Cell Biology Overview Quiz

Cytology and Histology: Cell Structure

• Cytology and histology are the study of cells and tissues, respectively
• Cell theory is the unifying foundation of cell biology
• Cell theory's three principles:
  • All organisms are composed of one or more cells, and life processes of metabolism and heredity occur within these cells
  • Cells are the smallest living things; the basic units of organization of life
  • Cells arise only by division of a previously existing cell
• Each cell is a complete living being; a cell can:
  • Absorb and process food
  • Reject waste
  • Breathe
  • Secret substances
  • Reproduce
  • Repair damaged parts
• The cells of all living organisms are very similar to each other with few differences between species
• A cell represents:
  • A structural unit: supporting biological activities
  • A reproduction unit: development of organs or repairing of organs
  • An information unit: contains hereditary information
  • A unit of function: ensures the realization of biological activities necessary for life

Prokaryotes vs. Eukaryotes

• The world of cells is divided into two main groups fundamental different in internal structure and general organization
• Two main types:
  • Prokaryotic cells (bacteria, archaebacteria)
    • 1 to 10 µm in general size
    • Unicellular organisms
    • Genetic material not enclosed in a membrane (nucleoid)
    • Lack an internal membrane system
    • Cell division mainly by binary fission, which does not utilize a spindle
  • Eukaryotic cells
    • 10 to 100 µm in general size
    • Uni or multicellular organisms
    • Genetic material enclosed in a membrane (nucleus)
    • Possess numerous membrane-bounded organelles
    • Cell division by mitosis, which utilizes a spindle

Prokaryotic Cell Division by Binary Fission

• Prokaryotic cells divide by binary fission. This process involves:
  • DNA replication
  • Segregation of DNA
  • Cell splitting into two identical daughter cells

Mitosis

• Mitosis is a type of cell division in eukaryotic cells.

All Cells Share Many Structural Features

• All cells share four major features in common
  • Nucleoid (prokaryotes) or Nucleus (eukaryotes): The DNA contains genes coding for proteins
  • Cytoplasm: A semifluid matrix containing sugars, amino acids, and proteins necessary for the cell's daily function
  • Ribosomes: Synthesizing proteins
  • Plasma membrane: A phospholipid bilayer with proteins embedded in it for functions like receptors, enzymes, channels, pumps

Prokaryotic Cells: Simple Organization

• Prokaryotes lack nuclei and linear chromosomes, instead possessing a circular DNA ring (nucleoid region)
• Many prokaryotes use plasmids, which are small, independently replicating DNA circles containing genes for selective advantage but not essential for survival
• Prokaryotic cells have a cell wall (except mycoplasma)
  • The cell wall consists of peptidoglycan, a carbohydrate matrix cross-linked to short polypeptide units, protecting the cell, maintaining its shape, and preventing water loss
  • Cell wall protects the cell and gives it shape. Gram-negative have a thinner peptidoglycan layer and an outer membrane, leading to greater antibiotic resistance
• The plasma membrane resembles eukaryotic cells made from a phospholipid bilayer with embedded intrinsic and peripheral proteins, necessary for control over intake and elimination of waste
  • Possesses protein components with multiple functions (transporters, receptors, respiratory chain proteins)
• Prokaryotic cells come in three basic shapes
  • Cocci (spheres)
  • Bacilli (rods)
  • Spirilla (spirals)

S-layer, Capsule, Flagella and Pili

• S-layer: An additional glycoprotein layer providing surfaces or protection
• Capsule: A gelatinous layer on the cell wall that enhances cell adhesion, evasion of immune response and virulence.
• Flagella: Slender flexible protein filaments assisting the movement in liquid environments
• Pili: Hair-like structures that assist in adhesion and bacterial conjugation (genetic exchange)

Bacterial Conjugation

• Bacterial conjugation is a process of genetic transfer between two bacteria through a pilus

Cell Division: Binary Division

• The genetic material is first duplicated. The daughter cells are identical to the mother cell, forming a colony of genetically identical cells.

Archaea

• Archaea have unusual membrane lipids distinct from bacteria. Peptidoglycan is not found on the cell wall.

Eukaryotic Cells

• Eukaryotic cells are distinctly more complex than prokaryotes. Their defining feature is compartmentalization (endomembrane systems and organelles).
• Eukaryotic organisms include animals, plants, fungi and protists (as well as most algae)

Shape

• Examples of eukaryotic cell shapes include globular (isolated cells), lenticular (erythrocytes), flagellate (spermatozoids) and some that extend in form (nervous cells)

Size

• Some eukaryotic cells are visible to the naked eye (hen's egg is 3cm) and others need magnification (leukocytes are ~5μm and muscle cells are ~250μm)

Turnover Rate

• Turnover rate varies depending on the cell type and its function: for example, WBCs have a turnover of 1–2 weeks, platelets typically last 10- 12 days and hepatocytes or pancreatic cells can last for a few months.

Organization Eukaryotic Cell

• Eukaryotic cells have components of a plasma membrane and a cytosol, containing individualized and highly specialized organelles
• These organelles include the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes and cytoskeleton

The Nucleus

• Round or spherical shape with a typically central location in the cell
• Houses the genetic material (DNA)
• Exhibits a dark-staining area called the nucleolus, involved in ribosome synthesis
• Nucleus and nucleolus surrounded by the nuclear envelope
• Composed of DNA (deoxyribonucleic acid) wrapped around protein "histones"

The Nuclear Envelope

• Double phospholipid bilayer
• Contains pores that allow molecules to diffuse between the nucleoplasm (inside the nucleus) and the cytoplasm (outside the nucleus)
• Active and passive transport mechanisms for specific materials

Nucleolus

• Highly visible dark-staining region within the nucleus involved in ribosome production
• Contains the nucleolar organizer (containing rRNA genes) which is a part of certain chromosomes

Chromatin: DNA Packaging

• DNA is packaged by being wrapped around protein histones, forming chromatin
• Two types of chromatin:
  • Heterochromatin: generally inactive
  • Euchromatin: active

Primary Structure of DNA

• Definition: nucleotide polymer
• Composed of:
  • Phosphate
  • Sugar (deoxyribose)
  • Nitrogenous bases: A, T, C, G
    • Pyrimidine: C, T, U (Uracil)
    • Purine: A and G

Secondary Structure of DNA

• Double helix structure
• Two complementary strands joined together by hydrogen bonds.
• A-T (2 hydrogen bonds) and C-G (3 hydrogen bonds)
• Antiparallel strands

Structure of Chromatin

• Composed of DNA wrapped around protein histones.
• Two forms:
  • Heterochromatin: generally not active
  • Euchromatin: active

Chromatin Compaction Levels

• Condenses further into chromatin fibers, which forms distinct structures called chromosomes (at the time of cell division)

From Chromatin → Chromosome

• Chromosome is the result of strong spiralization of chromatin, allowing for physical division and distribution

Ribosomes

• Ribosomes are complex molecular assemblies
• Comprised of ribosomal RNA (rRNA) and proteins
• They synthesize proteins
• Free ribosomes form proteins used in the cytoplasm, nucleus and other components
• Some ribosomes associate with the endoplasmic reticulum which synthesize membrane proteins found in the endomembrane system or for export from the cell

• Messenger RNA (mRNA): carries genetic information from DNA to ribosomes • Transfer RNA (tRNA): carries amino acids to ribosomes

mRNA

• Carries the genetic information from DNA in a cell to the protein synthesis machinery in the cytoplasm
• Serves as a template for protein synthesis

Transcription

• A process happening in the nucleus
• DNA strands separate, with one strand serving as a template.
• The rule of complementarity of the nitrogenous base is followed, producing a complementary RNA strand using RNA polymerase enzyme
• Synthesized mRNA passes into cytoplasm for translation to protein

Translation

• Happens in the cytoplasm
• mRNA migrates to a ribosome
• mRNA is read successively by codons to add amino acids by specific tRNA that carries the anticodon sequence.
• AUG: start codon and UGA UAA UAG: stop codon

Expression of Genetic Information

• The DNA triplet (nucleotides) corresponds to mRNA codon
• Each codon corresponds to a specific anti-codon of tRNA
• Each anti-codon corresponds to a specific amino acid

The Endomembrane System

• Internal eukaryotic membrane system
• Includes:
  • Nuclear envelope
  • Endoplasmic reticulum (rough and smooth)
  • Golgi apparatus
  • Lysosomes
  • Vacuoles
• Fills the cell and divides it into compartments
• Channels the passage of molecules
• Provides synthesis surfaces for lipids and proteins

Endoplasmic Reticulum (ER)

• Involved in protein and lipid synthesis.
• Two types:
  • Rough ER (RER): has ribosomes, makes and modifies proteins destined for secretion and for membrane
  • Smooth ER (SER): no ribosomes, synthesizes lipids and steroids, stores calcium ions, detoxification

Golgi Apparatus

• A flattened stack of membranes (cisternae), involved in modifying, sorting, and packaging proteins and lipids
• Three zones:
  • Cis face: receiving end (closest to RER)
  • Medial / Intermediate
  • Trans face: shipping end

Functions of the Golgi

• Modifies proteins and lipids made in ER
• Directs molecules to their proper destinations (secreted or for endomembrane system use)

Transport Through the Golgi

• Vesicles transport proteins and lipids through cisternae.
• Vesicles bud from cis, pass through cisternae, and fuse with trans face
• The final contents are dispatched to the proper cell locations in vesicles, transporting molecules

Lysosomes

• Membrane-bound vesicles containing digestive enzymes
• Break down biomolecules
• Recycle cellular components and expel waste.
• Two types
  • Primary: Newly formed; non-active
  • Secondary: fuse with other vesicles (phagocytic) containing ingested material, activating the digestive enzymes

Peroxisomes

• Microbodies rich in oxidative enzymes (e.g., catalase)
• Involved in oxidation reactions (e.g., breakdown of fatty acids)
• Detoxify harmful substances
• Produce hydrogen peroxide (byproduct), which catalase breaks down

Mitochondria

• Sausage-shaped organelles
• Two membranes:
  • Outer membrane
  • Inner membrane (cristae)
• Divide by fission
• Contain their own DNA and machinery for protein synthesis, but most enzymes required for division are encoded in the cell nucleus
• Central role in cellular respiration producing ATP
• Key compartments:
  • Matrix: inside the inner membrane
  • Intermembrane space: between outer and inner membrane

Mitochondrial Functions

• Cellular respiration (ATP synthesis)
• Steroid hormone synthesis (works with ER)
• Thermogenesis
• Calcium regulation
• Cell death (apoptosis)

The Cytoskeleton

• Network of protein fibers that supports cell shape and organizes internal structures
• Three main types
  • Actin filaments (microfilaments)
  • Microtubules
  • Intermediate filaments

Centrosomes and Centrioles

• Microtubule-organizing center (MTOC)
• Pair of barrel-shaped centrioles
• Involved in microtubule formation, cell division, and other cellular activities

Intermediate Filaments

• Durable protein fibers
• Provide structural support to the cell
• Composed of overlapping proteins, which provides mechanical stability to the cell

Summary of Eukaryotic Organelles

• Nucleus: contains genetic material
• Ribosomes: synthesize proteins
• Endoplasmic reticulum: synthesizes and modifies proteins and lipids
• Golgi apparatus: modifies, sorts, and packages proteins and lipids
• Mitochondria: produce ATP through cellular respiration
• Lysosomes: degrade waste materials
• Peroxisomes: break down toxic molecules
• Cytoskeleton: maintains cell shape, enables movement and facilitates transport