BMI3705 - VIROLOGY
Rubbia Khalid
@TEACHME2 https://www.teachme2.com/tutors/rubbia-29398
, BMI3705 - VIROLOGY
BMI3705 Virology
QUESTION 1 [5]
Describe, with specific examples, the Baltimore system for the classification of human
viruses according to whether the direction of translation of viral mRNA(s) is positive and
negative in relation to their genome.
The Baltimore classification system is a widely used method for categorizing viruses based on
the type of genome they possess and the mechanism of mRNA synthesis during their replication.
One of the key criteria in this classification is whether the direction of translation of viral mRNA
is positive or negative in relation to their genome. The Baltimore system divides viruses into
seven distinct classes (I to VII) based on this and other characteristics. Here, we'll focus on
Classes IV and V, which pertain to the direction of mRNA translation.
Class IV - Positive-sense single-stranded RNA viruses (ssRNA+): In this class, the viral
genome is single-stranded RNA that is directly translatable into functional proteins. Examples of
viruses in this class include:
1. Picornaviruses: Enteroviruses like poliovirus and rhinovirus. These viruses have a
single-stranded RNA genome that serves as a template for the translation of viral proteins.
2. Flaviviruses: Hepatitis C virus (HCV) is an example. HCV has a single-stranded RNA
genome that is directly translated into proteins, including viral enzymes and structural
proteins.
Class V - Negative-sense single-stranded RNA viruses (ssRNA-): In this class, the viral
genome is single-stranded RNA, but it is not directly translatable into proteins. Instead, it serves
as a template for the synthesis of complementary positive-sense RNA, which is then translated
into proteins. Examples of viruses in this class include:
1. Orthomyxoviruses: Influenza viruses are classic examples. They have a segmented,
single-stranded RNA genome that is negative-sense. The viral RNA is used as a template
to produce positive-sense mRNA strands, which are then translated to make viral
proteins.
, BMI3705 - VIROLOGY
2. Rhabdoviruses: The rabies virus is an example. It has a single-stranded RNA genome
with a negative-sense orientation. The viral RNA is transcribed into positive-sense mRNA,
which is translated to produce viral proteins.
In summary, the Baltimore classification system is a way to categorize viruses based on the
nature of their genetic material and how that material is used for protein synthesis. Class IV
viruses have positive-sense RNA genomes that can be directly translated into proteins, while
Class V viruses have negative-sense RNA genomes that require transcription into positive-sense
RNA before translation. These distinctions are crucial for understanding the replication strategies
and life cycles of different viruses.
QUESTION 2 [40]
2.1 “Under normal physiological conditions, proto-oncogenes and tumour suppressors
help control cell growth and proliferation. When mutated, these proteins contribute to the
development of cancer”. Briefly explain, the molecular basis of this phenomenon by
providing an example of one virus and its mechanism of action leading to cancer
development. (15)
The molecular basis of cancer development often involves mutations in proto-oncogenes and
tumor suppressor genes, disrupting their normal functions in controlling cell growth and
proliferation. Here's a brief explanation of this phenomenon, along with an example of a virus
and its mechanism of action contributing to cancer development:
Proto-oncogenes are genes that play a crucial role in promoting cell growth and division when
functioning normally. They encode proteins involved in signaling pathways that regulate cell
cycle progression. When these genes undergo mutations or alterations, they can become
oncogenes, which lead to uncontrolled cell growth and cancer development.
Tumor suppressor genes, on the other hand, are responsible for inhibiting cell growth and
division. They act as "brakes" on the cell cycle, preventing cells from dividing too rapidly or
inappropriately. Mutations in these genes can reduce their inhibitory function, allowing
unregulated cell growth.
, BMI3705 - VIROLOGY
Example: Human Papillomavirus (HPV) and Cervical Cancer: One well-known example of a
virus contributing to cancer development is the Human Papillomavirus (HPV) and its association
with cervical cancer.
Mechanism:
1. Infection: HPV infects the cervical epithelial cells and integrates its DNA into the host
genome.
2. E6 and E7 Proteins: HPV produces oncoproteins called E6 and E7.
• E6: Interferes with the function of the tumor suppressor gene p53. Normally, p53
helps regulate the cell cycle and initiate apoptosis (cell death) in cells with
damaged DNA. However, E6 binds to p53 and promotes its degradation,
preventing it from carrying out its tumor-suppressive functions.
• E7: Targets another tumor suppressor gene called pRb (retinoblastoma protein).
pRb controls the cell cycle by inhibiting the activity of the transcription factor E2F.
E7 interacts with pRb, leading to its inactivation. This allows E2F to promote the
expression of genes involved in cell cycle progression.
3. Uncontrolled Cell Growth: The inactivation of p53 and pRb by HPV's E6 and E7 proteins
disrupts the normal regulatory mechanisms of the cell cycle. As a result, infected cervical
cells can divide and proliferate uncontrollably.
Over time, this uncontrolled cell growth can lead to the development of cervical cancer. Mutations
in the viral genome or the accumulation of additional genetic mutations in the host cell can further
exacerbate the cancerous transformation.
In summary, the example of HPV and cervical cancer illustrates how viruses can contribute to
cancer development by interfering with the normal function of proto-oncogenes and tumor
suppressor genes. HPV's E6 and E7 oncoproteins disrupt key regulatory pathways, leading to
uncontrolled cell growth and the progression of cancer.
