Transposons, Genomes, Plasmids, and Metagenomics in Microbiology

Transposons (Jumping Genes)

Definition and Mechanism

Transposons, also known as "jumping genes," are DNA sequences that can change their position within the genome. This movement can disrupt or modify the function of genes at the insertion site.

• Transposase: An enzyme that binds to the inverted repeats at the ends of the transposon and catalyzes the movement of the transposon.

• Staggered Cut: Transposase makes a staggered cut in the target DNA sequence, creating protruding ends.

• Insertion: The transposon is moved from its original site into the target site, attaching to the protruding ends. These ends eventually become duplicated, marking the insertion site.

• Duplication: Each time a transposable element "jumps," a new duplication forms in the target DNA.

Types of Transposons

• Nonreplicative Transposons: Move from one site to another without increasing their copy number.

• Replicative Transposons: Copy themselves to a new site, so both the original and new sites contain the transposon.

Cointegrate Formation

During replicative transposition, a cointegrate structure forms, temporarily linking the donor and recipient DNA molecules.

Example

Transposons are important in bacterial genetics for spreading antibiotic resistance genes.

Genome Organization in Prokaryotes

What is a Genome?

A genome is the complete set of genetic material in an organism, including all of its genes and noncoding sequences.

• DNA Function: Depends on its chemical structure, which allows for storage, replication, and expression of genetic information.

• Bacterial Chromosomes: Are compacted into a region called the nucleoid.

• DNA Supercoiling: Compacts the chromosome further, aided by enzymes called topoisomerases.

Genome Size and Content

• Bacterial and archaeal chromosomes range from 490 to 9,400 kilobase pairs (kb).

• Most of the prokaryotic genome is coding, meaning it encodes proteins or RNA molecules.

• There are relatively few noncoding nucleotides between genes in prokaryotes.

Chromosome Number and Structure

• Most bacteria have a single, circular chromosome, but some may have additional secondary chromosomes or large plasmids.

• Secondary chromosomes can evolve from plasmids that acquire essential genes.

Plasmids

Characteristics of Plasmids

• Plasmids are small, usually circular DNA molecules found in archaea, bacteria, and some eukaryotic microbes.

• They replicate autonomously, independent of the chromosomal DNA, but require host proteins for replication.

• Plasmids vary in size, copy number, and the genes (cargo) they carry.

Transmission of Plasmids

• Plasmids can be transmitted between cells by several mechanisms:

  • Conjugation: Direct transfer from a donor to a recipient cell via cell-to-cell contact.

  • Transformation: Uptake of plasmid DNA from dead cells in the environment.

  • Transduction: Transfer via a bacteriophage (virus that infects bacteria).

Plasmid Inheritance

• Plasmids have evolved "tricks" to ensure their inheritance during cell division, such as partitioning systems and toxin-antitoxin modules.

Metagenomics

Definition and Applications

Metagenomics is the study of genetic material recovered directly from environmental samples, allowing for the analysis of microbiomes (the collective genomes of microorganisms in a particular environment).

• Enables the study of unculturable microbes and the diversity of microbial communities.

• Applications include environmental monitoring, human health, and biotechnology.

Review Questions

1. What is a transposon?

2. What "marks" a place where a transposon enters a chromosome?

3. What is a common form of a composite transposon?

4. What are genomic islands?

5. What experiments allowed us to know DNA is the home of the genome?

6. What form can bacterial chromosomes take?

7. How do secondary chromosomes evolve?

8. How are plasmids transmitted?

9. What is metagenomics?

Additional info: Genomic islands are large DNA segments in a genome that have been acquired from another organism through horizontal gene transfer. They often contain genes for specialized functions such as antibiotic resistance or pathogenicity.