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