2.1: A Microscopic Look at Life’s Organization
pg. 52 – 58
Key Terms: Virus, capsid, replication, lytic cycle, lysogenic cycle, and prion.
Introduction:
Prokaryotes and Eukaryotes
- Bacteria and Archaea are prokaryotes, lacking a true membrane bound
nucleus.
- Protista, Fungi, Plants, and Animals are eukaryotes, large and more
complex with membrane organelles and a nucleus.
Table 2.1: Two Types of Cells
pg. 53
Viruses
Virus: a structure that contains strands of DNA or RNA surrounded by a
protective protein coat; it cannot live independently outside of cells.
Viruses are different from prokaryotes and eukaryotes in many ways:
Viruses are functionally dependent on the internal workings of host cells,
either prokaryotic or eukaryotic. Considered non-living when found outside
of a cell and lie dormant. Viruses must invade a host cell to reproduce. They
are not cellular therefore do not contain organelles, cytoplasm, or a cell
membrane.
Viruses do affect cells. Viruses cause diseases in plants, animals,
populations, species and ecosystems. (plants – wheat, oats, and barley,
animal – polio, HIV, and H1N1) Scientists develop vaccines and new
treatments to fight viral infections
Viruses can be used in biotechnology research, as a viral vector or gene
cloning.
Classifying Viruses
Capsid: the outer protein layer that surrounds the genetic material of a virus.
Replication: the fundamental process of all cells, in which the genetic
material is copied before the cell reproduces.
Lytic Cycle: the replication process in viruses in which the virus’ genetic
material uses the copying machinery of the host cell to make new viruses.
Lysogenic Cycle: the replication process in viruses, in which the viral DNA
enters the host cell’s chromosome; it may remain dormant and later activate
and instruct the host cell to produce more viruses.
Although a virus does not express all the characteristics of life (considered
non-living), they do contain genetic material (either DNA or RNA) and
reproduce (host cell’s machinery) which allows a consideration for living.
Viruses can be classified by their size and shape of the capsid. They can also
be classified by what disease they cause.
Viruses that affect humans are divided into 21 groups, which differ in their
genomes, and method or replication.
Figure 2.2: Viruses have a variety of shapes. The viruses shown here include (A) the
polio virus, (B) HIV, (C) the tobacco mosaic virus, and (D) the T4 virus. Pg. 54
Reproduction in Viruses
The virus uses the host cell and its machinery to produce multiple copies of
its self. The copies are built, assembled inside the cell, and released
Lytic Cycle:
1. Attachment: proteins on the surface of the virus bind to protein
receptors on the surface of the host cell’s membrane.
2. Entry: the virus injects the genetic material (RNA or DNA) into the
host cell.
3. Replication: the host cell makes more viral DNA or RNA and proteins.
4. Assembly: new viral particles are assembled.
5. Lysis and Release: the host cell breaks open and releases new viral
particles.
Lysogenic Cycle:
1. Attachment: proteins on the surface of the virus bind to protein
receptors on the surface of the host cell’s membrane.
2. Entry: the virus injects the genetic material (RNA or DNA) into the
host cell.
3. Provirus Formation: viral DNA becomes part of the host cell’s
chromosome.
4. Cell Division: provirus replicates with host’s chromosome.
5. *Provirus Release: the provirus leaves the host’s chromosome.
6. Replication: the host cell makes more viral DNA or RNA and proteins.
7. Assembly: new viral particles are assembled.
8. Lysis and Release: the host cell breaks open and releases new viral
particles.
Figure 2.3 In the lytic cycle, the entire replication process occurs in the cytoplasm of the
host cell. The virus’s genetic material enters the host cell, and the cell replicates the viral
DNA or RNA. The host cell makes new capsids, and assembles new viral particles. The
host cell lyses, or breaks open, and the new viruses leave the cell. In the Lysogenic cycle,
the virus’s genetic material enters the host cell’s chromosome. In many cases, the genes
are not activated until later. Activation results in a continuation of the lytic cycle. pg. 55
Viruses and Diseases
During the lytic cycle, the viruses that are released form the host cell also
kill the cell by destroying the cell membrane.
These new viruses are free to infect other neighbouring cells, in multicellular organisms.
During the lysogenic cycle, affects are delayed as the provirus is replicated
as the cells replicate, and is known as a Retrovirus, for example HIV.
Retroviruses contain an enzyme known as transcriptase.
The enzyme causes the host cell to copy the viral RNA to be converted into
DNA, allowing it to combine with the cell’s DNA strand.
During Mitosis the viral DNA is replicated along with the cells DNA.
This process can continue for years before the provirus separates and
assembles new viruses.
Since the viral DNA is part of the host cell’s DNA it can not be detected.
Patterns of Disease
pg. 56
The replication of viruses can explain the patterns of diseases.
Virus, such as; the herpes virus for cold sores comes and goes.
This is because it appears during the viral cycle as cells die, but lies dormant
when it enters the provirus stage.
The HIV virus enters the provirus stage, and still produces viruses, as the
cell functions normally.
Figure 2.4 Retroviruses contain an enzyme that causes the host cell to copy viral RNA
into DNA. This DNA becomes a provirus that continues to produce new viruses without
destroying the cell. Pg. 56
Retroviruses carry RNA and an enzyme called reverse transcriptase that
causes the host cell to copy the viral RNA into DNA.
Then it embeds into the host’s chromosomes and becomes a provirus. Every
descendent cell then has HIV DNA copied within its genome.
Prions: Non-viral Disease-causing Agents pg. 57
Prion – an infectious particle that causes damage to nerve cells in the brain,
and that appears to consist mostly or entirely of a single protein.
Stanley Prusiner discovered an entirely new type of disease-causing agent
called a prion.
Prions are proteins found normally in the body.
Prions do not have RNA or DNA.
Diseases are caused when the prions change from a non-harmful form to a
harmful form.
May cause many different types of brains diseases, such as; CreutzfeldtJacob disease (CJD).
In sheep, scrapie and cows bovine spongeiform encephalopathy (BSE).
(Mad cow disease)
Activity 2.1: Comparing Prion Diseases
Viruses and Biotechnology
Viruses can be useful tools for genetic engineers. Genetic engineers will
remove the viral DNA and insert a genetic code they wish to reproduce.
The virus can be used as a viral vector to deliver genetic information to be
copied in a host cell.
Viruses are high specific to the type of cell they interact with.
Learning Check: questions 1 – 6
pg. 55
Section 2.1 Review: questions 1 – 15
pg. 58