33 Questions
What is the role of reverse transcriptase in a retroviral infection of a mammalian cell?
Facilitates immediate conversion of viral RNA to double-stranded DNA
Which enzyme catalyzes the integration of the viral DNA into the host genome?
Integrase
What is the function of the tRNA carried by viral particles entering the host cell?
Aids in immediate conversion of viral RNA to DNA
Which gene in the Rous sarcoma virus encodes a non-receptor tyrosine kinase enzyme that affects cell division?
src gene
What is the function of the enzyme encoded by the src gene in the Rous sarcoma virus?
Cell division regulation
What is the role of integrase in the life cycle of a retrovirus?
Catalyzes integration of viral DNA into host genome
How do new viruses get released from a host cell during a retroviral infection?
Via cell lysis
What is the function of capsid proteins in the viral particles formed after transcription and translation of integrated viral DNA?
Enclose the viral RNA
What happens to the polyprotein derived from the translation of the gag and pol genes in retroviruses?
It is cleaved into six distinct proteins
How does reverse transcriptase enable the conversion of viral RNA to double-stranded DNA?
By utilizing a cellular tRNA base-paired to the viral RNA
In what way does the Rous sarcoma virus contribute to cancer development?
Enhancing cell division through src gene expression
Which enzyme is responsible for synthesizing the complementary strand of telomeres?
DNA polymerases
Which retrovirus contains an oncogene known as the src gene?
Rous sarcoma virus
How is the synthesis of new RNA strands by RNA replicase different from DNA-dependent RNA polymerases?
Has no proofreading capabilities
Which type of genomes do some phages and viruses like flu and Covid19 possess?
ssRNA
What is the function of RNA replicase in viral RNA replication?
Synthesis of new RNA strands from an RNA template
What type of polymerase is responsible for replicating viral RNA genomes like Covid19?
RNA-dependent RNA polymerase
How does telomerase contribute to telomere maintenance?
Counteracts shortening of telomeres during DNA replication
Which enzyme is responsible for making DNA from RNA in retroviruses?
Reverse Transcriptase
What does the long terminal repeat (LTR) sequences in retroviruses facilitate?
Integration of virus genome into host DNA
Which gene in retroviruses encodes the viral envelope?
env
What catalytic activity is lacking in Reverse Transcriptase that makes it error-prone?
3' to 5' proofreading
Which sequence is required for packaging retroviral RNAs into mature viral particles?
ψ sequence
What is the role of integrase in retroviruses?
Inserting DNA into host genome
What type of synthesis is catalyzed by Reverse Transcriptases?
RNA-dependent DNA synthesis
Which Retroviral gene encodes protease that cleaves polypeptide and other proteins?
pol
What is the role of telomerase in chromosome replication?
Adding telomeres to the ends of chromosomes
How does HIV-encoded reverse transcriptase complicate the development of a vaccine against HIV?
It introduces errors in viral RNA during replication
What is the significance of alternative splicing in HIV genome?
It produces multiple proteins from a small genome
Which type of drugs target reverse transcriptase in HIV?
Nucleotide or nucleoside analogs
Why are telomeres important in chromosome replication?
To prevent chromosome shortening in each cell generation
How does HIV affect the immune system?
Suppresses the immune system by killing T lymphocytes
Why does the error-prone nature of HIV-encoded reverse transcriptase complicate vaccine development?
Results in diverse viral RNA sequences
Study Notes
Retroviral DNA and Transcription
- Transcription of retroviral DNA produces a primary transcript containing the gag, pol, and env genes
- Splicing of the primary transcript yields an mRNA derived largely from the env gene
- Translation produces a polyprotein, a single long polypeptide derived from the gag and pol genes, which is cleaved into six distinct proteins
Retroviruses and Cancer
- Some retroviruses contain an oncogene, such as the src gene in Rous sarcoma virus
- The src gene encodes a non-receptor tyrosine kinase, which affects cell division and can contribute to unregulated cell division and cancer
RNA-Dependent Synthesis of RNA and DNA
- Retroviruses can make DNA from RNA using the enzyme reverse transcriptase (RT)
- RT catalyzes three reactions: RNA-dependent DNA synthesis, RNA degradation, and DNA-dependent DNA synthesis
- RT contains Zn2+ and uses a primer of tRNA, but lacks 3' à 5'-proofreading, making it error-prone
Structure and Gene Products of an Integrated Retroviral Genome
- Retroviruses typically contain three genes: gag, pol, and env
- The gag gene encodes a long polypeptide that is cleaved into six smaller proteins that make up the viral core
- The pol gene encodes protease that cleaves the long polypeptide, reverse transcriptase, and an integrase to insert DNA into the host genome
- The env gene encodes viral envelope proteins
- Long terminal repeat (LTR) sequences facilitate integration of the virus genome into the host DNA
HIV and AIDS
- HIV genome has genes for killing host cells, resulting in suppression of the immune system
- HIV-encoded reverse transcriptase is unusually error-prone, complicating the push for a vaccine
- The genome of HIV is small (9.7 x 103 nucleotides) but contains several small genes with various functions
Pharmaceutical Targets for HIV
- Reverse transcriptase inhibitors, such as zidovudine (AZT) and didanosine (Videx), are used to target HIV
- Protease inhibitors, such as indinavir and saquinavir, are used to target HIV proteases
Telomerase and Telomeres
- Telomerase is a specific reverse transcriptase that extends the ends of linear chromosomes
- Telomeres are structures at the ends of eukaryotic chromosomes, consisting of tandem repeats of T1-4G1-4 and A-C on the opposing strand
- Telomerase solves the problem of replicating the ends of linear chromosomes by adding telomeres to chromosome ends
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