Viral noncoding RNAs: more surprises
However, just because a ncRNA expressed by a virus exhibits hallmarks of a In particular, surprising insights into the evolutionary relationships between .. (A) KSHV PAN RNA contains two key stabilization elements: the ORF57/MTA. Gene cloning and PCR allow scientists to make a large amount of DNA from only Revolution: PCR and the Use of Reverse Transcriptase to Clone Expressed Genes by the central dogma, it effectively mimics the process by which RNA viruses In , however, the two experiments mentioned in the Nature quote-- one. Here we describe a two-component genome yellow fever virus (YFV) . Nile and yellow fever viruses) and expressed capsid to a concentration sufficient for RNA .. Therefore, this factor(s) created a profound difference in the replication of.
KSHV miR-K also down-regulates p21, an inhibitor of cell cycle progression, to promote virally induced oncogenesis Gottwein and Cullen Similarly, genetic deletion and complementation studies of KSHV miRNAs have shown that they promote cell cycle progression and inhibit apoptosis, necessary for cellular transformation Moody et al.
These viral miR mimics target the same set of host mRNAs as cellular miR to promote virally induced cell transformation.
The critical role of viral miR has been demonstrated in genetic studies using MDV-1 as a model. Viral miRNAs attenuate host antiviral immune mechanisms, enhancing viral survival. However, more recent studies in an animal model of SV40 infection suggest that the SV40 miRNAs control viral replication rather than host immune evasion and cellular transformation Zhang et al.
In many of the above studies, it is not clear whether the reported effects of the viral miRNAs are direct or indirect. These act by binding to transcription factors which then cause the DNA to loop so that the regulatory sequence and bound transcription factor become close to the RNA polymerase binding site. The sequences at the ends of the introns, dictate the splice sites to generate the final mature mRNA which encodes the protein or RNA product.
The term cistron in this context is equivalent to gene. The products of operon genes typically have related functions and are involved in the same regulatory network. Similarly, a gene's introns can be much larger than its exons. Regulatory regions can even be on entirely different chromosomes and operate in trans to allow regulatory regions on one chromosome to come in contact with target genes on another chromosome.
This concept originally called the one gene-one enzyme hypothesis emerged from an influential paper by George Beadle and Edward Tatum on experiments with mutants of the fungus Neurospora crassa. In actuality they proved to be the opening gun in what became molecular genetics and all the developments that have followed from that.
Gene expression In all organisms, two steps are required to read the information encoded in a gene's DNA and produce the protein it specifies. Genetic code[ edit ] Schematic of a single-stranded RNA molecule illustrating a series of three-base codons. Each three- nucleotide codon corresponds to an amino acid when translated to protein The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the genetic code.
Sets of three nucleotides, known as codonseach correspond to a specific amino acid. Additionally, a " start codon ", and three " stop codons " indicate the beginning and end of the protein coding region.
The correspondence between codons and amino acids is nearly universal among all known living organisms.
Viral Genetics - Medical Microbiology - NCBI Bookshelf
The mRNA matches the sequence of the gene's DNA coding strand because it is synthesised as the complement of the template strand. To initiate transcription, the polymerase first recognizes and binds a promoter region of the gene. Thus, a major mechanism of gene regulation is the blocking or sequestering the promoter region, either by tight binding by repressor molecules that physically block the polymerase, or by organizing the DNA so that the promoter region is not accessible.
Antigenically altered viruses may be able to cause disease in previously resistant or immune hosts. Vaccine Strains from Mutations Mutations can produce viruses with a reduced pathogenicity, altered host range, or altered target cell specificity but with intact antigenicity.
Such viruses can sometimes be used as vaccine strains. Recombination Recombination involves the exchange of genetic material between two related viruses during coinfection of a host cell.
Viral noncoding RNAs: more surprises
Recombination by Independent Assortment Recombination by independent assortment can occur among viruses with segmented genomes. Genes that reside on different pieces of nucleic acid are randomly assorted. This can result in the generation of viruses with new antigenic determinants and new host ranges.
Development of viruses with new antigenic determinants through independent assortment is called antigenic shift.David Baltimore (Caltech): Introduction to Viruses
Recombination of Incompletely Linked Genes Genes that reside on the same piece of nucleic acid may undergo recombination. The closer two genes are together, the rarer is recombination between them partial linkage.
Are Viruses Living?
Phenotypic Variation from Recombination Development of viruses with new antigenic determinants by either type of recombination may allow viruses to infect and cause disease in previously immune hosts. Vaccines through Recombination Vaccine strains of viruses can be used to create recombinant viruses that carry extra genes coding for a specific immunogen. During viral vaccination, the replicating virus will express the specific immunogen.
Specific antibody production will be stimulated, and the host will be protected from the immunogen as well as from the vaccine virus. Introduction Viruses are simple entities, lacking an energy-generating system and having very limited biosynthetic capabilities. The smallest viruses have only a few genes; the largest viruses have as many as Genetically, however, viruses have many features in common with cells.
Viruses are subject to mutations, the genomes of different viruses can recombine to form novel progeny, the expression of the viral genome can be regulated, and viral gene products can interact.
By studying viruses, we can learn more about the mechanisms by which viruses and their host cells function. Genetic Change in Viruses This chapter covers the mechanisms by which genetic changes occur in viruses.
Two principal mechanisms are involved: Alterations in the genetic material of a virus may lead to changes in the function of viral proteins.
Gene - Wikipedia
Such changes may result in the creation of new viral serotypes or viruses of altered virulence. Mutations Mutations arise by one of three mechanisms: The first two mechanisms act similarly in all viruses; hence, the effects of physical mutagens and the natural behavior of nucleotides are relatively constant. However, viruses differ markedly in their mutation rates, which is due primarily to differences in the fidelity with which their enzymes replicate their nucleic acids.