrounded by protein.

Question 13

Give some examples of viruses.

Answer 13

HIV, which causes AIDS.
Influenza, which causes the flu.
Rhinoviruses, which cause colds.

Question 14

What are the cells called in which viruses reproduce?

Answer 14

Host cells.

Question 15

How are viruses different to bacteria?

Answer 15

They have no cell-surface membrane, no cytoplasm and no ribosomes.

Question 16

What is the capsid and what is one of its key features?

Answer 16

The capsid is a protein coat around a virus.

It has attachment proteins sticking out of it. These allow a virus to bind to a suitable host cell.

Question 17

How big are viruses compared to bacteria?

Answer 17

Much smaller e.g. HIV is only 0.1um across.

Question 18

Describe how general viral replication occurs.

Answer 18

1. The virus attachment proteins bind it to host cell receptor proteins.
2. Genetic material is released into the host cell.
3. Genetic material and proteins are replicated by host cell enzymes and ribosomes. Then these components assemble.
4. Replicated viruses are released from the host cell.

Question 19

Why do viruses only affect certain host cells and not others?

Answer 19

Different viruses have different attachment proteins, and therefore require specific receptor proteins only found on certain cells.
As a result every virus can only affect a certain number of cells.