BIOLOGY
Lesson 21.1: Viruses
Guided Notes
OBJECTIVES
1. Describe how ___________________________ reproduce.
2. Explain what happens after a virus _________________________ a cell.
3. Describe how viruses cause _________________________________.
4. Determine if viruses can be considered ________________________________.
WHAT IS A VIRUS?
➢ Discovery of viruses
○ 1892: Russian biologist Dmitri Ivanovski showed that the cause of tobacco
mosaic disease, a plant disease, was found in the liquid extracted from infected
_____________________________.
■ Dutch scientist Martinus Beijerinck suggested that tiny
__________________________ in the juice caused the disease and he
named these particles ____________________________, after a Latin
word for “______________________”.
○ In 1935, American biochemist Wendell Stanley isolated crystals of tobacco
mosaic virus. Since living organisms don’t ________________________,
Stanley said that viruses were ______________ truly alive, which is still
recognized as valid today.
○ Virus: nonliving particle made of ____________________, ________________
__________________________, and sometimes ________________________;
viruses can only reproduce by infecting ___________________ cells
➢ Structure & composition
○ Viruses are so small they can only be seen with a powerful _________________
_______________________________ and they vary widely in terms of size and
structure.
○ Viruses contain genetic information in the form of ___________ or ___________,
surrounded by a protein coat, or ____________________.
■ Some viruses, such as ___________________________, also have a
membrane surrounding the capsid.
VIRAL INFECTIONS
➢ To enter a host cell, most viruses have _________________________ on their surfaces
that bind to ________________________ on a cell. Those proteins “____________” the
cell to take in the virus. When the virus is inside the cell, it makes_________________
of itself so it can _____________________ to other cells. In doing this, it sometimes
___________________________ the host cell.
○ When in a living cell, viruses use their genetic information to reproduce, some
immediately (__________________ infection). Others temporarily stay in an
inactive state within the host (_______________________________ infection).
➢ Lytic infection: a virus enters a ________________________________ cell, makes
copies of itself, and causes the cell to _________________, or lyse.
○ T4, a bacterial virus, or _________________________________, causes this
type of infection— the virus binds to the surface of a bacterium, injects its
_______________ into the cell, then begins to make ___________________
from its own genes. The mRNAs are translated into proteins that act like a
molecular ________________________________________,
__________________________________ the cell’s DNA. The host cell makes
thousands of copies of ____________________________________________
and ____________________ proteins. Then the cell ______________, releasing
hundreds of virus particles that can go on to infect other cells.
➢ Lysogenic infection: a type of infection where a host cell is ______________
immediately taken over, but instead a _______________________________________,
along with host DNA, is replicated without _____________________ the ____________.
○ Example: _________________________ lamda
○ Prophage: bacteriophage DNA that becomes _________________________ in
the bacterial host’s DNA
■ The DNA may remain in the host genome for many
____________________. Influences from the___________________,
like radiation, heat, and certain chemicals, trigger the prophage to
become ______________________. It then removes itself from the host
cell DNA and reproduces by forming new virus particles— The lysogenic
infection now becomes an active _______________________________.
➢ A closer look at two RNA viruses
○ About 70% of viruses contain _________________ rather than DNA.
○ In humans, RNA viruses cause a wide range of ________________________,
from mild colds to influenza and AIDS.
■ Certain kinds of _____________________ also begin with an infection by
viral RNA.
○ The common cold
■ ____________________________ attack with a very simple, fast-acting
infection. The virus settles on a cell, and is brought inside. The host cell’s
ribosomes translate the _________________________ into
___________________________ and other viral proteins. These proteins
assemble around copies of viral RNA and within 9 hours, the host cell
releases _____________________ of new virus particles to infect other
cells.
○ HIV: the deadly disease called acquired immune deficiency syndrome (_______)
is caused by an RNA virus called human immunodeficiency virus (________)
■ HIV belongs to a group of RNA viruses called _____________________.
The genetic info of a retrovirus is copied from __________ to _________
and may become inserted into the DNA of the host cell.
● Retroviral infections are similar to _______________ of bacteria—
the viral genes may remain inactive for many cell cycles before
making new virus particles and damaging the cells of the host’s
________________________________.
VIRAL DISEASES
➢ Viruses produce disease by disrupting the body’s normal ________________________.
➢ Disease mechanisms
○ Viruses cause disease by directly destroying ________________________ or by
affecting cellular processes in ways that upset homeostasis.
■ Example: ___________________________________ destroys cells in
the nervous system, producing ___________________________.
➢ Prevention and treatment
○ Many viral diseases can be prevented by ________________________
prepared from ______________________ or __________________________
virus particles. Vaccines stimulate the body’s immune system to recognize and
________________ such viruses before they can cause disease.
○ Cold and flu viruses are often transmitted by __________-to- ______________
contact, so hand-washing and controlling coughs and sneezes can help prevent
the spread of viruses.
VIRUSES & CELLS
➢ Viruses can be considered _____________________________ because they must
infect living cells in order to ________________ and _____________________, taking
advantage of the _______________________ and cellular machinery of their hosts.
