CONJUGATION, TRANSFORMATION, TRANSDUCTION (HORIZONTAL GENE TRANSFER)

Summary

This script explores horizontal gene transfer in bacteria, a process that allows for rapid genetic diversity. It details three mechanisms: conjugation, where bacteria exchange DNA directly; transformation, the uptake of extracellular DNA; and transduction, involving bacteriophages transferring DNA. These methods enable bacteria to adapt quickly to new environments and pressures, such as acquiring antibiotic resistance. Despite continuous gene acquisition, genome size remains stable as non-beneficial genes are often lost.

Takeaways

• 🔬 Bacteria can exchange genes horizontally between cells of the same generation, a process known as horizontal gene transfer, which is distinct from vertical gene transfer where genes are passed from parents to offspring.
• 🔄 The three primary methods of horizontal gene transfer in bacteria are conjugation, transformation, and transduction, with conjugation being the most prevalent.
• 🔗 Conjugation involves direct cell-to-cell contact and is facilitated by conjugative plasmids or transposons, which are self-transmissible genetic elements.
• 🧬 Plasmids are circular DNA molecules that replicate independently of the bacterial chromosome, while transposons, or 'jumping genes', are mobile DNA segments capable of moving within or between genomes.
• 🤝 Conjugation requires the formation of a 'mating pair' where one bacterium transfers a single-stranded DNA to another, which is then replicated to restore double-stranded DNA in both bacteria.
• 🌐 Transformation is the process where bacteria take up extracellular DNA from their environment and incorporate it into their own genomes, facilitated by 'competent' bacteria with increased DNA uptake capabilities.
• 🧵 Recombination rearranges the donor and recipient genomes to form new hybrid genomes, potentially leading to new phenotypes such as pathogenicity or antibiotic resistance.
• 🦠 Transduction involves the transfer of bacterial DNA via bacteriophages, which are viruses that infect bacteria and can exist in a lysogenic state or undergo a lytic cycle.
• 🌀 Generalized transduction occurs when a phage accidentally packages bacterial DNA instead of its own, while specialized transduction involves the incorrect excision of bacterial DNA during the viral life cycle.
• 🌿 Horizontal gene transfer, along with mutations, contributes to genetic diversity in bacteria, allowing them to adapt rapidly to new environments and selective pressures.

Q & A

What is horizontal gene transfer in bacteria?

Horizontal gene transfer is a process where genes are exchanged between cells of the same generation, as opposed to vertical gene transfer where genes are passed from parents to offspring.

What are the three methods of horizontal gene transfer in bacteria?

The three methods of horizontal gene transfer in bacteria are conjugation, transformation, and transduction.

Why is conjugation the most common method of horizontal gene transfer?

Conjugation is the most common method of horizontal gene transfer because it allows for direct cell-cell contact and efficient transfer of genetic material, often facilitated by conjugative plasmids or transposons.

What is a conjugative plasmid and how does it facilitate conjugation?

A conjugative plasmid is a self-transmissible, circular DNA section that replicates independently of chromosomes and contains all the genes needed to connect with another bacterium and transmit itself via conjugation.

How does the formation of a conjugation pilus aid in the process of conjugation?

The conjugation pilus, also known as an F pilus or sex pilus, aids in conjugation by binding to another bacterium and retracting to pull the two cells together, forming a 'mating pair' that allows for DNA transfer.

What is the role of the enzyme transposase in horizontal gene transfer?

The enzyme transposase catalyzes the cutting and resealing of DNA during transposition, which is a part of the horizontal gene transfer process involving nonconjugative transposons and plasmids.

How does transformation differ from conjugation and transduction?

Transformation involves bacteria taking up extracellular DNA and incorporating it into their genomes, which is different from conjugation that requires cell-cell contact and transduction that involves bacteriophage-mediated DNA transfer.

What is the significance of bacterial competency in the process of transformation?

Bacterial competency is significant in transformation as it refers to the state of increased cell wall and cell membrane permeability that allows cells to uptake DNA from their environment.

How does transduction contribute to horizontal gene transfer?

Transduction contributes to horizontal gene transfer by transferring DNA from one cell to another via a bacteriophage, which can switch between lysogenic and lytic cycles to facilitate the transfer.

What are the two types of transduction and how do they differ?

The two types of transduction are generalized and specialized. Generalized transduction occurs when a phage accidentally packages bacterial DNA instead of its own, while specialized transduction involves the incorrect excision of bacterial DNA during the lysogenic cycle of the virus.

How does horizontal gene transfer contribute to bacterial adaptation and genetic diversity?

Horizontal gene transfer contributes to bacterial adaptation and genetic diversity by allowing bacteria to rapidly acquire beneficial traits such as antibiotic resistance or pathogenicity from other bacteria, thus responding to selective pressures and adapting to new environments.

Why doesn't the genome size of bacteria increase indefinitely despite continuous horizontal gene transfer?

The genome size of bacteria does not increase indefinitely because if the transferred genes do not provide a selective advantage, they are usually lost by deletion, thus maintaining a relatively constant genome size over time.

Outlines

00:00

🧬 Horizontal Gene Transfer in Bacteria

This paragraph explains the concept of horizontal gene transfer in bacteria, contrasting it with vertical gene transfer. It details three methods of horizontal gene transfer: conjugation, transformation, and transduction, with a focus on conjugation being the most common. The paragraph describes how plasmids and transposons facilitate conjugation, the process of direct cell-cell contact, and the formation of a 'mating pair'. It also discusses the role of enzymes like nucleases and transposase in the process. Additionally, it touches upon transformation, where bacteria incorporate extracellular DNA, and transduction, where bacteriophages transfer DNA between bacteria. The paragraph concludes by noting that horizontal gene transfer, along with mutations, contributes to genetic diversity in bacteria.

05:02

🌿 Adaptation and Genome Size in Bacteria

The second paragraph delves into how horizontal gene transfer allows bacteria to rapidly adapt to new environments by acquiring beneficial DNA from other bacteria. It contrasts this with the slower process of mutation, which often results in neutral or harmful changes. The paragraph addresses the concern of genome size increasing due to gene transfer by explaining that non-beneficial genes are usually lost, thus maintaining a stable genome size over time. This process ensures that bacteria can efficiently respond to selective pressures without an unwarranted increase in genome size.

Keywords

💡Horizontal Gene Transfer

Horizontal gene transfer (HGT) refers to the process by which genetic material is transferred between organisms other than parent to offspring. In the context of the video, HGT is a key mechanism that allows bacteria to exchange genes between cells of the same generation, which is crucial for their rapid adaptation and evolution. The video mentions three primary methods of HGT in bacteria: conjugation, transformation, and transduction.

💡Conjugation

Conjugation is a type of horizontal gene transfer where genes are transferred directly between two bacterial cells through direct cell-cell contact. The video explains that conjugation is often facilitated by conjugative plasmids or transposons, which are self-transmissible genetic elements. An example from the script is the use of a conjugation pilus, which helps in connecting two bacteria and forming a 'mating pair', allowing the transfer of genetic material.

💡Plasmids

Plasmids are circular DNA molecules that can replicate independently of the bacterial chromosome. They play a significant role in HGT as they often carry genes that facilitate conjugation. The video mentions that conjugation with plasmids is more common than with transposons, highlighting their importance in bacterial gene exchange.

💡Transposons

Transposons, also known as jumping genes, are mobile genetic elements that can move within or between genomes. They are involved in conjugation, as mentioned in the video, and can carry genes that provide a selective advantage to bacteria, such as antibiotic resistance.

💡Transformation

Transformation is another method of horizontal gene transfer where bacteria take up extracellular DNA from their environment and incorporate it into their genomes. The video describes this process as occurring when one bacterium lyses, releasing its DNA, which can then be taken up by another bacterium. This process can lead to the acquisition of new traits, such as pathogenicity or antibiotic resistance.

💡Competent Bacteria

Competent bacteria are those that have increased cell wall and cell membrane permeability, allowing them to take up DNA from their surroundings. This state is crucial for transformation, as it enables bacteria to bind and incorporate extracellular DNA into their genomes, as explained in the video.

💡Transduction

Transduction is a process where DNA is transferred between bacteria via a bacteriophage, a virus that infects bacteria. The video outlines two types of transduction: generalized and specialized. Generalized transduction occurs when a phage accidentally packages bacterial DNA instead of its own, while specialized transduction involves the incorrect excision of bacterial DNA during the viral life cycle.

💡Lysogeny

Lysogeny is a state in which a bacteriophage integrates its genome into that of a bacterial host and replicates along with the host. The video explains that during lysogeny, the viral genome remains dormant within the bacterial chromosome, only to be activated later, leading to the lytic cycle where the virus replicates and eventually lyses the host cell.

💡Genetic Diversity

Genetic diversity refers to the variety of genetic material within a species or population. The video emphasizes that horizontal gene transfer, along with mutations, contributes to genetic diversity in bacteria, allowing them to adapt to new environments and respond to selective pressures.

💡Selective Pressure

Selective pressure is a force in nature that favors the survival and reproduction of organisms with certain heritable traits. In the context of the video, selective pressures drive the evolution of bacteria by favoring the retention of genes that provide a survival advantage, such as those acquired through horizontal gene transfer.

💡Genome Size

Genome size refers to the total amount of genetic material within an organism. The video clarifies that while horizontal gene transfer might initially increase the size of bacterial genomes, genes that do not provide a selective advantage are often lost, maintaining a relatively constant genome size over time.

Highlights

Bacteria engage in horizontal gene transfer, exchanging genes between cells of the same generation.

Vertical gene transfer occurs when parents pass genes to their offspring.

Three methods of horizontal gene transfer in bacteria: conjugation, transformation, and transduction.

Conjugation is the most common method of horizontal gene transfer.

Transformation and transduction typically occur between closely-related bacterial species.

Conjugation is encoded by plasmids or transposons.

Plasmids are circular DNA sections that replicate independently of chromosomes.

Transposons are mobile DNA sections that can move within or between genomes.

Conjugation with plasmids is more common than with transposons.

Conjugation requires direct cell-cell contact for gene transfer.

Conjugative plasmids or transposons are self-transmissible, facilitating direct gene transfer.

Gram-negative bacteria use conjugation pili for cell surface association.

Once a bridge is formed, bacteria form a 'mating pair' for DNA transfer.

A nuclease breaks plasmid DNA, allowing one strand to enter the recipient bacterium.

Transformation involves bacteria taking up extracellular DNA and incorporating it into their genomes.

Competent bacteria are capable of binding and incorporating extracellular DNA.

Recombination rearranges donor and recipient genomes into new hybrid genomes.

Transduction is DNA transfer via bacteriophages, which are viruses that infect bacteria.

Bacteriophages can switch between lysogeny and the lytic cycle.

Generalized transduction occurs when phage capsids accidentally package bacterial DNA.

Specialized transduction involves incorrect excision of DNA, packaging bacterial DNA into phage capsids.

Horizontal gene transfer and mutations contribute to genetic diversity in bacteria.

Bacteria can adapt to new environments through horizontal gene transfer and mutations.

If transferred genes don't provide a selective advantage, they are usually lost, maintaining genome size.

Transcripts

Bacteria engage in horizontal, or lateral, gene transfer, meaning that genes are exchanged

between cells of the same generation. In contrast, vertical gene transfer occurs when parents

pass genes to their progeny. The three methods of horizontal gene transfer that bacteria

employ are conjugation, transformation, and transduction, with conjugation being the most

common. Transformation and transduction typically take place between bacteria of the same or

closely-related species. Conjugation is encoded by plasmids or transposons. Plasmids are circular

DNA sections that replicate independently of chromosomes. Transposons, also known as

transposable elements, or jumping genes, are mobile sections of DNA that can move within

or between genomes. Conjugation with plasmids is more common than conjugation with transposons.

Conjugation requires direct cell-cell contact. A conjugative plasmid or conjugative transposon

is self-transmissible – in other words, it has all the genes need to connect with

another bacterium and transmit itself to another bacterium via conjugation. We still don’t

know how the majority of gram-positive bacteria achieve the intimate association of cell surfaces

required, however, in gram-negative bacteria, this typically involves a conjugation pilus,

also known as an F pilus, or a sex pilus. The conjugation pilus binds the other bacterium,

then retracts to pull the two cells together. Once a bridge is formed with an opening between

the two bacteria, they are now called a “mating pair”. A nuclease breaks one strand of the

plasmid DNA at the oriT, or origin of transfer site. The nicked strand enters the other bacterium

while the other strand remains behind in the donor. They can now both produce a complementary

copy of the plasmid so the DNA is double stranded again. Now each bacterium has a copy of the

plasmid and both can make a conjugation pilus! Note that in a population of bacteria in which

some have a conjugative plasmid and others do not, eventually all cells will acquire

a conjugative plasmid. The enzyme transposase catalyzes cutting and resealing of DNA during

transposition. Note that once mating pairs are formed by conjugative plasmids or transposons,

this also allows nonconjugative transposons and plasmids to be transferred to another

bacterium. Transformation occurs when bacteria take up extracellular DNA and incorporate

it into their genomes. Typically, this occurs when one bacteria lyses, or splits open, releasing

its genetic contents, and then another bacteria comes by and acquires it. Bacteria able to

bind large amounts of DNA are termed “competent”. Competency is a state of increased cell wall

and cell membrane permeability that allows cells to uptake DNA. Many bacteria are naturally

competent, and so actively bind environmental DNA. After transport into their cytoplasm,

the bacterium can incorporate the new DNA into its genome through the process of “recombination”.

Recombination is the rearrangement of donor and recipient genomes into new, hybrid genomes.

This can result in new phenotypes – for example, the bacteria can acquire pathogenicity

or antibiotic resistance. Some competent bacteria actually kill noncompetent bacteria to release

DNA for transformation! Transduction occurs when DNA is transferred from one cell to another

by a bacteriophage – a type of virus that infects bacteria. Viruses cannot replicate

on their own – they are obligate parasites that rely on host machinery. Many bacteriophages

can switch between a state of lysogeny and a lytic cycle. When the bacteriophage is in

a state of lysogeny, the virus combines its genome with the bacterial chromosome. The

viral genome hangs out there for many generations as the bacterium replicates. When induction

occurs, the virus switches to the lytic cycle. The cell becomes a virus-producing factory

until it gets so full of virus that it lyses, or bursts open, releasing virus particles

into its surroundings. There are two kinds of transduction – generalized and specialized.

During generalized transduction, the phage capsid accidentally assembles around a fragment

of bacterial DNA or a plasmid. When the assembled viral particle infects a new bacterium, it

injects its previous host’s DNA into its new host. Specialized transduction occurs

during the lysogenic life cycle of the virus. Incorrect excision of DNA during induction

results in a fragment of bacterial DNA being picked up instead of a part of the viral genome,

which then stays in the bacterial nucleoid. Now, the bacterial DNA replicates as part

of the bacteriophage genome, is packaged into phage capsids, and is injected into new bacteria.

Horizontal gene transfer, along with mutations, allows bacteria to achieve genetic diversity.

These processes allow bacteria to adapt to new environments because they enable them

to respond to selective pressures. Mutation is a slow process, and most mutations are

harmful or neutral, not beneficial to the bacterium. Meanwhile, horizontal gene transfer

is a rapid way to acquire large chunks of DNA from another bacterium all at once. So

if bacteria keep acquiring new snippets of DNA from other bacteria, do their genomes

keep getting larger and larger? No! If the transferred genes don’t provide a selective

advantage, they are usually lost by deletion, so the genome size remains constant over time.