Bacteria and viruses

Biology Unit 4

Bacteria and viruses

HideShow resource information
Preview of Bacteria and viruses

First 506 words of the document:

Biology Unit 4
Revision Notes
Topic 6: Infection, Immunity and Forensics
8. Distinguish between the structure of bacteria
and viruses.
Bacteria Viruses
Cell surface membrane, cytoplasm, cell wall, No cell wall, cell surface membrane, cytoplasm
ribosomes, plasmids, and sometimes or organelles. Nucleic acid enclosed in protein
Mesosomes, flagellum and pili coat
Circular strand of DNA is the genetic material DNA or RNA can be the genetic material
Can live independently Must have a living organism as host
Average diameter 0.5 - 5µm 20 ­ 400 nm, wide range of shapes and sizes
Often have mucus-based outer capsule May have outer membrane of host cell surface
membrane, but containing glycoproteins from
the virus
Viruses are arrangements of genetic material and protein that invade other living cells and take over
their biochemistry to make more viruses. Viruses can exist and reproduce as parasites only in the
cells of other living organisms.
Viruses are usually geometric shapes and have similar basic structures. There is considerable variation
in the genetic material they possess, the structure of their protein coat and whether or not they have
an envelope. Their protein coat or capsid is made up of simple repeating protein units called
capsomeres, arranged in different ways. In some viruses, the genetic material and protein coat are
covered by a lipid envelope, produced from the host cell. This makes it easier for the viruses to pass
from cell to cell, but makes them vulnerable to substances which dissolve the lipid membrane.
Viral genetic material can be DNA or RNA, and the nucleic acid is sometimes double stranded and
sometimes single. Viral DNA acts directly as a template for both new viral DNA and for the mRNA
needed to induce synthesis of viral proteins. Viral RNA directs the synthesis of reverse transcriptase
which is an enzyme that proceeds to make DNA molecules corresponding to the viral genome. This
DNA is used as a template for new viral proteins and ultimately a new viral RNA genome.
Once a virus is in the host cells, there are two different routes of infection:
Lysogenic pathway: Many viruses are non-virulent when they get to the host cell. The
insert their DNA into the host DNA so it is replicated every time the host cell divides. This is a
provirus. mRNA is not produced from the viral DNA because one of the viral genes causes the
production of a repressor protein which makes it impossible to translate the rest of the viral
genetic material. During the period of lysogeny, the virus is dormant.
Lytic pathway: Sometimes the viral genetic material is replicated independently of the
host DNA straight after entering the host. Mature viruses are made and eventually the host
cell bursts, releasing large numbers of new virus particles to invade other cells. The virus is
virulent (disease-causing).
Text Book: p. 82 ­ 88

Other pages in this set

Page 2

Preview of page 2

Here's a taster:

Biology Unit 4
Revision Notes
Retroviruses (including HIV and Rous sarcoma) have a different and complex lifecycle. Their genetic
material is viral RNA.
1. The retrovirus attacks an animal cell
2. Viral RNA enters the host cell. This RNA cannot be used as mRNA
3. Viral RNA is translated into viral DNA by reverse transcriptase in the cytoplasm
4. Viral DNA is incorporated into the host DNA in the nucleus. It directs the production of new
viral genome RNA, mRNA and coat proteins
5.…read more

Page 3

Preview of page 3

Here's a taster:

Biology Unit 4
Revision Notes
Binary fission ­ once a bacterium reaches a certain size, the DNA is replicated and the old
cell wall begins to break down around the middle of the cell
In certain situations, some types of bacteria can reproduce using what appear to be different
forms of sexual reproduction. These are very rare events and none involve true sexual
reproduction with the formation and transfer of gametes.…read more


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all resources »