Bacterial Genetics and Variation

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Questions and Answers

Which component is NOT typically part of the bacterial genome?

  • Bacteriophage DNA (prophage)
  • Mitochondrial DNA (correct)
  • Plasmids
  • The bacterial chromosome

The bacterial chromosome replicates conservatively, where the original DNA molecule serves entirely as the template for the new DNA, leaving the original intact.

False (B)

Describe the crucial difference between how bacterial chromosomes and plasmids replicate and are inherited during bacterial cell division.

Bacterial chromosomes replicate and are inherited vertically to ensure essential genes are passed to offspring, while plasmids replicate autonomously and are generally dispensable, carrying non-essential genes that may be lost or transferred horizontally to other bacteria.

________ are extrachromosomal DNA molecules that can replicate independently of the bacterial chromosome and often carry genes that confer antibiotic resistance or virulence factors.

<p>Plasmids</p> Signup and view all the answers

Match the following plasmid functions with their descriptions:

<p>Conjugation and sex pilus formation = Mediate the transfer of genetic material between bacterial cells. Antibiotic resistance = Encode enzymes that inactivate antibiotics or alter their targets. Virulence plasmids = Carry genes for toxins, adhesins, or other factors that enhance pathogenicity. Bacteriocin production = Enable the synthesis of bactericidal substances that kill or inhibit other bacterial strains.</p> Signup and view all the answers

What is the primary consequence of the insertion of a transposable genetic element (TGE) into a gene?

<p>Gene inactivation (C)</p> Signup and view all the answers

Bacteriophage DNA, when integrated into the bacterial chromosome, always leads to the immediate lysis of the host cell.

<p>False (B)</p> Signup and view all the answers

Explain the key differences between phenotypic and genotypic variation in bacteria concerning heritability and the underlying causes.

<p>Phenotypic variation is due to environmental conditions without changes in the genetic constitution and is non-heritable, while genotypic variation involves changes in the bacterial genetic material and is heritable, occurring through mutation or gene transfer.</p> Signup and view all the answers

Lateral gene transfer, also known as ________ gene transfer, refers to the transmission of genetic material between bacteria that are not in a direct line of descent.

<p>horizontal</p> Signup and view all the answers

Match the following descriptions to the appropriate method of horizontal gene transfer in bacteria:

<p>Conjugation = Transfer of genetic material via direct cell-to-cell contact and a conjugative plasmid. Transformation = Uptake of naked DNA from the environment by a competent bacterial cell. Transduction = Transfer of bacterial DNA mediated by bacteriophages.</p> Signup and view all the answers

What is the primary difference between vertical and horizontal gene transfer in bacteria?

<p>Vertical gene transfer occurs from parent to offspring, horizontal gene transfer occurs between unrelated organisms. (C)</p> Signup and view all the answers

Mutations are always detrimental to bacterial survival.

<p>False (B)</p> Signup and view all the answers

Distinguish between spontaneous and induced mutations, detailing their causes and implications for bacterial evolution.

<p>Spontaneous mutations occur naturally during DNA replication due to errors, whereas induced mutations result from exposure to mutagens like radiation or chemicals. Both types contribute to bacterial evolution by creating genetic diversity, enabling adaptation to new environments or challenges.</p> Signup and view all the answers

A ________ mutation results in a change in the DNA sequence that does not alter the amino acid sequence of the encoded protein.

<p>silent</p> Signup and view all the answers

Match each type of point mutation with its description:

<p>Silent mutation = Alters a base but does not change the amino acid. Missense mutation = Results in an amino acid substituition. Nonsense mutation = Substitutes a stop codon for an amino acid.</p> Signup and view all the answers

What is the direct effect of a frameshift mutation on a protein?

<p>It alters the reading frame, leading to a completely different amino acid sequence downstream of the mutation. (C)</p> Signup and view all the answers

Conjugation is the only method by which bacteria can acquire antibiotic resistance genes.

<p>False (B)</p> Signup and view all the answers

Explain the role of the F factor in bacterial conjugation and its significance in the spread of antibiotic resistance.

<p>The F factor is a plasmid that carries genes for the synthesis of the sex pilus, facilitating the transfer of genetic material during conjugation. Its role is significant in spreading antibiotic resistance because it can carry resistance genes between bacterial cells, leading to widespread resistance.</p> Signup and view all the answers

In bacterial transformation, the ability of a bacterium to take up DNA from its environment is known as ________.

<p>competence</p> Signup and view all the answers

Match the component to its function in the conjugation process:

<p>Sex pilus = Mediates initial contact and draws cells together for DNA transfer. F factor = Carries the genes necessary for conjugation and its transfer. Relaxosome/Transferosome = Facilitates the unwinding and transfer of the plasmid DNA strand.</p> Signup and view all the answers

Which of the following describes generalized transduction?

<p>Transfer of any bacterial DNA fragment by a bacteriophage. (A)</p> Signup and view all the answers

Specialized transduction involves the transfer of any random fragment of the bacterial chromosome.

<p>False (B)</p> Signup and view all the answers

Compare and contrast generalized and specialized transduction, highlighting the types of genes transferred and mechanisms involved.

<p>Generalized transduction involves the transfer of any bacterial DNA fragment, packaged randomly into a phage head during the lytic cycle. Specialized transduction only transfers specific genes adjacent to the prophage integration site, through imprecise excision during the lysogenic cycle.</p> Signup and view all the answers

In specialized transduction, the bacteriophage integrates into the host chromosome as a ________, which may later excise and carry adjacent bacterial genes.

<p>prophage</p> Signup and view all the answers

Match the following descriptions with the corresponding process:

<p>Lytic cycle = Viral reproduction involving host cell lysis to release new virions. Lysogenic cycle = Viral integration into the host chromosome, replicating with the host without lysis. Prophage = Viral DNA integrated into the host chromosome during lysogeny.</p> Signup and view all the answers

Which of the following is LEAST likely to contribute to genetic variation within a bacterial population?

<p>High fidelity DNA replication mechanisms. (C)</p> Signup and view all the answers

The presence of a nuclear membrane in bacteria facilitates genetic recombination through meiosis.

<p>False (B)</p> Signup and view all the answers

Describe how bacterial conjugation can lead to the rapid dissemination of antibiotic resistance genes within a bacterial population, including the roles of plasmids and selective pressure.

<p>Conjugation facilitates direct transfer of plasmids carrying antibiotic resistance genes between bacteria. Under antibiotic selective pressure, bacteria without resistance die, while those with resistance survive and continue to transfer the resistance genes via conjugation, leading to rapid dissemination of resistance.</p> Signup and view all the answers

________ is a process by which cell-to-cell contact is needed for the transfer of plasmids.

<p>Conjugation</p> Signup and view all the answers

Match the following genetic elements with their characteristic properties:

<p>Bacterial chromosome = Essential genes, vertically inherited. Plasmid = Extrachromosomal, may carry antibiotic resistance genes, horizontally transferred. Transposable element = Mobile DNA sequence that can insert into different locations.</p> Signup and view all the answers

What is the role of calcium chloride ($CaCl_2$) in artificial transformation?

<p>It permeabilizes the cell membrane, increasing DNA uptake. (D)</p> Signup and view all the answers

Transformation always results in the stable integration of the incoming DNA into the recipient chromosome.

<p>False (B)</p> Signup and view all the answers

Describe how specialized transduction contributes to bacterial virulence evolution by focusing on toxin-encoding genes.

<p>Specialized transduction transfers specific genetic material, including toxin-encoding genes, adjacent to the prophage integration site. This promotes bacterial virulence as non-virulent bacteria can acquire toxin-producing genes through transduction, leading to a more harmful phenotype within the population.</p> Signup and view all the answers

A mutation that changes a codon for an amino acid to a stop codon is called a ________ mutation.

<p>nonsense</p> Signup and view all the answers

Match each genetic term with its definition:

<p>Gene = A segment of DNA that carries the genetic information for a specific biochemical or physiological property. Genome = The total set of genes present inside the bacterial cell. Mutation = A sudden change in the nucleotide sequence of DNA.</p> Signup and view all the answers

A bacterial cell exhibits increased resistance to multiple antibiotics after receiving a plasmid from another bacterium. Which function is most likely encoded by this plasmid?

<p>Synthesis of efflux pumps that expel antibiotics. (C)</p> Signup and view all the answers

Transposable genetic elements (TGEs) always excise cleanly from the DNA, restoring the original gene sequence.

<p>False (B)</p> Signup and view all the answers

Explain how the interplay between mutation, horizontal gene transfer, and natural selection can lead to the rapid evolution of multidrug-resistant bacteria in a hospital environment.

<p>Mutations can generate new resistance genes, while horizontal gene transfer allows bacteria to spread these genes rapidly among themselves. Exposure to different antibiotics in hospitals creates a selective pressure, where only the most resistant bacteria survive and proliferate, resulting in multidrug-resistant strains.</p> Signup and view all the answers

The enzyme ________, encoded by some plasmids, can break down beta-lactam antibiotics, rendering them ineffective.

<p>beta-lactamase</p> Signup and view all the answers

Match each description with the correct gene transfer mechanism.

<p>Conjugation = The transfer of genetic material from donor into recipient cell involving direct contact. Generalized Transduction = The transfer of any bacterial genetic material from donor into recipient cell using a bacteriophage. Specialized Transduction = The transfer of a specific piece of bacterial genetic material from donor into recipient cell using a bacteriophage. Transformation = The uptake and incorporation of naked DNA into a recipient cell.</p> Signup and view all the answers

Flashcards

What is genetics?

The science that defines and analyzes heredity.

What is a gene?

A segment of DNA that carries genetic information for a specific biochemical or physiologic property.

What is the bacterial genome?

The total set of genes present inside a bacterial cell, including the bacterial chromosome, plasmids, transposable genetic elements and bacteriophage DNA.

Bacterial DNA Location

The bacterial cell lacks a nuclear membrane; instead, DNA is concentrated in the cytoplasm as a nucleoid.

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What is bacterial chromosome structure?

A circular, supercoiled, double-stranded DNA structure found in bacteria.

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What is semi-conservative replication?

Strands separate, each acts as a template to form a complementary strand.

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What is a plasmid?

Extra-chromosomal, double-stranded, circular DNA molecule, smaller than the chromosome.

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What is plasmid copy number?

Multiple copies of the same plasmid that may exist in the same cell.

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Are plasmids generally dispensable?

Carry non-essential genes that encode properties not essential for growth, replication, or survival of bacteria.

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Plasmid Function: Conjugation

Some plasmids carry fertility (F) factors that code for the formation of a sex pilus which mediates the process of conjugation.

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Plasmid Function: Antibiotic resistance

Some plasmids carry genes for resistance (R-factors) to one or several antibiotics.

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Plasmid Function: Virulence

Some plasmids may code for exotoxins, adhesins, or invasion factors.

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Plasmid Function: Bacteriocin production

Bacteriocins are bactericidal substances produced by certain bacterial strains and are active against other strains.

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Plasmid Function: Cloning vector

A plasmid that may act as a cloning vector in recombinant DNA technology.

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Transposable Genetic Elements (TGE)

Extra-chromosomal, single-stranded, linear DNA, smaller than the chromosome. Can move locations.

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Effect of TGE insertion

The insertion of a transposable genetic element into a gene usually leads to its inactivation.

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Bacteriophage DNA

The DNA of a temperate (lysogenic) bacteriophage that is inserted in the chromosome of a lysogenic bacterial cell (i.e., prophage) is considered part of the bacterial genome.

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What is bacterial variation?

Changes in bacterial characters due to phenotypic or genotypic causes.

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What is phenotypic variation?

Due to changes in the environmental conditions without change in the genetic constitution; it is reversible and not heritable.

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What is genotypic variation?

Due to a change in the underlying genetic constitution; it is irreversible and heritable.

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What is Horizontal (Lateral) Gene Transfer?

When there is gene exchange that can be intracellular or intercellular.

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What is vertical gene transfer?

When there is change in genes from parental generation to offspring during bacterial reproduction.

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What is conjugation?

When there is exchange of genetic material via a sex pilus.

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What is bacterial transformation?

The uptake of naked DNA molecules and/or plasmid and their stable maintenance in bacteria.

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What is competence?

The ability of the recipient bacterial cell to take up DNA.

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What is transduction?

The transfer of DNA from one bacterial cell to another by means of a bacteriophage.

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What is generalized transduction?

During the lytic phage cycle, the bacterial DNA is fragmented & any fragment of DNA may be incorporated into the phage head instead of the phage DNA.

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What is specialized transduction?

When a prophage contained in a lysogenized bacterial cell is induced to detach, it may carry with it the adjacent piece of the chromosomal DNA & transfer it to another bacterial cell.

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What is a mutation?

A sudden change in the nucleotide sequence of DNA.

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What is spontaneous mutation?

Occurs spontaneously during replication of DNA.

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What is induced mutation?

Caused by radiation or chemical agents.

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Study Notes

  • Lecture 7 covers bacterial genetics and bacterial variation.

Important Definitions

  • Genetics is the science defining and analyzing heredity.
  • A gene is a DNA segment that carries genetic information for a specific biochemical or physiological property.
  • The bacterial genome is the total set of genes inside a bacterial cell.
    • It includes the bacterial chromosome, plasmids, transposable genetic elements, and bacteriophage DNA (prophage).

Bacterial Chromosome

  • Bacterial cells lack a nuclear membrane; DNA (A, T, C, G) concentrates in the cytoplasm as a nucleoid, a single chromosome.
  • The bacterial chromosome is circular, supercoiled, and double-stranded.
  • Replication happens through a semi-conservative method where the two strands separate and each acts as a template for forming a complementary strand.

Plasmids

  • Plasmids are extra-chromosomal, double-stranded, circular DNA molecules
  • Plasmids are smaller than the chromosome and replicate autonomously (independently of the bacterial chromosome)
  • Plasmids can transfer from one bacterium to another via conjugation.
  • Multiple copies of the same plasmid can exist in a single cell.
  • Plasmids are generally dispensable, carrying non-essential genes for growth, replication, or survival.

Functions Encoded by Plasmids

  • Plasmids mediate conjugation and sex pilus formation with fertility (F) factors.
  • They confer antibiotic resistance (R-factors) to one or more antibiotics, such as β-lactamase for penicillin and cephalosporin resistance.
  • Virulence plasmids code for exotoxins, adhesins, or invasion factors.
  • Bacteriocins, bactericidal substances, can be produced to act against other strains.
  • Plasmids can serve as cloning vectors in recombinant DNA technology.

Transposable Genetic Elements (TGE)

  • TGEs are extra-chromosomal, single-stranded, linear DNA smaller than the chromosome.
  • TGEs cannot replicate autonomously
  • TGEs transfer from one location to another on the chromosome or between chromosome and plasmids.
  • Insertion of a TGE into a gene usually inactivates it.

Bacteriophage DNA

  • The DNA of a temperate (lysogenic) bacteriophage inserted into a lysogenic bacterial cell's chromosome (prophage) is part of the bacterial genome.

Bacterial Variation

  • Variations are changes in bacterial characters due to phenotypic or genotypic causes.

Phenotypic Variation

  • Phenotypic variation's due to environmental conditions without a change in the genetic constitution.
  • Phenotypic variation is reversible (transient) and not heritable.
  • L forms of bacteria and loss of flagella upon exposure to phenol exemplify phenotypic variation.

Genotypic Variation

  • Genotypic variation's due to a change in the underlying genetic constitution.
  • Genotypic variation is irreversible (permanent) and heritable.
  • Genotypic variation occurs through mutation and gene transfer: transformation, transduction, and conjugation.

Gene Transfer

  • Horizontal (lateral) gene transfer occurs when there is gene exchange.
    • Intracellular happens via transposition by transposons.
    • Inter-cellular happens from one cell to another via:
      • Conjugation (mediated by plasmids).
      • Transformation.
      • Transduction (mediated by bacteriophages).
  • Vertical gene transfer involves a change in genes from parent to offspring.
    • It occurs during bacterial reproduction (i.e., mutation or error.)

Mutation

  • Mutation Definition: a sudden change in the nucleotide sequence of DNA.
  • Spontaneous mutation occurs spontaneously during DNA replication.
  • Induced mutation results from radiation or chemical agents.

Mechanisms of Gene Exchange Among Bacteria

  • Mechanisms of genetic exchange among bacteria involve:
    • Conjugation
    • Transformation
    • Transduction

Conjugation

  • Conjugation is the most frequently observed mechanism of DNA transfer.
  • It involves two cell types: donors (F+) with a fertility (F) factor and recipients (F-) lacking the F factor.
  • The F factor carries genes for the synthesis of the sex pilus (conjugation tube).
  • DNA strands of the F factor separate, and one strand transfers to the recipient cell.
  • Each strand forms a complementary strand, and the recipient cell becomes an F+ cell.

Transformation

  • Transformation Definition: the uptake of naked DNA molecules and/or plasmid and their stable maintenance in bacteria.
  • Transformation depends on competence, the recipient bacterial cell's ability to take up DNA.
  • Naked DNA comes into the environment by lysis of bacterial cells
  • The artificial process makes cells competent and uptakes naked DNA with calcium chloride.

Transduction

  • Transduction is the transfer of DNA from one bacterial cell to another via a bacteriophage.
  • Generalized transduction: During the lytic phage cycle, bacterial DNA fragments may incorporate via the phage head instead of phage DNA.
    • The phage particle then transfers the incorporated bacterial DNA into another bacterial host like generalized transduction.
  • Specialized transduction:
    • When a prophage is in a lysogenized bacterial cell it can detach, carrying an adjacent piece of the chromosomal DNA and transfer it to another bacterial cell.

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