DNA Viruses and Reverse Transcriptase: Animal and Plant Viruses

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DNA Genome Viruses Using Reverse Transcriptase

Key Enzymes in Viral Replication

Some DNA viruses utilize reverse transcriptase (RTase) during their replication cycle, in addition to other essential enzymes.

DNA polymerase: Synthesizes DNA from a DNA template.
Reverse transcriptase: Synthesizes DNA from an RNA template.
Primase: Produces short RNA primers to initiate DNA synthesis.

Hepadnaviruses — Hepatitis B Virus (HBV)

General Features and Replication

Hepatitis B virus (HBV) is a DNA virus that infects liver cells and is notable for its use of reverse transcription during replication.

Genome: DNA
Envelope: Present
Target cells: Hepatocytes (liver cells)
Transmission: Body fluids
Diseases: Liver inflammation, jaundice, acute/chronic hepatitis

Replication Sequence:

HBV enters the liver cell.
Viral DNA enters the nucleus.
DNA is transcribed to RNA.
RNA is exported to the cytoplasm.
Reverse transcriptase converts RNA back to DNA.
New DNA is packaged into new virions.
Virions exit the cell to infect others.

Key Concept: HBV replication involves a DNA → RNA → DNA sequence, which is unusual for DNA viruses.

HBV Genome — Overlapping ORFs

HBV has a compact genome with overlapping open reading frames (ORFs), allowing multiple proteins to be encoded from the same DNA region.

S gene: Encodes surface proteins (envelope components)
C gene: Encodes core proteins (capsid)
P gene: Encodes polymerase (including reverse transcriptase)
X gene: Encodes X protein (regulates gene expression and cellular processes)

Significance: Overlapping ORFs enable efficient use of a small genome.

HIV — Reverse Transcription and Integration

Genome and Replication Cycle

Human Immunodeficiency Virus (HIV) is an RNA virus that uses reverse transcription and integrates its genome into the host chromosome.

Genome: Single-stranded positive-sense RNA (+RNA)
Replication: +RNA → DNA → integrated DNA → RNA/proteins → new viruses

HIV enters the host cell and uncoats.
Reverse transcriptase converts viral RNA to DNA.
Viral DNA enters the nucleus and integrates into the host genome (via integrase).
Host machinery transcribes viral DNA to RNA.
Host ribosomes translate viral RNA into polyproteins.
Viral protease cleaves polyproteins into functional proteins.
New virions assemble and bud from the cell.

Key Enzymes in HIV Replication

Enzyme	Function
Reverse transcriptase	Converts RNA to DNA
Integrase	Inserts viral DNA into host chromosome
Protease	Cuts viral polyproteins into functional proteins

Memory Aid: RT (Reverse Transcriptase) → IN (Integrase) → PRO (Protease)

Polyomaviruses — SV40

Structure and Oncogenic Potential

Simian Virus 40 (SV40) is a polyomavirus with a double-stranded DNA genome and the ability to integrate into host DNA.

Capsid: Icosahedral
Envelope: Absent (unenveloped)
Genome: dsDNA
Integration: Can integrate into host chromosome

Oncogenesis: The Large T-antigen protein inhibits the tumor suppressor protein p53, potentially leading to uncontrolled cell growth.

p53: Prevents division of cells with damaged DNA
SV40 Large T-antigen: Inhibits p53, contributing to cell transformation in experimental settings

Adenoviruses

Characteristics and Applications

Adenoviruses are non-enveloped, icosahedral dsDNA viruses that commonly cause respiratory infections in humans.

Genome: dsDNA (~36 kb)
Envelope: Absent
Diseases: Respiratory infections (cough, fever, sore throat)
Severity: Usually mild, but can be severe in some cases

Biotechnological Use: Adenoviruses are used as vectors for gene therapy due to their ability to deliver DNA into human cells.

Human Papillomavirus (HPV)

Structure and Cancer Connection

Human papillomaviruses are small, non-enveloped dsDNA viruses, some of which are associated with cancer development.

Genome: dsDNA (~8 kb)
Envelope: Absent
Capsid: Icosahedral
Oncogenic types: Certain high-risk types interfere with tumor suppressor pathways

Cancers Associated:

Cervical cancer
Other anogenital cancers
Some head and neck cancers

Herpesviruses

General Features and Latency

Herpesviruses are large, enveloped dsDNA viruses capable of establishing latent infections in host cells.

Genome: dsDNA (about 150 kb or larger)
Envelope: Present
Transcription: Use host RNA polymerase II
Latency: Viral genome persists in host without active replication; can reactivate

Important Human Herpesviruses:

Virus	Disease/Condition
Herpes simplex virus (HSV)	Oral/genital herpes
Varicella-zoster virus (VZV)	Chickenpox, shingles
Epstein-Barr virus (EBV)	Infectious mononucleosis

Key Concept: Latency is a hallmark of herpesvirus infections.

Mimivirus

Giant Virus Characteristics

Mimivirus is a giant virus notable for its large size and complex genome, yet it remains dependent on host cells for essential functions.

Size: ~0.5 μm (approaching small cellular organisms)
Genome: Very large DNA genome (~900 protein-coding genes)
Host: Infects amoebas
Metabolic genes: Includes genes for amino acid/tRNA synthesis and metabolic functions
Limitations: Lacks ribosomes; cannot perform independent metabolism

Key Concept: Despite their complexity, giant viruses like Mimivirus are still obligate parasites.

Comparison Table: Key Animal DNA and RNA Viruses

Virus	Genome	Envelope?	Key Feature
HBV	DNA	Yes	Uses reverse transcription
HIV	+RNA	Yes	RNA → DNA; integrates into chromosome
SV40	dsDNA	No	Large T-antigen inhibits p53
Adenovirus	dsDNA	No	Respiratory infections; gene-therapy vectors
HPV	dsDNA	No	Some types associated with cancer
Herpesviruses	dsDNA	Yes	Latency; HSV, VZV, EBV
Mimivirus	Very large DNA genome	No	Giant virus; infects amoebas

Key Concepts and Exam Points

Reverse Transcriptase: Enzyme that synthesizes DNA from an RNA template. Used by HBV and HIV.
HBV: DNA virus using reverse transcription; infects liver; overlapping ORFs; S, C, P, X genes.
HIV: RNA virus; reverse transcriptase, integrase, and protease are essential; polyproteins processed by protease.
SV40: dsDNA, non-enveloped; Large T-antigen inhibits p53, affecting cell cycle regulation.
Adenovirus: dsDNA, non-enveloped; causes respiratory infections; used in gene therapy.
HPV: Small dsDNA, non-enveloped; some types cause cancer.
Herpesviruses: dsDNA, enveloped; establish latency; HSV, VZV, EBV are key examples.
Mimivirus: Giant DNA virus; infects amoebas; large genome but lacks ribosomes.

Definitions and Key Terms

Reverse Transcriptase: Enzyme that synthesizes DNA from an RNA template.
Open Reading Frame (ORF): A sequence of DNA or RNA that can be translated into a protein.
Polyprotein: A large protein that is cleaved into several functional proteins by proteases.
Latency: A state in which a virus remains dormant within a host cell.
p53: A tumor suppressor protein that regulates the cell cycle and prevents cancer.

Sample Equations and Processes

HBV Replication:
- DNA → RNA → DNA
HIV Replication:
- +RNA → DNA → integrated DNA → RNA/proteins → new viruses

Summary Table: Major Animal Viruses Discussed

Virus	Genome Type	Envelope	Key Features	Associated Diseases
HBV	DNA (uses RTase)	Yes	Overlapping ORFs, reverse transcription	Hepatitis, liver cancer
HIV	+RNA	Yes	Reverse transcription, integration, polyprotein processing	AIDS
SV40	dsDNA	No	Large T-antigen, p53 inhibition	Cell transformation (experimental)
Adenovirus	dsDNA	No	Gene therapy vector	Respiratory infections
HPV	dsDNA	No	Oncogenic types	Cervical, anogenital, head/neck cancers
Herpesviruses	dsDNA	Yes	Latency	Herpes, chickenpox, mononucleosis
Mimivirus	Very large DNA	No	Giant virus, complex genome	Infects amoebas

Additional Info

Some details about the number of protein-coding genes in Mimivirus and the exact size of its genome are based on current scientific descriptions and may vary between strains.
SV40's role in human cancer is primarily experimental; its Large T-antigen is a model for studying cell transformation.