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Capsa - a box capsid Lyto – loosen lytic -oid - like, form viroid Retro – backward retrovirus Virul – poisonous virulent
AP Biology Plans for the weeks of January 30th, 2017 through February 10th, 2017
o Monday 1-30-17: Genetics of Viruses lesson o Tuesday 1-31-17: Transcription and Translation Quiz; Genetics of Bacteria lesson o Wednesday 2-1-17: Genetics of Bacteria Lesson o Thursday 2-2-17: Paper Plasmid Lab o Friday 2-3-17: genetic engineering background research o Monday 2-6-17: Pre-lab information o Tuesday 2-7-17: Practice with equipment o Wednesday 2-8-17: Practice with equipment o Thursday 2-9-17: Per. 1 Biotech lab day, Per. 2 to… o Friday 2-10-17: Per. 2 Biotech lab day, Per. 1 to…
Ski week: Complete lab reports for biotech labs
For Tuesday’s Quizplease turn in all work by this day, as well
• Definitions/Concepts:– Transcription– Translation– How is pre-mRNA
different from functional mRNA
– Promoter region– Three types of point
mutations– Redundant and
unambiguous
The Genetics of Viruses & Bacteria
E. coli being attacked by a T4 virus
Viruses and viral genomes
Different types of viruses
Viruses Very tiny - 20 nm Not cells Infectious
“particles” made up of nucleic acids and a protein coat
Note the relative size of an animal cell, a bacterium, and a virus
Viral Genomes May have:
Double or single stranded DNA Double or single stranded RNA
DNA/RNA linear or circular Very few genes (4 to a few hundred) Viruses are called DNA viruses or RNA viruses
The protein coat Called a capsid Can take several shapes Built from subunits called capsomeres Most complex capsids found in phages
Viral envelopes Some viruses have
these - help infect cells Come from the host
cell Made up of host cell
phospholipids and membrane proteins
Also made up of proteins and glycoproteins from the virus
Viral Reproduction Viruses are obligate intracellular parasites -
they can only reproduce within a host cell Viruses can infect only certain types of cells -
called its host range
The host range Viruses can infect only certain types
of cells - called its host range Viruses recognize their host cells with
a “lock & key” fit between proteins on the virus and receptor molecules on the surface of the host cell
Why would cells have receptor molecules for a virus? The hypothesis is that sometime in the
distant past, the viruses were useful to cells
CD4 receptor molecule for HIV
Some viruses have a broad host range West Nile virus - infects birds, mosquitoes, horses
and humans Some viruses have a very narrow host range
Measles - only infects humans Viruses usually target specific tissues in
animals HIV only infects helper T cells Cold virus only infect cells lining the respiratory tract
measles
West Nile
Infection1. Begins when the virus
injects its nucleic acid into the host cell
2. Viral genome takes over cell
3. Uses cell to copy the viral genome and make the viral proteins
4. Once viral components are produced, they will self-assemble into new viruses
Several thousand fully formed viruses break out of host cell - damaging or killing it in the process
Alternative reproductive mechanisms in viruses
Double stranded DNA phage viruses have two different infection mechanisms: The lytic cycle The lysogenic cycle
Phages are the most complex and best understood viruses
The Lytic Cycle This ends with the death of the host
because the virus lyses open the host cell when it is released
Viruses that reproduce only in this way are called virulent phages
This step causes the destruction of the host DNA
100-200 new phages released, can go on to infect more cells
Bacterial Defense against Phages Some bacteria have evolved
to not have the receptor molecules for the phage
Some bacteria have restriction enzymes that cut up the foreign DNA before it can damage the cell
But…some phages have evolved resistance to the restriction enzymes…
The Lysogenic Cycle Viral DNA is replicated
without killing the host Viruses that do this
called temperate phages
(lambda) is a temperate phage
inserts its DNA into E.coli
DNA forms a circle If lytic cycle is
entered, the viral DNA immediately takes over the cell and starts producing viruses
If lysogenic cycle is entered, the DNA is incorporated into the bacterial genome It is now known as a
prophage It sits there mostly silent
Whenever bacteria divides, the viral DNA is copied along with it and passed on to daughter cells
So the virus can spread without killing the host cell
Eventually, the viral DNA will leave the host genome, and the lytic cycle will begin
External cues (chemicals, radiation) can trigger this switch
How do viruses reproduce?
Use DNA/RNA, lytic, lysogenic, protein, ribosome in your answer
Animal Viruses Nearly all RNA animal
viruses, and many DNA animal viruses, have a viral envelope
They use these envelopes to enter the host cell
When the new viruses emerge, they envelop themselves in the hosts cell membrane (like exocytosis)
Some viruses have envelopes derived from the host’s nuclear membrane Herpesvirus does this
RNA viruses RNA viruses are most common in animal viruses There are three types of single stranded RNA
viral genomes: Class IV: Single stranded RNA (ssRNA) that serves
directly as the mRNA - can be translated into new viruses immediately
Class V: ssRNA that serves as the template for the synthesis of mRNA
VI: ssRNA that serves as a template for DNA synthesis, aka retroviruses
Retroviruses Have the enzyme reverse transcriptase
Transcribes RNA into DNA
HIV HIV is a retrovirus
• Contains two identical molecules of ssRNA and two molecules of reverse transcriptase
HIV enters helper T-cells It’s reverse transcriptase enzyme catalyzes
the formation of viral DNA Viral DNA is integrated into the host DNA
Are viruses living?
They meet some of the characteristics of life, but not all Has nucleic acid (but not always DNA) Cannot reproduce independently (but they
can reproduce)
So…are they very complex molecules Or are they very simple life forms?
Evolution of viruses - where did they come from?
Because they rely on cells to reproduce, they must have evolved AFTER cells One hypothesis is that they
started out as pieces of nucleic acid that could move from cell to cell
We observe that the genome of a virus is more similar to its host cell that to the genomes of other viruses
Where did viruses come from? Plasmids -
small circular DNA molecules found in bacteria
Exist and replicate apart from the main bacterial chromosome
Can be transferred from one cell to another
Where did viruses come from? Transposons -
DNA segments that can move from one location to another within the genome of an organism
Where did viruses come from? Plasmids, transposons, viruses -
all types of mobile nucleic acid - called mobile genetic elements