6.6.2 Reproduction in Bacteria

Introduction

Bacteria reproduce by two main methods: asexual reproduction and sexual reproduction (genetic recombination). The primary method of asexual reproduction in bacteria is binary fission. Sexual reproduction in bacteria involves the transfer of genetic material between bacteria, leading to genetic recombination.

Definition

Reproduction in Bacteria: The two types of reproduction in bacteria are asexual reproduction and sexual reproduction.

In simple words: Bacteria can reproduce asexually (binary fission) or sexually (genetic recombination).

Core Concept

• The two types of reproduction in bacteria are:
• Asexual reproduction primarily occurs by binary fission.
• Sexual reproduction in bacteria is genetic recombination, i.e., DNA of two bacteria combine to give rise to a new type of bacteria called recombinant.
• The genetic recombination in bacteria occurs by:

Detailed Explanation

Asexual Reproduction: Binary Fission

Definition: The primary method of asexual reproduction in bacteria is known as binary fission.

Key Steps:

• Binary fission is the primary method of asexual reproduction in bacteria.
• It produces two genetically identical daughter cells.
• It is a rapid process.
• Examples: Escherichia coli, Staphylococcus aureus.

• The doubling generation time of bacteria ranges from as little as 20 minutes for Escherichia coli to more than 24 hours for Mycobacterium tuberculosis.
• The doubling time varies not only with the species but also with the amount of nutrients, the temperature, the pH, and the environmental factors.

Sexual Reproduction: Genetic Recombination

Definition: The sexual reproduction in bacteria is genetic recombination, i.e., DNA of two bacteria combine to give rise to a new type of bacteria called recombinant.

• Conjugation
• Transduction
• Transformation

1. Conjugation

Definition: Conjugation is the transfer of genetic material from one bacterium (donor) to another bacterium (recipient) through a bridge-like connection.

• For a long time, people believed that bacteria simply split in two to make more bacteria, and all bacteria were the same.
• But in 1946, Joshua Lederberg and Edward Tatum showed that bacteria can change their genes, kind of a way similar to that of sexual reproduction seen in more complex organisms.
• They discovered that bacteria could share their DNA with each other, passing pieces of it through a bridge-like connection.
• They were awarded Nobel Prize for this and other research works.

Experiment of Lederberg and Tatum:

• They took wild type of Escherichia coli (E. coli) bacteria that were able to synthesize all substances necessary for growth and reproduction when grown on a minimal medium containing inorganic salts and glucose.
• They then irradiated wild type E. coli with X-rays to cause mutations.
• They selected two types of mutants from these:
• Ones that could not produce substance "A" for growth.
• The others that require substance "B" in the minimal medium for growth.
• Both these mutant types were grown in nutrition medium containing both the substances "A" and "B".
• After a certain period of growth, three types of bacteria were obtained from the growth culture:
• (a) One like parent that could not produce substance "A"
• (b) Other like the parent that could not produce substance "B"
• (c) Third group was like the wild type, as it could grow in the minimal medium without requiring substances "A" and "B".
• From this experiment, it was concluded that the offspring of the two mutant types were wild type, and it is only possible when actual genetic recombination takes place.

Direct Proof of Genetic Recombination:

• With the invention of the electron microscope, the direct proof of genetic recombination was obtained.
• Mixture of the two mutants was observed using the electron microscope.
• The mutants are easily distinguished by their structure.
• The bacterium that will give the DNA is called donor.
• The bacterium that will receive the DNA is called recipient.
• Bacterial conjugation involves a plasmid that can be transferred during the conjugation process from donor to recipient.
• In some instances, however, a part of bacterial chromosomal genes is also transferred.

• Conjugation requires direct contact between donor and recipient.
• The donor cell has a plasmid (F factor) that encodes the proteins needed for conjugation.
• The plasmid is transferred from donor to recipient.
• Sometimes, chromosomal genes are also transferred.

2. Transduction

Definition: Transduction is a genetic transfer process in bacteria facilitated by bacteriophages, which are viruses that infect bacterial cells.

• During infection, phages inject their genetic material into bacteria.
• The process of transduction is explained in Fig. 6.11.

Steps of Transduction:

• Transduction is mediated by bacteriophages.
• It can transfer bacterial genes from one bacterium to another.
• It is a form of horizontal gene transfer.

3. Transformation

Definition: When bacteria die or when they are reproducing very rapidly, they release fragments of their DNA into their immediate environment. Transformation is the absorption of DNA into a cell.

• As a result, the cell is transformed into a new type of cell.
• These cells are called transformed cells.
• Bacterial cells that release DNA fragment are called donor cells.
• If one of the released DNA fragments contacts a cell of a species of bacteria that is capable of transformation, the DNA fragment may bind to the recipient and be taken inside.

Steps of Transformation:

• Transformation does not require direct contact between cells.
• It involves the uptake of free DNA from the environment.
• It is a form of horizontal gene transfer.

Mutations and Genetic Recombination in Bacteria

• Bacteria, as asexual organisms, inherit identical copies of their parent's genes (i.e., they are clonal).
• However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations.
• Mutations come from:
• Errors made during the replication of DNA
• Exposure to mutagens

Summary Table of Reproduction in Bacteria

Summary Table of Sexual Reproduction Methods

Intermediate Level

Binary Fission in Detail

Sexual Reproduction in Detail

Experiment of Lederberg and Tatum

Comparison of Conjugation, Transduction, and Transformation

Important Points

• 1. The two types of reproduction in bacteria are asexual reproduction and sexual reproduction.
• 2. Asexual Reproduction:
• - The primary method of asexual reproduction in bacteria is known as binary fission.
• - The key steps in the process are as follows:
• - DNA Replication: Before bacterial cells can divide, they need to replicate their genetic material. Bacterial DNA is typically a single, circular chromosome located in the nucleoid region of the cell. DNA replication begins at the origin of replication and proceeds bidirectionally, resulting in the formation of two identical DNA molecules.
• - Elongation of the Cell: As DNA replication progresses, the bacterial cell elongates. This elongation is facilitated by the continuous addition of new cell wall material and membrane components. The growth and elongation ensure that there will be enough cellular components to distribute between the two daughter cells.
• - Cell Division: Once DNA replication is complete and the cell has elongated sufficiently, the actual division process begins. The cell's plasma membrane invaginates or pinches inward at the midpoint of the cell, creating two distinct compartments. As this division progresses, a septum forms, separating the two daughter cells.
• - Formation of Daughter Cells: The final result of binary fission is the production of two genetically identical daughter cells. Each daughter cell contains its own complete set of cellular components.
• - Examples of bacteria that reproduce through binary fission include Escherichia coli (E. coli), and Staphylococcus aureus.
• 3. Sexual Reproduction:
• - The sexual reproduction in bacteria is genetic recombination i.e., DNA of two bacteria combine to give rise to a new type of bacteria called recombinant.
• - The genetic recombination in bacteria occurs by conjugation, transduction and transformation.
• 4. Conjugation:
• - For a long time, people believed that bacteria simply split in two to make more bacteria, and all bacteria were the same.
• - But in 1946, Joshua Lederberg and Edward Tatum showed that bacteria can change their genes, kind of a way similar to that of sexual reproduction seen in more complex organisms.
• - They discovered that bacteria could share their DNA with each other, passing pieces of it through a bridge-like connection, and were awarded Nobel Prize for this, and other research works.
• - Experiment of Lederberg and Tatum:
• - They took wild type of Escherichia coli (E. coli) bacteria that were able to synthesize all substances necessary for growth and reproduction when grown on a minimal medium containing inorganic salts, glucose.
• - They then irradiated wild type E. coli with X-rays to cause mutations. They selected two types of mutants from these, ones that could not produce substance "A" for growth and the others that require substance "B" in the minimal medium for growth. Both these mutant types were grown in nutrition medium containing both the substances "A" and "B". After certain period of growth, three types of bacteria were obtained from the growth culture (a) one like parent that could not produce substance "A" (b) other like the parent that could not produce substance "B" and (c) third group was like the wild type, as it could grow in the minimal medium without requiring substances "A" and "B".
• - From this experiment it was concluded that the offspring of the two mutant types were wild type, and it is only possible when actual genetic recombination takes place.
• - Direct Proof of Genetic Recombination:
• - With the invention of electron microscope, the direct proof of genetic recombination was obtained.
• - Mixture of the two mutants was observed using electron microscope.
• - The mutants are easily distinguished by their structure.
• - The bacterium that will give the DNA is called donor and the bacterium that will receive the DNA is called the recipient.
• - Bacterial conjugation involves a plasmid that can be transferred during the conjugation process from donor to recipient.
• - In some instances, however a part of bacterial chromosomal genes is also transferred.
• 5. Transduction:
• - Transduction is a genetic transfer process in bacteria facilitated by bacteriophages, which are viruses that infect bacterial cells.
• - During infection, phages inject their genetic material into bacteria.
• - The process of transduction is explained in Fig 6.11.
• - Step 1: The DNA of a temperate phage penetrates the bacterial cell.
• - Step 2: The phage DNA may become integrated with host cell DNA as a prophage.
• - Step 3: When the prophage becomes lytic, bacterial DNA is degraded and new phages are produced. The new phages may contain some bacterial DNA.
• - Step 4: The bacterial cell lyses and releases many phages, which can then infect other cells.
• - Step 5: A phage infects a new host cell.
• - Step 6: Bacterial genes introduced into the new host cell are integrated into the host's DNA. They become a part of the bacterial DNA and are replicated along with it.
• 6. Transformation:
• - When bacteria die or when they are reproducing very rapidly, they release fragments of their DNA into their immediate environment.
• - Transformation is the absorption of DNA into a cell.
• - As a result, the cell is transformed into a new type of cell.
• - These cells are called transformed cells.
• - Bacterial cells that release DNA fragment are called donor cells.
• - If one of the released DNA fragments contact a cell of a species of bacteria that is capable of transformation, the DNA fragment may bound to the recipient and take inside.
• 7. Mutations and Genetic Recombination in Bacteria:
• - Bacteria, as asexual organisms, inherit identical copies of their parent's genes (i.e., they are clonal).
• - However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations.
• - Mutations come from errors made during the replication of DNA or from exposure to mutagens.

Remember

• - Binary fission = asexual reproduction.
• - Conjugation = direct contact.
• - Transduction = through bacteriophage.
• - Transformation = uptake of free DNA.
• - Recombinant = new type of bacteria.
• - Lederberg and Tatum = discovered conjugation.
• - Nobel Prize = awarded for this discovery.
• - Generation time: E. coli = 20 minutes; M. tuberculosis = 24 hours.
• - Mutations = errors in DNA replication or exposure to mutagens.

Common Mistakes

• 1. Confusing binary fission with conjugation – Binary fission = asexual; Conjugation = sexual.
• 2. Thinking conjugation requires a virus – Conjugation requires direct contact; Transduction requires a virus.
• 3. Confusing transduction and transformation – Transduction = through bacteriophage; Transformation = uptake of free DNA.
• 4. Thinking transformation requires direct contact – Transformation does not require direct contact.
• 5. Forgetting the scientists – Lederberg and Tatum discovered conjugation.
• 6. Confusing the generation time – E. coli = 20 minutes; M. tuberculosis = 24 hours.
• 7. Thinking binary fission produces genetically different cells – Binary fission produces genetically identical cells.
• 8. Forgetting the role of plasmids – Plasmids are involved in conjugation.
• 9. Thinking mutations are always harmful – Mutations can be harmful, beneficial, or neutral.
• 10. Confusing the three methods of genetic recombination – Conjugation, transduction, transformation.
• 11. Thinking all bacteria can undergo transformation – Only some bacteria are capable of transformation.
• 12. Forgetting the role of the electron microscope – The electron microscope provided direct proof of genetic recombination.

Quick Revision

• The two types of reproduction in bacteria are asexual reproduction and sexual reproduction.
• Asexual Reproduction:
• The primary method of asexual reproduction in bacteria is known as binary fission.
• The key steps in the process are as follows:
• Examples of bacteria that reproduce through binary fission include Escherichia coli (E. coli), and Staphylococcus aureus.
• Sexual Reproduction:
• The sexual reproduction in bacteria is genetic recombination i.e., DNA of two bacteria combine to give rise to a new type of bacteria called recombinant.
• The genetic recombination in bacteria occurs by conjugation, transduction and transformation.
• Conjugation:
• For a long time, people believed that bacteria simply split in two to make more bacteria, and all bacteria were the same.
• But in 1946, Joshua Lederberg and Edward Tatum showed that bacteria can change their genes, kind of a way similar to that of sexual reproduction seen in more complex organisms.
• They discovered that bacteria could share their DNA with each other, passing pieces of it through a bridge-like connection, and were awarded Nobel Prize for this, and other research works.
• Experiment of Lederberg and Tatum:
• They took wild type of Escherichia coli (E.coli) bacteria that were able to synthesize all substances necessary for growth and reproduction when grown on a minimal medium containing inorganic salts, glucose.
• They then irradiated wild type E.coli with X-rays to cause mutations. They selected two types of mutants from these, ones that could not produce substance "A" for growth and the others that require substance "B" in the minimal medium for growth. Both these mutant types were grown in nutrition medium containing both the substances "A" and "B". After certain period of growth, three types of bacteria were obtained from the growth culture (a) one like parent that could not produce substance "A" (b) other like the parent that could not produce substance "B" and (c) third group was like the wild type, as it could grow in the minimal medium without requiring substances "A" and "B".
• From this experiment it was concluded that the offspring of the two mutant types were wild type, and it is only possible when actual genetic recombination takes place.
• Direct Proof of Genetic Recombination:
• With the invention of electron microscope, the direct proof of genetic recombination was obtained.
• Mixture of the two mutants was observed using electron microscope.
• The mutants are easily distinguished by their structure.
• The bacterium that will give the DNA is called donor and the bacterium that will receive the DNA is called the recipient.
• Bacterial conjugation involves a plasmid that can be transferred during the conjugation process from donor to recipient.
• In some instances, however a part of bacterial chromosomal genes is also transferred.
• Transduction:
• Transduction is a genetic transfer process in bacteria facilitated by bacteriophages, which are viruses that infect bacterial cells.
• During infection, phages inject their genetic material into bacteria.
• The process of transduction is explained in Fig 6.11.
• Step 1: The DNA of a temperate phage penetrates the bacterial cell.
• Step 2: The phage DNA may become integrated with host cell DNA as a prophage.
• Step 3: When the prophage becomes lytic, bacterial DNA is degraded and new phages are produced. The new phages may contain some bacterial DNA.
• Step 4: The bacterial cell lyses and releases many phages, which can then infect other cells.
• Step 5: A phage infects a new host cell.
• Step 6: Bacterial genes introduced into the new host cell are integrated into the host's DNA. They become a part of the bacterial DNA and are replicated along with it.
• Transformation:
• When bacteria die or when they are reproducing very rapidly, they release fragments of their DNA into their immediate environment.
• Transformation is the absorption of DNA into a cell.
• As a result, the cell is transformed into a new type of cell.
• These cells are called transformed cells.
• Bacterial cells that release DNA fragment are called donor cells.
• If one of the released DNA fragments contact a cell of a species of bacteria that is capable of transformation, the DNA fragment may bound to the recipient and take inside.
• Mutations and Genetic Recombination in Bacteria:
• Bacteria, as asexual organisms, inherit identical copies of their parent's genes (i.e., they are clonal).
• However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations.
• Mutations come from errors made during the replication of DNA or from exposure to mutagens.

60-Second Revision

• Binary fission = asexual reproduction.
• Conjugation = direct contact.
• Transduction = through bacteriophage.
• Transformation = uptake of free DNA.
• Recombinant = new type of bacteria.
• Lederberg and Tatum = discovered conjugation.
• Nobel Prize = awarded for this discovery.
• Generation time: E. coli = 20 minutes; M. tuberculosis = 24 hours.
• Mutations = errors in DNA replication or exposure to mutagens.
• Warning: Binary fission produces genetically identical cells. Conjugation requires direct contact. Transduction requires a bacteriophage. Transformation involves uptake of free DNA. E. coli generation time = 20 minutes; M. tuberculosis = 24 hours. Mutations come from errors in DNA replication or exposure to mutagens. Lederberg and Tatum discovered conjugation in 1946 and were awarded the Nobel Prize.