Biology - AQA - Unit 2 - A2. Prokaryotic Cells and Viruses

• Prokaryotes are single-celled organisms. Bacteria (like E. coli) are examples of prokaryotes.

• Prokaryotic cells are much smaller and simpler than eukaryotic cells

• they don’t have any membrane-bound organelles (like a nucleus) in their cytoplasm.

• Cytoplasm contains 70s ribosomes - but they’re smaller than those in a eukaryotic cell.

• Just like in a eukaryotic cell, the cell-surface membrane is mainly made of lipids and proteins. It controls the movement of substances into and out of the cell.

• The cell wall supports the cell and prevents it from changing shape. It’s made of a polymer called murein. Murein is a glycoprotein (a protein with a carbohydrate attached).

• The flagellum (plural flagella) is a long hair-like structure that rotates to make the prokaryotic cell move. Not all prokaryotes have a flagellum. Some have more than one.

• Some prokaryotes (e.g. bacteria) also have a capsule made up of secreted slime. It helps to protect the bacteria from attack by cells of the immune system.

• Unlike a eukaryotic cell, a prokaryotic cell doesn’t have a nucleus. Instead, the DNA floats free in the cytoplasm. It’s circular DNA, present as one long coiled-up strand. It’s not attached to any histone proteins

• Plasmids are small loops of DNA that aren’t part of the main circular DNA molecule. Plasmids 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.

Step 1
The circular DNA and plasmid(s) replicate. The main DNA loop is only replicated once, but plasmids can be replicated loads of times.

Step 2
The cell gets bigger and the DNA loops move to opposite ‘poles’ (ends) of the cell.

Step 3
The cytoplasm begins to divide (and new cell walls begin to form).

Step 4
The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but can have a variable number of copies of the plasmid(s).

• Viruses are acellular—they’re not cells. In fact, viruses are just nucleic acids surrounded by protein-they’re not even alive.

• Examples of viruses include HIV (which causes AIDS), influenza (which causes the flu) and rhinoviruses (which cause colds).

• All viruses invade and reproduce inside the cells of other organisms. These cells are known as host cells.

• Unlike bacteria, viruses have no cell-surface membrane, no cytoplasm and no ribosomes.

• They do have a protein coat, called a capsid, with attachment proteins sticking out from it. The attachment proteins let the virus cling onto a suitable host cell.

• Viruses are even smaller than bacteria

Because they’re not alive, viruses don’t undergo cell division. Instead, they inject their DNA or RNA into the host cell-this hijacked cell then uses its own ‘machinery’ (e.g. enzymes, ribosomes) to do the virus’s dirty work and replicate the viral particles.

Different viruses have different attachment proteins and therefore require different receptor proteins on host cells. As a result, some viruses can only infect one type of cell (e.g. some viruses can only infect one species of bacteria), while others can infect lots of different cells (e.g. influenza).

AESAR

A - Attachment

E - Entry

S - Synthesis

A - Assembly

R - Release

1. Virus attaches to host cell receptor proteins. (Attachment)

2. Genetic material is released into the host cell. (Entry)

3. Genetic material and proteins are replicated by host cell ‘machinery’. (Synthesis)

4. Viral components assemble. (Assembly)

5. Replicated viruses released from host cell. (Release)