Structural features of viral genome

Viruses are the simplest organisms. Complete virus particles include coat proteins and internal genomic DNA or RNA (some viruses have an envelope formed by the host cell outside the coat protein. The envelope contains glycoproteins encoded by viral genes. Viruses cannot replicate independently, they must enter the host cell with the help of some enzymes and organelles in the cell to allow the virus to replicate. The function of the coat protein (or envelope) is to recognize and invade a specific host cell and protect the viral genome from nuclease of destruction.

Structural function

Structural characteristics of the viral genome

Bovine papillomavirus genome structure and function

RNA bacteriophage genome structure and function

Structural characteristics and functions of hepatitis B virus genome

Structural features

1. The size of the virus genome varies greatly. Compared with bacteria or eukaryotic cells, the genome of the virus is very small, but the genomes of different viruses also differ greatly. For example, the HBV DNA is only 3kb in size and contains less information, and can only encode 4 kinds of proteins. The poxvirus genome is 300kb, which can encode hundreds of proteins, not only for the enzymes involved in virus replication, and even encodes enzymes that metabolize nucleotides, so poxviruses are much less dependent on the host than HBV.


2. The viral genome can be composed of DNA or RNA. Each virus particle contains only one kind of nucleic acid, either DNA or RNA. The two generally do not coexist in the same virus particle. The DNA and RNA that make up the viral genome can be single-stranded, double-stranded, closed-loop molecules, or linear molecules. For example, papilloma virus is a closed-loop double-stranded DNA virus, while the genome of adenovirus is linear double-stranded DNA, poliovirus is a single-stranded RNA virus, and the genome of reovirus is double-stranded. RNA molecule. In general, the genome of most DNA viruses is double-stranded DNA molecules, while the genome of most RNA viruses is single-stranded RNA molecules.


3. The genome of most RNA viruses is composed of continuous ribonucleic acid chains, but some viral genomic RNAs are composed of discrete nucleic acid chains. For example, the genomic RNA molecules of influenza viruses are segmental, consisting of eight RNA molecules, Each RNA molecule contains information that encodes a protein molecule; and the reovirus genome consists of double-stranded segmented RNA molecules, with a total of 10 double-stranded RNA fragments, each of which also encodes a protein. No viral genome composed of segmented DNA molecules has been found.


4. Gene overlap means that the same DNA segment can encode two or even three protein molecules. This phenomenon is only found in mitochondria and plasmid DNA in other biological cells, so it can also be considered as the structural characteristics of the viral genome. This structure allows smaller genomes to carry more genetic information. The overlapping genes were discovered by Sanger in 1977 when he studied ΦX174. ΦX174 is a single-stranded DNA virus, the host is E. coli, so it is also a bacteriophage. After infecting E. coli, it synthesizes 11 protein molecules with a total molecular weight of about 250,000, which is equivalent to the amount of information held by 6,078 nucleotides. The virus DNA itself has only 5375 nucleotides and can encode protein molecules with a total molecular weight of 200,000 at most. Before Sanger figured out that some of the 11 genes of ΦX174 overlapped, such a contradiction could not be resolved for a long time.

There are several cases of overlapping genes:

(1) One gene is completely inside another gene. For example, genes A and B are two different genes, and B is contained in gene A. Similarly, gene E is within gene D.

(2) Partial overlap. For example, gene K overlaps part of genes A and C.

(3) Only one base of the two genes overlap. For example, the last base of the stop codon of gene D is the first base of the start codon of J gene (such as TAATG). Although these overlapping genes are mostly the same in DNA, due to the different reading frames when translating mRNA into protein, the resulting protein molecules are often not the same. Some overlapping genes have the same reading frame, but different starting positions. For example, in the SV40 DNA genome, there are 122 base overlaps between the genes encoding the three coat proteins VP1, VP2, and VP3, but the reading frames of the codons are different. The small t antigen is completely in the large T antigen gene, and they have a common start codon.

1. Most of the viral genome is used to encode proteins, only a very small part is not translated, which is different from the phenomenon of eukaryotic DNA redundancy. For example, the untranslated part in ΦX174 only accounts for 217/5375 G4DNA accounts for 282/5577, less than 5%. The untranslated DNA sequence is usually the control sequence for gene expression. For example, the sequence between the H gene and the A gene of ΦX174 (3906-3973), a total of 67 bases, includes RNA polymerase binding sites, transcription termination signals and ribosome binding sites and other gene expression control regions. Papillomavirus is a type of virus that infects humans and animals. The genome is about 8.0Kb, and the untranslated part is about 1.0kb. This region is also a regulatory region for other gene expression.

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