BIO 204 Lecture 7: Bacterial Gene Transfer & Evolution

What is vertical gene transfer?

The generational passing of genes from parent cells to daughter cells during cell division.

What is horizontal gene transfer?

Transfer of genes from one organism to another coexisting organism.

What is the main difference between vertical and horizontal gene transfer?

Vertical = parent to daughter cells; horizontal = one coexisting organism to another.

A bacterium passes DNA to its daughter cells during cell division. What type of gene transfer is this?

Vertical gene transfer.

A bacterium receives DNA from another bacterium living alongside it. What type of gene transfer is this?

Horizontal gene transfer.

Why is horizontal gene transfer important?

It can give bacteria new properties and contribute to rapid evolution.

What are the three main mechanisms of horizontal gene transfer?

Transformation, conjugation, and transduction.

What is transformation?

The process of importing free DNA into bacterial cells.

Where can the free DNA used in transformation come from?

Short DNA fragments released by nearby dead cells.

What does it mean for bacteria to be naturally competent?

They can recognize and import free DNA.

What complex imports free DNA during transformation?

The transformasome complex.

Where does the transformasome form and what does it do?

Forms at the membrane and uses ATP to import extracellular free DNA.

A bacterium takes up a free DNA fragment from its surroundings. What mechanism occurred?

Transformation.

What is conjugation?

DNA transfer caused by direct contact between a donor and recipient cell.

What structure pulls cells together during conjugation?

The sex pilus.

What carries the genes transferred during conjugation?

Transferable plasmids.

What are the main steps of conjugation?

Sex pilus pulls cells together → cells fuse → plasmid is nicked and transferred.

Two bacterial cells directly contact each other and transfer a plasmid. What mechanism occurred?

Conjugation.

What is used to transfer DNA during transduction?

Bacteriophages.

What are bacteriophages?

Bacterial viruses that inject their DNA into living bacterial cells.

How can bacterial DNA become involved in transduction?

Host bacterial DNA can accidentally be packaged into newly formed phages.

What happens to bacterial DNA accidentally packaged into a phage?

It can be carried to and injected into another bacterial cell.

What can happen to DNA delivered through transduction?

It can integrate into the new host cell's chromosome.

A virus carries bacterial DNA from one bacterial cell to another. What mechanism occurred?

Transduction.

How can you quickly distinguish the three mechanisms of horizontal gene transfer?

Transformation = free DNA; conjugation = direct contact/plasmid; transduction = bacteriophage.

What are the two potential outcomes for new DNA entering a bacterial cell?

It can be degraded or incorporated.

Why might bacteria degrade new DNA?

To prevent a negative impact on fitness.

Why might bacteria incorporate new DNA?

To potentially gain new enzymes/proteins and increase fitness.

What do restriction endonucleases do?

Recognize and cut specific DNA sequences.

How is bacterial "self DNA" protected from restriction endonucleases?

Methylation inhibits endonuclease activity and protects the DNA.

How do restriction endonucleases and methylation protect bacteria from foreign DNA?

They eliminate foreign DNA before it integrates while protecting self DNA.

A bacterium wants to prevent potentially damaging foreign DNA from integrating. What mechanism can it use?

Restriction endonucleases cut foreign DNA, while methylation protects self DNA.

What is recombination?

An enzyme-mediated process in which two DNA molecules exchange portions.

Why can plasmids remain separate from the bacterial chromosome?

They can replicate independently.

What must happen to other transferred DNA for it to be replicated and expressed by the host?

It must integrate into the chromosome.

What are two advantages of recombination?

It can incorporate genes from other organisms and repair DNA damage.

What must recombining DNA molecules have?

Long sequences of homology.

What is the main enzyme involved in finding and aligning homologous regions during recombination?

RecA.

A bacterium incorporates transferred DNA into its chromosome. What process is involved?

Recombination.

What are the three main drivers of genetic divergence and evolution?

Random mutations, natural selection, and reductive/degenerative evolution.

What can DNA be used to track?

Divergence and evolution of organisms over time.

What do phylogenetic trees illustrate?

Genetic distance between species.

What is a mutation?

A change in the DNA genome of an organism.

What can cause mutations?

DNA polymerase mistakes during replication or mutagens like radiation and ROS.

Why can a small DNA mutation have a large effect?

It can alter a codon or disrupt promoters and regulator binding sites.

What is a point mutation?

A single nucleotide is changed.

What is an insertion?

Addition of one or more nucleotides.

What is a deletion?

Subtraction of one or more nucleotides.

What is a silent mutation?

It encodes the same amino acid, causing no effect.

What is a missense mutation?

It encodes a different amino acid, leading to a range of effects.

What is a frameshift mutation?

An insertion or deletion that shifts the reading frame.

Why can a frameshift mutation have a huge effect?

All downstream amino acids are affected.

What is a nonsense mutation?

It encodes a stop codon, causing protein truncation.

Same amino acid vs. different amino acid vs. stop codon = what mutations?

Same = silent; different = missense; stop codon = nonsense.

What is natural selection?

It occurs when a changed trait or mutation affects survival.

How does survival affect reproduction during natural selection?

Surviving variants produce more offspring than less fit variants.

How does natural selection lead to bacterial evolution?

Bacteria adapt over time by passing beneficial traits to offspring.

Can bacterial evolution occur over short time periods?

Yes, such as antibiotic resistance developing over days to weeks.

What is degenerative/reductive evolution?

Traits are lost over time in the absence of selective pressure.

How can degenerative/reductive evolution affect endosymbionts?

They lose genes for synthesizing amino acids and vitamins, relying on hosts.