## Learning Outcomes

- Distinguish prokaryotic cells, eukaryotic cells, and viruses by structure, genome organization, and metabolism.
- Identify major bacterial structural features, including cell walls, capsules, flagella, pili, plasmids, and ribosomes.
- Explain bacterial growth by binary fission and interpret the phases of a bacterial growth curve.
- Compare bacterial genetic exchange mechanisms: transformation, conjugation, transduction, and transposition.
- Describe basic prokaryotic gene regulation using operons as a model.
- Explain viral structure, host dependence, and lytic, lysogenic, and retroviral life cycles.
- Avoid common MCAT traps involving antibiotics, viral replication, and horizontal gene transfer.

## Medical College Admission Test (MCAT) Syllabus

In the Biological and Biochemical Foundations of Living Systems section, Foundational Concept 2 tests how organized cellular and molecular assemblies support life. Content Category 2B focuses on prokaryotes and viruses: how they are structured, how they grow, how they regulate genes, and how genetic information moves between organisms or viral particles. Expect both discrete questions and passage-based questions using bacterial growth curves, antibiotic experiments, plasmid maps, viral life-cycle diagrams, or gene-transfer assays.

- Cell theory and the distinction between cellular life and acellular viruses.
- Classification of prokaryotes, especially Bacteria and Archaea.
- Bacterial morphology and structures: cell wall, membrane, capsule, flagella, pili, ribosomes, nucleoid, and plasmids.
- Bacterial growth, binary fission, growth phases, and oxygen requirements.
- Prokaryotic physiology, including metabolism and antibiotic targets.
- Prokaryotic genetics, including operons, plasmids, mutations, and horizontal gene transfer.
- Viral structure, host specificity, lytic and lysogenic cycles, and retroviral replication.
- Subviral infectious particles, including prions and viroids, at a recognition level.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why can many antibiotics harm bacteria without directly damaging human cells?
2. Which process transfers bacterial DNA by a bacteriophage: transformation, conjugation, or transduction?
3. How does the log phase of a bacterial growth curve differ from the stationary phase?
4. What is the key difference between the lytic and lysogenic cycles of a bacteriophage?
5. Why are viruses not considered cells?

## Introduction

Prokaryotes and viruses are small, fast-replicating biological systems that show many core principles of cell biology and genetics in simplified form. Prokaryotes are cells: they have plasma membranes, cytoplasm, ribosomes, genetic material, and the ability to carry out metabolism. Viruses are not cells: they package genetic information in protein coats and must use host-cell machinery to reproduce.

For the MCAT, this subtopic often tests contrast. A passage may describe an unfamiliar organism, antibiotic, plasmid, or phage, then ask you to infer whether a structure is bacterial or viral, whether a process is vertical or horizontal gene transfer, or how a mutation affects growth. Your task is less about memorizing obscure species and more about recognizing the logic of prokaryotic and viral systems.

> **Key Term: prokaryote**
> A single-celled organism lacking a membrane-bound nucleus and membrane-bound organelles; Bacteria and Archaea are prokaryotes.

### Prokaryotic classification and cell structure

Prokaryotes include two major domains: Bacteria and Archaea. Both lack a nucleus, so their DNA is located in a nucleoid region rather than enclosed by a nuclear membrane. Most prokaryotic chromosomes are circular, and many prokaryotes also carry smaller circular DNA molecules called plasmids. Prokaryotes do not have mitochondria, endoplasmic reticulum, Golgi apparatus, or other membrane-bound organelles. Their ribosomes are smaller than eukaryotic ribosomes: prokaryotic ribosomes are 70S, made from 30S and 50S subunits, while eukaryotic cytoplasmic ribosomes are 80S.

Bacteria and Archaea differ in cell-wall and membrane chemistry. Bacteria commonly have cell walls containing peptidoglycan, while Archaea do not have peptidoglycan and often have membrane lipids with ether linkages rather than the ester linkages typical of bacterial and eukaryotic membranes. On the MCAT, the bacterial–archaeal distinction is usually tested at this level rather than through detailed taxonomy.

> **Key Term: peptidoglycan**
> A polymer of sugars and short peptides that forms a strong mesh-like bacterial cell wall.

Bacterial shapes are also high-yield recognition facts. Cocci are spherical, bacilli are rod-shaped, and spirilla or spirochetes are spiral-shaped. Bacteria may occur singly, in chains, or in clusters. These names are often embedded in clinical-sounding passages, but the MCAT usually expects recognition rather than diagnosis.

The bacterial cell envelope is one of the most tested structural topics. Gram-positive bacteria have a thick peptidoglycan wall outside the plasma membrane. Gram-negative bacteria have a thinner peptidoglycan layer plus an outer membrane containing lipopolysaccharide. The outer membrane can make Gram-negative bacteria harder to treat because it limits drug entry and contains molecules that may stimulate strong immune responses.

Bacteria may also possess accessory structures. A capsule is a sticky outer layer that can help bacteria avoid host immune defenses and adhere to surfaces. Flagella allow motility, often in response to chemical gradients. Pili and fimbriae are short projections used for attachment; sex pili are involved in conjugation, a form of DNA transfer. The nucleoid contains the main chromosome, and plasmids may carry genes for antibiotic resistance, toxins, or metabolism of unusual nutrients.

> **Test Tip:**
> If a question asks why a drug selectively affects bacteria, look for bacterial-specific targets: peptidoglycan cell-wall synthesis, 70S ribosomes, bacterial DNA gyrase, folate synthesis pathways, or bacterial membrane features.

A major exam trap is assuming that all cells have the same internal compartments. Prokaryotes can perform respiration, photosynthesis, DNA replication, transcription, and translation, but they do so without mitochondria, chloroplasts, or a nucleus. In bacteria, transcription and translation can occur at the same time because there is no nuclear envelope separating DNA from ribosomes.

### Bacterial growth, metabolism, and antibiotic targets

Bacteria reproduce mainly by binary fission. The circular chromosome is replicated, the cell elongates, copies of DNA separate, and a septum forms to divide one cell into two genetically similar daughter cells. This process is not mitosis. Mitosis requires eukaryotic chromosomes, a spindle, and nuclear division; bacteria lack these structures.

> **Key Term: binary fission**
> Asexual division in prokaryotes in which one cell replicates its DNA and splits into two daughter cells.

Under ideal conditions, bacterial growth can be rapid and exponential. If a bacterial population doubles every generation, the number of cells after $$n$$ generations is:

$$N = N_0 2^n$$

where $$N_0$$ is the starting population and $$N$$ is the final population. MCAT passages may give a doubling time and ask you to estimate population size. For example, if 100 bacteria double three times, the population becomes $$100 \times 2^3 = 800$$ cells.

In a closed culture, bacterial growth follows four classic phases. During the lag phase, bacteria adjust to the environment and synthesize enzymes, but cell number changes little. During the log or exponential phase, cells divide at a maximum rate. During the stationary phase, nutrient depletion and waste accumulation cause the rate of cell division to equal the rate of cell death. During the death phase, viable cell number decreases.

Bacteria also vary in oxygen use. Obligate aerobes require oxygen for growth. Obligate anaerobes are harmed by oxygen and use anaerobic metabolism. Facultative anaerobes can use oxygen when present but can switch to fermentation or anaerobic respiration when oxygen is absent. Aerotolerant anaerobes do not use oxygen but can survive in its presence. Microaerophiles require oxygen at lower-than-atmospheric levels.

> **Exam Warning:**
> Do not equate “anaerobic” with “cannot make ATP.” Anaerobic organisms still produce ATP, but they do so without oxygen as the final electron acceptor, often by fermentation or anaerobic respiration.

Some bacteria form endospores, highly resistant dormant structures that survive heat, drying, and chemical stress. Endospores are not reproductive spores in the fungal sense; one bacterial cell forms one endospore, which can later germinate back into one active bacterial cell. Classic endospore-forming genera include Bacillus and Clostridium.

Antibiotics exploit differences between bacterial and human cells. Beta-lactam antibiotics, such as penicillin-like drugs, inhibit bacterial cell-wall synthesis. Other antibiotics bind bacterial ribosomal subunits and inhibit protein synthesis. Some interfere with DNA replication, RNA transcription, or metabolic pathways such as folate synthesis. Bacterial resistance can arise by mutation or by acquiring resistance genes, often on plasmids. Resistance mechanisms include drug-degrading enzymes, altered drug targets, reduced membrane permeability, and efflux pumps that export the drug.

Bactericidal drugs kill bacteria; bacteriostatic drugs inhibit growth. The distinction can matter in passage logic, especially if an experiment measures living cell number versus cell division rate.

### Prokaryotic genetics and horizontal gene transfer

Prokaryotic genetics is high-yield because bacteria can change quickly. Vertical gene transfer passes DNA from parent cell to daughter cells during reproduction. Horizontal gene transfer moves DNA between organisms in the same generation. This allows traits such as antibiotic resistance to spread through a bacterial population much faster than by mutation alone.

> **Key Term: plasmid**
> A small, usually circular DNA molecule separate from the main bacterial chromosome, often carrying nonessential but useful genes.

Transformation is uptake of naked DNA from the environment. If a bacterium dies and releases DNA, another bacterium may take up fragments and incorporate them into its genome. In classic experiments, transformation showed that DNA is genetic material.

Conjugation is direct DNA transfer between bacterial cells through a sex pilus. A donor cell with an F plasmid can transfer plasmid DNA to a recipient. In some cases, the F plasmid integrates into the bacterial chromosome, creating an Hfr cell. When an Hfr cell conjugates, some chromosomal genes may be transferred along with plasmid sequence.

Transduction is DNA transfer mediated by bacteriophages, which are viruses that infect bacteria. In generalized transduction, random bacterial DNA is accidentally packaged into phage particles during a lytic infection. In specialized transduction, a prophage excises incorrectly from the bacterial chromosome and carries nearby bacterial genes with it.

> **Key Term: transduction**
> Transfer of bacterial DNA from one bacterium to another by a bacteriophage.

Transposons, or “jumping genes,” are DNA sequences that can move from one location to another within or between DNA molecules. They can disrupt genes, alter regulation, or carry antibiotic resistance genes. If a transposon moves onto a plasmid, that resistance gene can spread by conjugation.

Bacterial gene expression is often organized into operons. An operon contains multiple related genes controlled by a single promoter and regulatory region, allowing coordinated transcription into one polycistronic mRNA. The lac operon is an inducible system involved in lactose metabolism: lactose or allolactose reduces repressor binding, allowing transcription when lactose is available. The trp operon is a repressible system: high tryptophan levels activate a repressor, reducing transcription of genes needed for tryptophan synthesis.

> **Key Term: operon**
> A prokaryotic gene-regulation unit in which multiple genes are controlled together and transcribed from one promoter.
>
> **Test Tip:**
> For gene-transfer questions, identify the vehicle of DNA movement. Naked environmental DNA means transformation. Cell-to-cell transfer through a pilus means conjugation. Phage-mediated transfer means transduction. Movement within DNA by mobile elements means transposition.

Because bacteria lack a nucleus, transcription and translation are coupled. A ribosome can begin translating an mRNA while RNA polymerase is still transcribing it. This differs from eukaryotes, where pre-mRNA processing and nuclear export separate transcription from translation.

### Viral structure and reproductive cycles

Viruses are acellular infectious agents. They contain genetic material, either DNA or RNA, surrounded by a protein capsid. Some viruses also have a lipid envelope derived from host membranes. Enveloped viruses often enter cells by membrane fusion or endocytosis and tend to be more sensitive to drying, detergents, and heat than nonenveloped viruses. Nonenveloped viruses are often more resistant in the external environment.

> **Key Term: virion**
> A complete infectious viral particle outside a host cell, consisting of a viral genome packaged in a capsid and sometimes an envelope.

Viruses do not have ribosomes, do not generate ATP by their own metabolism, and do not reproduce by binary fission, mitosis, or meiosis. They depend on host cells to synthesize proteins, replicate genomes, and assemble new particles. This is why viruses are called obligate intracellular parasites.

Viral genomes vary. They may be DNA or RNA, single-stranded or double-stranded, linear or circular, segmented or nonsegmented. Positive-sense RNA can often function directly as mRNA. Negative-sense RNA must first be copied into a complementary positive-sense strand by an RNA-dependent RNA polymerase. Retroviruses are RNA viruses that use reverse transcriptase to make DNA from RNA; the viral DNA can integrate into the host genome.

The lytic cycle produces new virions and lyses the host cell. A bacteriophage attaches to a bacterium, injects its genetic material, uses host machinery to make viral components, assembles new phage particles, and breaks open the cell to release them. This increases phage number rapidly and kills the host bacterium.

The lysogenic cycle involves integration or maintenance of the viral genome without immediate host-cell lysis. In bacteriophages, the integrated viral DNA is called a prophage. The host cell replicates the prophage along with its own genome. Environmental stress can induce the prophage to enter the lytic cycle.

> **Key Term: provirus**
> Viral DNA integrated into a host-cell genome, especially in retroviral infections.

Be careful with terminology: a prophage refers to bacteriophage DNA integrated into a bacterial genome, while provirus is commonly used for viral DNA integrated into a eukaryotic host genome, especially retroviral DNA. The underlying idea is similar: viral genetic information is copied along with host DNA.

Viruses also show host specificity, meaning they infect particular cell types or species. Specificity often depends on viral attachment proteins binding host-cell receptors. For example, a virus that cannot bind a receptor on a given cell usually cannot enter that cell. MCAT passages may describe altered receptor expression and ask you to predict whether infection increases or decreases.

Subviral particles may also appear. Prions are infectious misfolded proteins that induce misfolding of normal proteins; they contain no nucleic acid. Viroids are small circular RNA molecules that infect plants and do not encode proteins. For MCAT purposes, recognize that prions and viroids are not cells and do not fit the standard virus structure of genome plus capsid.

> **Exam Warning:**
> Antibiotics do not treat viral replication directly. If a passage describes a viral infection improving after an antibacterial drug, look for a secondary bacterial infection or an experimental effect on bacteria, not direct killing of viruses.

A final common trap is confusing viral replication with cell division. A virus does not “grow” and split. Instead, its parts are synthesized separately inside a host cell and then assembled into virions. This assembly-line pattern is why viral replication can produce many particles from one infected cell.

## Key Point Checklist

This article has covered the following key knowledge points:

- Prokaryotes lack a nucleus and membrane-bound organelles, but they are living cells with membranes, ribosomes, DNA, and metabolism.
- Bacteria usually have peptidoglycan cell walls; Archaea do not.
- Prokaryotic ribosomes are 70S, made of 30S and 50S subunits, which helps explain selective antibiotic action.
- Bacteria reproduce by binary fission, not mitosis.
- Bacterial growth curves include lag, log, stationary, and death phases.
- Oxygen relationships include obligate aerobes, obligate anaerobes, facultative anaerobes, aerotolerant anaerobes, and microaerophiles.
- Horizontal gene transfer can occur by transformation, conjugation, transduction, or transposition.
- Plasmids often carry antibiotic resistance genes and can spread between bacteria.
- Operons allow coordinated regulation of multiple prokaryotic genes.
- Viruses are acellular obligate intracellular parasites with genomes packaged in capsids, sometimes with envelopes.
- Lytic cycles produce and release virions rapidly, while lysogenic cycles maintain viral DNA in the host before possible induction.
- Retroviruses use reverse transcriptase and may integrate viral DNA into the host genome as a provirus.

## Key Terms and Concepts

- prokaryote
- peptidoglycan
- binary fission
- plasmid
- transduction
- operon
- virion
- provirus
- Bacteria
- Archaea
- nucleoid
- capsule
- flagella
- pili
- Gram-positive bacteria
- Gram-negative bacteria
- endospore
- transformation
- conjugation
- transposon
- bacteriophage
- lytic cycle
- lysogenic cycle
- prophage
- retrovirus
- reverse transcriptase
- prion
- viroid


Foundational concept 2: Cellular and viral assemblies - 2B: Structure, growth, physiology, and genetics of prokaryotes and viruses