A: Question - Glucose is able to cross the cell membrane with relatively little difficulty while both Q: What is the relationship of the bats' status hibernation in the physiological mechanism with the oth A: Hibernation involves an extreme reduction in metabolic rate , heart rate and respiratory rate that a Q: Joe is a counselor at a summer camp for children ages What should Joe do next?
During a A: Ans. Emergencies need some kind of first-aid help. Compression is one of the life-saving techniques Q: Compare an autotetraploid and an allotetraploid to each other with respect to the characteristics li Q: In a population of baby kitties, 40 are white because of a recessive allele b.
Non-white cats A: In this question, we are given: Population of baby kitties: Baby kitties with recessive phenot A: Bats hibernate in caves, mines, rock formations and even in your home.
These locations, referred to Q: Which of the following are NOT processes that can change allelic frequencies? Group of answer choic Mutations can result from DNA copying errors A: Cranial capacity can be said to be limited by all of the following except intelligence. The crania A: Trp operon in E.
Q: A researcher is isolating proteins from a cell via different techniques. They find that when the cel A: Establishing protein expression is important in the study of cells, because proteins often carry out Q: A: As per our policy, We are answering only 2 questions as all are different questions. For the rest of Q: Aminoglycosides inhibit protein synthesis in bacteria.
These class of antibiotics are considered to A: Aminoglycosides -- Aminoglycosides are member of group of antibiotic. An antibiotic is defined as a Direction: Compare and contrast aerobic with anaerobic respliration using the Venn diagram below B Herpes simplex virus DNA consists of two covalently linked components, long L and short S , each of which consists of a large unique sequence U L and U S , respectively flanked by inverted repeats.
In a viral population, four isomeric forms differing in the orientation of the unique regions relative to each other occur in equimolar amounts.
Intact single strands anneal as shown on the right. C Vaccinia virus DNA has inverted terminal repeats and each end is covalently closed, so that on denaturation it forms a large, single-stranded circular molecule.
This has an essential function in replication of the genome. The DNA of certain iridoviruses genus Ranavirus contains a high proportion of 5-methylcytosine instead of cytosine. The size of viral DNA genomes ranges from 4. As 1 kb or 1 kbp contains enough genetic information to code for about one average-sized protein, we recognize as an approximation that viral DNAs contain from about 4 to genes and code for 4 to proteins.
However, the relationship between any particular nucleotide sequence and its protein product is not as straightforward as this. First, the DNA of most of the larger viruses—like that of cells—contains what appears to be redundant information, in the form of 1 repeat reiterated sequences and 2 introns, i.
Furthermore, a given mRNA sequence may be read in two different reading frames theoretically, up to three, because each codon is a triplet , giving rise to two or three proteins with different amino acid sequences. These fascinating examples of genetic economy are well illustrated by the papovaviruses see Fig. Viral DNAs contain several kinds of noncoding sequences, in addition to introns and various types of terminal repeat sequences, described above.
Consensus sequences, which tend to be conserved through evolution because they serve vital functions, include those of RNA splice sites, polyadenylation sites, RNA polymerase recognition sites and promoters, initiation codons for translation, and termination codons. The genome of RNA viruses may also be single-stranded or double-stranded. Furthermore, while some occur as a single molecule, others are segmented. Arenavirus and birnavirus RNAs consist of 2 segments, bunyavirus RNA of 3, orthomyxovirus RNA of 7 or 8 in different genera , and reovirus 10, 11, or 12 in different genera.
All viral RNAs are linear; none is a covalently closed circle. However, the ssRNAs of arenaviruses and bunyaviruses have sticky ends, hence these molecules occur as circles. Single-stranded viral nucleic acid, which is generally RNA, can also be defined according to its sense also known as polarity.
This is the case with picornaviruses, caliciviruses, togaviruses, flaviviruses, coronaviruses, and retroviruses. If, on the other hand, its nucleotide sequence is complementary to that of mRNA, it is said to have negative — sense. Such is the case with the paramyxoviruses, orthomyxoviruses, rhabdoviruses, arenaviruses, and bunyaviruses, all of which have an RNA-dependent RNA polymerase transcriptase in the virion, in order that mRNA can be transcribed.
With the arenaviruses and at least one genus of bunyaviruses one of the RNA segments is ambisense, i. The size of ssRNA viral genomes varies from 7. Accordingly they code, in general, for fewer than a dozen proteins. In the case of the segmented RNA genomes of orthomyxoviruses and reoviruses, one can consider most of the segments to be individual genes, each coding for one unique protein. No such simple relationship applies to the other RNA viruses. The essential features of the genomes of viruses of vertebrates are summarized in Table Their remarkable variety is reflected in the diverse ways in which the information encoded in the viral genome is transcribed to RNA, then translated into proteins, and the ways in which the viral nucleic acid is replicated see Chapter 4.
Viral preparations often contain some particles with an atypical content of nucleic acid see Chapter 5. Host cell DNA is found in some papovavirus particles, and cellular ribosomes are incorporated in arenaviruses. Several copies of the complete viral genome may be enclosed within a single particle, or viral particles may be formed that contain no nucleic acid empty particles or that have an incomplete genome defective interfering particles.
Some virus-coded proteins are structural, i. A major role of structural proteins is to provide the viral nucleic acid with a protective coat. The virions of all viruses of vertebrates contain several different proteins, the number ranging from 3 in the case of the simplest viruses to over in the case of the complex poxviruses.
In isometric viruses, the structural proteins form an icosahedral capsid which sometimes encloses a polypeptide core that is intimately associated with the nucleic acid. Some virions, e. The capsid proteins are assembled in the virion in groups, to form the capsomers visible in electron micrographs. Each capsomer is composed of one to six molecules of polypeptide, usually of the same kind homopolymers but sometimes different heteropolymers.
Capsomers from the vertices and the faces are usually composed of different polypeptides. A few viruses have a double capsid, each being composed of a different set of polypeptides. Other proteins, invariably glycoproteins, make up the peplomers projecting from the envelope; a second type of envelope protein is the nonglycosylated matrix protein that occurs as a layer at the inner surface of the lipid envelope of orthomyxoviruses, paramyxoviruses, and rhabdoviruses.
One or more of the proteins on the surface of the virion has a specific affinity for complementary receptors present on the surface of susceptible cells; the same viral protein contains the antigenic determinants against which neutralizing antibodies are made. Virions of several families carry a limited number of enzymes, transcriptases being the most important Table As a consequence, the composition of lipids of particular viruses differs according to the composition of the membrane lipids of the cells in which they have replicated.
The poxviruses, ranaviruses, and African swine fever virus contain cellular lipid in their envelopes, and other lipids in the inner part of the virion. Lipid occurs in the outer membrane of poxviruses, and has a different composition from that of host cell lipids. In ranaviruses and African swine fever virus the additional viral lipid occurs within the icosahedral capsid.
Apart from that associated with viral nucleic acid, carbohydrate occurs as a component of viral glycoproteins, which usually occur as peplomers, with their hydrophobic ends buried in the lipid bilayer of the envelope, while their glycosylated hydrophilic ends project into the medium. Poxviruses also contain internal glycoproteins, in the membrane of the core, and one of the outer capsid proteins of rotaviruses is glycosylated. In general, viruses are more sensitive than bacteria or fungi to inactivation by physical and chemical agents.
A knowledge of their sensitivity to environmental conditions is therefore important for ensuring the preservation of the infectivity of viruses as reference reagents, and in clinical specimens collected for diagnosis, as well as for their deliberate inactivation for such practical ends as sterilization, disinfection, and the production of inactivated vaccines see Chapter 14 and Chapter The principal environmental condition that may adversely affect the infectivity of viruses in clinical specimens is too high a temperature; other important conditions are pH and lipid solvents.
Viruses vary considerably in heat stability. At ambient temperature the rate of decay of infectivity is slower but significant, expecially in hot summer weather or in the tropics in any season. The enveloped viruses are more heat labile than nonenveloped viruses. Some enveloped viruses, notably respiratory syncytial virus, tend to be inactivated by the process of freezing and subsequent thawing, probably as a result of disruption of the virion by ice crystals.
Because they can't reproduce by themselves without a host , viruses are not considered living. The pace of evolution The RNA polymerase that copies the virus's genes generally lacks proofreading skills , which makes RNA viruses prone to high mutation rates—up to a million times greater than the DNA-containing cells of their hosts. Teeny, single-cell creatures floating in the ocean may be the first organisms ever confirmed to eat viruses. Scientists scooped up the organisms, known as protists , from the surface waters of the Gulf of Maine and the Mediterranean Sea off the coast of Catalonia, Spain.
Scientists were studying viruses that infect and kill bacteria, called bacteriophages or phages as a therapeutic for bacterial infections over a hundred years ago. Antibiotics came along, however, and we no longer needed these viruses. The two genomes also differ by indels, most of them are insertions or deletions of single bases in non-coding regions. However, 26 indels are in frame additions or deletions of codons. Such studies will increase the scope for better identification of the genetic attributes of KOS and its contributions to its pathogenesis.
Viruses with single stranded DNA genomes infect hosts that belong to all three domains of life and are considered to be economically, medically and environmentally important pathogens. Recent studies have shown that these single stranded DNA viruses exist in great numbers in highly diverse habitats, ranging from extreme geothermal springs to the gut of humans and other animals.
International Committee on Taxonomy of Viruses currently classified single stranded DNA viruses into 10 different taxa. However, several viruses that can be classified into additional groups have been isolated and many of their genomes were sequenced. All single stranded DNA viruses are pathogenic on eukaryotes, possess non-enveloped, icosahedral capsids, along with Microviridae family members, which infects bacteria.
Single stranded DNA viruses are the group comprising of smallest viruses and their genomes are as small as 1—2 kb, encoding two proteins; one for capsid formation and the other for genome replication. Such irreducible simplicity of single stranded DNA viruses epitomizes their essence of being a virus and makes them an attractive model for investigating virus origins and evolution. Numerous metagenomic studies have revealed a high range of genetic diversity existing in single stranded DNA viruses in the environment, suggesting a highly dynamic interaction between these viruses and their respective hosts.
Also, single stranded DNA viruses with the smallest genomes and simplest proteomes were found to be widespread in cellular chromosomes, providing new important insight into the evolution of these viral. Bacteriophages are the smallest viruses with simple genomes. The Density of phage viruses present in the oceans is 10 6 —10 7 particles per ml.
It was estimated that the total population of the bacteriophages is 10 31 particles and the ratio of environmental virus and bacteria are 5—, after the validation of 10 30 bacterial cells in the biosphere. It has been hypothesized that oceanic bacteriophages infect bacterial cells at the rate of 10 29 phage infections per day, which releases over 10 11 kg of carbon from the biological pool per day.
Over the past three decades, research on bacteriophages has revealed their abundance in nature, genome diversity, impact on the evolution of microbial diversity, their utilization in control of infectious diseases and their influence in regulating the microbial balance in the ecosystem has been explored, leading to a resurgence of interest in the phage research.
Research on phages has played a pivotal role in the most significant discoveries, that were made in biological sciences right from the identification of DNA as the genetic material, in the elucidation of the genetic code, leading to the development of the molecular biology. Research on phages has continuously broken new grounds in our understanding of the basic molecular mechanisms of gene expression and their structure.
In recent times, phage genomics has revealed novel biochemical mechanisms for replication, maintenance, and expression of the genetic material and is providing new insights into the origins of infectious diseases, utilization of phage gene products and even whole phage as an agent for the gene therapy.
In addition to the killing of bacterial cells, temperate phage genomes also carry toxins and other critical virulence factor genes that are important for many bacterial pathogens to infect human beings.
Phages also contribute to the diversity of the bacterial community by serving as vectors for the transduction of different genetic alleles, such as antibiotic resistance genes, between bacterial cells. Phages also have great medical and nanotechnological potential.
Strategies for using tailed phages for detecting bacteria, curing bacterial diseases through phage therapy or decontaminating surfaces have been implemented for almost years in Russia and Georgia.
These phages are currently being used to treat agricultural diseases as well as in the prevention of food contamination in western countries. Phage virions are being developed as nanocontainers for specific chemical cargoes that can be delivered to specific targets. Small size and the simplicity of isolation have made bacteriophages as the primary choice for the complete genome sequencing.
Further, the sequencing of the bacteriophage genomes are propelled exponentially with two main objectives;. To understand the relationship between the phage genomes the evolutionary mechanisms that shaped these bacteriophage populations. For increased utilization of bacteriophages in the development of tools, utilities, and techniques related to genetics and biotechnology. Phage genomes display a considerable amount of variation in their size, varying from Leuconostoc phage L5 bp to Pseudomonas phage , b.
The main reason for the absence of large Siphoviruses is still unknown. Bacteriophages are estimated to be the most widely distributed biological entity of the biosphere. They are found in all habitats of the world, where bacteria proliferate. However, phages belonging to other groups also occur abundantly in the biosphere, such as phages with different virions, genomes, and lifestyles.
Two key approaches were made for studying the viral diversity are metagenomics of total concentrated phage samples collected from the environment and a genome-by-genome strategy of individually isolated phages. These two approaches are compatible, having distinct outcomes. Metagenomics generates a large amount of sequence data, which provides a good insight into their diversity. Sequencing and analysis of individually isolated phages generate small data sets, which are structured into whole genomes.
As phage genomes are architecturally mosaic, the availability of complete genomes contextualizes the complexities of their relationships. The nucleotide sequences of phage genomes with non-overlapping hosts rarely share sequence similarity, as noticed in the published genomes of four Streptomyces phages and available collection of 50 mycobacteriophage genomes. Phages infecting a common bacterial host are in genetic contact with each other, and they share common nucleotide sequences.
Genomes of over 30 phages with common host have been isolated and sequenced from Pseudomonas , Staphylococcus, and Mycobacterium containing related sequences, with a few exceptions. Most of these phages share a very low or no sequence similarity, as illustrated by the nucleotide sequence comparisons of mycobacteriophages and Pseudomonas phages.
Phages were evolved not only by the accumulation of mutations but also through the recombination events, during which they exchanged genetic material with other phages. These events have been suggested to explain the mosaic structure of the phages, arisen by comparison of two or more phage genomes. During the comparison of the genomes, nearly identical sequences alternate with merely similar sequences or completely divergent sequences.
Such type of exchanges in bacteriophages was obtained by heteroduplex mapping in the early s. Since then, numerous mosaics have been identified by sequence comparison, and the mosaic structure of bacteriophages is now a well-documented phenomenon.
This mosaicism is also found to be ubiquitous among bacteria, where the genes are acquired through horizontal genetic exchange mostly through transduction, transformation, and conjugation. But, the extent of mosaicism is highly remarkable in phage genomes as evidenced by the increasing number of genomes available for comparative genomics analysis. The mechanism of genome mosaicism in bacteriophages can be understood at two levels; 1.
There are two models which explain the recombination mechanisms that are responsible for these patterns. Model 1 describes the role of short conserved boundary sequences that are located at gene junctions in targeting various exchange events that are catalyzed by homologous recombinations, by using the recombinases synthesized by either host-or phages. Model 2 attempts to explain that the homologous recombination events are not specifically targeted and occur randomly with the preference of a few short sequences so that most of the events results in non-functional genomic trash.
Comparison of the predicted amino acid sequences encoding phage gene products is an alternative manifestation of mosaicism. This is an informative approach, since many phages including those that infect common hosts may not share any nucleotide sequence information. In that case, protein sequence data reveals genes that share much older ancestry.
M13 Enterobacteria phage infects E. The genome of M13 phage consists of 6. Unlike most icosahedral virions, the capsid of M13 phage is filamentous, which can be expanded by the addition of further protein subunits.
Hence, the genome size can also be increased by the addition of extra sequences in the nonessential intergenic region without becoming incapable of being packaged into the capsid Fig. In a newly infected cell, the gaps on either side of the cos site are closed by DNA ligase, and resulting circular DNA undergoes vegetative replication and integration into the bacterial chromosome.
Bacteriophages T2 and T4 are the model organisms playing an instrumental role in the development of modern genetics and molecular biology since the s. They were involved in the development of many salient concepts related to biological sciences, including the recognition of nucleic acids as genetic material, identification of a gene through structural, mutational, recombinational, and functional analyses, in the demonstration triplet genetic code, in the identification of mRNA and establishing the importance of recombination in the replication of DNA, in the light-dependent and light-independent DNA repair mechanisms, restriction and modification of DNA, self-splicing introns in prokaryotes, etc.
Analysis of the T4 capsid assembly and functioning of its nucleotide-synthesizing complex, replisome, and recombination complexes has led to important insights into macromolecular interactions, substrate channeling, and co-operation between phage and host proteins within such complexes. The genome of T4 phage is considered as the best avenue for understanding and evaluating the complete genome of a well organized biological system.
This genome comprises expressing genes, 8 tRNA genes, and a minimum of 2 genes that encodes small, stable RNAs with unknown function. Genes 16, 17, and 49 contains multiple coding regions that encode more than one protein. T4 phage genome is four times higher than that of Herpesviruses and yeast, two times higher than E.
Regulatory regions in this phage genome are compact, occasionally with overlapping coding regions. T4 phage has several groups of nested genes. Mutants generated by altering a few other genes produced very small plaques under similar standard laboratory conditions. Many of the 62 essential genes are larger than an average T4 gene, occupying half of the genome.
Essential genes encode proteins of the replisome and nucleotide precursor complex, transcriptional regulatory factors, and proteins involved in the structure and assembly of the phage particle. The genome of T4 phage illustrates another rare molecular feature of certain linear viral genomes, terminal redundancy. Replication this phage genome produces long concatemers of DNA, which are cleaved by a specific endonuclease, gets incorporated into the particle with the length exceeding its complete genome due to the repetition of some genes at each end of the genome.
Resulting T4 phage genome containing reiterated information is packed into the phage head. Three T4 phage genes that encode for thymidylate synthase td , subunit of the aerobic ribonucleotide reductase nrdB and the anaerobic ribonucleotide reductase nrdD are found to contain introns that are later spliced out of these transcripts.
A possibility of an unusual relationship between the nucleic acid sequence and protein sequence occurring through translational bypassing is demonstrated in gene 60 of the T4 phage genome. A 50 bp mRNA segment in the coding region of this gene is not translated by the regular mechanism.
This mRNA segment is the only known and unique high-efficiency translational bypass site in the entire T4 phage genome. DNA in the genome of this phage contains only Capsid proteins, which are the most widely conserved among the T4-related phages have the lowest AT contents. A substantial decrease in the pairing of G against C in the coding strands of translated regions has been identified.
A and T are equally divided between the coding strands. However, some AT bias has been identified in the T4 phage genome, which is stronger in the third position of codons, as expected in genomes with a high amount of AT-rich regions. The ultimate size of the RNA viral genomes is affected by the fragility of RNA and the tendency of their long strands to break.
In addition, RNA genomes tend to have higher mutation rates than those composed of DNA because they are copied less accurately. This tendency might have tended to drive RNA viruses towards smaller genomes. Genomes of RNA viruses encode for a limited number of proteins. In monopartite ssRNA viruses, the genome encodes a single polyprotein, which is further processed into a number of small molecules, critical for the completion of the viral life cycle.
In multipartite ssRNA genomes, each segment encodes for a single gene. These genomes contain cis-acting RNA elements that direct different viral processes, such as protein translation, genome replication, and subgenomic mRNA transcription mRNAs. Picornaviruses are the etiologic agents of numerous diseases with medical and veterinary importance such as Poliomyelitis, common cold, flu, hepatitis, foot-and-mouth disease all are caused by picornaviruses.
These viruses have a single-stranded RNA genome of positive polarity in the size of nucleotides in human rhinoviruses to nucleotides in foot and mouth disease virus, containing a number of features conserved in all picornaviruses. The rest of the genome encodes a single polyprotein between and amino acids. Alphavirus genus has 27 members, many of which can be transmitted via insect vectors.
Rubella virus is the only known member of the rubivirus. The envelope contains 2 virus-encoded glycoproteins, E1 and E2. Translation of the non-structural proteins will be done from genomic RNA, resulting in the synthesis of a polyprotein, which is cleaved into the matured proteins.
The capsid protein is synthesized on free cytoplasmic ribosomes. It is cleaved off co-translationally, exposing a signal sequence which directs the ribosome to the ER membrane where the translation of the remaining E1 and E2 proteins gets completed.
E1 and E2 are synthesized in association with the rough ER membrane and are then processed through the Golgi apparatus before being transported to the plasma membrane. The family Flaviviridae consisting of three genera, 1 Flaviviruses yellow fever virus , 2 Pestiviruses Bovine viral diarrhea virus and 3 Hepatitis C Viruses. Many of the Flaviviruses are transmitted via insects.
The entire viral genome is translated as a single polyprotein, which is cleaved into the mature proteins. No subgenomic RNA is formed. The family Coronaviridae is comprised of 2 genera , coronaviruses, and toroviruses. Inclusion of arteriviruses into this family is recent and is not still widely accepted.
ORF 1b is translated via ribosome frame-shifting. Each of these transcripts is monocistronic and contains a single translation unit, which starts at the first AUG after the leader. The mechanism of the nested transcript synthesis is not completely clear, but it is not via RNA splicing since it occurs in the nucleus and coronaviruses replicate in the cytoplasm.
Translation of the subgenomic mRNAs yields structural proteins along with some additional non-structural proteins. These viruses have a unique crown-like structure as identified in the electron microscope.
The envelopes contain two viral glycoproteins; M protein membrane protein , which binds the viral nucleocapsid to the viral envelope during budding and S spike protein , which facilitates receptor binding and cell fusion.
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