Cell Biology: Discovery and Properties

Introduction to Cell Biology

• Cells are intensely studied and require creative techniques.
• Cell biology operates on reductionism, explaining the whole character through studying its parts.

The Discovery of Cells

• The invention of the microscope by Robert Hooke, refined by Anton Leeuwenhoek, led to cell discovery.
• In the mid-1800s, Schleiden, Schwann, and Virchow articulated cell theory.
• Key tenets of cell theory: all organisms consist of one or more cells, the cell is life's structural unit, and cells arise from pre-existing cells via division.

Basic Properties of Cells

• Life is the basic property of cells.
• Cells can be grown and reproduce in extended culture, like HeLa cells, which are key tools for cell biologists.
• HeLa cells are immortal and derived from Henrietta Lacks, an African-American mother who died of cancer.
• Cellular processes are highly regulated.
• Cells from different species share consistent structure and metabolic processes, conserved via evolution.
• Genes encode information to build cells, organisms, and dictate reproduction, activity, and structure.
• Daughter cells receive genetic instructions.
• Photosynthesis fuels all living organisms.
• Animal cells use photosynthesis products, primarily glucose; cells convert glucose to ATP for energy.
• Cells carry out chemical reactions, engage in mechanical activities, respond to stimuli, self-regulate, and evolve.

Classes of Cells

• Prokaryotic and eukaryotic cells differ in size and organelles.
• Prokaryotes are all bacteria, originating approximately 3.7 billion years ago.
• Eukaryotes comprise protists, animals, plants, and fungi.
• Prokaryotes and Eukaryotes similarities are:
  • Plasma membrane construction
  • Genetic information encoded in DNA using identical genetic code
  • Similar mechanisms for transcription and translation of genetic information, including similar ribosomes
  • Shared metabolic pathways
  • Similar apparatus for conservation of chemical energy as ATP
  • Similar mechanism of photosynthesis
  • Similar mechanism for synthesizing and inserting membrane proteins
  • Proteasomes of similar construction
• Features of eukaryotic cells not found in prokaryotes include:
  • Division of cells into nucleus and cytoplasm, separated by a nuclear envelope containing complex pore structures
  • Complex chromosomes composed of DNA and associated proteins that are capable of compacting into mitotic structures
  • Complex membranous cytoplasmic organelles
  • Specialized cytoplasmic organelles for aerobic respiration
  • Complex cytoskeletal system
  • Complex flagella and cilia
  • Ability to ingest particulate material by enclosure within plasma membrane vesicles
  • Cellulose-containing cell walls
  • Cell division using a microtubule-containing mitotic spindle that separates chromosomes
  • Presence of two copies of genes per cell
  • Presence of three different RNA synthesizing enzymes
  • Sexual reproduction requiring meiosis and fertilization
• Distinguishing Characteristics:
  • Complexity: Prokaryotes are simple, eukaryotes are complex in structure/function.
  • Cytoplasm: Eukaryotes have membrane-bound organelles, complex cytoskeletal proteins.
  • Cellular Reproduction: Eukaryotes use mitosis, prokaryotes use simple fission.
  • Locomotion: Eukaryotes use cytoplasmic movement, cilia, and flagella; prokaryotes have flagella that differ in form and mechanism.
  • Genetic Material: Prokaryotes have a nucleoid region; eukaryotes have a membrane-bound nucleus and more genetic material.

Cell structure

• Cells structure varies between an animal, plant and bacterial cell, especially regarding the size and the organelles present

Cellular Reproduction

• Cellular reproduction occurs in both eukaryotes and prokaryotes.
• Eukaryotic division show DNA and microtubules of daughter cells.
• Bacterial conjugation shares DNA through the F pilus.

Flagella

• Flagella differs between prokaryotes and eukaryotes

Types of Prokaryotes

• Cells are divided into two domains; Archaea which includes methanogens, halophiles, acidophiles, and thermophiles.
• Cells are divided into two domains; Bacteria which includes the smallest know cells, mycoplasma, cyanobacteria.
• Cyanobacteria contains types that:
  • Gave rise to green plants and an oxygen-rich atmosphere.
  • Are capable of nitrogen fixation.
• Prokaryotes are classified by specific DNA sequences, showing they are diverse and numerous.

Eukaryotic cell types

• Eukaryotes are both unicellular and multicellular:
  • Unicellular eukaryotes are complex single-celled organisms.
  • Multicellular have different cell types for different functions.
• Cell differentiation occurs during the embryonic development through different arrangements of organelles.
• Despite differentiation, cells all share many features in common.

Cell size

• Cells are measured in micrometers (1 µm = 10-6 meter) and nanometers (1 nm = 10-9 meter).
• Cell size is limited by the volume of cytoplasm supported by the genes, nutrient exchange, and substance diffusion.

Synthetic Biology

• Synthetic biology creates living cells in the laboratory.
• Developing novel life forms is an avenue, starting with existing organisms.
• Modest applications include medicine, industry, and more.
• There is a good prospect to replace the genome of one bacterium with that of a closely related species.

Cell Replacement Therapy

• Stem cells are undifferentiated with self-renewal and differentiation capacity.
• Adult stem cells can replace damaged/diseased adult tissue.
• Hematopoietic stem cells produce blood cells in bone marrow; neural stem cells treat neurodegenerative disorders.
• Limitations for adult stem cells are overcome in embryonic stem cells (ES).
• The use of ES cells involves Ethical considerations.
• Induced pluripotent cells (iPS) can be reprogrammed from fully differentiated cells into pluripotent stem cells.

Viruses

• Viruses are pathogens and intracellular parasites.
• Virion structure includes genetic material, a protein capsid, and sometimes a lipid envelope.
• Viroids are naked RNA pathogens that interfere with gene expression.
• Viruses have surface proteins, host specificity, and various infection types.
• Lytic infection redirects hosts to create virus particles; host cells lyse and release viruses.
• Integration inserts viral DNA (provirus) into host chromosomes.

How Eukaryotic Cells Originated

• Prokaryotic cells came first, eventually giving rise to eukaryotic cells.
• Endosymbiont Theory: eukaryotic cell organelles (mitochondria, chloroplasts) evolved from smaller prokaryotic cells.
• Evidence of the endosymbiont theory:
  • Absence of known eukaryotes in an intermediate stage of evolution.
  • Many symbiotic relations among different organisms.
  • Organelles have their own DNA.
  • rRNA nucleotide sequences resembled that of prokaryotes.
  • Organelles can duplicate independently of the nucleus.