All You Need to Know: MCAT Cells and Viruses
Organisms rely on cells as their fundamental building blocks, playing a crucial role in the MCAT’s biology section. Cells, being versatile subjects, are not only directly testable but also form the core of various concepts explored in biology passages and experiments. Therefore, establishing a solid understanding of cellular biology is essential for effective MCAT preparation.
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Understanding Cells
A cell, the tiniest unit in the body, is the fundamental building block of life, responsible for both structure and function. Various organisms have different cell numbers; some, like amoebas, are single-celled, while humans have trillions of cells. Cells also vary in size and shape, with examples ranging from the smallest, Mycoplasma gallicepticum, to the largest, the ostrich egg. Shapes can be spherical (like red blood cells), elongated (nerve cells in humans), or spindle-shaped (muscle cells).
The Journey of Cell Discovery
In 1665, Robert Hook discovered cells by examining a small cork slice under a microscope, noting honeycomb-like patterns. This marked the beginning of understanding cells. The cell theory, proposed by German scientists Theodor Schwann and Matthias Jakob Schleiden in 1838, established three principles: all living things consist of cells, cells are life’s basic units, and new cells originate from existing ones.
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Inside the Cell: Components and Types
All cells share fundamental features like the plasma membrane, cytosol, chromosomes, ribosomes, and nucleus. Yet, cells differ based on being eukaryotic or prokaryotic, primarily in DNA placement. Eukaryotic cells have a double-membraned nucleus housing DNA, while prokaryotic cells have non-membranous nucleoids.
Prokaryotic and Eukaryotic Cells: Structure and Function
Prokaryotic cells are simpler, lacking a true nucleus and membrane-bound organelles. They possess a circular chromosome, plasmids, a cell wall made of peptidoglycan, plasma membrane, glycocalyx, and fimbriae for attachment and flagella for movement. Binary fission drives their reproduction.
Eukaryotic cells, in contrast, are more complex with a true nucleus and diverse organelles suspended in cytosol. The plasma membrane’s fluid mosaic structure supports selective permeability, protection, and cellular transport through endocytosis and exocytosis. Plant cells additionally have a rigid cell wall regulating shape and preventing excessive water absorption. Specialized organelles perform various functions within eukaryotic cells.
The Nucleus: Control Center of the Cell
At the heart of the cell lies the nucleus, often referred to as the cell’s brain. Enclosed by a double-membrane nuclear envelope, it processes information and contains chromosomes and the nucleolus. Chromosomes, composed of DNA and histone proteins, form thread-like structures known as chromatin. The nucleolus, a dense structure, serves as a site for ribosomal RNA (rRNA) synthesis. The number of nuclei in a cell depends on its species and reproductive stage, with DNA duplication and mRNA synthesis occurring within the nucleus.
Ribosomes: Cell’s Protein Factories
Ribosomes, the cell’s protein factories, consist of RNA and protein. Divided into free and bound types, they synthesize various proteins. Free ribosomes float in the cytoplasm, while bound ribosomes are attached to the nuclear envelope or endoplasmic reticulum. Each type produces specific proteins, such as membrane proteins and secreted enzymes by bound ribosomes, and cytosol-retained proteins by free ribosomes.
Endoplasmic Reticulum (ER): Cellular Network of Membranes
Comprising cisternae, membrane sacs, the endoplasmic reticulum (ER) is an extensive network in the cytoplasm. The rough ER, with associated ribosomes, is involved in protein synthesis, modification, and transportation. The smooth ER supports metabolic processes like calcium ion storage, glucose metabolism, detoxification, and lipid synthesis.
Golgi Apparatus: Cell’s Processing and Transport Center
Functioning as the cell’s processing and transport center, the Golgi apparatus consists of connected membrane sacs (cisternae). It processes, stores, and transports proteins and ER products, with vesicles aiding material transport.
Vacuoles: Cell’s Storage Units
Produced by the Golgi apparatus and endoplasmic reticulum, vacuoles are large vesicles with diverse functions. Animal cells contain small vacuoles, while plant cells have a single large vacuole providing rigidity. Vacuoles store food, remove excess water (contractile vacuoles), and perform various cell-specific tasks.
Lysosomes and Peroxisomes: Cell’s Cleanup Crew
Lysosomes, known as the cell’s suicidal bags, house hydrolytic enzymes to break down macromolecules in an acidic medium. Peroxisomes are specialized compartments conducting oxidation reactions and breaking down hydrogen peroxide.
Mitochondria: Powerhouses of the Cell
Mitochondria serve as the cell’s powerhouses, facilitating cellular respiration and ATP generation. Enclosed by a double-membrane envelope with inner membrane folds (cristae), they house mitochondrial DNA, ribosomes, and enzymes in the matrix.
Plastids: Food Production and Storage Centers
Present in algae and plant cells, plastids are double-membrane organelles for food production and storage. Chloroplasts, a type of plastid, conduct photosynthesis with thylakoids forming granum and stroma containing enzymes, ribosomes, and chloroplast DNA. Amyloplasts accumulate starch, while chromoplasts determine colors in fruits and flowers.
Cytoskeleton: Cell’s Support System
Supporting cells, the cytoskeleton is a fiber network vital for animal cells lacking cell walls. Microtubules, microfilaments, and intermediate filaments provide structural support.
Flagella and Cilia: Cellular Locomotion
Present in some eukaryotic cells, flagella and cilia are cellular extensions made of microtubules. They act as locomotor appendages, with flagella in sperm and cilia performing various functions like mucus movement in the trachea. Flagella are longer and less abundant than cilia, which are smaller and more numerous on cell surfaces.
Cell Signaling: Chemical Conversations
Cells engage in communication through chemical signals, a process involving signal reception, transduction, and cell response. Both multicellular and unicellular organisms employ diverse pathways for this purpose. Cell communication occurs either locally or over long distances.
Local Signaling: Communication Up Close
Cell communication at a close range involves local signaling between adjacent cells. Signals acting locally, known as paracrine signals, include growth factors in animals that regulate the growth of neighboring target cells. Synaptic signaling, specific to the nervous system in animals, involves an electric signal passing along nerve cells. Neurosecretory cells release neurotransmitters (e.g., acetylcholine), acting as chemical signals that cross the synapse (the narrow junction between nerve and target cells) to reach effectors (neurons or muscles) and elicit a response.
Long-Distance Signaling: Hormones in Action
Extended signaling over prolonged distances, termed endocrine signaling, is achieved by animals and plants using hormones. Specialized cells release hormones that travel through the bloodstream in animals or via other cells in plants to reach target cells. Common animal hormones include insulin, glucagon, prolactin, oxytocin, and thyroxine, while plant hormones encompass ethylene, auxin, cytokinin, among others.
Cell Division: The Two Paths
Cells undergo division through two mechanisms: mitosis and meiosis.
Mitosis: Building Blocks of Growth
Mitosis is the primary process forming new body cells. DNA replication occurs during interphase (G1, S, and G2 phases), followed by the four stages: prophase, metaphase, anaphase, and telophase. Prophase involves the disappearance of the nuclear membrane, chromosome condensation, and spindle formation. In metaphase, chromosomes align at the metaphase plate, transitioning to anaphase with sister chromatid separation. Telophase sees chromosome decondensation and nuclear envelope reformation, followed by cytokinesis, splitting the cytoplasm into genetically similar daughter cells. Mitosis is crucial for normal body development and growth; uncontrolled mitosis may lead to health issues like cancer.
Meiosis: Specialized Division
Meiosis, occurring only in germ cells, produces four daughter cells with haploid chromosomes in two rounds: meiosis I and meiosis II. Similar to mitosis, meiosis involves phases like prophase, metaphase, anaphase, and telophase, with differences such as homologous chromosome separation in meiosis.
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Meiosis I: The First Round of Division
Before embarking on meiosis I, a cell undergoes interphase, ensuring the replication of chromosomes in the diploid germ cell. This replication results in the formation of paired duplicate chromatids.
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Prophase I: Chromosome Dance
During Prophase I, visible changes unfold. Chromosome constriction initiates, with chromosomes pairing up to form tetrads. Each chromosome aligns with its homologous partner, facilitating crossing over—a process where chromosome segments are exchanged. This interchange leads to the creation of recombinant chromosomes.
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Metaphase I: Chromosome Capture
Moving into Metaphase I, the spindle attaches to the chromosomes, relocating them to the cell’s center known as the metaphase plate. Here, homologous pairs, not individual chromosomes, align for separation. The random positioning of each pair results in gametes with diverse sets of homologs.
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Anaphase I: Homologs on the Move
The onset of Anaphase I sees homologs separating and migrating to opposite ends of the cell. This movement marks the beginning of cell division.
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Telophase I: Nucleus Redux
As chromosomes reach their respective poles, Telophase I commences. The nuclear membrane reforms, and chromosomes decondense, culminating in the formation of two haploid daughter cells through cytokinesis.
Meiosis II: The Final Act
The haploid daughter cells from meiosis I proceed to meiosis II, where their chromosomes have two sister chromatids. Following a pathway similar to mitosis, the division takes place. Ultimately, meiosis concludes with the formation of four haploid daughter cells. In humans, this outcome manifests as either sperm or egg cells. Through reproduction, the cell cycle initiates once more.
Understanding Viruses: Microscopic Intruders
The term “virus,” rooted in the Latin word for “poison,” refers to an infectious agent comprising a protein-coated nucleic acid segment, either DNA or RNA. Unlike independent entities, viruses depend on host cells for reproduction, utilizing the host’s machinery and cellular metabolism. Among the tiniest microbes, most viruses measure between 20 to 400 nanometers in diameter. Notably, the largest known viruses, termed Mimivirus, exceed 400 nm in diameter.
Viruses: Historical Journey
In 1883, German scientist Adolf Mayer delved into the study of tobacco mosaic disease. Initially unsuccessful in finding microorganisms in diseased leaves, he proposed a bacterium as the culprit. Subsequently, Dmitri Ivanowsky’s experiment with sap from infected tobacco plants revealed a microorganism present in the filtrate. Martinus Beijerinck furthered the research, observing that this infectious agent could replicate but couldn’t be cultured on nutrient media like bacteria. In 1935, American scientist Wendell Stanley successfully studied the tobacco mosaic virus (TMV), crystallizing it from infected plant leaves and revealing its composition of RNA and protein, marking the commencement of the era of virus exploration.
Genome: Viral Genetic Diversity
Viruses possess either DNA or RNA as their genetic material, and this can be single-stranded or double-stranded. The genome may consist of a single nucleic acid molecule, either linear or circular. The smallest viruses may have only three genes, while the largest can have several hundred to up to 2,000 genes.
Classification of viruses is based on their genome, categorizing them as DNA or RNA viruses. RNA viruses are prevalent in plant viruses and some bacteriophages, while both RNA and DNA viruses can infect animals.
To read more on this topic, check out Jack Westin’s MCAT Content Hub on Viral Structures.
DNA Viruses: The Genetic Code
DNA viruses encompass both single-stranded and double-stranded variants. Among them, double-stranded DNA viruses exhibit distinct characteristics:
Virus Replication: The Lytic and Lysogenic Cycles Simplified
When viruses replicate, it happens in three steps: starting the infection, copying the genetic material, and releasing new viruses. The infection begins with the virus entering the host. The way this happens depends on the type of virus. Most viruses use a process called endocytosis, while bacteriophages use their tails to attach to the host. Once inside, the virus directs the host machinery to make viral proteins and copy the viral genetic material. After this, new viruses are put together, breaking the host cell in the process and spreading the infection.
Bacteriophages have two ways to replicate. In the lytic cycle, they burst the bacterium, spreading to infect other cells. This type is called a virulent phage and causes the host cell to die.
On the flip side, in the lysogenic cycle, the phage genetic material replicates without harming the bacterium. Here, the viral and host genetic materials coexist peacefully. These are called temperate phages.
Sample MCAT Questions
Question 1
Which cellular structure is responsible for the synthesis, modification, and transportation of proteins?
- a) Endoplasmic Reticulum (ER)
- b) Golgi Apparatus
- c) Ribosomes
- d) Nucleus
Correct Answer:
a) Endoplasmic Reticulum (ER)
Explanation:
The endoplasmic reticulum (ER) is an extensive network of membranes involved in protein synthesis, modification, and transportation. The rough ER, in particular, is associated with ribosomes and plays a crucial role in these cellular processes.
Question 2
During which phase of meiosis does crossing over occur, leading to the formation of recombinant chromosomes?
- a) Metaphase I
- b) Anaphase I
- c) Prophase I
- d) Telophase I
Correct Answer:
c) Prophase I
Explanation:
Crossing over occurs during Prophase I of meiosis, where chromosomes pair up to form tetrads, and segments are exchanged between homologous chromosomes, leading to the creation of recombinant chromosomes.
Conclusion
In summary, a thorough grasp of cellular biology and viruses is indispensable for success in the MCAT exam. Cells, the fundamental units of life, form the basis for numerous concepts tested in the biology section. Understanding cell discovery, components, division, and the microscopic world of viruses is essential for effective MCAT preparation. Proficiency in these topics not only ensures success in the exam but also establishes a solid foundation for broader comprehension in medical studies.
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