Biochem Lecture 11

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Questions and Answers

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 (correct)

Causes cell lysis to release new viruses

Transcribes the integrated viral DNA

Directly integrates the viral DNA into the host genome

Which enzyme catalyzes the integration of the viral DNA into the host genome?

Reverse transcriptase

Polymerase

Telomerase

Integrase (correct)

What is the function of the tRNA carried by viral particles entering the host cell?

Aids in immediate conversion of viral RNA to DNA (correct)

Integrates viral DNA into host genome

Facilitates translation of integrated viral DNA

Forms capsid proteins for new viruses

Which gene in the Rous sarcoma virus encodes a non-receptor tyrosine kinase enzyme that affects cell division?

src gene Signup and view all the answers

What is the function of the enzyme encoded by the src gene in the Rous sarcoma virus?

Cell division regulation Signup and view all the answers

What is the role of integrase in the life cycle of a retrovirus?

Catalyzes integration of viral DNA into host genome Signup and view all the answers

How do new viruses get released from a host cell during a retroviral infection?

Via cell lysis Signup and view all the answers

What is the function of capsid proteins in the viral particles formed after transcription and translation of integrated viral DNA?

Enclose the viral RNA Signup and view all the answers

What happens to the polyprotein derived from the translation of the gag and pol genes in retroviruses?

It is cleaved into six distinct proteins Signup and view all the answers

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 Signup and view all the answers

In what way does the Rous sarcoma virus contribute to cancer development?

Enhancing cell division through src gene expression Signup and view all the answers

Which enzyme is responsible for synthesizing the complementary strand of telomeres?

DNA polymerases Signup and view all the answers

Which retrovirus contains an oncogene known as the src gene?

Rous sarcoma virus Signup and view all the answers

How is the synthesis of new RNA strands by RNA replicase different from DNA-dependent RNA polymerases?

Has no proofreading capabilities Signup and view all the answers

Which type of genomes do some phages and viruses like flu and Covid19 possess?

ssRNA Signup and view all the answers

What is the function of RNA replicase in viral RNA replication?

Synthesis of new RNA strands from an RNA template Signup and view all the answers

What type of polymerase is responsible for replicating viral RNA genomes like Covid19?

RNA-dependent RNA polymerase Signup and view all the answers

How does telomerase contribute to telomere maintenance?

Counteracts shortening of telomeres during DNA replication Signup and view all the answers

Which enzyme is responsible for making DNA from RNA in retroviruses?

Reverse Transcriptase Signup and view all the answers

What does the long terminal repeat (LTR) sequences in retroviruses facilitate?

Integration of virus genome into host DNA Signup and view all the answers

Which gene in retroviruses encodes the viral envelope?

env Signup and view all the answers

What catalytic activity is lacking in Reverse Transcriptase that makes it error-prone?

3' to 5' proofreading Signup and view all the answers

Which sequence is required for packaging retroviral RNAs into mature viral particles?

ψ sequence Signup and view all the answers

What is the role of integrase in retroviruses?

Inserting DNA into host genome Signup and view all the answers

What type of synthesis is catalyzed by Reverse Transcriptases?

RNA-dependent DNA synthesis Signup and view all the answers

Which Retroviral gene encodes protease that cleaves polypeptide and other proteins?

pol Signup and view all the answers

What is the role of telomerase in chromosome replication?

Adding telomeres to the ends of chromosomes Signup and view all the answers

How does HIV-encoded reverse transcriptase complicate the development of a vaccine against HIV?

It introduces errors in viral RNA during replication Signup and view all the answers

What is the significance of alternative splicing in HIV genome?

It produces multiple proteins from a small genome Signup and view all the answers

Which type of drugs target reverse transcriptase in HIV?

Nucleotide or nucleoside analogs Signup and view all the answers

Why are telomeres important in chromosome replication?

To prevent chromosome shortening in each cell generation Signup and view all the answers

How does HIV affect the immune system?

Suppresses the immune system by killing T lymphocytes Signup and view all the answers

Why does the error-prone nature of HIV-encoded reverse transcriptase complicate vaccine development?

Results in diverse viral RNA sequences Signup and view all the answers

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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