MCAT Biology Review
Chapter 1: The Cell
1.4 Genetics and Growth of Prokaryotic Cells
As we have already explored, prokaryotic cells differ from eukaryotic cells both structurally and biochemically. Prokaryotes reproduce via asexual reproduction in the form of binary fission. In addition, prokaryotes are capable of acquiring genetic material from outside the cell and using that genetic material.
Binary fission, shown in Figure 1.10, is a simple form of asexual reproduction seen in prokaryotes. The circular chromosome attaches to the cell wall and replicates while the cell continues to grow in size. Eventually, the plasma membrane and cell wall begin to grow inward along the midline of the cell to produce two identical daughter cells. Because binary fission requires fewer events than mitosis, it can proceed more rapidly. In fact, some strains of E. coli can replicate every 20 minutes under ideal growth conditions.
Figure 1.10. Stages of Binary Fission
This single circular chromosome of a prokaryotic cell contains the information that is necessary for the cell to survive and reproduce. However, many bacteria also contain extrachromosomal (extragenomic) material known as plasmids. Plasmids often carry genes that impart some benefit to the bacterium, such as antibiotic resistance, some mechanisms of which are shown in Figure 1.11. Plasmids may also carry additional virulence factors, or traits that increase how pathogenic a bacterium is, such as toxin production, projections that allow the bacterium to attach to certain kinds of cells, or evasion of the host’s immune system. A subset of plasmids called episomes are capable of integrating into the genome of the bacterium.
Figure 1.11. Mechanisms of Antibiotic Resistance
Bacterial genetic recombination helps increase bacterial diversity and thus permits evolution of a bacterial species over time. These recombination processes include transformation, conjugation, and transduction.
Transformation results from the integration of foreign genetic material into the host genome. This foreign genetic material most frequently comes from other bacteria that, upon lysing, spill their contents in the vicinity of a bacterium capable of transformation. Many gram-negative rods are able to carry out this process.
Conjugation is the bacterial form of mating (sexual reproduction). It involves two cells forming a conjugation bridge between them that allows for the transfer of genetic material. The transfer is unidirectional, from the donor male (+) to the recipient female (–). The bridge is made from appendages called sex pili that are found on the donor male. To form the pili, bacteria must contain plasmids known as sex factors that contain the necessary genes. The best-studied sex factor is the F (fertility) factor in E. coli. Bacteria possessing this plasmid are termed F+ cells; those without are called F– cells. During conjugation between an F+ and an F– cell, the F+ cell replicates its F factor and donates the copy to the recipient, converting it to an F+ cell. This enables the cell obtaining the new plasmid to then transfer copies to other cells. This method of genetic recombination allows for rapid acquisition of antibiotic resistance or virulence factors throughout a colony because other plasmids can also be passed through the conjugation bridge. The process of conjugation is illustrated in Figure 1.12.
Figure 1.12. Bacterial Conjugation
The sex factor is a plasmid, but through processes such as transformation, it can become integrated into the host genome. In this case, when conjugation occurs, the entire genome replicates because it now contains the sex factor. The donor cell will then attempt to transfer an entire copy of its genome into the recipient; however, the bridge usually breaks before the full DNA sequence can be moved. Cells that have undergone this change are referred to by the abbreviation Hfr for high frequency of recombination.
Transduction is the only genetic recombination process that requires a vector—a virus that carries genetic material from one bacterium to another. Viruses are obligate intracellular pathogens, which means that they cannot reproduce outside of a host cell. Because of this, bacteriophages(viruses that infect bacteria) can accidentally trap a segment of host DNA during assembly. When the bacteriophage infects another bacterium, it can release this trapped DNA into the new host cell. This transferred DNA can then integrate into the genome, giving the new host additional genes. The process of transduction is shown in Figure 1.13.
Figure 1.13. Bacterial Transduction
Transposons are genetic elements capable of inserting and removing themselves from the genome. This phenomenon is not limited to prokaryotes; it has been seen in eukaryotes as well. If a transposon is inserted within a coding region of a gene, that gene may be disrupted.
One of the biggest challenges a doctor faces is that of patient compliance with treatment, especially antibiotics. Many patients fail to complete an entire course of antibiotics, often discontinuing the treatment because they feel better. Unfortunately, this breeds antibiotic resistance by killing off the bacteria that are nonresistant and leaving behind bacteria that are more resistant. These resistant bacteria then reproduce, resulting in recurrence of the infection. Over time, this practice has led to bacteria that are resistant to multiple antibiotics, making common infections more difficult to treat.
As discussed previously, bacteria reproduce via binary fission. This implies that all of the bacteria are exactly the same in a local colony (assuming no mutations or genetic recombination), and no bacteria will be dividing faster than the others. Bacteria can be said to grow in a series of phases, as shown in Figure 1.14. In a new environment, the bacteria first adapt to the new local conditions during the lag phase. As the bacteria adapt, growth increases, causing an exponential increase in the number of bacteria in the colony during the exponential phase, which can also be called the log phase. As the number of bacteria in the colony grows, resources are often reduced. The reduction of resources slows reproduction, and the stationary phase results. After the bacteria have exceeded the ability of the environment to support the number of bacteria, a death phaseoccurs as resources in the environment have been depleted.
Figure 1.14. Bacterial Growth Curve
The bacterial growth curve is an example of a semilog plot. The fact that the y-axis is logarithmic means that a straight line (as seen during the exponential phase) actually represents an exponential increase in the number of bacteria, not a linear increase. Semilog and log–log plots are discussed in Chapter 12 of MCAT Physics and Math Review.
MCAT Concept Check 1.4:
Before you move on, assess your understanding of the material with these questions.
1. Briefly describe the three mechanisms of bacterial genetic recombination:
2. What are the four phases of the bacterial growth curve? What are the features of each phase?