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Unit 7 Notes, Part 3: Viral and Bacterial Genetics
AP Biology, Mrs. Krouse
What are Viruses?
1) Viruses have a very simple structure ; they are made of a nucleic acid
genome (either RNA or DNA) and are surrounded by a protein coat
2) They are tiny (smaller than ribosomes)
3) The protein shell is called a capsid ; some viruses also have a lipid-based
envelope surrounding the capsid that aids in host infection
4) One type of virus = bacteriophage (a virus that infects bacteria)
5) Are they alive? – Most scientists say no because they are not made of cells
and can only reproduce when in host cell, which makes them obligate
intracellular parasites
6) Viruses use the enzymes, ribosomes, etc. of their hosts to create “baby
viruses” by copying their genetic material and capsids
Two Methods of Viral Reproduction
7) The two viral reproductive cycles are called the lytic cycle and lysogenic cycle (see comparison below)
Lytic Cycle
-Type of reproduction that results in the death of the host
-Virus injects DNA into the host cell
-Takes over the enzymes (ex: DNA and RNA polymerase,
ribosomes, etc.) to make copies of viral DNA and capsid
proteins
-Progeny (baby) viruses are assembled
-Cell is lysed (cell membrane and cell wall are digested /
broken), which releases multiple copies of the virus
Lysogenic Cycle
-Does not destroy the host
-Viral DNA is incorporated into host DNA and replicates
along with the host DNA
-The incorporated viral DNA is called a prophage or
provirus
-Can stay in host DNA for years
-Certain conditions can cause prophage to leave host
DNA and enter lytic cycle
A Unique Kind of Virus: the Retrovirus
8) Viruses with RNA genomes are typically those that infect animal cells
9) These viruses are called retroviruses (ex: HIV)
10) They contain the enzyme reverse transcriptase, which uses viral RNA to
make a complementary DNA strand
11) Reverse transcriptase can then synthesize the other DNA strand from the
cDNA  full DNA double helix
12) The DNA can be incorporated into the host cell genome as a provirus
(before virus) and later transcribed into RNA to make protein capsids and
RNA genomes for the next generation
How do Viruses Increase Genetic Variation?
13) Mutations occur during DNA replication when the host cell’s DNA polymerase makes errors, which results in
increased genetic variation within the viral genome
14) RNA viruses do not have ways to “proofread” the creation of their cDNA from RNA, so they have higher rates
of mutation. Recall, HIV has an RNA genome, which mutates so quickly that we cannot come up with
treatments quickly enough to “catch up” with the changes in the virus.
15) If two viruses infect the same host cell, they can mix/recombine their genetic material.
How do Viruses Harm Cells?
16) They can cause the release of digestive (hydrolytic) enzymes from lysosomes, which break down the host cell
and eventually kill it
17) They can cause infected cells to produce toxins that lead to disease symptoms
18) Vaccines = harmless derivatives of viruses that stimulate the immune system to mount defenses against the
actual pathogen (in the form of antibodies) … can be used to prevent certain viral illnesses
Why are bacteria good model organisms for geneticists?
19) Bacteria reproduce quickly and have high rates of mutation / genetic recombination
20) They are also a very simple single-celled organism
21) The bacterial chromosome is usually a circular DNA molecule with a few associated proteins… eukaryotic
cells, in comparison, typically have multiple linear chromosomes instead of a single circular chromosome
22) In addition to the chromosome, many bacteria have plasmids, smaller circular DNA molecules that can
replication independently of the bacterial chromosome
Characteristics of Bacterial Plasmid DNA
23) Carry 2-30 genes and often carry genes for antibiotic resistance (R
plasmids) or fertility (F factor)
24) They are often used as a genetic tool because they can be cut using
restriction enzymes and used to incorporate foreign DNA (from another
organism) into the bacteria. For example, researchers can insert the
human insulin gene into bacterial plasmids. When the bacteria transcribe and translate their own plasmid
DNA, they also make the human insulin protein, which can be used to treat diabetes.
Bacterial Reproduction
25) Bacteria divide by binary fission, which starts with replication of the bacterial
chromosome
26) They reproduce very quickly  mutations caused by errors in DNA
replication can very quickly accumulate and increase a population’s genetic
diversity
27) Further genetic recombination can arise from two bacteria “switching” DNA
Mechanisms of Gene Transfer / Genetic Recombination in Bacteria
28) Three processes bring bacterial DNA from different individuals together: Transformation, Transduction, and
Conjugation
29) Transformation = Is the alteration of a bacterial cell’s genetic material by the uptake of naked, foreign DNA
from the surrounding environment
30) Transduction = bacteriophage viruses (viruses that infect bacteria) can pick up and transfer bacterial DNA to a
new host along with viral DNA
31) Conjugation = bacterial “sex” ; bacteria with F factor plasmids can form sex pili; sex pili are structures that are
used to directly transfer DNA to another bacterium ; F+ cells (with F plasmid) are the donor cells during
conjugation and F- cells (without F plasmid) are receivers of plasmid DNA
32) Shifting of DNA within one bacterium involves transposons (aka jumping genes), which are genes that can
move around within the genome from one chromosome to another
Notes Questions
You must complete one page (front and back) of notes annotations on a separate piece of paper!
1. How is the lytic cycle different from the lysogenic cycle of viral reproduction?
2. Explain how infection by a retrovirus is different from lytic infection by a normal virus.
3. Why do retroviruses tend to have higher mutation rates than normal viruses?
4. How does transduction increase genetic variation in bacteria?
5. Describe the difference between bacterial conjugation and transformation.