Virology Basics

Study Notes

A virus may contain a sheath, base plate, and tail fibers in addition to a lipid bilayer picked up when the virus leaves the host cell.

A viral genome refers to the genetic material of the virus, also known as the viral chromosome.
Viral genomes can be DNA or RNA, single-stranded or double-stranded, and circular or linear.
The size of viral genomes varies from a few thousand to over a hundred thousand nucleotides.

The bacterial chromosome is found in the nucleoid region, which is not membrane-bounded.
Bacteria can have one to four identical copies of the same chromosome, depending on the species and growth conditions.
Bacterial chromosomal DNA is usually a circular molecule, typically a few million nucleotides in length, containing a few thousand different genes.

To fit within the bacterial cell, the chromosomal DNA must be compacted about 1000-fold, involving the formation of loop domains.
The looped structure compacts the chromosome about 10-fold, with the number of loops varying according to the size of the bacterial chromosome and the species.

Repetitive sequences in the genome can be classified into three types: unique or non-repetitive, moderately repetitive, and highly repetitive.
Unique or non-repetitive sequences are found once or a few times in the genome, including structural genes and intergenic areas.
Moderately repetitive sequences are found a few hundred to a few thousand times and include genes for rRNA and histones, origins of replication, and transposable elements.
Highly repetitive sequences are found tens of thousands to millions of times and are relatively short, sometimes interspersed throughout the genome or clustered together in tandem arrays.

DNA compaction involves the formation of radial loop domains, which are anchored to the nuclear matrix using matrix-attachment regions (MARs) or scaffold-attachment regions (SARs).
Radial loops are important for gene regulation and organizing chromosomes within the nucleus, with each chromosome occupying a discrete and non-overlapping chromosome territory.

Euchromatin is less condensed, transcriptionally active, and forms 30 nm fiber radial loop domains.
Heterochromatin is tightly compacted, transcriptionally inactive, and has radial loop domains compacted even further.
There are two types of heterochromatin: constitutive heterochromatin, which is always heterochromatic and permanently inactive, and facultative heterochromatin, which can interconvert between euchromatin and heterochromatin.

During M phase, the level of compaction changes dramatically, with sister chromatids becoming entirely heterochromatic by the end of prophase.
Metaphase chromosomes are highly condensed, have an overall diameter of 1,400 nm, and undergo little gene transcription.
Histones are necessary for the compaction of radial loops, and the scaffold formed from the nuclear matrix anchors the highly compacted radial loops in metaphase chromosomes.