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Questions and Answers
What is the function of helicase in DNA replication?
What is the function of helicase in DNA replication?
What is the role of RNA polymerase during transcription?
What is the role of RNA polymerase during transcription?
Which of the following accurately describes the leading and lagging strands during DNA replication?
Which of the following accurately describes the leading and lagging strands during DNA replication?
What are Okazaki fragments?
What are Okazaki fragments?
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Which proteins stabilize the unwound DNA helix during replication?
Which proteins stabilize the unwound DNA helix during replication?
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In which cellular location does protein synthesis occur in bacteria?
In which cellular location does protein synthesis occur in bacteria?
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Which stage follows initiation in protein synthesis?
Which stage follows initiation in protein synthesis?
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How do transcription and translation processes differ in prokaryotes?
How do transcription and translation processes differ in prokaryotes?
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What is the unique characteristic of plasmids compared to chromosomes in bacteria?
What is the unique characteristic of plasmids compared to chromosomes in bacteria?
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In the central dogma theory, which process involves converting DNA into RNA?
In the central dogma theory, which process involves converting DNA into RNA?
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What enzyme is responsible for DNA replication in bacterial cells?
What enzyme is responsible for DNA replication in bacterial cells?
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Which type of RNA is involved in directing protein synthesis?
Which type of RNA is involved in directing protein synthesis?
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What does reverse transcription involve?
What does reverse transcription involve?
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Which of the following statements about translation is true?
Which of the following statements about translation is true?
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What is the main role of ribosomal RNA (rRNA) in the cell?
What is the main role of ribosomal RNA (rRNA) in the cell?
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Which best describes the semi-conservative nature of DNA replication?
Which best describes the semi-conservative nature of DNA replication?
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What is the typical size of DNA fragments produced by nuclease enzymes during transformation?
What is the typical size of DNA fragments produced by nuclease enzymes during transformation?
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What role do Rec A proteins play in the transformation process?
What role do Rec A proteins play in the transformation process?
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What is required for a recipient bacterium to effectively take up DNA from the environment?
What is required for a recipient bacterium to effectively take up DNA from the environment?
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Which of the following bacteria is generally known to be competent for transformation?
Which of the following bacteria is generally known to be competent for transformation?
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What distinguishes a virulent phage from a temperate phage?
What distinguishes a virulent phage from a temperate phage?
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Why are DNA fragments sensitive to nucleases considered to be at least 5 X 105 daltons?
Why are DNA fragments sensitive to nucleases considered to be at least 5 X 105 daltons?
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What is the process called by which genetic material is transferred between bacteria through a bacteriophage?
What is the process called by which genetic material is transferred between bacteria through a bacteriophage?
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What is the defining characteristic of temperate phages?
What is the defining characteristic of temperate phages?
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What is the integrated state of phage DNA within a host bacterium called?
What is the integrated state of phage DNA within a host bacterium called?
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What advantage does lysogeny provide to the host bacterium?
What advantage does lysogeny provide to the host bacterium?
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Which process involves the integration of phage genome into the bacterial chromosome?
Which process involves the integration of phage genome into the bacterial chromosome?
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What happens during the activation of a prophage?
What happens during the activation of a prophage?
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How do defective phages differ from traditional phages during generalized transduction?
How do defective phages differ from traditional phages during generalized transduction?
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What factors can induce a prophage to enter the vegetative state?
What factors can induce a prophage to enter the vegetative state?
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In which scenario would you expect cells to be referred to as lysogenic?
In which scenario would you expect cells to be referred to as lysogenic?
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Which of the following methods can utilize bacteriophages in therapy and prevention?
Which of the following methods can utilize bacteriophages in therapy and prevention?
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What is the main feature of specialized transduction?
What is the main feature of specialized transduction?
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Which step involves the encapsidation of host DNA during generalized transduction?
Which step involves the encapsidation of host DNA during generalized transduction?
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What allows for the high frequency of gene transfer in conjugative plasmids?
What allows for the high frequency of gene transfer in conjugative plasmids?
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What type of DNA can conjugative elements primarily transfer?
What type of DNA can conjugative elements primarily transfer?
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Which of the following describes the outcome when a phage injects DNA into a recipient bacterium during specialized transduction?
Which of the following describes the outcome when a phage injects DNA into a recipient bacterium during specialized transduction?
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What is lysogenic conversion responsible for in bacteria?
What is lysogenic conversion responsible for in bacteria?
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What is NOT a characteristic of conjugative plasmids?
What is NOT a characteristic of conjugative plasmids?
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In what type of bacterial interactions is conjugation most commonly observed?
In what type of bacterial interactions is conjugation most commonly observed?
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Study Notes
Microbial Genetics
- Bacteria contain two genetic structures: chromosomes and plasmids.
- Chromosomes are double helix DNA structures twisted counterclockwise.
- Plasmids are autonomous DNA molecules found in the cytoplasm.
- Plasmids carry genes responsible for specific phenotypic traits in the host cell.
Central Dogma Theory
- Explains the flow of genetic information in living cells.
- DNA → RNA → protein represents the central dogma of molecular biology.
- The major processes involved are: replication, transcription, and translation.
DNA Replication
- DNA polymerase replicates the entire nuclear genome in a semi-conservative manner.
- The double-stranded DNA is separated into two strands, serving as templates for DNA polymerase.
- Replication ensures the duplication of genomic material for equal partitioning between daughter cells during division.
Transcription
- DNA is copied into RNA by RNA polymerase.
- Three types of RNA are produced: mRNA, rRNA, and tRNA.
Translation
- mRNA directs protein synthesis with the assistance of tRNA.
Reverse Transcription
- RNA information is copied into DNA using reverse transcriptase.
- Retroviruses utilize this process to transcribe DNA from an RNA template, creating cDNA.
- The viral DNA integrates into the host cell's nucleus, where it is transcribed and translated into proteins.
DNA Replication in Bacteria
- Bacteria possess one chromosome and many plasmids.
- Both strands of the bacterial chromosome are duplicated during replication.
- Each strand acts as a template for replication.
- DNA polymerases transport complementary nucleotides from the cytoplasm to the template strands, resulting in two strands, one parental and one new.
DNA Synthesis in Bacteria
- Replication initiation occurs at a specific site on the DNA molecule known as the origin of replication.
- Enzymes unwind the double helix, making it accessible for replication.
- Helicase unwinds the helix to form replication forks.
- SSB proteins and DNA topoisomerases stabilize the unwound helix.
- Primase synthesizes RNA primers (10 bases) to initiate the synthesis of both leading and lagging strands.
- The leading strand is synthesized continuously in the 5′ to 3′ direction by DNAP III.
- The lagging strand is synthesized discontinuously in the 5′ to 3′ direction through the formation of Okazaki fragments.
- DNAP I removes the RNA primers and replaces the gap with deoxynucleotides.
- DNA ligase seals the breaks between Okazaki fragments and around the primers, creating continuous strands.
Transcription in Bacteria
- Transcription is the synthesis of RNA by RNA polymerase.
- RNA polymerase binds to DNA at a promoter site near the gene to be transcribed.
- RNA polymerase travels along the DNA, using it as a template to duplicate.
- Transcription continues until a termination site is reached, at which point the process is complete.
Protein Synthesis in Bacteria
- Protein synthesis occurs in the cytoplasm.
- It begins with DNA duplication into mRNA (transcription).
- mRNA then migrates to the ribosome, where tRNA transfers information from mRNA to rRNA (translation).
- Protein synthesis is continuous and occurs in three stages: initiation, elongation, and termination.
- In prokaryotes, transcription and translation happen simultaneously in the cytoplasm, allowing for rapid cellular responses.
Transformation
- The transfer of genetic material from a donor bacterium to a recipient bacterium through direct uptake of DNA from the environment.
- Donor bacteria are usually dead and degraded, releasing DNA fragments.
- The DNA fragments bind to DNA binding proteins on the recipient's cell wall and are translocated into the cytoplasm.
- Rec A proteins facilitate the exchange of donor DNA with the recipient's DNA.
Factors Affecting Transformation
- DNA size and state (at least 5 x 10⁵ daltons).
- Sensitivity to nucleases.
- Competence of the recipient bacteria (e.g., Bacillus, Haemophilus, Neisseria, Streptococcus).
- Competence refers to the ability to take up DNA from the environment.
- Only DNA from closely related bacteria can be successfully transformed.
- Competence factor is a specific protein produced by competent bacteria, enabling DNA uptake.
- Induced competence can be achieved through chemical manipulation in lab conditions.
Transduction
- The transfer of genetic material from a donor to a recipient bacterium through a bacteriophage.
- A bacteriophage is a virus that infects bacteria.
- There are two types of transduction: Generalized and Specialized.
Phage Composition
- Contains nucleic acid (DNA/RNA), protein, and a tail structure.
- The genome size varies, and modified bases protect the nucleic acid from host nucleases.
- The protein coat provides protection and facilitates infection.
- The tail structure contains a contractile sheath, base plate, and tail fibers.
Types of Bacteriophages
- Virulent phage: Multiplies in the host cell, causing lysis and releasing progeny phages (e.g., T4). This is known as the lytic cycle.
- Temperate phage: Can multiply via the lytic cycle or enter a quiescent integrated state in the bacterial cell, known as the lysogenic cycle.
Lysogeny
- Integration of the phage genome into the host chromosome by region-specific recombination using an integrase.
- The integrated phage genome is called a prophage.
- The prophage can switch to the vegetative state spontaneously or upon induction by physical and chemical factors (e.g., UV light).
- The process involves excision of the phage genome, replication of phage DNA, synthesis of phage structure proteins, and finally, host cell lysis.
- Cells carrying a prophage are called lysogenic because they contain the genetic information for lysis.
- Lysogeny offers benefits for both the phage and the bacterium. It prevents immediate lysis and allows concurrent replication of the phage genome with the host cell.
Importance of Bacteriophages
- Biological research: Studying DNA replication, gene expression, gene regulation, and viral morphogenesis.
- Genetic engineering: Used as vectors for gene cloning and adjuvants in sequencing.
- Therapy and prevention: Treating gastrointestinal infections and animal husbandry (e.g., EHEC infections).
- Epidemiology: Bacterial typing and DNA typing for identifying bacterial strains (e.g., Salmonella typhi, Salmonella paratyphi B., Staphylococcus aureus, Pseudomonas aeruginosa).
Types of Transduction
- Generalized Transduction: Any gene of the donor bacteria can be transferred to the recipient bacteria.
- Specialized Transduction: Only certain donor genes can be transferred to the recipient bacteria.
Generalized Transduction
- Occurs during the replication of a lytic phage.
- The phage capsid can enclose a bacterial DNA fragment instead of phage DNA.
- This is a defective phage, unable to lyse another bacterium because it lacks the genetic information for phage genome and protein production.
- Upon infecting another bacterium, the defective phage injects the bacterial DNA fragment into the recipient, where it can be exchanged for a piece of recipient DNA if their sequences are homologous.
Specialized Transduction
- Occurs during the lysogenic life cycle of a temperate phage.
- A small piece of bacterial DNA may be exchanged for a piece of phage genome during lysogeny induction.
- This bacterial DNA replicates as part of the phage genome and is incorporated into the capsid of progeny phages.
- Upon infecting a recipient bacterium, the phage DNA carrying donor bacterial genes is injected, allowing those genes to be exchanged for a piece of recipient DNA.
- Specialized transduction can transfer specific genes, and different phages may transfer different genes.
- Lysogenic (phage) conversion can occur in nature, resulting in virulent strains of bacteria (e.g., toxin production in Cl.botulinum, C.diptheriae, STEC).
Conjugation
- The transfer of DNA from a donor to a recipient bacterium through cell-to-cell contact.
- Requires two genetic elements: conjugative plasmids and conjugative transposons.
- The conjugative elements are transferred during the process.
- They can also mobilize chromosomal genes or non-transferable plasmids.
- Conjugation is common in gram-negative rods and enterococci.
Conjugative Plasmids
- Extra-chromosomal DNA elements capable of horizontal transmission, found in many bacteria.
- While beneficial to the bacterial host, they can also impose a fitness cost.
- These plasmids are efficiently spread due to:
- High frequency of gene transfer through the "transfer replication" mechanism.
- Wide range of hosts, allowing transfer across diverse species.
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Description
This quiz covers key concepts in microbial genetics, including the structures of chromosomes and plasmids, as well as the central dogma of molecular biology. You'll learn about DNA replication, transcription, and the roles of different types of RNA. Test your understanding of how genetic information flows in living cells!
