Gene Expression and Genetic Transfer in Bacteria

Posted on Feb 6, 2025 in Biology

Transcription and Translation in Gene Expression

Transcription and translation are two key processes in gene expression, where the information in a gene is used to produce a functional product, typically a protein.

Transcription

  1. Initiation: The process begins when RNA polymerase binds to a specific region of the DNA called the promoter, signaling the start of a gene.
  2. Unwinding: The DNA double helix unwinds, exposing the template strand of the DNA that will be used to synthesize RNA.
  3. Elongation: RNA polymerase moves along the template strand, synthesizing a single strand of RNA by adding complementary RNA nucleotides (A, U, C, G) to the growing RNA chain. Adenine (A) in the DNA pairs with uracil (U) in RNA, while cytosine (C) pairs with guanine (G).
  4. Termination: Transcription continues until RNA polymerase reaches a termination signal in the DNA, causing it to stop synthesizing the RNA strand. The newly formed RNA, known as messenger RNA (mRNA), detaches from the DNA.

Translation

  1. Initiation: The mRNA molecule leaves the nucleus and enters the cytoplasm, where it binds to a ribosome. The ribosome reads the mRNA sequence in sets of three nucleotides called codons.
  2. tRNA Binding: Transfer RNA (tRNA) molecules, each carrying a specific amino acid, recognize and bind to the corresponding codon on the mRNA through their anticodon.
  3. Elongation: The ribosome facilitates the formation of peptide bonds between the amino acids brought by the tRNA, creating a growing polypeptide chain. The ribosome moves along the mRNA, reading each codon and adding the appropriate amino acid to the chain.
  4. Termination: The process continues until the ribosome reaches a stop codon on the mRNA. At this point, the completed polypeptide chain is released, and the ribosome disassembles.

Genetic Transfer in Bacteria: Transduction and Transformation

Transduction and transformation are two ways bacteria can exchange genes.

Transduction

This occurs when a virus (bacteriophage) transfers DNA from one bacterium to another. There are two types:

  • Generalized Transduction: Any bacterial gene can be transferred. When a bacteriophage infects a bacterium, it sometimes accidentally picks up some of the bacterial DNA. When it infects another bacterium, it injects this DNA.
  • Specialized Transduction: This is more specific. When a bacteriophage integrates into a bacterial chromosome and then exits, it might take some nearby bacterial genes with it. These genes then get transferred to the next bacterium it infects.

Transformation

This is when bacteria take up free-floating DNA from their surroundings (from dead cells or plasmids). Bacteria can naturally absorb this DNA. If it integrates into their genome, it can lead to new traits.

Both methods help bacteria gain new abilities, like antibiotic resistance or new metabolic pathways, increasing adaptability and diversity.

Conjugation in Bacteria

Conjugation in bacteria is a process where they share genetic material directly.

  1. Formation of a Pilus: The donor bacterium, which has a plasmid (a small, circular DNA piece), creates a structure called a pilus. This pilus reaches out from the donor cell to connect with the recipient cell.
  2. Contact and Mating Pair Formation: Once the pilus is attached, it pulls the two bacteria closer, forming a mating pair.
  3. Transfer of Genetic Material: The plasmid DNA is transferred from the donor to the recipient. This often happens through rolling circle replication, where one strand of the plasmid is cut and sent to the recipient while the other strand stays in the donor.
  4. Recombination: After the transfer, the recipient cell can incorporate the new genetic material into its own DNA, potentially gaining new traits, like antibiotic resistance.

Conjugation is important because it increases genetic diversity in bacterial populations and allows beneficial traits to spread quickly.

DNA Replication

DNA replication is when a cell makes an exact copy of its DNA. The double helix unwinds, and the two strands separate. Each strand serves as a template for a new strand. DNA polymerase adds the correct nucleotides to form the new strands, working from the 5′ end to the 3′ end.

First, the strands unwind at specific spots called origins of replication. Then, primase lays down an RNA primer to start the process. DNA polymerase then adds nucleotides to build the new strand. Once the entire DNA is copied, the RNA primers are replaced with DNA, and the strands wind back into a double helix. This process is essential for cell division, ensuring each new cell gets an exact copy of the DNA.

Regulation of Gene Expression in Bacteria

The regulation of gene expression in bacteria is crucial for adapting to environmental changes and using resources effectively. Here are the key mechanisms:

  1. Transcriptional Regulation: This is the primary control method. Bacteria manage gene transcription through regulatory proteins that attach to specific DNA sequences. These proteins can be repressors (block transcription) or activators (encourage transcription).
  2. Operons: Genes are frequently organized into operons, groups of genes transcribed together from a single promoter. A well-known example is the lac operon in E. coli, which is involved in lactose metabolism. When lactose is available, it binds to the repressor protein, allowing transcription of the genes needed for lactose processing.
  3. Post-Transcriptional Regulation: After transcription, bacteria can control gene expression through methods like RNA degradation or by using small RNA molecules that can inhibit translation.
  4. Post-Translational Regulation: This involves changes to proteins after they have been made, such as phosphorylation or methylation, which can alter their function or stability.
  5. Environmental Signals: Bacteria can sense environmental changes (nutrient availability, stress) and modify gene expression in response. This is often mediated by two-component regulatory systems, where a sensor protein identifies a change and activates a response regulator to adjust gene expression.

These regulatory mechanisms enable bacteria to swiftly adapt to their environment, which is vital for survival and effective resource management.

Mutations in DNA

Mutations are changes in the DNA sequence and can occur for various reasons. Here are the main types:

  1. Point Mutations: These involve small changes where just one base pair in the DNA is altered. They can be:
    • Silent Mutations: These don’t affect the protein.
    • Missense Mutations: These change one amino acid in the protein, potentially impacting its function.
    • Nonsense Mutations: These introduce a stop signal too early, resulting in a shortened, usually nonfunctional protein.
  2. Insertions and Deletions: These involve adding or removing one or more bases, disrupting the entire reading frame of the DNA and significantly altering the protein.
  3. Duplications: A segment of DNA is copied, leading to more of a gene than normal, potentially giving it new functions.
  4. Inversions: A segment of DNA is flipped, changing how genes are expressed.
  5. Translocations: Segments of DNA move from one location to another, either within the same chromosome or to a different one. This can interfere with gene function.

Mutations can arise spontaneously during DNA replication or due to environmental factors like radiation or chemicals. They play a crucial role in evolution by creating the genetic diversity that natural selection acts upon.