What Do Viral Genomes Look Like?
- The viral genome is the nucleic acid-based repository of the information needed to build, reproduce, and transmit a virus
- A universal function of viral genomes is to specify proteins.
- The compositions and structures of virus genomes are more varied than any of those seen in the entire bacterial, plant, or animal kingdoms.
- The nucleic acid comprising the genome may be DNA or RNA with single-stranded or double-stranded strands and may have a linear, circular, segmented or gapped configuration.
- Single-stranded virus genomes may be either positive (+) sense (i.e., of the same polarity, or nucleotide sequence, as the mRNA), negative (-) sense, or ambisense (a mixture of the two)
- The genomes of viruses range from those that are extraordinarily small (2 kb) to those that are extraordinarily large (2,500 kbp); the diversity in size likely provides advantages in the niche in which particular viruses exist. Some newly discovered viruses like Mimivirus (approximately 1.2 million bp) are larger than the smallest bacterial genomes (e.g., Mycoplasma).
- Unlike the genomes of all cells, which are composed of DNA, virus genomes may contain their genetic information encoded in either DNA or RNA.
- Evolution has led to the formation of only seven major types of viral genomes.
Terminologies:
Important conventions: plus (+) and minus (-) strands
- mRNA is defined as the positive (+) strand because it can be translated.
- If the ss sense or polarity is the same sense as mRNA is said to be a positive or positive plus (+)
- If the ss sense or polarity is complementary to mRNA is said to be negative or minus sense (-)
- A strand of DNA of the equivalent polarity is also designated as a (+) strand; i.e., if it were mRNA, it would be translated into protein.
- The RNA or DNA complement of the (+) strand is called the (-) strand.
- The (-) strand cannot be translated; it must first be copied to make the (+) strand.
- Ambisense RNA contains both (+) and (-) sequences.