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Lecture 5

BIOC 4403 Lecture Notes - Lecture 5: Virus Classification, Reoviridae, Poliovirus

Biochem & Molecular Biology
Course Code
BIOC 4403
Archibald John

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Lecture 5
Viruses: entities whose genomes are elements of nucleic acids that replicate inside living cells
using the cellular synthetic machinery, and cause the synthesis of specialized elements (virions)
that can transfer the genome to other cells
Virus classification: By the nature of the genome (Baltimore classification), shape and capsid
structure, presence of envelope, type of cellular organisms they parasitize
Baltimore classification:
- dsDNA (Herpes virus)
- ssDNA (Adenovirus)
- dsRNA (Reovirus)
- (+) ssRNA (Poliovirus)
- (-) ssRNA (Influenza, Ebola)
- ssRNA-RT (DNA intermediate) (Retroviruses like HIV)
- dsDNA-RT (RNA intermediate) (Hepatitis B)
Virus abundance: VERY abundant, 10 million per mL of seawater
Viral genomes: do not have typical features can be linear, circular, alternating
- gene density is high, often compositionally biased (high AT or GC)
- they are fast evolving and divergent gene sequences
- extremely diverse in coding capacity
5 Types of viral genes
- Genes with readily detectable homologs in cellular life forms
1. Genes with closely related homologs and present in only a small subset of viruses
2. Genes that are present within a major group or several groups of viruses, and have distant
related homologs
- Virus-specific genes
3. ORFans without detectable homologs larger genomes have more ORFans
4. Virus specific genes conserved in a group of viruses no cellular homologs
- Viral Hallmark Genes
5. Genes shared by many diverse groups of viruses, with only very distant homologs in
cellular organisms
NCLDV: Nucleo-Cytoplasmic Large DNA Virus family, have the largest genomes known, some
genomes overlap with sizes of eukaryotes and prokaryotes
- Examples: Pandoravirus, Mimivirus
Mimivirus: ‘mimicking microbe’ and has a gram-stain positive, has remarkable genome
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