Cell Biology 1 - Prokaryotes and Viruses
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How many people die from infections each year?
Where do most deaths occur?
- Millions die from infections each year.
- According to data from the world health organisation, in 2004, 13.8 million people died from communicable infectious diseases caused by
- Bacteria: Tuberculosis
- Viruses: Aids
- Protozoa: Malaria
- Parasitic worms:
Most deaths occur in developing countries, where clean water, good nutrition and sewage facilities are lacking.
How can living organisms be classified?
Eukaryotes and Prokaryotes.
have a membrane-bound nucleus and a double stranded DNA genome
have a double stranded DNA genome but lack a nuclear membrane.
- This can be seen in a phylogenetic tree of life based on comparative rRNA sequencing.
- This tree can also show the LUCA - Last Common Universal Ancestor.
Comment on the morphological types of prokaryote.
Morphology is linked to environment, so similar shape doesn't necessarily mean they are closely related.
- Bacillus (bacilli): rod shaped 2μm
- Spirillum (spirilla): spiral shaped 5 μm
- Coccus (cocci): 1μm
What two ways can prokaryote cells exist?
They may exist singly, or associate into clusters with a characteristic shape.
How do the sizes of prokaryotic and eukaryotic cells differ?
Typical prokaryotic cells are much smaller than typical eukaryotic cells.
- Typical prokaryotes - 1μm - 5μm
- Typical Eukaryotes - 10μm - 30μm
How does the size of the cell affect it?
The metabolic rate of a cell varies inversely with the square of its size.
Small cells tend to grow faster than large cells.
As cell size increases, surface area:volume ratio decreases.
Limitations in rates of exchange and diffusion dictate the upper size limit for prokaryotic cell size
What size is E-coli?
What structural features to prokaryote cells have?
They have DNA in the form of a neucleoid (an irregularly-shaped region within the cell of a prokaryote that contains all or most of the genetic material)
They have a cell wall which gives in shape, protects it, and resists osmotic pressure.
They have a cytoplasmic membrane.
They can also have flagella (movement) or fimbriae (for adhering to one another) or a pilus (for transferring DNA between bacteria), or a slime layer
What are two different types of bacteria?
How can you distinguish between them?
The two different types are gram positive and gram negative.
You can distinguish between the two by performing a gram stain.
- 1. Flood the heat-fixed smear with crystal violet for 1 minute.
- 2. Add iodine solution for 1 minute
- 3. Decolourize with alcohol briefly - about 20 seconds.
- 4. Counterstain with safranin for 1-2 minutes.
bacteria will retain the crystal violet stain and so will look purple; Gram negative
bacteria do not retain the stain
What is the prokaryotic cell membrane like?
It is a phospholipid bilayer which also contains protein molecules.
What difference in apperance is there between gram positive and gram negative bacteria under an electron microscope?
- Gram positive looks smooth,
- Gram negative looks bumpy/rough/textured
What does the prokaryotic cell wall contain and where is it found?
It is found outside the plasma membrane and it contains Peptidoglycan.
What is peptidoglycan?
Peptidoglycan is an alternating copolymer of N-acetylglucosamine and N-acetylmuramic acid (joined by β1-4 glycosidic bonds), crosslinked with shord peptide bonds, giving a stong 3D mesh with high tensile strength.
The peptide interbridges are between the peptide side chains of N--acetylmuramic acid
What is the gram positive cell wall like?
It is a thick layer of petidoglycan which contains lipoteichoic acid, teichoic acid, and wall-associated proteins.
The main function of teichoic acids is to provide rigidity to the cell-wall by attracting cations such as magnesium and sodium.
The peptidoglycan wraps round the cell like a cable
What is the gram negative cell wall like?
It has a thin peptidoglycan layer sandwiched between the inner cell membrane and the bacterial outer membrane.
The bacterial outer membrane contains porins, proteins and lipopolysacharides. The lipid portion of lipopolysacharides (Lipid A) acts as an Endotoxin (salmonella)
Lipid A is fatty acids linked by amine ester bond to N-Acetylglucosamine.
Why does the Gram staining procedure work?
- Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in a thick layer in gram-positive bacteria.
- In a Gram stain test, gram-positive bacteria retain the crystal violet dye, while a counterstain (commonly safranin or fuchsine) added after the crystal violet gives all gram-negative bacteria a red or pink coloring.
For gram positive bacteria, the thick peptidoglycan layer in the cell wall that encases their cell membrane retains the stain, making definitive identification possible.
Gram-negative bacteria cannot retain the violet stain after the decolorization step; alcohol used in the decolorization process degrades the outer membrane of gram-negative cells making the cell wall more porous and incapable of retaining the crystal violet stain. Their peptidoglycan layer is much thinner and sandwiched between an inner cell membrane and a bacterial outer membrane, causing them to take up the counterstain (safranin orfuchsine) and appear red or pink.
What is a common bacteria found in dental plaque?
Streptcocci: eg. Streptococci sobrinus in chains
What must all prokaryotes have?
All prokaryotes have a membrane and a nucleoid; most also have a cell wall.
What are viruses?
A virus is a genetic element that subverts normal cellular processes for its own replication and has a mature infectious extracellular form.
It is essentially a nucleic acid (DNA or RNA) surrounded by a protective coat: non-cellular
It has no metabolism (no respiration, no biosynthesis) so it is non living
What size are viruses
0.02 - 0.3 μm
What type of structure do viruses have?
What can viruses be classified as?
Obligate intracellular parasites: They can reproduce only by infecting a host cell.
What is a virion?
A virus outside a cell.
What is a naked virus and what is an enveloped virus?
A naked virus is a nucleic acid contained within a capsid (protein shell made of capsomers)
An enveloped virus is a naked virus which is inside a viral envelope.
- The envelopes typically are derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins.
- Functionally, viral envelopes happen to help viruses enter host cells and may help them avoid the host immune system.
- Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane. The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host.
What are rod-shaped viruses like?
Rod shaped viruses have a hollow core and show helical symmetry.
- E.g. Tobacco mosaic virus.
- Rabies virus (has an envelope)
What are polyhedral viruses?
Polyhedral (many sided) viruses are approximately spherical and show icosahedral symmetry.
- The icosahedral formation uses the smallest number of subunits to build the closed shell.
- It is the optimum method of forming a closed shell from identical subunits.
- The minimum number of identical capsomers required is twelve, each composed of five identical sub-units
e.g Human papillomavirus , herpes virus
What are complex viruses?
Complex viruses are composed of an icosahedral head and helical tail.
They generally infect bacteria, so are called bacteriophages.
How can viruses be categorised?
Viruses are categorised according to their type of genome.
- - DNA (linear or circular) or RNA (linear, may be fragmented)
- - Single or double stranded (ss or ds)
The type of genome has important implications for how the virus replicates, but there are features common to all viral lifecycles
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