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Studied by 73 peopleBiology 8 Comps
Energy and Cells (The Chemistry of Life)
Water's unique properties arise from its polarity and the ability to form hydrogen bonds, and these properties are fundamental to its biological significance.
Polarity of Water Molecules
Water is a polar molecule because of the unequal sharing of electrons between oxygen and hydrogen atoms. Oxygen is more electronegative, creating a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.
This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.
Hydrogen Bonding
Hydrogen bonds are weak, transient bonds that form between the partial positive hydrogen of one water molecule and the partial negative oxygen of another.
Despite being weak individually, hydrogen bonds are collectively strong, giving water its cohesion and other properties.
Biological Properties and Their Significance to Living Systems
Cohesion and Adhesion
Cohesion: Water molecules stick together due to hydrogen bonding, which results in surface tension.
Significance: Surface tension allows small organisms (like water striders) to move across water surfaces. It also facilitates water transport in plants through capillary action.
Adhesion: Water molecules stick to other polar or charged surfaces.
Significance: Adhesion helps water cling to plant cell walls, aiding in the upward movement of water during transpiration.
High Specific Heat
Water can absorb and store large amounts of heat energy without significant temperature changes because of hydrogen bonding.
Significance: This property helps stabilize the internal temperature of organisms and moderate Earth’s climate, providing a stable environment for life.
High Heat of Vaporization
A large amount of energy is needed to convert liquid water into vapor due to hydrogen bonds.
Significance: This property enables evaporative cooling (e.g., sweating in humans), helping organisms maintain homeostasis.
Ice Density and Insulation
In solid form (ice), water molecules form a lattice due to hydrogen bonds, making ice less dense than liquid water.
Significance: Ice floats, insulating aquatic ecosystems in winter and allowing life to survive in liquid water beneath the ice layer.
Universal Solvent
Due to its polarity, water dissolves a wide range of substances, including salts, sugars, and gases.
Significance: Water serves as a medium for biochemical reactions, nutrient transport, and waste elimination in living systems.
Role in Chemical Reactions
Water participates directly in biochemical processes, such as hydrolysis and condensation reactions.
Significance: Hydrolysis breaks down complex molecules like proteins and polysaccharides, while condensation helps build them.
Transparency
Water is transparent, allowing light to penetrate aquatic ecosystems.
Significance: This transparency is essential for photosynthesis in aquatic plants and algae, supporting the food chain.
Macromolecules
Living organisms rely on four main types of macromolecules—carbohydrates, lipids, proteins, and nucleic acids—for structure, energy, and biological functions.
Carbohydrates
Composition: Composed of carbon (C), hydrogen (H), and oxygen (O) in a typical ratio of C: H_2: C.
Examples include sugars (glucose, sucrose), starches, and cellulose.
Structure:
Monosaccharides: Single sugar molecules (e.g., glucose, fructose).
Disaccharides: Two monosaccharides joined by glycosidic bonds (e.g., sucrose = glucose + fructose).
Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).
Function:
Energy Storage: Starch (plants) and glycogen (animals).
Structural Support: Cellulose in plant cell walls; chitin in fungal cell walls and arthropod exoskeletons.
Quick Energy: Glucose fuels cellular respiration.
Lipids
Composition: Primarily composed of carbon (C), hydrogen (H), and oxygen (O), with fewer oxygen atoms than carbohydrates. May include phosphorus (P) in phospholipids.
Structure:
Fats and Oils: Made of glycerol and three fatty acids (triglycerides).
Phospholipids: Glycerol, two fatty acids, and a phosphate group.
Steroids: Four fused carbon rings (e.g., cholesterol).
Waxes: Long chains of fatty acids bonded to alcohols or carbon rings.
Function:
Energy Storage: Long-term energy reserves in fats.
Membrane Structure: Phospholipids form the bilayer of cell membranes.
Insulation and Protection: Fat cushions organs and insulates against heat loss.
Signaling: Steroids (e.g., hormones like testosterone and estrogen).
Proteins
Composition: Composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulfur (S). Made of amino acid monomers.
Structure:
Primary Structure: Sequence of amino acids in a polypeptide chain.
Secondary Structure: Alpha helices and beta sheets formed by hydrogen bonds.
Tertiary Structure: Three-dimensional folding due to interactions between R-groups.
Quaternary Structure: Multiple polypeptide chains (e.g., hemoglobin).
Function:
Enzymes: Catalyze biochemical reactions (e.g., amylase, DNA polymerase).
Structural Support: Collagen in connective tissues; keratin in hair and nails.
Transport: Hemoglobin transports oxygen; membrane proteins facilitate molecule movement.
Defense: Antibodies help the immune response.
Signaling: Hormones (e.g., insulin) regulate physiological processes.
Nucleic Acids
Composition: Composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P). Made of nucleotide monomers, each consisting of:
A sugar (ribose in RNA, deoxyribose in DNA).
A phosphate group.
A nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA).
Structure:
DNA (Deoxyribonucleic Acid): Double-helix structure with complementary base pairing (A-T, G-C).
RNA (Ribonucleic Acid): Single-stranded.
Function:
Genetic Information: DNA stores and transmits hereditary information.
Protein Synthesis: RNA transcribes and translates genetic instructions to make proteins.
Energy Transfer: ATP (adenosine triphosphate), a modified nucleotide, provides energy for cellular processes.
Enzymes
Enzymes are biological catalysts that speed up biochemical reactions by lowering the activation energy.
They are typically proteins (though some RNA molecules, called ribozymes, can act as enzymes).
Active Site is the region where the substrate binds. It has a specific shape that is complementary to the substrate’s shape.
Enzyme Function
Catalysis: Enzymes speed up reactions by lowering the activation energy required for the reaction to proceed.
Specificity: Enzymes are highly specific to the substrates they bind to. The lock-and-key model and induced-fit model explain how enzymes recognize their substrates:
Lock-and-Key Model: The enzyme's active site has a specific shape that fits perfectly with the substrate.
Induced-Fit Model: The enzyme’s active site changes shape slightly upon substrate binding, enhancing the fit.
Factors Affecting Enzyme Activity
Temperature
Optimum Temperature: Each enzyme has an optimal temperature at which it is most active.
Denaturation: At high temperatures, enzymes lose their shape and function (denaturation).
pH
Enzymes have an optimal pH range, and deviations from this range can disrupt the enzyme’s structure, reducing its activity.
Substrate Concentration
Saturation Point: As substrate concentration increases, enzyme activity increases, but eventually, all active sites are occupied, and the rate of reaction levels off (enzyme saturation).
Enzyme Concentration
Increasing enzyme concentration increases the reaction rate, assuming there is enough substrate to bind to.
Examples
Amylase: Breaks down starch into simpler sugars. Found in saliva.
Lipase: Breaks down lipids (fats) into fatty acids and glycerol.
DNA Polymerase: Catalyzes the synthesis of DNA.
Protease: Breaks down proteins into amino acids.
How Changes to Enzyme Structure Affect Function
Denaturation
High temperatures, extreme pH levels, or harsh chemicals can disrupt the enzyme’s structure, breaking hydrogen and ionic bonds.
This changes the shape of the active site, rendering the enzyme non-functional.
Mutation
Changes to the gene encoding the enzyme can alter the sequence of amino acids, leading to a different protein structure.
A mutation might:
Enhance or reduce enzyme activity.
Prevent substrate binding if the active site is altered.
Competitive Inhibition
If molecules resembling the substrate bind to the active site, they block substrate access, reducing the enzyme's effectiveness.
Cofactor or Coenzyme Absence
Many enzymes require non-protein helpers (cofactors or coenzymes) for proper function.
Without these, the enzyme’s active site may not properly form or function.
Energy and Cells (Cell Structure, Function and Cell Transport)
Cell Membrane
Structure: The cell membrane is composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
Phospholipids: Hydrophilic (water-attracting) heads face outward toward the aqueous environment, while hydrophobic (water-repelling) tails face inward, forming a semi-permeable barrier.
Proteins: Integral (span the membrane) and peripheral (attached to the surface) proteins help with transport, signaling, and structural support.
Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), they are involved in cell recognition and signaling.
Cholesterol: Helps maintain membrane fluidity by preventing the membrane from becoming too rigid or too fluid.
Functions:
Separates the contents of the cell from the outside environment.
Serves as a barrier for which substances can enter and exit a cell.
Recognizes chemical signals (messages) which will trigger the cell to react in a particular way
It is a selective permeable membrane.
Structure Details:
A phospholipid is made of two parts:
Phosphate Head - hydrophilic, or “water-loving”
Lipid Tails- hydrophobic, or “water-fearing” (think oil, a substance that does not dissolve in water)
Cholesterol – prevents fatty acid (of the lipid tails) from sticking together
Membrane Proteins
Transport Proteins – form a channel through the cell membrane
Receptor Proteins - Typically span the membrane and have a portion that sticks out from the cell membrane. When a molecule of the same shape fits into the receptor, it causes a response inside the cell to occur.
Marker Proteins - Proteins that have a carbohydrate chain attached to them. The carbohydrate chain acts as an identifier and distinguishes types of cells from each other. Also referred to as “glycoproteins” (glycol = sugar/glucose).
Membrane Permeability
The membrane is selectively permeable, meaning it allows certain substances to pass while blocking others.
Small nonpolar molecules (e.g., oxygen, carbon dioxide) can pass freely through the lipid bilayer.
Small polar molecules (e.g., water, urea) can pass but at a slower rate.
Ions and large polar molecules (e.g., glucose) typically cannot pass directly through the lipid bilayer and require transport proteins.
Types of Transport Across Membranes
Transport across the cell membrane can be passive or active.
Passive Transport
Does not require energy (ATP) because substances move down their concentration gradient (from high to low concentration).
Diffusion
Movement of molecules from an area of high concentration to an area of low concentration.
Works for small nonpolar molecules (e.g., oxygen, carbon dioxide).
Facilitated Diffusion
Diffusion of molecules via specific transport proteins (e.g., channel proteins, carrier proteins).
Works for larger or polar molecules (e.g., glucose, amino acids).
Channel Proteins:
Form water-filled pores that allow ions or small polar molecules to pass.
Carrier Proteins:
Bind to the molecule, change shape, and transport it across the membrane.
Osmosis:
A type of facilitated diffusion specifically for water molecules through aquaporins (specialized channel proteins).
Water moves from areas of low solute concentration to areas of high solute concentration.
Tonicity: Describes the relative concentration of solutes in the external environment compared to the cell.
Hypotonic: Lower solute concentration outside the cell (cell swells).
Hypertonic: Higher solute concentration outside the cell (cell shrinks).
Isotonic: Equal solute concentration (no net movement of water).
Active Transport
Active transport requires energy (usually ATP) to move substances against their concentration gradient (from low to high concentration).
Primary Active Transport:
Direct use of energy (ATP) to transport ions or molecules via a pump protein.
Example: The sodium-potassium pump (Na^+/K^+ pump) moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.
Secondary Active Transport:
Uses the energy created by primary active transport.
Example: The sodium-glucose transport protein (SGLT) uses the sodium gradient created by the Na^+/K^+ pump to transport glucose into the cell.
Bulk Transport (Vesicular Transport)
Bulk transport involves the movement of large molecules or large quantities of materials across the plasma membrane via vesicles. This requires energy.
Endocytosis:
The process by which cells take in substances from the external environment by engulfing them in vesicles.
Types of endocytosis:
Phagocytosis: (cell eating) engulfment of large particles or cells.
Pinocytosis: (cell drinking) uptake of small droplets of extracellular fluid.
Receptor-Mediated Endocytosis: specific molecules are taken into the cell after binding to receptors on the cell surface.
Exocytosis:
The process by which cells release substances to the external environment by fusing vesicles with the plasma membrane.
Used for secretion of proteins, waste removal, and cell signaling.
Cell Communication
Cells communicate with each other through various signaling mechanisms, which are essential for coordinating cellular activities and maintaining homeostasis in multicellular organisms.
Types of Signaling
Direct Contact
Gap Junctions:
Channels that connect the cytoplasm of adjacent cells, allowing small molecules and ions to pass directly between them.
Significance: Facilitates rapid communication and coordination between cells (e.g., heart muscle cells).
Local Signaling
Paracrine Signaling:
A signaling cell releases molecules that affect nearby target cells.
Significance: Important in development and tissue repair (e.g., growth factors).
Synaptic Signaling:
A nerve cell releases neurotransmitters that bind to receptors on a target cell (e.g., another nerve cell, muscle cell).
Significance: Allows for rapid and specific communication in the nervous system.
Long-Distance Signaling
Endocrine Signaling:
Endocrine cells release hormones that travel through the bloodstream to target cells throughout the body.
Significance: Regulates long-term physiological processes (e.g., growth, metabolism, reproduction).
Stages of Cell Signaling
Cell signaling typically involves three main stages:
Reception:
A signaling molecule (ligand) binds to a specific receptor protein on the target cell surface or inside the cell.
Receptor Types:
Plasma Membrane Receptors:
Bind to water-soluble ligands.
Examples: G protein-coupled receptors, receptor tyrosine kinases, ion channel receptors.
Intracellular Receptors:
Located in the cytoplasm or nucleus.
Bind to lipid-soluble ligands that can cross the plasma membrane (e.g., steroid hormones).
Transduction:
The signal is converted into a form that can bring about a specific cellular response.
Signal transduction often involves a cascade of molecular interactions known as a signaling pathway.
Signaling Pathways:
Phosphorylation Cascade:
A series of protein kinases add phosphate groups to the next protein in the sequence, activating it.
Significance: Amplifies the signal and provides multiple points for regulation.
Second Messengers:
Small, non-protein molecules that relay signals from the receptor to other molecules in the cell.
Examples: Cyclic AMP (cAMP), calcium ions (Ca^{2+}), inositol trisphosphate (IP3), diacylglycerol (DAG).
Response:
The transduced signal triggers a specific cellular response.
Types of Responses:
Gene Expression Changes:
Activation or repression of specific genes, leading to changes in protein synthesis.
Significance: Controls cell growth, differentiation, and development.
Enzyme Activation:
Activation of metabolic enzymes, leading to changes in cellular metabolism.
Significance: Regulates energy production, nutrient utilization, and waste removal.
Cytoskeletal Rearrangement:
Changes in the organization of the cytoskeleton, leading to changes in cell shape or movement.
Significance: Important in cell migration, cell division, and cell signaling.
Photosynthesis
Photosynthesis is the process by which autotrophic organisms, such as plants, algae, and cyanobacteria, convert light energy into chemical energy in the form of glucose.
Overview of Photosynthesis
Equation:
6CO2 + 6H2O + Light Energy \rightarrow C6H{12}O6 + 6O2
Two Main Stages:
Light-Dependent Reactions (in the thylakoid membrane):
Light energy is absorbed by chlorophyll and converted into chemical energy in the form of ATP and NADPH.
Water is split, releasing oxygen as a byproduct.
Light-Independent Reactions (Calvin Cycle) (in the stroma):
ATP and NADPH are used to convert carbon dioxide into glucose.
Light-Dependent Reactions
Light Absorption:
Pigments, such as chlorophyll, absorb light energy.
Chlorophyll a: primary photosynthetic pigment.
Chlorophyll b and Carotenoids: accessory pigments (expand the range of light that can be absorbed).
Photosystems:
Pigments are organized into photosystems (PSI and PSII) within the thylakoid membrane.
Photosystem II (PSII):
Absorbs light energy and passes it to the reaction center, where water is split (photolysis).
Electrons are transferred to the electron transport chain.
Photosystem I (PSI):
Absorbs light energy and passes it to the reaction center, where electrons are re-energized.
Electrons are used to reduce NADP+ to NADPH.
Electron Transport Chain (ETC):
Electrons are passed through a series of electron carriers, releasing energy that is used to pump protons (H^+) into the thylakoid lumen.
This creates a proton gradient, which drives ATP synthesis by chemiosmosis.
Chemiosmosis:
Protons flow down their concentration gradient from the thylakoid lumen to the stroma through ATP synthase, generating ATP.
ATP and NADPH are then used in the Calvin cycle to fix carbon dioxide.
Light-Independent Reactions (Calvin Cycle)
The Calvin cycle occurs in the stroma of the chloroplast and involves three main phases:
Carbon Fixation:
Carbon dioxide is added to ribulose-1,5-bisphosphate (RuBP), a five-carbon molecule, catalyzed by the enzyme RuBisCO.
This forms an unstable six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
Reduction:
ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
For every six molecules of carbon dioxide fixed, twelve molecules of G3P are produced.
Two molecules of G3P are used to make one molecule of glucose.
Regeneration:
The remaining ten molecules of G3P are used to regenerate RuBP, allowing the cycle to continue.
Factors Affecting Photosynthesis
Light Intensity:
As light intensity increases, the rate of photosynthesis increases up to a certain point, after which it plateaus.
Carbon Dioxide Concentration:
As carbon dioxide concentration increases, the rate of photosynthesis increases up to a certain point, after which it plateaus.
Temperature:
Photosynthesis has an optimal temperature range, typically between 25°C and 35°C. High temperatures can denature enzymes and reduce
Genetics
Basic Principles of Heredity
Genes: Units of heredity made of DNA that encode specific traits.
Alleles: Different versions of a gene.
Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).
Phenotype: The observable characteristics of an organism (e.g., blue eyes, tall).
Dominant Allele: An allele that expresses its phenotype even when paired with a recessive allele (represented by uppercase letters).
Recessive Allele: An allele that only expresses its phenotype when paired with another recessive allele (represented by lowercase letters).
Homozygous: Having two identical alleles for a gene (e.g., AA or aa).
Heterozygous: Having two different alleles for a gene (e.g., Aa).
Mendelian Genetics
Mendel's Laws:
Law of Segregation: Each individual has two alleles for each gene, and these alleles separate during gamete formation.
Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation (if the genes are on different chromosomes).
Monohybrid Cross: A cross between individuals that are heterozygous for one gene (e.g., Aa x Aa).
Phenotypic Ratio: Typically 3:1 (dominant:recessive) in the offspring.
Dihybrid Cross: A cross between individuals that are heterozygous for two genes (e.g., AaBb x AaBb).
Phenotypic Ratio: Typically 9:3:3:1 in the offspring.
Chromosomal Basis of Inheritance
Chromosomes: Structures made of DNA and proteins that contain genes.
Homologous Chromosomes: Pairs of chromosomes that have the same genes but may have different alleles.
Meiosis: Cell division that produces haploid gametes (sperm and egg cells) with half the number of chromosomes as somatic cells.
Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis, leading to genetic variation.
Non-Mendelian Genetics
Incomplete Dominance: Heterozygous individuals show an intermediate phenotype (e.g., a red flower crossed with a white flower produces pink flowers).
Codominance: Both alleles are fully expressed in heterozygous individuals (e.g., human blood types AB).
Multiple Alleles: More than two alleles exist for a gene within a population (e.g., human blood types A, B, and O).
Polygenic Inheritance: Traits are determined by the interaction of multiple genes (e.g., height, skin color).
Sex-Linked Traits: Genes located on sex chromosomes (X and Y in humans).
X-linked traits are more commonly expressed in males because they have only one X chromosome.
DNA Structure and Function
DNA Structure:
Double helix composed of nucleotides.
Nucleotides consist of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, or thymine).
Complementary base pairing: A with T, and G with C.
DNA Replication: The process by which DNA is copied.
DNA polymerase is the enzyme that adds nucleotides to the new DNA strand.
Transcription: The process by which RNA is synthesized from a DNA template.
RNA polymerase is the enzyme that synthesizes RNA.
Translation: The process by which proteins are synthesized from an RNA template.
Ribosomes are the site of translation.
tRNA molecules bring amino acids to the ribosome based on the mRNA sequence.
Mutations
Definition: Changes in the DNA sequence.
Types of Mutations:
Point Mutations: Changes in a single nucleotide.
Substitution: One nucleotide is replaced with another.
Insertion: A nucleotide is added to the sequence.
Deletion: A nucleotide is removed from the sequence.
Frameshift Mutations: Insertions or deletions that alter the reading frame of the mRNA, leading to a different protein sequence.
Causes of Mutations:
Errors during DNA replication.
Exposure to mutagens (e.g., radiation, chemicals).
Effects of Mutations:
Mutations can be harmful, beneficial, or neutral, depending on their effect on protein function.
Evolution
Darwin's Theory of Evolution
Natural Selection: The process by which individuals with traits that are better suited to their environment survive and reproduce at a higher rate than individuals with less suited traits.
Variation: Individuals within a population have different traits.
Inheritance: Traits are passed from parents to offspring.
Differential Survival and Reproduction: Individuals with certain traits are more likely to survive and reproduce.
Adaptation: A trait that increases an organism's survival and reproduction in a particular environment.
Descent with Modification: The idea that species change over time, giving rise to new species that share a common ancestor.
Evidence for Evolution
Fossil Record: Fossils provide evidence of the history of life on Earth and show how species have changed over time.
Comparative Anatomy:
Homologous Structures: Structures that have a common evolutionary origin but may have different functions (e.g., the bones in the forelimbs of mammals).
Analogous Structures: Structures that have similar functions but do not have a common evolutionary origin (e.g., the wings of birds and insects).
Vestigial Structures: Structures that have lost their original function and are reduced in size (e.g., the human appendix).
Comparative Embryology: Similarities in the development of different species provide evidence of common ancestry.
Molecular Biology: Similarities in DNA and protein sequences provide evidence of common ancestry.
Mechanisms of Evolution
Mutation: Creates new genetic variation.
Gene Flow: The movement of genes between populations.
Genetic Drift: Random changes in allele frequencies in a population.
Bottleneck Effect: A sudden reduction in population size that can lead to a loss of genetic variation.
Founder Effect: A small group of individuals colonizes a new area, and the new population has a different allele frequency than the original population.
Natural Selection: Acts on existing genetic variation to favor individuals with certain traits.
Speciation
Definition: The process by which new species arise.
Types of Speciation:
Allopatric Speciation: Occurs when populations are geographically separated.
Sympatric Speciation: Occurs when populations are not geographically separated.
Patterns of Evolution
Divergent Evolution:
Occurs when two or more species evolve from a common ancestor and become increasingly different over time.
Adaptive Radiation: A type of divergent evolution in which a single ancestral species evolves into a wide array of descendant species, each adapted to a different ecological niche.
Convergent Evolution:
Occurs when two or more species that are not closely related evolve similar traits because they are exposed to similar environmental pressures.
Coevolution:
Occurs when two or more species reciprocally affect each other's evolution.
DNA Replication:
The process by which DNA is copied.
DNA polymerase is the enzyme that adds nucleotides to the new DNA strand.
Steps:
Initiation: Replication begins at specific sites called origins of replication, where the DNA double helix unwinds.
Elongation: DNA polymerase adds nucleotides to the 3' end of the new strand, using the existing strand as a template. Leading and lagging strands are synthesized differently.
Leading Strand: Synthesized continuously in the 5' to 3' direction.
Lagging Strand: Synthesized discontinuously in short fragments (Okazaki fragments) that are later joined together.
Termination: Replication ends when the entire DNA molecule has been copied.
Meiosis:
Cell division that produces haploid gametes (sperm and egg cells) with half the number of chromosomes as somatic cells.
Stages:
Meiosis I:
Prophase I: Chromosomes condense, and homologous chromosomes pair up to form tetrads. Crossing over occurs.
Metaphase I: Tetrads align at the metaphase plate.
Anaphase I: Homologous chromosomes separate and move to opposite poles.
Telophase I and Cytokinesis: Chromosomes arrive at the poles, and the cell divides into two haploid daughter cells.
Meiosis II:
Prophase II: Chromosomes condense.
Metaphase II: Chromosomes align at the metaphase plate.
Anaphase II: Sister chromatids separate and move to opposite poles.
Telophase II and Cytokinesis: Chromosomes arrive at the poles, and the cell divides into four haploid daughter cells.
Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis, leading to genetic variation.
Mitosis:
Cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.
Stages:
Prophase: Chromosomes condense, and the nuclear envelope breaks down.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Chromosomes arrive at the poles, and the nuclear envelope reforms.
Cytokinesis: The cell divides into two identical daughter cells.
Cell Cycle:
An ordered sequence of events in the life of a cell, from its origin in the division of a parent cell until its own division into two.
Phases:
Interphase (G1, S, G2):
G1 Phase: Cell growth and preparation for DNA replication.
Cell increases in size.
Organelles duplicate.
Synthesis of proteins and RNAs necessary for DNA replication.
S Phase: DNA replication occurs.
Each of the 46 chromosomes (23 pairs) is duplicated by the cell.
G2 Phase: Further growth and preparation for cell division.
Cell continues to grow and produce new proteins.
The cell checks the duplicated chromosomes for errors and makes any needed repairs.
M Phase (Mitosis and Cytokinesis):
Mitosis: Nuclear division.
Prophase, metaphase, anaphase, and telophase
Cytokinesis
Division of the cytoplasm, resulting in two separate cells.
Checkpoints:
G1 Checkpoint:
Determines whether the cell is ready to divide based on factors like size, nutrients, and DNA integrity.
If conditions are not favorable, the cell may enter a resting phase (G0).G2 Checkpoint:
Ensures that DNA replication is complete and that there are no errors before mitosis begins.
M Checkpoint (Spindle Checkpoint):Occurs during metaphase to ensure that chromosomes are correctly aligned on the spindle before anaphase proceeds.
Regulation:
The cell cycle is regulated by a complex network of proteins, including:
Cyclins: Proteins that fluctuate cyclically during the cell cycle.
Cyclin-Dependent Kinases (CDKs): Enzymes that are activated by cyclins and regulate the progression through the cell cycle.