Pointers to Review in STEM 4: General Biology 1 (Midterm Examination)

CELL DISCOVERY AND CELL THEORY

• The cell theory is a postulated and widely accepted hypothesis of how most life on Earth operates. It states that cells comprise all organisms. Organs, tissues, and organisms are made up of groups of cells.

• Fundamental assumptions of cell theory include:

  • The cell is the fundamental unit of structure and function in living things.
  • Examples: Red blood cells transport oxygen throughout the body; a single-celled amoeba performs all life processes within a single cell. This is considered the main principle of cell theory.
  • All organisms are made up of one or more cells.
  • Examples: The human body is composed of trillions ($10^{12}$) of cells that form tissues, organs, and organ systems; a microscope reveals that a leaf is made up of many tiny cells.
  • Cells came from other pre-existing cells through cellular division.
  • Examples: A scientist observes that bacteria reproduce by dividing into two ($2$) new bacteria cells; during wound healing, skin cells divide to replace damaged cells.
  • This concept means new cells are produced by the division of existing cells, not by spontaneous generation from nonliving matter (the idea of life on Earth evolving from inorganic molecules).

• The expanded version of the cell theory includes additional points:

  • Cells carry genetic material passed to daughter cells during cellular division.
  • All cells are essentially the same in chemical composition.
  • Energy flow, including metabolism and biochemistry, occurs within cells.

• Key contributors to Cell Theory:

  • Robert Hooke: A scientist who observed cork cells and named the structures "cells." His discovery led to the understanding of cells as the smallest units of life.
  • Anton Van Leeuwenhoek: A fabric merchant and the first person to observe and describe microorganisms using handmade microscopes. His single most important discovery was the existence of single-cell organisms, which he called "animalcules."
  • Matthias Schleiden: A German botanist who concluded that all plant tissues are composed of cells and that an embryonic plant arises from a single cell. He reached this conclusion while studying different kinds of plants.
  • Theodor Schwann: A German zoologist who concluded that animals were composed of cells and is credited with establishing the idea that animals are made of cells.
  • Rudolf Virchow: A scientist who concluded and proposed that every cell comes from a pre-existing cell. He used this theory to lay the groundwork for cellular pathology, or the study of disease at the cellular level.
  • Janssen Brothers (Hans and Zacharias Janssen): Spectacle makers who invented the compound microscope and are credited with inventing the first microscope. The discovery of the lens caught their attention, leading to this invention.

QUESTIONS & DISCUSSION

• Question: How would our lives be different today if scientists had never discovered and studied cells?
• Response: If scientists had never discovered and studied cells, lives today would look very different and be significantly more confusing regarding the understanding of health, life, and disease. We would not know that all living things are made of cells or how the body grows, repairs itself, and functions. Microscopes would not have been used to visualize cells, leaving important structures like the nucleus and cell membrane unknown. The contributions of scientists such as Hooke, Leeuwenhoek, Schleiden, and Virchow would not have led to the development of cell theory. Consequently, modern medicine, disease treatment, and scientific understanding of life processes would not exist as they are known today.

CELL STRUCTURES AND THEIR FUNCTIONS

• A cell is the smallest living unit capable of performing all life processes. It contains specific structures that allow it to carry out essential functions for survival.

• Reasons why the cell is the fundamental unit of life:

  • Cells are the smallest unit of life that can function independently.
  • All known organisms have one ($1$) or more cells.
  • Some cells are separate living entities that can survive individually.

• Comparison of Cell Types (Eukaryotic vs. Prokaryotic):

  • Eukaryotic Cells:
  • Have a nucleus.
  • Found in multicellular organisms.
  • DNA is located in the nucleus.
  • Some have a cell wall.
  • Larger in size.
  • Prokaryotic Cells:
  • Have no nucleus.
  • Typically single-celled organisms.
  • DNA is located in the nucleoid region.
  • Most have a cell wall.
  • Small in size.
  • Shared Characteristics: Both prokaryotic and eukaryotic cells contain DNA, which stores genetic information.

• Organelles are known as "little organs." Each organelle has a unique function, such as producing energy, making proteins, or removing waste.

• Three major parts of a Eukaryotic cell:

  • Nucleus
  • Plasma Membrane
  • Cytoplasm

• Specific Cell Structures and Their Functions:

  • Nucleus: The control center of an animal cell containing the cell’s DNA/chromosomes. During the cell cycle, chromosome replication occurs inside the nucleus.
  • Plasma Membrane (Cell Membrane): Controls what enters and leaves the cell; protects the cell and maintains its shape.
  • Cytoplasm: A jelly-like substance that holds organelles and is the site of many cell activities.
  • Nucleolus: Located inside the nucleus; produces ribosomes needed for making proteins.
  • Ribosomes: Major site of protein synthesis. Synthesis occurs in the cytoplasm or on the rough ER. Ribosomes make proteins for cell growth, repair, and daily functions.
  • Endoplasmic Reticulum (ER): A network that transports materials inside the cell.
  • Rough Endoplasmic Reticulum: Has ribosomes; makes and transports proteins.
  • Smooth Endoplasmic Reticulum: The site of lipid synthesis; makes lipids (fats), detoxifies harmful substances, and helps in metabolism.
  • Golgi Apparatus (Golgi Body): Modifies, packages, and transports proteins and other materials.
  • Mitochondria: Produces energy (ATP) through cellular respiration; known as the ‘powerhouse of the cell.’
  • Lysosomes: Break down waste materials, old cell parts, and harmful substances.
  • Vacuoles: Store water, food, nutrients, and waste products. They are large in plant cells.
  • Cytoskeleton: Provides support, maintains cell shape, and helps movement of cell parts.
  • Centrioles: Help in cell division by organizing spindle fibers; mainly found in animal cells.
  • Chloroplasts: Found in plant cells only; contain chlorophyll and perform photosynthesis to make food using sunlight.
  • Cell Wall: Found in plant cells only; provides support, protection, and a fixed shape for the cell.
  • Cilia: Found in some animal cells (such as cells lining the respiratory tract and reproductive system); small hair-like structures that help move the cell or move substances across the surface of cells in coordinated waves.
  • Flagellum: A whip-like structure used for movement.

• Specialized cell structures:

  • Villi: Finger-like projections in the small intestine of animals/humans that increase surface area for faster and more efficient absorption of nutrients into the bloodstream.
  • Pili: Short, hair-like structures made of protein found in bacterial cells; they help bacteria attach to surfaces and exchange genetic material/DNA.

• General Cell Functions:

  • Nutrition: Taking in and processing nutrients (e.g., cells in the intestines absorbing nutrients).
  • Respiration: Breaking down food to release energy (ATP) (e.g., a cell breaking down glucose).
  • Excretion: Removing waste products (e.g., lung cells removing carbon dioxide from the bloodstream).
  • Growth and Development: Increasing in size and complexity (e.g., cells producing proteins to build and repair tissues).
  • Reproduction: Producing new cells through cell division (e.g., skin cell division).

• Cell Part Analogies:

  • Brain of the cell: Nucleus
  • Subway System: Endoplasmic Reticulum
  • Garbage collector: Lysosome
  • Security guard: Plasma membrane
  • Protein factory: Ribosome
  • Packaging area: Golgi body
  • Transformer: Mitochondria
  • Water tanks: Vacuoles
  • Cafeteria: Chloroplast

• Cell as a Factory Analogy and Drawing Layout:

  • Factory Building / Walls: Cell Membrane (Controls entry/exit).
  • Factory Manager's Office: Nucleus (Controls activities; contains DNA instructions).
  • Blueprint Room: DNA (Instructions for making products).
  • Machines on the Production Line: Ribosomes (Make proteins).
  • Assembly Line: Endoplasmic Reticulum (Transports/processes materials).
  • Packaging Department: Golgi Apparatus (Prepares products for export).
  • Delivery Trucks: Vesicles (Carry products to locations).
  • Power Generator: Mitochondria (Provides energy).
  • Waste Disposal Area: Lysosomes (Breaks down waste).
  • Storage Room: Vacuole (Stores materials/waste).
  • Factory Support Beams/Rails: Cytoskeleton (Structure/movement).
  • Factory Gate: Transport Proteins (Part of Cell Membrane; controls movement of materials).

• Drawing Layout Suggestions:

  • $1$. Large factory building labeled Cell Membrane.
  • $2$. Office in the center labeled Nucleus.
  • $3$. Machines/assembly lines labeled Ribosomes + ER.
  • $4$. Packaging area with boxes labeled Golgi Apparatus.
  • $5$. Trucks leaving the factory labeled Vesicles.
  • $6$. Power station labeled Mitochondria.
  • $7$. Storage room labeled Vacuole.
  • $8$. Trash/recycling area labeled Lysosomes.

• Cells are like factories or cities because organelles work together to keep the cell functioning, much like workers, machines, and shipping departments in a factory or power plants, roads, and government offices in a city.

CELL CYCLE REGULATION

• The cell cycle must be carefully controlled to regulate healthy cell growth. It is the process by which cells grow, copy their DNA, and divide to produce new cells.

• Purposes of the Cell Cycle:

  • Growth: Increases cell count for organism development (e.g., a child growing taller).
  • Repair and Healing: Replaces damaged cells and repairs tissues (e.g., a cut skin cell dividing to close a wound).
  • Cell Replacement: Replaces old or dead cells (e.g., old skin flaking off and being replaced).
  • Reproduction: In single-celled organisms, one cell produces a new organism (e.g., an amoeba dividing).
  • Maintain Homeostasis: Keeps tissues balanced with the right number of cells (e.g., bone marrow producing blood cells).

• Two Main Stages of the Cell Cycle:

  • Interphase: Longest stage; cell grows, performs normal functions, and prepares for division.
  • G$_1$ (Gap $1$): Cell increases in size, makes proteins and organelles.
  • S (Synthesis): Cell replicates/copies its DNA (e.g., human cell duplicating all $46$ chromosomes).
  • G$_2$ (Gap $2$): Continued growth, final check for DNA errors, and protein synthesis for division.
  • Mitotic (M) Phase: The cell divides into two ($2$) identical daughter cells.
  • Mitosis: Division of the nucleus (Prophase, Metaphase, Anaphase, Telophase).
  • Cytokinesis: Division of the cytoplasm.

• Cell Death and Checkpoints:

  • Human cells are programmed to undergo many divisions and then die.
  • Apoptosis: Orderly, programmed cell death process.
  • Necrosis: Uncontrolled cell death caused by injury or damage.

• Cancer and Tumors:

  • Cancer is caused by uncontrolled cell growth/division due to DNA mutations in genes controlling the cell cycle.
  • Normal cells follow checkpoints; cancer cells ignore them and divide uncontrollably.
  • Tumor: An abnormal mass of cells.
  • Benign tumor: Non-cancerous, does not spread, usually grows slowly, easier to remove.
  • Malignant tumor: Cancerous, invades nearby tissues, and can spread through metastasis.

MITOSIS

• Mitosis occurs in somatic (body) cells for growth, repair, and replacement.

• Results of Mitosis: One parent cell produces two ($2$) genetically identical diploid daughter cells/nuclei.

• Genetic Materials in Division:

  • DNA: Molecule carrying genetic info; replicates before division.
  • Chromatin: Uncoiled DNA and proteins; allows replication during Interphase.
  • Chromosomes: Condensed chromatin; ensures accurate DNA distribution.
  • Sister Chromatids: Two ($2$) identical copies of a chromosome.
  • Centromere: Region joining sister chromatids; attaches to spindle fibers.

• Stages of Mitosis:

  • Prophase: Chromatin condenses into visible chromosomes; sister chromatids join at the centromere; nuclear membrane breaks down; spindle fibers form.
  • Metaphase: Chromosomes line up at the equator (center); spindle fibers attach to centromeres.
  • Anaphase: Sister chromatids separate at the centromere and are pulled to opposite poles.
  • Telophase: Chromosomes reach poles and uncoil into chromatin; new nuclear membranes form; spindle fibers disappear.
  • Cytokinesis: Cytoplasm divides to form two ($2$) separate identical daughter cells.

MEIOSIS

• Meiosis is the cell division type that produces gametes (egg and sperm cells).

• Results of Meiosis: Four ($4$) haploid daughter cells/nuclei. It reduces the chromosome number from diploid ($2n$) to haploid ($n$).

• A normal human zygote has $46$ chromosomes (diploid), receiving $23$ from each parent.

• Meiosis Functions:

  • Produces Gametes: Forms sperm in testes and eggs in ovaries.
  • Reduces Chromosome Number: Changes diploid cells into haploid cells (human gametes = $23$ chromosomes).
  • Creates Genetic Variation: Produces genetically different gametes; reason why siblings differ.
  • Maintains Chromosome Number: Ensures the correct count ($46$) after fertilization.

• Stages of Meiosis I and II:

  • Prophase I: Homologous chromosomes pair up; crossing over occurs for genetic variation.
  • Metaphase I: Homologous pairs line up at the center.
  • Anaphase I: Homologous chromosomes separate; chromosome number is reduced.
  • Telophase I & Cytokinesis: Two ($2$) haploid cells are formed.
  • Prophase II: Spindle fibers form again.
  • Metaphase II: Chromosomes line up individually.
  • Anaphase II: Sister chromatids separate.
  • Telophase II & Cytokinesis: Four ($4$) haploid daughter cells are formed.

• Genetic Disorders:

  • Down Syndrome (Trisomy $21$): Caused by an extra copy of chromosome $21$. Results in intellectual disability and distinctive facial features.
  • Turner Syndrome ($45, X$): Caused by a missing X chromosome in females. Leads to short stature and infertility.
  • Klinefelter Syndrome ($47, XXY$): Extra X chromosome in males. Leads to reduced muscle mass and infertility.
  • Trisomy X Syndrome ($47, XXX$): Extra X chromosome in females. Often mild symptoms; may include taller height or learning difficulties.
  • Patau Syndrome (Trisomy $13$): Extra copy of chromosome $13$. Severe defects in brain and heart; shortened lifespan.
  • Edwards Syndrome (Trisomy $18$): Extra copy of chromosome $18$. Severe developmental delays and heart defects.
  • Cri-du-chat Syndrome: Deletion of part of chromosome $5$. Characterized by a high-pitched, cat-like cry in infancy and intellectual disability.

QUESTIONS & DISCUSSION

• Question: Chemotherapy is a drug treatment using powerful chemicals to kill fast-dividing cells. Explain why people get bald after chemotherapy.

• Response: Chemotherapy drugs target and kill fast-dividing cells. While meant to destroy cancer cells, they also affect healthy cells that divide quickly, such as hair follicle cells. Hair follicle cells normally divide rapidly to produce hair. When damaged, hair growth stops, causing thinning or loss. Follicles often recover after treatment ends.

• Question: Describe how problems in the cell cycle, particularly during mitosis or meiosis, can lead to genetic disorders like Down syndrome.

• Response: Disorders like Down syndrome occur due to errors during division, specifically meiosis. A common error is nondisjunction, where chromosomes fail to separate properly. This leads to gametes with extra or missing chromosomes. If an abnormal gamete with an extra chromosome $21$ participates in fertilization, the baby has three ($3$) copies of chromosome $21$ (Trisomy $21$). Errors in mitosis checkpoints or chromosome separation can also lead to abnormal chromosome numbers, causing growth problems or cancer.