Cells as Basic Units of Life

• Humans consist of about 37 trillion cells of 200 different types.
• Cell biology, anatomy, chemistry, biochemistry, biophysics, genetics, molecular biology, histology, physiology, and pathology are all important for medicine.

Cells as Basic Units

• Living organisms are either single cells or composed of cells.
• Striated muscle and giant algae are atypical occurrences of cells.

History of Cell Biology

• 1595: First light microscope (Jansen).
• 1655: Hooke observed 'cells' in cork.
• 1674: Leeuwenhoek observed protista (animalcules).
• 1833: Brown described the cell's nucleus from the orchid.
• 1839: Schleiden & Schwann proposed cell theory, stating all organisms are comprised of cells.
• 1857: Pasteur discovered lactobacillus, noting bacteria are also cells.
• 1858: Rudolf Virchow stated cells develop only from pre-existing cells via cell division (omnis cellula e cellula).
• 1859: Darwin's evolution theory was published in "The Origin of Species".
• 1866: Mendel became known as the Father of Genetics.
• 1874: Flemming described chromosome behavior during mitosis.
• 1882: Koch discovered Mycobacterium tuberculosis and established the pathogenic role of bacteria.
• 1894: Altmann first described mitochondria.
• 1898: Golgi described the Golgi apparatus.
• 1951: HeLa cells were discovered
• 2010: Creation of a bacterial cell was controlled by a chemically synthesized genome.
• Biophysical and molecular biology tools are now used for observation and manipulation.

Modern Cell Theory

• All known living things are made up of cells, which are their structural and functional units.
• All cells come from pre-existing cells by division, meaning spontaneous generation does not occur.
• Cells contain hereditary information, which is passed from cell to cell during cell division.
• Cells are similar in chemical composition.
• Cells maintain their organized structure by investing energy, with catabolic and anabolic processes occurring inside.

Origins and Features of Life

• The Miller experiment simulates conditions of early Earth to test the chemical origin of life.
• Main flow of information in cells moves from DNA to RNA protein.
• "Phenotype" is determined primarily by proteins.

RNA and Evolution

• Life involves enzyme-catalyzed anabolic and catabolic processes with self-reproduction.
• The RNA world likely preceded the protein world.
• RNA molecules can function as enzymes, such as RNA polymerase, and reverse transcriptase.
• By coupling RNA and protein synthesis via aminoacyl-tRNA synthetases, the universal genetic code evolved.

Prokaryotes

• Prokaryotes include archea and eubacteria.
• Prokaryotes have a nucleoid region instead of a nucleus.
• Prokaryotic DNA is circular and typically ranges from 0.75 to 5 Mbp.
• Prokaryotes usually lack introns.
• They have one chromosome, which is membrane-attached.
• They lack inner compartmentalization and a cytoskeleton.
• Prokaryotes do not have endocytosis or exocytosis.

Eukaryotes

• Eukaryotes include protists, yeast, plants, and animals.
• Eukaryotes have a nucleus, where RNA and protein synthesis are separated.
• They have a nucleolus and histones.
• Eukaryotic DNA is linear and 15 Mbp or more.
• Eukaryotes have introns in genes.
• They have multiple chromosomes and a nucleoskeleton.
• Eukaryotes possess mitochondria, chloroplasts, ER, Golgi, lysosomes, and peroxisomes.
• They have microtubules, microfilaments, and intermediate filaments,
• Endocytosis and exocytosis are present in eukaryotes.

Prokaryotes vs. Eukaryotes

• Prokaryotes have only a nucleoid, while eukaryotes have a nucleus.
• Prokaryotes lack introns, while eukaryotes have them.
• Transcription and translation occur in one compartment in prokaryotes but in separate compartments in eukaryotes.
• Prokaryotes lack intracellular membrane systems, mitochondria and phagocytosis.
• Eukaryotes have: intracellular membrane systems, mitochondria and phagocytosis
• Eukaryotic chromosomes are linear with telomeres, while prokaryotic chromosomes are not llinear.

Endosymbiotic Theory

• Mitochondria are thought to have descended from close relatives of typhus-causing bacteria via endosymbiosis.
• During evolution, a eukaryotic organism with adequate cytoskeleton and internal membrane systems engulfed/phagocytosed a prokaryotic organism, leading to a mutually beneficial relationship.

Evolution of Life

• Earth is 4.5 billion years old.
• Life arose on Earth 3.5-4 billion years ago.
• Prokaryotes and eukaryotes emerged 3 billion years ago.
• Plants, animals, and yeasts appeared 1.5 billion years ago.
• Early life forms used RNA.
• The nucleus evolved.
• Mitochondria developed.
• The evolution of life involves mutations, selection, and endosymbiosis.

Evolution Insights

• Main features of evolution of life on Earth:
• Prebiological evolution led to "organic soup" then RNA and DNA.
• Catalyzed chemical reactions, mutability, and selection are key with the self-reproductive capacity of traits.
• Compartmentalization, changes in leaps, and metabolism shifts are also important.
• E. coli divides every 20-40 minutes.

Cell Size and Function

• Cells need to have a large surface area-to-volume ratio.
• Cell functions include self-maintenance, motility, growth, proliferation, cell-cell communication, differentiation, and morphogenesis, all determined by gene expression.

Genes

• Mycoplasma has ~300 genes.
• E. coli has ~4000 genes.
• Yeast has ~6000 genes.
• C. elegans has ~20000 genes.
• Humans have ~20000 or more genes.

Genome Size

• E. coli has a genome size of ~5 million base pairs.
• Yeast has a genome size of ~12 million base pairs.
• C. elegans has a genome size of ~100 million base pairs.
• Humans have a genome size of ~3000 million base pairs.

Gene Expression: Prokaryotes vs. Eukaryotes

• In prokaryotes, transcription and translation occur simultaneously.
• In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytosol.
• Eukaryotic genes contain introns that are removed during RNA processing.
• Eukaryotic mRNA undergoes splicing and export.
• Alternative splicing allows more proteins than genes.

Why More DNA?

• Humans have 30x more DNA than C. elegans, but about the same number of genes.
• Gene expression regulation in eukaryotes includes protein factors, RNAs, lncRNAs (long non-coding RNA), miRNAs (micro RNA), and circular RNAs.

Cell Dimensions

• Cell membrane: 4-10 nm.
• Average protein: 3-6 nm.
• Water molecule: 0.3 nm.

Important Molecule Dimensions

• Water molecule: 0.3 nm.
• DNA diameter: 2 nm.
• Average protein: 3-6 nm.
• Cell membrane thickness: 7 nm.
• HIV virus diameter: 100 nm.
• Protozoan length: 25 μm.
• Human red blood cell diameter: 7-8 μm.
• Bacterium diameter: 1-5 μm.
• Average cell diameter: 10-20 μm.
• Neurite length: <1.5 meter
• Average cell volume: 100-10000 μm³.

Questions to Consider

• Do plants have mitochondria?
• Which organelles originated by endosymbiosis?
• Do bacteria have ribosomes?
• What are introns and exons?

Revisions

• Know the size range of relevant organic entities.
• Know the approximate volume, time, speed and concentration ranges of biologically relevant items/processes.