The Cell MCAT

What are the four tenets of cell theory?

1. All living things are composed of cells.
2. The cell is the basic functional unit of life.
3. Cells arise only from preexisting cells.
4. Cells carry genetic information in the form of DNA.

Why are viruses not considered living organisms?

Because they are acellular, cannot reproduce without a host cell, and may use RNA as genetic material.

What are key features of eukaryotic cells?

They have membrane-bound organelles, a nucleus, and may form multicellular organisms.

What is the function of the nucleus?

Contains DNA organized into chromosomes and is surrounded by a nuclear membrane with nuclear pores.

What are the functions of mitochondria?

Perform cellular respiration, contain outer and inner membranes, divide independently via binary fission, and can initiate apoptosis.

What does the inner mitochondrial membrane contain?

It is folded into cristae and contains enzymes for the electron transport chain.

What are lysosomes and what do they do?

Organelles with hydrolytic enzymes that break down cellular waste; can trigger autolysis

What is the function of the rough endoplasmic reticulum (RER)?

crowded with ribosomes; responsible for translating proteins destined for secretion.

What is the function of the smooth endoplasmic reticulum (SER)?

Lipid synthesis and detoxification.

What does the Golgi apparatus do?

Modifies, packages, and directs cellular products to specific locations.

What do peroxisomes do?

Contain hydrogen peroxide, break down long fatty acids via β-oxidation, and participate in phospholipid synthesis and the pentose phosphate pathway

What is the cytoskeleton?

Provides structure and facilitates transport within the cell.

What are microfilaments made of and what is their role?

Made of actin; they support the cell and help in muscle contraction and cytokinesis.

What are microtubules made of and what do they do?

Made of tubulin; serve as tracks for kinesin and dynein and are components of cilia and flagella.

What are intermediate filaments?

Provide cell structure and anchor organelles; examples include keratin and desmin.

What is the parenchyma?

The functional part of an organ formed by epithelial cells.

What is the stroma in tissue organization

Connective tissue that supports epithelial tissue and produces the extracellular matrix.

What are prokaryotes missing that eukaryotes have?

Membrane-bound organelles and a true nucleus.

What are the two domains of prokaryotes?

Archaea and Bacteria.

How are bacteria classified by shape?

Cocci (spherical), Bacilli (rod-shaped), Spirilli (spiral-shaped).

What are the different oxygen requirements for bacteria?

Obligate aerobes, obligate anaerobes, facultative anaerobes, aerotolerant anaerobes.

What are Gram-positive bacteria characterized by?

Thick peptidoglycan cell wall and lipoteichoic acid.

What are Gram-negative bacteria characterized by?

Thin peptidoglycan layer and outer membrane with lipopolysaccharides.

What is bacterial binary fission?

Asexual reproduction where the chromosome replicates and splits into two daughter cells.

What are plasmids?

Small circular DNA molecules in bacteria carrying non-essential genes, like antibiotic resistance.

What are episomes?

Plasmids that integrate into the bacterial genome.

What is transformation in bacteria?

Uptake and incorporation of foreign DNA from the environment.

What is conjugation in bacteria?

Transfer of DNA via a conjugation bridge from F+ to F– cells or from an Hfr cell.

What is transduction in bacteria?

Transfer of DNA from one bacterium to another via bacteriophages.

What are transposons?

Genetic elements that can insert or remove themselves from the genome.

What are the phases of bacterial growth?

Lag phase, exponential (log) phase, stationary phase, death phase.

What are viruses composed of?

Genetic material (DNA or RNA), a protein coat (capsid), and sometimes an envelope

What is a virion?

A complete viral particle.

What are tail sheath and tail fibers in bacteriophages?

Tail sheath injects genetic material; tail fibers attach the phage to host cells.

What is the difference between positive-sense and negative-sense RNA viruses?

Positive-sense can be directly translated; negative-sense requires RNA replicase.

What do retroviruses contain?

Single-stranded RNA genome and reverse transcriptase

What is the lytic cycle in bacteriophages?

Virus replicates and lyses the host cell.

What is the lysogenic cycle?

Viral DNA integrates into the host genome as a prophage and can later enter the lytic cycle.

What are prions?

Infectious proteins that cause misfolding of other proteins into β-sheets.

What are viroids?

Plant pathogens with small circular RNA that disrupt gene expression.

A virus consists of:

• Genetic material (DNA or RNA, single- or double-stranded)

• Capsid (protein coat)

• Envelope (optional, lipid bilayer derived from host membrane)

• Enzymes (in some, like reverse transcriptase in retroviruses)

What is the function of the viral envelope?

Helps viruses evade the host immune system and enter host cells via fusion with the cell membrane. Enveloped viruses are more sensitive to heat, detergents, and desiccation.

What is a virion?

A fully formed infectious virus particle outside of a host cell.

How do bacteriophages infect bacteria?

• Tail fibers to attach to the host

• Tail sheath to inject genetic material into the host bacterium

Double-stranded DNA (dsDNA)

requires host RNA polymerase for transcription

Single-stranded DNA (ssDNA)

converted to dsDNA in the host

Single-stranded RNA (ssRNA)

positive or negative sense

double-stranded RNA (dsRNA)

requires RNA-dependent RNA polymerase

What is a positive-sense RNA virus?

Its genome acts like mRNA and can be directly translated into protein by host ribosomes.

What is a negative-sense RNA virus?

Its genome is complementary to mRNA and must be converted into a readable mRNA strand by RNA-dependent RNA polymerase (which the virus must carry in the virion).

What are retroviruses?

A class of positive-sense ssRNA viruses that use reverse transcriptase to make DNA from RNA, which then integrates into the host genome as a provirus (e.g., HIV).

What are the major steps in the viral life cycle?

1. Attachment (adsorption) – binds specific receptors on host cell

2. Entry – via fusion (enveloped) or endocytosis

3. Replication and transcription – genome replicated and transcribed

4. Translation and assembly – viral proteins synthesized and assembled

5. Release – by budding (enveloped), lysis, or cell death

Lytic cycle

virus replicates rapidly and lyses host cell → virulent

Lysogenic cycle

viral DNA integrates into host genome as a prophage → temperate virus; can later reactivate into lytic cycle

What are viroids and how are they different from viruses?

Viroids are small circular RNA molecules (no capsid) that infect plants by silencing host genes.

What are prions?

Infectious proteins that misfold other proteins, converting α-helices into β-pleated sheets, leading to neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease, mad cow).

How are viral genomes replicated?

• DNA viruses often use host DNA polymerase

• RNA viruses must use RNA-dependent RNA polymerase

• Retroviruses use reverse transcriptase to synthesize DNA from RNA

How are enveloped vs non-enveloped viruses released?

• Enveloped viruses bud from the plasma membrane

• Non-enveloped viruses typically cause cell lysis

Conjunction

Transfer of genetic information from one bacterial cell to another via direct contact. The donar cell contains the F plasmid, allowing it to form a sex pilus. During this process, the donor cell transfers a copy of the plasmid to the recipient cell through the sex pilus, which facilitates the exchange of genetic material.

define bacterial transformation.

Uptake of free, foreign DNA from the environment into a bacterial cell.

bacterial transduction.

DNA transfer from one bacterium to another via a bacteriophage (virus that infects bacteria).

Define "habitat" in terms of bacterial classification.

The natural environment in which a bacterium resides (e.g., thermophiles live in high temperatures).

Define bacterial virulence.

The degree to which a pathogen can cause harmful, potentially life-threatening disease

what are the two main phases of the cell cycle?

Interphase (G₁, S, G₂) and M phase (mitosis + cytokinesis).

What happens during interphase?

Cell growth, organelle production, and DNA replication (S phase).

What happens during mitosis?

Chromosomes segregate into two nuclei, followed by cytokinesis to form two daughter cells.

Why is synchronization of cells necessary for cell cycle studies?

It ensures most cells are at the same phase for monitoring progression through the cell cycle.

what are the four stages of mitosis in most eukaryotes?

• Prophase: Chromatin condenses, nuclear envelope breaks down

• Metaphase: Chromosomes align at metaphase plate

• Anaphase: Sister chromatids separate to opposite poles

• Telophase: Nuclear envelope reforms, chromatin decondenses

What feature distinguishes viruses from cells?

Viruses lack ribosomes, so they depend on host cells for transcription and translation.

Why can’t the presence of DNA alone distinguish viruses from prokaryotic cells?

Both viruses and prokaryotes contain DNA, so DNA presence alone is insufficient.

How does the presence of a cell wall help classify pathogens?

A cell wall indicates a pathogen is likely prokaryotic or eukaryotic (e.g., fungi); absence makes a prokaryotic classification unlikely.

What does animal cell plasma membranes contain ?

Cholesterol
Glycolipids ( lipids attached sugar groups)
Glycoproteins ( proteins with attached sugar groups )
other proteins

Cholesterol is found in ?

Eukaryotic but not Prokaryotic cell membranes.

What is Cholesterols function ?

Cholesterol helps maintain membrane fluidity and stability in eukaryotic cell membranes.
Has a unique structure that allows it to fit between phospholipids, preventing tight packing.
Decreases fluidity at high temperatures and increases fludity at low temperatures.

Cytoskeletal Filaments are found in ?

Cytoplasm of cells and can interact directly with various components of the cell membrane (eg. by anchoring membrane proteins in place), however they are not themselves part of the plasma membrane

Animals cells do not have ?

a cell well however, nearly all prokaryotic cells do have one, providing structural support and protection.

Northern Blotting

An assay used to detect and measure the concentration of specific RNA molecules within a cell or tissue sample

1. RNA is isolated from the tissue and denatured
2. The RNA is separated by size via gel electrophoresis
3.The RNA samples are transferred from the gel to a blotting membrane and immobilized
4. The membrane is incubated with a lableled RNA or single-stranded DNA probe complememntrary to the RNA sequence of interest. If the sequence of interest is present, the probe binds to it ( hybridizes) and the label can be detected through various methods, allowing for visualization and quantification of the target RNA.
5. the sample is washed to remove unbound probes

Southern blotting

a technique used to detect and measure the concentration of

DNA in a cell. Accordingly, it can could detect the presence of a viral DNA genome but not an RNA genome.

DNA sequencing

determining the specific nucleotide order of a DNA molecule. This technique could be used to analyze genome contents, but it is not typically used to detect the presence of RNA.

Gene cloning

technique used to generate many copies of a previously identified gene of interest, is not useful for the detection of specific DNA or RNA sequences.

The phospholipid bilayer of the cell membrane has an?

outer leaflet that interacts with the extracellular environment and an inner leaflet that interacts with the cytoplasm of the cell. Transmembrane proteins cross the entire phospholipid bilayer and therefore interact with both the outer and inner leaflets. Proteins that interact with only one leaflet are not transmembrane proteins.

Cell membranes are composed largely of?

phospholipids, which act as a fluid that allows other membrane components such as transmembrane proteins, glycoproteins, cholesterol, and glycolipids to migrate through this environment laterally. Accordingly, the structure of the animal cell membrane is known as the fluid mosaic model.

The membrane of an animal cell is described by the?

fluid mosaic model, in which transmembrane proteins and other components reside in a fluid bilayer composed largely of phospholipids.

Translation of mRNA to proteins always begins in the?

cytosol

In the RER, some post-translational modifications may occur, including?

glycosylation, disulfide bond formation, phosphorylation, and protein cleavage. The RER then packages proteins into vesicles and sends them to the Golgi apparatus, where they are further processed. Finally, the Golgi packages proteins destined for the plasma membrane into secretory vesicles which fuse with the plasma membrane. Proteins that were initially embedded in the RER membrane frequently end up embedded in the plasma membrane as receptor proteins.

The secretory pathway involves?

the processing of proteins as they go through the endoplasmic reticulum and the Golgi apparatus. After processing in the Golgi, proteins destined for the cell membrane are packed into secretory vesicles, which fuse with the plasma membrane. Other organelles, such as the nucleus, mitochondria, and peroxisomes, are not involved in the secretory pathway, and most of their proteins are translated in the cytosol.

active transport systems.

When molecules undergo net movement from a region of one concentration into a region of greater or equal concentration, they are moving against their concentration gradient. For this to occur, energy must be put into the system because the process is thermodynamically unfavorable. Transport systems that require energy input and move substances against their concentration gradient is this

protons from the cytosol can only enter the endosomal lumen through active transport because ?

Endocytic vesicles obtain their fluid from the extracellular space, which is at physiological pH

(7.4). This is the same as the pH of the cytosol. Consequently, acidification of the endosomal lumen involves the transport of protons against their concentration gradient from an area of relatively low concentration (cytosol; pH ~ 7.4) to one of equal or higher concentration (endosomal lumen; pH ~ 7.4-5.5).

Simple diffusion and facilitated diffusion are both forms of

passive transport, in which no energy input is required and molecules diffuse down their concentration gradient. In simple diffusion, molecules that are small or lipid-soluble travel across the cell membrane directly, without help from protein channels. In facilitated diffusion, molecules that cannot normally cross the membrane on their own due to charge or size are allowed to pass through protein channels or gates that do not require energy input to operate.

Microtubules also facilitate transport of vesicles such as?

endosomes and other organelles from one location in the cell to another. The ones involved in intracellular transport originate near the nucleus on small organelles called centrosomes. The centrosome consists of two smaller entities called centrioles, and microtubules radiate out from this organelle toward the plasma membrane.

kinetochore

a protein-DNA complex to which spindle fibers, made of microtubules, attach during metaphase of mitosis.

Microtubules

structural cytoplasmic filaments composed of tubulin subunits. These filaments have a number of critical functions, including serving as tracks for intracellular transport of organelles and vesicles. The movement of intracellular cargo along microtubules is mediated by two motor proteins: kinesin and Dynein

Microtubules ( Kinesin)

• Moves intracellular cargo along microtubules in anterograde axonal transport (ie, away from the nucleus and toward distal sites).

Microtubules ( Dynein)

• Participates in retrograde axonal transport of intracellular cargo (ie, from distal sites toward the nucleus)

The cytoskeleton is composed of three types of fibers:

Microfilaments, intermediate filaments, and microtubules. Together, these fibers function to organize cellular components, support cellular motility (eg, cell movement, intracellular transport), and give the cell its shape.

Movement along actin microfilaments occur via?

myosin motors (not kinesin or dynein). The primary functions of myosin movement along microfilaments are cellular locomotion, cellular division, and muscle contractions.

Cells can exist in many forms and sizes; some live as independent (free-living) organisms, and others live as a group in a colony or make up the body of a multicellular organism. Based on their shared characteristics, all living organisms can be classified into one of three domains:

Bacteria, Archaea, or Eukarya.

Telomeres

regions of repetitive nucleotide sequences at the ends of eukaryotic chromosomes that shorten with each round of cell division. They are present only on linear eukaryotic chromosomes and protect these ends from deteriorating. In contrast, both archaea and bacteria have circular chromosomes (lacking these)

Proofreading of DNA normally:

occurs during S phase.

DNA is replicated and proofreading occurs in S phase of mitosis and meiosis.

The tail fibers of the virus allows a virus to ?

identify and infect the host cell.