The Process of Bacterial Conjugation and Gene Transfer

Anne Kamiya, Angela Hartsock
  • Author
    Anne Kamiya

    Anne has experience in science research and writing. She has a graduate degree in nutrition (gut microbiome & nutritional microbiology) and undergraduate degrees in microbiology (immunology & medical microbiology) and English (myth & folklore). She has also worked as an ocean & Earth science educator.

  • Instructor
    Angela Hartsock

    Angela has taught college microbiology and anatomy & physiology, has a doctoral degree in microbiology, and has worked as a post-doctoral research scholar for Pittsburgh’s National Energy Technology Laboratory.

Learn about bacterial conjugation. Identify what conjugation in biology is and its definition. Discover about the conjugation process and sexual reproduction. Updated: 05/08/2022

What Is Bacterial Conjugation?

Ever wonder how bacteria reproduce? When bacteria create more bacteria, they do so independently and without the need for other bacteria. Bacteria create more bacteria by a method of asexual propagation called binary fission, where the bacterial cell grows very large, and then splits into two bacterial cells that are identical copies of each other.

Conjugation definition biology

However, bacteria also have a way of exchanging genetic material with each other without reproducing, called conjugation. What is bacterial conjugation and what happens during the conjugation process? When bacteria make physical contact with each other and transfer a tiny piece of DNA called a plasmid, this process of genetic exchange is called conjugation. The plasmid is not related to the primary chromosomal DNA of either bacteria; it is an unconnected piece of circular DNA that functions more like a downloaded app. The plasmid transferred during conjugation always gives the receiving bacteria some kind of functional advantage. For example, one advantage for bacteria that has been linked to conjugation is antibiotic resistance.


An illustration of a bacterial cell showing its primary genome and separate, circular plasmid. Plasmids are transferred during the parasexual reproductive process called bacterial conjugation.

Drawing of a bacteria cell showing its primary genome and separate circular plasmid visible.


Horizontal Gene Transfer

When genes are exchanged between organisms without the use of reproductive cells like sperm and egg, the process is called horizontal gene transfer (HGT). Bacteria are capable of horizontally transferring genetic material via three distinct methods: (1) transformation, (2) conjugation, and (3) transduction. Bacterial conjugation was defined in the previous section, and conjugation genetics and its details will be explained in upcoming sections. Briefly, the process of transduction is facilitated by a bacteriophage (a virus that infects bacteria) that inadvertently transfers genetic material between bacteria during the infective process. Transformation happens when bacteria scavenge some external piece of DNA it finds in the environment and incorporates that DNA into its primary genome, usually picked up from the leftover pieces of another dead bacteria.

Conjugation Process

In biology, the conjugation process describes a sort of pseudo-mating process between two non-sexually reproducing organisms. When the two organisms meet up, they make physical contact, and then one organism penetrates and injects genetic material into the other organism. There is no exchange of sperm, eggs, or any kind of reproductive cells, just DNA. For this reason, conjugation could be thought of more as a process of genetic exchange or DNA conjugation. Conjugation genetics usually happens between very simple organisms, like bacteria, fungi, or protozoa. The process always involves two organisms, and for bacteria, conjugation biology proceeds according to the following steps:

  • Two bacteria meet up with the intention of undergoing bacterial conjugation. One bacteria, called the donor, has a plasmid it wants to transfer to another bacteria that do not have the plasmid, called the recipient.
  • The donor bacteria gets very close to the recipient bacteria, and extends a long, hairlike sex structure from its body, called a pilus.
  • The pilus adheres to the recipient bacteria and draws it in like a fishing line until the two bacteria touch each other and a bridge forms between the cytoplasm of the two bacterial cells.
  • While the two bacteria are connected, the donor bacteria transfers a copy of the plasmid DNA into the recipient bacteria. When the process is complete, they break free from each other.
  • Now both bacteria have a copy of the same functional plasmids, and the recipient bacteria is no longer a recipient, but a potential donor for another bacteria.

Bacterial Sexual Reproduction

Have you ever had the bacterial version of the birds and the bees talk? No? Well, it is definitely time. I am sure this won't come as a big shock to you, but bacteria don't have sex - at least in the conventional sense.

Bacterial cells reproduce by making clones of themselves. The mother cell copies its DNA chromosome, then splits her cell in half, keeping one chromosome and giving one to the new daughter cell. By convention, these cells are called mother and daughter cells, but really they are actually clones. They have the exact same genetic material. In a bacterial population, this process continues, one cell dividing into two again and again and again, resulting in huge populations of cells that are all clones of one another. It is considered vertical gene transfer when DNA is passed from mother to offspring cell, and this is what happens in nature the vast majority of the time.

But bacteria are tricky little creatures, and they have some ingenious ways of generating genetic diversity. For instance, bacteria are able to swap genes with their neighbors. This would be kind of like you deciding you want your buddy's black hair, so he does you a favor and gives you his gene for black hair and VOILA! Now you have black hair. This is called horizontal gene transfer and refers to the ability of some bacterial cells to acquire new genes from neighboring cells in their environment. Now instead of being a clone daughter cell, the cell has new genetic diversity, a mix between the vertically transferred mother cell DNA and the horizontally transferred neighbor cell DNA.

Remember that we said that bacteria don't have sex? Well, in an evolutionary sense, sex is crucial for allowing mixing and matching of genes, resulting in genetic diversity within a species. We can now see that while bacteria don't have conventional sex, they do have horizontal gene transfer mechanisms for generating genetic diversity.

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  • 2:09 Horizontal Gene Transfer
  • 2:37 Conjugation
  • 3:25 Plasmid Transfer
  • 4:09 Chromosome Transfer
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Sexual Reproduction in Bacteria

When animals reproduce, genetic recombination between the DNA of two organisms occurs when their sperm and egg sex cells, called gametes, combine. Sexual reproduction between two organisms produces genetically unique offspring and is a process called vertical gene transfer in biology. (In rare cases, vertical gene transfer can also happen within a single hermaphroditic organism). The gametes in vertical gene transfer are created during a special type of cell division process that is specific to reproductive sex cells, called meiosis. The reason why meiosis is so unique is because parent cells do not copy themselves like they would in mitosis.

Horizontal Gene Transfer

There are three ways that bacteria are able to carry out horizontal gene transfer:

  • Transduction uses bacterial viruses, called bacteriophages, to transfer DNA from one infected cell to another.
  • Transformation is the ability of some cells to take up freely floating DNA found in the environment.
  • Conjugation allows for the transfer of DNA through a structure called a pilus from one cell to another.

Conjugation

For conjugation to take place, two live bacterial cells must come into direct contact with one another. Contact between the cells is accomplished using a conjugation pilus. The term pilus (plural: pili) refers to a protein-based, hair-like structure that extends from the bacterial cell. Some pili are used to attach to surfaces, but the special conjugation pilus is used specifically to attach to other cells and facilitate DNA transfer. The cell that will transfer DNA is called the donor cell and builds the conjugation pilus attached to its cell. The conjugation pilus is a hollow, pipe-like structure that connects the cytoplasm of the donor cell to the cytoplasm of the recipient cell.

Plasmid Transfer

Once the cytoplasm of the donor and recipient cell are physically linked, it is time for the transfer of the DNA. If the donor cell contains a plasmid, a circular piece of extrachromosomal DNA, the plasmid can be transferred to the recipient cell. This is done by copying the plasmid and sending the strand of copied DNA to the recipient through the conjugation pilus. The final result is a copy of the plasmid in both the donor and the recipient cell. Perhaps most interesting is the fact that the plasmid, in addition to other genes, carries genes that allow the recipient cell to become a conjugation donor itself! Now the recipient cell can also spread the plasmid to new cells it encounters in its environment.

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Video Transcript

Bacterial Sexual Reproduction

Have you ever had the bacterial version of the birds and the bees talk? No? Well, it is definitely time. I am sure this won't come as a big shock to you, but bacteria don't have sex - at least in the conventional sense.

Bacterial cells reproduce by making clones of themselves. The mother cell copies its DNA chromosome, then splits her cell in half, keeping one chromosome and giving one to the new daughter cell. By convention, these cells are called mother and daughter cells, but really they are actually clones. They have the exact same genetic material. In a bacterial population, this process continues, one cell dividing into two again and again and again, resulting in huge populations of cells that are all clones of one another. It is considered vertical gene transfer when DNA is passed from mother to offspring cell, and this is what happens in nature the vast majority of the time.

But bacteria are tricky little creatures, and they have some ingenious ways of generating genetic diversity. For instance, bacteria are able to swap genes with their neighbors. This would be kind of like you deciding you want your buddy's black hair, so he does you a favor and gives you his gene for black hair and VOILA! Now you have black hair. This is called horizontal gene transfer and refers to the ability of some bacterial cells to acquire new genes from neighboring cells in their environment. Now instead of being a clone daughter cell, the cell has new genetic diversity, a mix between the vertically transferred mother cell DNA and the horizontally transferred neighbor cell DNA.

Remember that we said that bacteria don't have sex? Well, in an evolutionary sense, sex is crucial for allowing mixing and matching of genes, resulting in genetic diversity within a species. We can now see that while bacteria don't have conventional sex, they do have horizontal gene transfer mechanisms for generating genetic diversity.

Horizontal Gene Transfer

There are three ways that bacteria are able to carry out horizontal gene transfer:

  • Transduction uses bacterial viruses, called bacteriophages, to transfer DNA from one infected cell to another.
  • Transformation is the ability of some cells to take up freely floating DNA found in the environment.
  • Conjugation allows for the transfer of DNA through a structure called a pilus from one cell to another.

Conjugation

For conjugation to take place, two live bacterial cells must come into direct contact with one another. Contact between the cells is accomplished using a conjugation pilus. The term pilus (plural: pili) refers to a protein-based, hair-like structure that extends from the bacterial cell. Some pili are used to attach to surfaces, but the special conjugation pilus is used specifically to attach to other cells and facilitate DNA transfer. The cell that will transfer DNA is called the donor cell and builds the conjugation pilus attached to its cell. The conjugation pilus is a hollow, pipe-like structure that connects the cytoplasm of the donor cell to the cytoplasm of the recipient cell.

Plasmid Transfer

Once the cytoplasm of the donor and recipient cell are physically linked, it is time for the transfer of the DNA. If the donor cell contains a plasmid, a circular piece of extrachromosomal DNA, the plasmid can be transferred to the recipient cell. This is done by copying the plasmid and sending the strand of copied DNA to the recipient through the conjugation pilus. The final result is a copy of the plasmid in both the donor and the recipient cell. Perhaps most interesting is the fact that the plasmid, in addition to other genes, carries genes that allow the recipient cell to become a conjugation donor itself! Now the recipient cell can also spread the plasmid to new cells it encounters in its environment.

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Frequently Asked Questions

What is conjugation in bacterial reproduction?

Conjugation is the process where two bacteria meet up, and one bacteria, called a donor, injects and latches onto another bacteria (called a recipient) with a sex appendage called a pilus. The pilus draws the two bacteria together, and then the donor bacteria gives the recipient bacteria a small piece of DNA, called a plasmid.

Why is bacterial conjugation important?

Conjugation is very important for bacteria because it allows them to exchange genetic material with each other, and the genes exchanged always confer some kind of functional advantage. Antibiotic resistance is one such advantage, so studying and understanding the processes of bacterial conjugation is important for both bacteria and humans.

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