Replication Cycles and Classification of Viruses (MICR215 Study Notes)

Genome-Based Classification of Viruses

Introduction to Virus Classification

Viruses are classified based on the nature of their genetic material. The genome-based classification system is fundamental in virology, as it determines how viruses replicate and express their genes. The most widely accepted system is the Baltimore Classification, which organizes viruses into seven groups according to their genome type and replication strategy.

• RNA viruses: Viruses with RNA as their genetic material.

• DNA viruses: Viruses with DNA as their genetic material.

• Further classification considers whether the genome is single-stranded (ss) or double-stranded (ds), and whether it is positive-sense (+) or negative-sense (-).

Baltimore Classification divides animal viruses into seven groups:

• Group I & II: DNA viruses

• Group III, IV & V: RNA viruses

• Group VI & VII: Reverse-transcribing (RT) viruses

Baltimore Classification: Replication Strategies

Each Baltimore group synthesizes mRNA using distinct strategies, which are hallmarks of their replication cycles. The classification enables prediction of genome replication and gene expression mechanisms.

• Group I (dsDNA): DNA-dependent RNA polymerase transcribes mRNA from double-stranded DNA.

• Group II (ssDNA): Single-stranded DNA is converted to double-stranded DNA before transcription.

• Group III (dsRNA): RNA-dependent RNA polymerase transcribes mRNA from double-stranded RNA.

• Group IV (+ssRNA): Positive-sense RNA serves directly as mRNA.

• Group V (-ssRNA): Negative-sense RNA is transcribed into mRNA by RNA-dependent RNA polymerase.

• Group VI (ssRNA-RT): Reverse transcriptase converts RNA to DNA, which is then transcribed.

• Group VII (dsDNA-RT): DNA is transcribed to RNA, then reverse transcribed back to DNA.

Key Equation:

$\text{dsDNA} \rightarrow \text{mRNA} \rightarrow \text{protein}$

$\text{ssRNA}^- \xrightarrow{\text{RNA-dependent RNA polymerase}} \text{mRNA}$

Classification Examples

Representative viruses from each Baltimore group:

• dsDNA: Smallpox, Herpes, Papilloma

• ssDNA: Parvovirus

• dsRNA: Rotavirus

• +ssRNA: Polio, Rhino, Corona

• -ssRNA: Measles, Mumps, Rabies, Influenza

• Retroviruses: HIV

DNA Viruses

Overview of DNA Virus Families

DNA viruses are classified based on their genome structure (single or double-stranded) and whether they are enveloped or nonenveloped. Major families include:

• Parvoviridae: Single-stranded DNA, nonenveloped. Example: Parvovirus (Fifth Disease).

• Adenoviridae: Double-stranded DNA, nonenveloped. Example: Adenovirus.

• Papovaviridae: Double-stranded DNA, nonenveloped. Associated with tumors, cancers, and warts.

• Poxviridae: Double-stranded DNA, enveloped. Examples: Smallpox, Cowpox.

• Herpesviridae: Double-stranded DNA, enveloped. Examples: Herpes simplex (cold sores, genital herpes), Kaposi's sarcoma.

• Hepadnaviridae: Partially double-stranded DNA, enveloped. Example: Hepatitis B.

Table: Major DNA Virus Families and Associated Diseases

Family	Genome Type	Envelope	Example Viruses	Associated Diseases
Parvoviridae	ssDNA	No	Parvovirus	Fifth Disease
Adenoviridae	dsDNA	No	Adenovirus	Respiratory infections
Papovaviridae	dsDNA	No	Papillomavirus	Tumors, cancers, warts
Poxviridae	dsDNA	Yes	Smallpox, Cowpox	Smallpox, Cowpox
Herpesviridae	dsDNA	Yes	Herpes simplex virus	Cold sores, genital herpes, Kaposi's sarcoma
Hepadnaviridae	Partially dsDNA	Yes	Hepatitis B virus	Hepatitis B

Additional info:

• The Baltimore classification is essential for predicting the replication and transcription strategies of viruses, which is crucial for understanding viral pathogenesis and for developing antiviral therapies.

• DNA viruses often utilize host cell machinery for replication and gene expression, but some encode their own enzymes.