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MCAT Content / Growth And Physiology Of Prokaryotic Cells / High Degree Of Genetic Adaptability Acquisition Of Antibiotic Resistance

High degree of genetic adaptability, acquisition of antibiotic resistance

Topic: Growth And Physiology Of Prokaryotic Cells

The short generation time, random mutations, and mechanisms of genetic recombination (transformation, transduction, and conjugation) enable prokaryotes to have a high degree of genetic adaptability and acquisition of antibiotic resistance.

Bacteria reproduce by splitting in two via binary fission which does not provide an opportunity for genetic recombination or genetic diversity (aside from the occasional random mutation). Still, genetic variation is key to the survival of a species, allowing groups to adapt to changes in their environment by natural selection. Prokaryotes such as bacteria can share genes by three mechanisms to promote genetic variation: conjugation, transformation, and transduction.

In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its chromosome, it, too, may become pathogenic.

Left: plasmid taken up by transformation. Right: linear DNA fragment taken up by transformation and swapped into the bacterial chromosome by homologous recombination.

In transduction, bacteriophages, the viruses that infect bacteria, sometimes also move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism. 

Virus infects cell by injecting its DNA. Bacterial DNA is fragmented and viral DNA is replicated. New viral particles are made and exit the cell. One contains host DNA instead of viral DNA. When this virus infects a new host, it injects the bacterial DNA, which can recombine with the chromosome of the new hows.

In conjugation, DNA is transferred from one prokaryote to another using a pilus, which brings the organisms into contact with one another. The DNA transferred can be in the form of a plasmid or as a hybrid, containing both plasmid and chromosomal DNA. For instance, in many cases, conjugation serves to transfer plasmids that carry antibiotic resistance genes.

Donor cells typically act as donors because they have a chunk of DNA called the fertility factor (or F factor). This chunk of DNA codes for the proteins that make up the sex pilus. It also contains a special site where DNA transfer during conjugation begins. 1. An F+ donor cell contains its chromosomal DNA and an F plasmid. It has a rodlike pilus. A recipient F- cell has only a chromosome and no F plasmid. 2. The donor cell uses its pilus to attach to the recipient cell, and the two cells are pulled together. 3. A channel forms between the cytoplasms of the two cells, and a single strand of the F plasmid is fed through. 4. Both of the cells now have an F plasmid and are F+. The former recipient cell is now a new donor and can form a pilus.

In bacteria, reproduction can be very fast, with a generation taking little more than a few minutes for some species. This short generation time, together with random mutations and the mechanisms of genetic recombination, allow bacteria (and other prokaryotes) to evolve very quickly. These enable them to respond to environmental changes (such as the introduction of an antibiotic) very rapidly and consequently become resistant.

 

Practice Questions

 

Khan Academy   

 

MCAT Official Prep (AAMC)

Official Guide B/B Section Passage 2 Question 9

Biology Question Pack, Vol 2. Passage 3 Question 12

Biology Question Pack, Vol 2. Passage 3 Question 15

Section Bank B/B Section Passage 2 Question 12

Practice Exam 2 B/B Section Passage 6 Question 30

 

Key Points

• Transformation is a type of prokaryotic reproduction in which a prokaryote can take up DNA found within the environment that has originated from other prokaryotes.

• Transduction is a type of prokaryotic reproduction in which a prokaryote is infected by a virus that injects short pieces of chromosomal DNA from one bacterium to another.

• Conjugation is a type of prokaryotic reproduction in which DNA is transferred between prokaryotes using a pilus.

• The short generation time, random mutations, and mechanism of genetic recombination allow some prokaryotes to have a high degree of genetic adaptability and acquisition of antibiotic resistance.


Key Terms

Generation time: the doubling time of the bacteria

Transformation: the alteration of a bacterial cell caused by the transfer of DNA from another, especially if pathogenic

Transduction: horizontal gene transfer mechanism in prokaryotes where genes are transferred using a virus

Conjugation: the temporary fusion of organisms, especially as part of sexual reproduction

Pilus: a hairlike appendage found on the cell surface of many bacteria

Binary fission: the process whereby a cell divides asexually to produce two daughter cells

Genetic recombination is the exchange of genetic material which leads to the production of offspring with combinations of traits that differ from those found in either parent

Genetic variation: the diversity in gene frequencies in a population

Prokaryotes: an organism whose cell (or cells) are characterized by the absence of a nucleus or any other membrane-bound organelles

Nonpathogenic: a non-harmful microorganism that does not cause a disease

Bacteriophage: a type of virus that infects bacteria

Recombinant organism: an organism in which a foreign DNA is recombined or added into It’s existing DNA

Plasmid: small circular DNA strand in the cytoplasm of a bacterium

Fertility factor: allows genes to be transferred from one bacterium carrying the factor to another bacterium lacking the factor by conjugation
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