Cell Biology:

1.1 Introduction to Cells

Cell theory:

• The cell theory was proposed by Mathias Schleiden, Theodore Schwaan, and Rudolf Virchow.

• All living organisms are composed of cells and its products.

• The cell is the basic structural, and functional unit of all living organisms.

• All cells arise from pre-existing cells that is they cannot be created from non-living material.

Functions of life:

A cell carries out many different functions of life which includes:

• Metabolism: 

  • It is the sum of all the chemical reactions in the body, including both anabolic (building) and catabolic (breakdown) processes.

    • Catabolism: These are the energy-releasing processes that break down complex molecules into simpler ones.

    • Anabolism: These are the energy-requiring processes that bild complex molecules from simpler ones.

  • It is very essential for maintaining various life functions.

  • It also controls and regulates the rate of chemical reactions in the body.

• Response:

  • It is a fundamental aspect of human behaviour.

  • It is basically an action, behaviour, or answer that follows a particular stimulus or situation.

  • It can be voluntary, involuntary, or depending on whether they are under conscious control.

• Homeostasis:

  • Homeostasis is the body's natural ability to maintain internal stability and balance.

  • Homeostasis is a fundamental biological process that ensures the body's internal environment remains stable despite external changes.

  • It involves the regulation of various physiological variables, including temperature, blood pressure, blood sugar levels, and pH, among others.

  • Homeostasis is essential for proper bodily function. 

    • When it fails, it can lead to health issues or even life-threatening conditions.

  • Homeostasis helps the body adapt to changing external conditions, such as temperature fluctuations, exercise, or dietary changes.

• Growth:

  • Growth is one of the essential characteristics of living organisms.

  • It is a fundamental biological process.

  • It can occur at the cellular, tissue, organ, and organismal levels.

  • Types of growth:

    • Primary Growth:

      • Typically seen in plants and involves the lengthening of stems and roots.

      • Driven by cell division in the apical meristems.

    • Secondary Growth:

      • Also in plants, it involves the thickening of stems and roots.

      •  Occurs due to the activity of lateral meristems, like the vascular cambium.

    • Limited Growth:

      • This type of growth is seen in animals where the growth happens upto a certain age.

    • Unlimited Growth:

      • This type of growth is seen in plants where the growth happens life time.

• Reproduction:

  • It is the biological process by which new individuals of the same species are produced.

  • The primary purpose of reproduction is to ensure the survival and continuation of species.

  • Two types of reproduction:

    • Sexual: It involves the fusion of gametes from two parents.

    • Asexual: It involves the production of offspring without the fusion of gametes.

• Excretion:

  • It is the process of eliminating waste products and excess substances from the body.

• Nutrition:

  • It refers to the process by which organisms obtain and utilize the nutrients and substances necessary for their growth.

Surface and Volume ratio:

• As, a cell grows in size its volume increases much more rapidly than its surface area. 

  • The surface area-to-volume ratio decreases as cell increases in size.

  • The surface allows the entry of oxygen. So, large cells cannot get as much oxygen as they would need to support themselves.

• Smaller single-celled organisms have a high surface area to volume ratio.

  • The higher the surface area to volume ratio, the more effective the diffusion process can be.

Magnification:

• Magnification is the process of enlarging an object or image to make it appear larger than its actual size.

• Commonly used in optics and microscopy to study fine details.

• Calculation of magnification:

  • Magnification = Image size (with ruler) ÷ Actual size (according to scale bar)

• Calculation of actual size:

  • Actual Size = Image size (with ruler) ÷ Magnification

Emergent Properties:

• These are the characteristics or behaviours that arise in complex systems but are not evident in their individual components.

• Due to this multicellular organisms are capable of completing functions that unicellular organisms could not undertake.

  • Multicellular organisms are living beings composed of multiple cells that work together to perform various functions.

  • Cell → Tissue → Organ → Organ system → Organism

Cell Differentiation:

• The development and maintenance of multicellular organisms,

• It refers to the transformation of unspecialized or stem cells into specialized cell types with distinct functions and characteristics.

• This process is critical for the growth, repair, and functioning of various tissues and organs in an organism.

Gene Packaging:

• It is a fundamental process in molecular biology that involves the organization and compaction of genetic material within the nucleus of eukaryotic cells.

• Packaging includes:

  • Euchromatin: 

    • Active genes are usually packaged as euchromatin.

    • Transcriptionally active.

  • Heterochromatin:

    • Inactive genes are usually packaged as heterochromatin.

    • Transcriptionally inactive.

Stem cells:

• It is a type of unspecialized, undifferentiated cell that has the unique ability to develop into various specialised cell types.

• Charactized by:

  • Self-renewal: The ability to create more stem cells.

  • Pluripotency/ Multipotency: The ability to differentiate into specific cell types.

Types of stem cells:

• Stem cells come in various types, each with its own unique characteristics and differentiation potential. Here are the main types of stem cells:

• Embryonic Stem Cells (ESCs):

  • Derived from the inner cell mass of early-stage embryos.

  • Pluripotent, meaning they can differentiate into almost any cell type in the body.

  • Used in research and regenerative medicine due to their high differentiation potential.

• Adult (Somatic) Stem Cells:

  • Found in various tissues and organs throughout the body.

  • Multipotent, meaning they can differentiate into a limited range of cell types specific to the tissue or organ where they are located.

  • Responsible for tissue maintenance and repair.

  • Examples include hematopoietic stem cells (found in bone marrow) and mesenchymal stem cells (found in various tissues).

• Induced Pluripotent Stem Cells (iPSCs):

  • Generated by reprogramming adult somatic cells, such as skin cells or blood cells, into a pluripotent state.

  • Similar to embryonic stem cells in terms of pluripotency.

  • Offer the advantage of patient-specific stem cells for personalized medicine, disease modeling, and drug testing.

• Fetal Stem Cells:

  • Derived from the tissues of developing fetuses, typically obtained during prenatal testing or elective abortion procedures.

  • Show a degree of multipotency, with potential applications in regenerative medicine and research.

• Perinatal Stem Cells:

  •  Isolated from various tissues associated with childbirth, such as the placenta, amniotic fluid, and umbilical cord.

  • Can differentiate into a range of cell types and are considered a valuable source of stem cells for research and therapy.

• Cancer Stem Cells:

  • Found within tumors and possess stem cell-like properties.

  • Thought to play a role in cancer growth, metastasis, and resistance to treatments.

Stem Cell Therapy:

• Stem cell therapy, also known as regenerative medicine that utilizes stem cells to treat various diseases, injuries, and medical conditions.

• It is  based on the remarkable ability of stem cells to sef-renew and differentiate into different cell types.

• Stem cell therapy has shown promise in treating a variety of medical conditions and diseases.

• Examples of stem cell therapy:

  • Bone Marrow Transplantation:

    • Hematopoietic stem cells from the bone marrow have been used for decades to treat blood-related disorders, such as leukemia, lymphoma, and aplastic anemia. 

    • They can re-establish the patient's blood and immune system.

  • Cardiovascular Diseases:

    • Stem cells, such as mesenchymal stem cells, have been investigated for their potential to repair damaged heart tissue in conditions like myocardial infarction (heart attack) and heart failure.

  • Neurodegenerative Diseases:

    • Stem cell therapy is being explored for conditions like Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis (ALS) to replace damaged or lost neurons.

  • Spinal Cord Injuries:

    • Researchers are investigating stem cell therapy as a means to regenerate damaged spinal cord tissue and restore function in individuals with spinal cord injuries.

  • Orthopedic Injuries:

    • Mesenchymal stem cells have been used in the treatment of orthopedic injuries, such as osteoarthritis and sports-related injuries, to promote cartilage and bone regeneration.

  • Autoimmune Diseases:

    • Mesenchymal stem cells have shown potential in modulating immune responses and have been explored as a treatment for autoimmune diseases like multiple sclerosis and systemic lupus erythematosus (SLE).

  • Type 1 Diabetes:

    • Efforts are underway to develop insulin-producing beta cells from stem cells for transplantation to treat type 1 diabetes.

  • Eye Diseases:

    •  Stem cell therapy is being researched for eye conditions like age-related macular degeneration (AMD) and retinal degenerative diseases to restore vision.

  • Burn Injuries:

    • Stem cell therapy, particularly with skin stem cells, has been used to promote skin regeneration in burn victims.

  • Cancer Treatment:

    • Hematopoietic stem cell transplantation is a critical component of treatment for certain cancers, such as leukaemia and lymphoma, allowing for high-dose chemotherapy and radiation therapy.

  • Autoimmune Diseases:

    • Stem cell therapy is under investigation for autoimmune diseases, like rheumatoid arthritis and Crohn's disease, to regulate the immune response and reduce inflammation.

Cell Theory History:

• Stem The cell theory is a fundamental concept in biology that describes the basic structural and functional unit of life. Its development is a significant milestone in the history of biology. Here's a brief overview in bullet points:

  • 17th Century: Early microscopes allowed scientists to observe cells. In 1665, Robert Hooke coined the term "cell" when he examined cork and saw small, box-like structures under his microscope.

  • Late 17th Century: Anton van Leeuwenhoek, a Dutch scientist, used a more advanced microscope to observe single-celled microorganisms, including bacteria and protists.

  • 19th Century: The cell theory was formulated as follows:

    • Matthias Schleiden:

      •  In 1838, Schleiden proposed that all plants are composed of cells.

    • Theodor Schwann:

      •  In 1839, Schwann extended the theory by suggesting that all animals are also made up of cells.

    • Rudolf Virchow: 

      • In 1855, Virchow added the concept that cells arise from pre-existing cells, completing the cell theory.

• Cell Theory Postulates: The cell theory comprises three main principles:

  • All living organisms are composed of one or more cells.

  • The cell is the basic structural and functional unit of life.

  • All cells arise from pre-existing cells through cell division.

Microscopes:

• Microscopes are essential scientific instruments that enable the observation of objects and structures at a microscopic level, revealing details that are not visible to the naked eye. 

• Types of Microscopes: There are several types of microscopes, each designed for specific applications. Common types include:

  • Optical Microscopes: These use visible light to magnify and observe specimens, including bright-field, dark-field, phase-contrast, and fluorescence microscopes.

  • Electron Microscopes: These use focused beams of electrons to achieve higher magnification and resolution, including transmission electron microscopes (TEM) and scanning electron microscopes (SEM).

  • Confocal Microscopes: These use lasers and pinhole apertures to create sharp, three-dimensional images, often used in biological and medical research.

• Magnification and Resolution:

  • Microscopes allow for both magnification and improved resolution. 

  • Magnification increases the apparent size of an object.

  • Resolution refers to the ability to distinguish between closely spaced objects. 

  • Higher magnification and resolution are essential for detailed observations.

Cell Scale:

• Cells are measured according to this metric:

1.2 Ultrastructure of Cells:

Organisms are divided based on cell structure:

• Prokaryotes:

  • Primitive; Small; Simple.

  • No distinct nuclear compartment.

  • No membrane-bound organelles.

  • Live in variable ecological habitats.

  • The prokaryotes are divided into two groups:

    • Bacteria/ Eubacteria

    • Archaebacteria

• Bacteria (the primitive organisms):

  • Bacteria are the sole members of Kingdom Monera.

  • Eubacteria are true bacteria characterized by the presence of a cell wall. Eg: Blue-green algae (Cyanobacteria)

  • Bacteria are classified based on shape into four types:

    • Cocci (spherical)

    • Bacilli (rod-shaped)

    • Spirillum (spiral like)

    • Vibrio (comma shaped)

• Archaebacteria:

  • They are special and survive in some of the harshest habitats.

  • Extreme salty (halophiles), marshy areas (methanogens), and hot springs (thermoacidophiles)

• Eukaryotes:

  • Advanced; Complex

  • Distinct nuclear compartment.

  • Membrane-bound organelles are present.

Structure and function of prokaryotic cell:

• Flagella: A slender whip like structure used for locomotion

• Pili and fimbrae: Attachment to substrate

• Cell wall: Structural support and protection against damage

• Cell membrane: Provide protection and allow movement of substance from in and out of cells.

• Ribosome: Protein synthesis

• Naked DNA (Nucleoid): Store genetic information and passed to daughter cell.

• Structure some time present in prokaryotic cell:

  • Plasmid: Extra chromosomal DNA. Contain antibiotic resistance gene.

    • Used as vector for genetic engineering.

  • Capsule: Protect cell from chemical and dry environment

Structure and function of eukaryotic cell:

• Cell membrane: Protect the cell from its surroundings and controls the exchange of substances with the outside.

• Nucleus: It stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction.

• Cytoplasm: Contents of the cell located between the plasma membrane and the nuclear membrane. 

  • Comprises a liquid medium or cytosol, comprising water and soluble substances, where the present other organelles.

• Mitochondria: Called as power house of the cells. Responsible for breakdown of sugar molecules for releasing ATP (the energy currency of cells).

  • It also regulate cellular metabolism.

• Endoplasmic reticulum: Structural frame work. 

  • Production and processing of protein (RER)

  • Synthesis of carbohydrates and lipid (SER).

• Ribosome: Ribosomes are a cell structure that makes protein. 

  • Protein is needed for many cell functions such as repairing damage or directing chemical processes. 

  • Ribosomes can be found floating within the cytoplasm or attached to the endoplasmic reticulum.

• Golgi apparatus: A number of proteins synthesized by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face. 

  • Golgi apparatus is the important site of formation of glycoproteins and glycolipids.

• Lysosomes: Contain lytic enzyme. Site for intracellular digestion and destruction of certain organelles at the time of development. 

  • Also called as suicidal sac.

• Cytoskeleton: Mechanical support, motility, maintenance of the shape of the cell.

• Centrosome and Centriole: The centrioles form the basal body of cilia, flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

• Chloroplast: Helps in photosynthesis. (only plant cell)

• Cell wall: Maintain cell shape and avoid cell bursting (only plant cell).

Differences between plant and animal cell:

• Plant Cell:

  • Plant cells have cell wall.

  • They contain chloroplast.

  • They donot have centriole.

  • Vacuole is large and present in centre of the cell.

  • Nucleus is present in the side of the plant cell.

• Animal Cell:

  • Animal cells don’t hve a cell wall.

  • They don’t have chloroplasts.

  • Centriole is present in them.

  • Vacuole is small.

  • Nucleus is present in the centre of the animal cell.

Differences between prokaryotic and eukaryotic cell:

• Prokaryotic cell:

  • Primitive cell

  • Generally small in size

  • Nucleus is absent

  • Chromosome is single, and circular.

  • Plasmid is present

  • Nucleolus is absent.

  • Membrane bound cell organelles are absent.

  • Pilli, and fimbrae helps in adhesion.

• Eukaryotic cell:

  • True nucleus

  • Size is generally large

  • Nucleus is present

  • No plasmid

  • Nucleolus is present

  • Membrane bound organelles are present

  • Cillia and flagella helps in motility

1.3 Membrane Structure:

• Cell Membrane:

  • Cell Membranes are the outermost layer in animals whereas in plants the second most layer after the cell wall.

  • It is the universal structure and structural cell membrane of prokaryotes that are similar to eukaryotes.

  • It encloses the boundaries and all the cellular interactions between cytosol and the external environment.

  • Cell Membranes are dynamic, fluid structures comprising lipid bilayers and protein molecules embedded in them.

  • They are held together by non-covalent interactions.
    
    

• Lipid Bilayer:

  • The lipid bilayer comprises phosphoglycerides, sphingolipids, and sterols in the cell membrane.

  • All lipid molecules have a hydrophilic head and two hydrophobic tails. This makes nature to be amphillic.

  • Lipids are the main component of the cell membrane because it forms the continuous structural frame of the cell membrane.

  • The phospholipid layer provides fluidity to the plasma membrane because phospholipids are rich in unsaturated fatty acids.

  • The main phospholipids in most animal cell membranes are phosphoglycerides.

  • Glycolipids and cholesterol are also part of the lipid bilayer.

  • The reason behind the formation of the bilayers is their shape and amphiphilic nature.

  • The lipid bilayer is a two-dimensional fluid.

  • Liposomes (spherical vesicles) provide fluidity to the lipid bilayer.

  • When the lipid molecules in the plasma membrane of living cells segregate in their specialized domains called lipid rafts.

  • The excess lipids often get stored as fat droplets and these are often termed to be adipocytes.

  • As the lipid bilayer is known to be asymmetrical which is functionally very important when it comes to converting extracellular signals into intracellular ones.
    
    

• Membrane Proteins:

  • Membrane proteins are responsible for performing most of the membrane’s specific tasks.

  • “A typical plasma membrane is somewhere in between, with protein accounting for about half of its mass”.

  • Membrane proteins are amphiphilic having both hydrophobic and hydrophilic regions.

  • Many of the membrane proteins also extend through the lipid bilayer called transmembrane proteins.

  • Other membrane proteins are located entirely in the cytosol.

  • There are membrane-associated proteins that do not extend into the hydrophobic interior of the lipid bilayer at all. These sorts of proteins are often referred to as peripheral membrane proteins.

  • Only transmembrane proteins can function on both sides of the bilayer or transport the molecules across it.

  • There are two types of protein that are present in the plasma membrane.

• Carbohydrates:

  • Oligostructures of the glycolipids and glycoproteins on the outer surface of the plasma membrane are involved in cell to cell recognization mechanism.

1.4  Membrane Transport:

• Transportation of molecules across lipid membrane:

  • The lipid layer restricts the passage of most polar molecules.

  • This barrier often allows the balance in concentrations of solutes in the cytosol and the external environment.

  • To regulate and transport materials across the membrane. It uses specialized membrane proteins.

  • Cells often transfer larger molecules (macromolecules), that take place with a different mechanism.

• Principles of Membrane Transport:

  • The main classes of membrane proteins include:

• Transporters: These undergo sequential conformational changes to transport the small molecules.

• Channels: It forms narrow pores allowing passive transmembrane movement.

• Transport through channels occurs much faster than transport mediated by transporters.

• Protein-Free Lipid bilayers are impermeable to ions. The diffusion rate depends on the size of the molecule.

• Transportation across a gradient:

  • Passive transport: All channels and many transporters allow solute to cross the membrane only passively.

  • Active transport: Here, it requires a certain form of energy to catalyze and
    actively help the solute cross the membrane.

  • Endocytosis: (bulk transport)

    • Pinocytosis: ingestion of liquid material.

    • Phagocytosis: ingestion of solid complex materials.

  • Exocytosis: egestion of waste materials from cells through the plasma membrane.

• Due to the concentration gradient and the potential difference across the membrane, there exists the membrane’s potential difference which also influences the transport.

• Membrane Structure:

  • Cell membranes are composed of a phospholipid bilayer with embedded proteins.

  • The hydrophobic interior of the membrane acts as a barrier to the movement of polar molecules.

• Selective Permeability:

  • Membranes are selectively permeable, allowing certain substances to pass while restricting others.

  • Permeability is determined by factors such as molecular size, charge, and lipid solubility.

• Passive Transport:

  • Passive transport occurs without the input of energy.

  • Diffusion is the primary mechanism of passive transport, allowing molecules to move from an area of high concentration to an area of low concentration.

  • Simple diffusion involves the movement of small, non-polar molecules directly through the lipid bilayer.

  • Facilitated diffusion relies on carrier proteins or channel proteins to facilitate the movement of specific molecules across the membrane.

• Active Transport:

  • Active transport requires the expenditure of energy (usually ATP) to move molecules against their concentration gradient.

  • Primary active transport involves the direct use of ATP to pump molecules across the membrane, often against a concentration gradient.

  • Secondary active transport utilizes the energy stored in an ion gradient to transport molecules against their gradient.

Endocytosis and Exocytosis:

• Endocytosis is the process by which cells engulf substances from the extracellular environment by forming vesicles.

• Exocytosis is the reverse process, where vesicles fuse with the membrane, releasing their contents into the extracellular space.

Membrane Potential:

• Membrane potential refers to the voltage difference across the cell membrane, usually maintained by ion concentration gradients.

• It is crucial for various cellular functions, such as nerve impulse transmission and muscle contraction.

Transporters:

• Transporters are integral membrane proteins that facilitate the movement of specific molecules across the membrane.

• They can operate through active or passive mechanisms, depending on the energy requirement and directionality of transport.

Specialized Transport Processes:

• Some specialized transport processes include symport, antiport, and uniport.

• Symport involves the transport of two different molecules in the same direction.

• Antiport involves the transport of two different molecules in opposite directions.

• Uniport refers to the transport of a single molecule or ion.

Channels and Electrical Properties of Membranes

Ion Channels

• Ion channels are specialized membrane proteins that allow the selective passage of ions across the cell membrane.

• They play a crucial role in maintaining the electrical properties of membranes.

Types of Ion Channels:

• Voltage-gated ion channels: These channels open or close in response to changes in the membrane potential.

• Ligand-gated ion channels: These channels open or close in response to the binding of specific molecules (ligands) to the channel.

• Mechanically-gated ion channels: These channels open or close in response to mechanical stimuli such as pressure or stretch.

• Leak channels: These channels are always open, allowing a small and constant flow of ions across the membrane.

Ion Selectivity:

• Ion channels exhibit selectivity for specific ions based on their size, charge, and hydration.

• For example, potassium channels are highly selective for potassium ions, while sodium channels preferentially allow the passage of sodium ions.

Conductance and Permeability:

• Conductance refers to the ability of an ion channel to allow the flow of ions.

• Permeability is the measure of the ease with which ions can pass through a channel.

• Channels with high conductance and permeability facilitate the movement of ions more effectively.

Resting Membrane Potential:

• Resting membrane potential is the electrical potential difference across the cell membrane when the cell is at rest.

• In most cells, the resting membrane potential is around -70 millivolts (mV).

• It is primarily determined by the differential distribution of ions (such as potassium, sodium, and chloride) across the membrane.

Action Potentials:

• Action potentials are rapid and transient changes in the membrane potential that allow for long-range communication in excitable cells such as neurons and muscle cells.

• They are initiated by a depolarization event that reaches a threshold, leading to the opening of voltage-gated ion channels.

Depolarization and Repolarization:

• Depolarization refers to a change in the membrane potential towards a more positive value, typically caused by the influx of positively charged ions.

• Repolarization is the process of returning the membrane potential back to its resting state after depolarization.

• This is achieved by the efflux of positively charged ions or the influx of negatively charged ions.

Refractory Period:

• After an action potential, there is a refractory period during which the membrane is temporarily unresponsive to further depolarization stimuli.

• The refractory period ensures the proper propagation of action potentials and prevents overlapping signals.

Saltatory Conduction:

• In myelinated neurons, action potentials jump from one node of Ranvier to the next, a process known as saltatory conduction.

• This increases the speed and efficiency of electrical signal transmission along the axon.

Synaptic Transmission:

• Synaptic transmission involves the release of neurotransmitters from the presynaptic neuron, which then bind to receptors on the postsynaptic neuron.

• This binding can result in the opening or closing of ion channels, leading to changes in the postsynaptic membrane potential.

1.5 The Origin of Cells:

• Abiogenesis: 

  • It is also known as spontaneous generation.

  • It is the scientific theory that proposes the natural, non-biological origin of life from simple organic compounds.

  •  It suggests that life could emerge from inanimate matter under specific conditions, without the need for pre-existing life.

• Historical Context: 

  • Abiogenesis was a prevailing belief for centuries, with the notion that life could spontaneously arise from decaying matter or non-living substances.

  •  This idea was challenged by the work of Louis Pasteur in the mid-19th century, who demonstrated through experiments that life only arises from pre-existing life (biogenesis).

• Oparin-Haldane Hypothesis: 

  • The modern scientific concept of abiogenesis gained prominence in the early 20th century with the work of Alexander Oparin and J.B.S. Haldane. 

  • They proposed that life could have originated on Earth from simple organic molecules in a reducing atmosphere rich in methane, ammonia, and water vapor.

• Miller-Urey Experiment: 

  • In 1953, Stanley Miller and Harold Urey conducted a famous laboratory experiment that simulated the conditions proposed by Oparin and Haldane. 

  • They produced amino acids, the building blocks of proteins, from simple inorganic compounds, providing experimental support for the idea that the building blocks of life could form abiotically.

• RNA World Hypothesis: 

  • Another hypothesis related to abiogenesis is the "RNA world."

  •  It suggests that self-replicating ribonucleic acid (RNA) molecules could have preceded DNA as the genetic material. 

  • RNA can both store genetic information and catalyze chemical reactions, making it a potential candidate for the first self-replicating molecule.

• Hydrothermal Vent Hypothesis: 

  • Some researchers propose that life could have originated in the high-temperature, high-pressure environment near hydrothermal vents on the ocean floor. 

  • These conditions may have provided the necessary energy and chemical gradients for life to emerge.

• Biogenesis:

  • It states that living organisms can only arise from pre-existing living orgranisms.

  • Louis Pasteur’s experiments provided strong evidence for biogenesis.

  • The acceptance of biogenesis was a significant factor in the development of cell theory.

  • It remains as a very fundamental concept in both modern biology and microbiology.

1.6 Cell Division:

• Overview of the Cell Cycle:

  • The cell cycle is a highly regulated process that ensures proper cell growth, DNA replication, and cell division, leading to the production of two daughter cells with identical genetic material.

  • The cell cycle is the series of events that occur in a cell leading to its division and the production of two daughter cells.

  • It consists of distinct phases, including interphase and mitotic phase.

Interphase

• Interphase is the longest phase of the cell cycle, where the cell grows, carries out its normal functions, and replicates its DNA.

• Interphase is further divided into three stages: G1 (Gap 1), S (Synthesis), and G2 (Gap 2).

  • G1 phase is characterized by cell growth, protein synthesis, and preparation for DNA replication.

  • S phase involves DNA replication, where the genetic material is duplicated, resulting in two identical copies of the genome.

  • G2 phase is a period of growth and preparation for cell division, during which the cell checks the accuracy of DNA replication and repairs any errors.

Mitotic Phase

• The mitotic phase (M phase) is the shortest phase and involves the actual division of the cell into two daughter cells.

• The M phase is subdivided into four stages: prophase, metaphase, anaphase, and telophase.

  • During prophase, the chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and the mitotic spindle begins to form.

  • In metaphase, the chromosomes align at the equator of the cell, facilitated by microtubules attached to the kinetochores.

  • Anaphase is characterized by the separation of sister chromatids, which are pulled to opposite poles of the cell by the shortening of microtubules.

  • Telophase marks the end of mitosis, where the nuclear envelope reforms around the separated chromosomes, and the cell prepares for cytokinesis.

Cytokinesis

• Cytokinesis is the final stage of the cell cycle, where the cytoplasm divides, and two daughter cells are formed.

• Cytokinesis differs in animal and plant cells, with animal cells undergoing cell cleavage through the formation of a contractile ring, while plant cells form a cell plate to divide the cytoplasm.

• The completion of cytokinesis marks the end of one cell cycle and the beginning of the next, resulting in the formation of two genetically identical daughter cells.

Cell-Cycle Control System

• The cell-cycle control system is a complex network of molecular checkpoints and regulatory mechanisms that ensure the orderly progression of the cell cycle.

• It functions to monitor and regulate the timing and coordination of cell-cycle events, including DNA replication, chromosome segregation, and cell division.

• The cell-cycle control system consists of various proteins, enzymes, and signaling pathways that interact to regulate the cell cycle.

• Key components of the control system include cyclins, cyclin-dependent kinases (CDKs), and checkpoints.

  • Cyclins are proteins that fluctuate in concentration during the cell cycle and bind to specific CDKs to activate them.

  • CDKs are enzymes that regulate the progression of the cell cycle by phosphorylating target proteins involved in cell-cycle events.

  • Checkpoints are critical control points that monitor the integrity of DNA, ensure proper replication and repair, and verify accurate chromosome alignment.

    • Checkpoints include the G1 checkpoint (restriction point), G2 checkpoint, and spindle assembly checkpoint (SAC) during mitosis.

    • At each checkpoint, the cell-cycle control system assesses the conditions and signals from the environment and other cellular processes before deciding whether to proceed or halt the cell cycle.

• If the cell-cycle control system detects DNA damage, replication errors, or other abnormalities, it can activate DNA repair mechanisms or induce cell-cycle arrest to allow time for repair or prevent the transmission of damaged DNA.

• Failure of the cell-cycle control system can lead to uncontrolled cell growth, genomic instability, and the development of cancer.

• Abnormalities or mutations in cell-cycle control genes, such as tumor suppressor genes (e.g., p53) or oncogenes (e.g., cyclins, CDKs), can disrupt the normal functioning of the control system.

• Studying the cell-cycle control system provides insights into the mechanisms underlying cell division, DNA replication, and cell-cycle regulation, with implications for understanding diseases like cancer and potential targets for therapeutic interventions.

Meiosis

• Meiosis is a specialized form of cell division that occurs in sexually reproducing organisms.

• It involves the division of a diploid (2n) cell into four haploid (n) daughter cells, resulting in the production of gametes (sperm and eggs).

• Meiosis consists of two successive divisions, known as meiosis I and meiosis II.

  • Meiosis I is the reduction division, where homologous chromosomes pair up and exchange genetic material through a process called crossing over.

    • Crossing over leads to genetic recombination and promotes genetic diversity among offspring.

    • During meiosis I, homologous chromosomes separate, resulting in the formation of two haploid cells with duplicated chromosomes (consisting of sister chromatids).

  • Meiosis II is similar to mitosis, where the sister chromatids of each chromosome separate, resulting in the formation of four haploid daughter cells.

• The four daughter cells produced at the end of meiosis are genetically distinct due to the independent assortment of chromosomes during meiosis I and the random segregation of sister chromatids during meiosis II.

Meiosis I

• Meiosis I is the first division of meiosis and consists of several distinct steps, including prophase I, metaphase I, anaphase I, and telophase I.

  • Prophase I is the longest phase of meiosis I and can be further divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

    • During leptotene, the chromosomes condense, becoming visible as thread-like structures, and the nuclear envelope starts to break down.

    • In zygotene, homologous chromosomes pair up and form structures called synaptonemal complexes. This pairing is called synapsis.

    • Pachytene is characterized by the crossing over of genetic material between non-sister chromatids of homologous chromosomes. 

      • This genetic exchange increases genetic diversity.

    • Diplotene is marked by the separation of homologous chromosomes but with the physical connection points known as chiasmata still present. 

      • This is when the synaptonemal complexes dissolve partially.

    • During diakinesis, the chromosomes further condense, and the nuclear envelope completely disintegrates. The spindle apparatus starts to form.

• Metaphase I is the stage where homologous chromosome pairs line up along the equator of the cell called the metaphase plate.

• The orientation of the chromosomes is random, which contributes to genetic diversity through a process called independent assortment.

• Anaphase I follows metaphase I, and it involves the separation of homologous chromosomes, with one member of each pair moving towards opposite poles of the cell.

• Telophase I marks the end of meiosis I, where the chromosomes arrive at the poles and start decondensing. 

  • Nuclear envelopes may form around each group of chromosomes, and cytokinesis typically occurs, resulting in the formation of two haploid daughter cells.

Meiosis II

• Meiosis II is the second division of meiosis and follows meiosis I. It is similar to a mitotic division, consisting of four stages: prophase II, metaphase II, anaphase II, and telophase II.

  • Prophase II initiates with the condensation of the chromosomes, and the nuclear envelope disintegrates.

    • The centrosomes, containing the centrioles, duplicate and move towards opposite poles of the cell, forming the spindle apparatus.

  • Metaphase II is the stage where individual chromosomes line up along the equator of the cell, called the metaphase plate.

    • Unlike meiosis I, the chromosomes align individually in metaphase II, without homologous pairs.

  • Anaphase II follows metaphase II and is characterized by the separation of sister chromatids. 

    • The connections between the sister chromatids break, and each chromatid is pulled towards opposite poles of the cell.

  • Telophase II marks the end of meiosis II, where the chromosomes arrive at the poles and start decondensing.

    • Nuclear envelopes may form around each set of chromosomes, and the spindle apparatus disintegrates.

• Cytokinesis typically occurs, resulting in the division of the cytoplasm, and the formation of four haploid daughter cells.

• Each daughter cell produced at the end of meiosis II contains a single set of chromosomes, making them genetically distinct from one another and the parent cell.

• Meiosis II completes the process of meiosis, resulting in the production of four haploid daughter cells from the original diploid (2n) parent cell.

                                        Meiotic phases