Cellular Biology - Notes and Summaries PDF

# Cellular Biology - Notes and Summaries ## Life Biology is a science: - Scientific method: 1. Observation 2. Hypothesis 3. Testing hypothesis through controlled experimentation 4. Interpretation and conclusion 5. Report if hypothesis is supported or not supported 6. New...

# Cellular Biology - Notes and Summaries ## Life Biology is a science: - Scientific method: 1. Observation 2. Hypothesis 3. Testing hypothesis through controlled experimentation 4. Interpretation and conclusion 5. Report if hypothesis is supported or not supported 6. New hypothesis - Biostatistics: a group of procedures used by biologists to interpret data - Descriptive - Inferential ## Introduction ### Chapter 1 1. Properties of life: There is unity in the living world, for all organisms-cells have the following properties in common. - All organisms are made of cells; cells are the smallest unit of life. (Cell theory see point 4 below) - Cells exhibit order, made of parts/organelles with specific functions - They are assembled from the same kinds of atoms and molecules, macromolecules (see point 3 below) - They are capable of processing energy and obey the same laws of energy. - 1st law of thermodynamics: Law of conservation of energy: Energy cannot be created or destroyed; it can only be converted from one form to another. - 2nd law of thermodynamics: The total amount of entropy or disorder tends to increase in the Universe and this correlates with release of heat an unusable form of energy, and small molecules (carbon dioxide and water). Cells need to constantly counteract this tendency and therefore require continual energy input from food (cellular respiration) or from sunlight (photosynthesis). - These two laws help explain: energy flows but does not cycle. Sunlight → chemical energy → kinetic energy - Cells metabolize (perform chemical activities). Cells need to regulate these chemical activities in order to maintain a balanced internal environment, called homeostasis. - They can respond to specific conditions in the environment, by changing their metabolism, patterns of behavior. - Cells are adapted to their environment through selection, evolution; eg macro-phago-bast bacteria evolved to ingest - by natural selection. - Most importantly they are able to reproduce and pass on their characteristics or traits to offspring based on the heritable instructions encoded in the molecular structure of their DNA to divide. - They have the capacity for growth and development determined by their genes. ## Chapter 2 - In this course, we study cells. Cells are positioned within the hierarchical levels of biological organization: - atoms → molecules → macromolecules → organelles → cells → tissues → organs → organ systems → organism - Organs and organ systems: studied in BIO 3 Human Physiology - Population - Community - Ecosystems: studied in BIO 2 Ecology and Evolution ## Chapter 3 - All organisms are made of the same macromolecules: - Carbohydrates (sugars) - Lipids (fats) - Proteins - Nucleic acids (DNA, RNA) ## Chapter 4 - The cell theory: - All organisms consist of cells which are the smallest units of life. - Cells can only be made from pre-existing cells, the basis of biogenesis - The purpose of cellular division is: - reproduction in uni-and multi-cellular organisms - growth and repair of multicellular organisms. - Two types of cells exist: - Prokaryotes (bacteria and archaebacteria) - Small 10-6 m; no nucleus - Eukaryotes (protists, fungi, plants and animals) - Large 100 µm; nucleus, organelles ## Chapter 5 **Carbon Compounds - Macromolecules in Cells** - Organic compounds consist of H hydrogen covalently bonded to C carbon atoms that commonly form near and ring-shaped backbones. The following most common elements, O, N, P, and S are also linked to the carbon backbone of cellular organic compounds. CHNOPS - CO2 is not considered an organic carbon compound. It is inorganic. - Functional groups attached to the carbon backbone impart diverse properties to organic compounds that are the characteristics of life. - In nature, only living cells can assemble the large organic compounds called macromolecules or polymers: - Complex carbohydrates - Lipids - Proteins - Nucleic acids - Organisms draw from small pools of organic compounds - monomers. The accompanying Table on page 14 summarizes the major categories of these compounds and their functions. **Carbohydrates** - Most abundant macromolecules; general composition: CHO (N in chitin) - Monosaccharides or sugars generally (CH2O)n; n being the number of C in the sugar - Glucose, fructose, galactose, glucosamine - The presence of O =hydrophilic. The numbers of carbons are counted with 6 carbons at the top. - Disaccharides (2 sugars) store energy: maltose, sucrose, lactose. Note: Dehydration synthesis takes place to create disaccharides from monosaccharides. - Complex carbohydrates - Polysaccharides are true polymers: - They are made of multiple monomers by dehydration synthesis. They function either in storage of energy or have a structural role in the cell. - Structural: - Cellulose in cell wall (plant) - β glucose - Chitin (chain of glucosamine found in cell wall fungi) - α alpha β beta - Energy and chemical storage: - Starch -plants - chloroplasts - Glycogen - animal - liver **Lipids** - Not true polymers but considered macromolecules due to their size - Heterogeneous group of macromolecules; hydrophobic - water fearing; not soluble in water (non polar) - Composition: CHO, (N and P in phospholipids) - Monomers: fatty acids and glycerol (for fats, oils and phospholipids) - Energy storage: triacylglycerides = triglycerides - Fats - animals - Oils - plants (also insulation & protection) - Structural: phospholipids in cell membrane – bilayer - Amphipathic: - Hydrophilic head - Hydrophobic tails - Multiple functions: cholesterol see Table on page 14 for functions - Cofactors required for absorbing light: retinal, carotenoids **Properties of fatty acids:** - Saturated: - No C=C bond - Linear - Solid at Room temperature - Animal fats / butter - Unsaturated: - 1 or more C=C bond - Bent - Kinked. - Liquid at Room temperature - Oils (vegetable-plant) **Cell membrane Bilayer** - Hydrophilic head - Hydrophobic tails **Relationship between cell membrane and cell wall** - Cell membrane (lipid) - Cell wall (carbohydrate) **Cholesterol** - 3 functions. - Membrane - Bile salts - Other steroids ## Chapter 6 **A Tour of the Cell: Cell Structure and Function** **Biogenesis** - All living things are composed of one or more cells. Cells are made of the same macromolecules. - The cell is the smallest unit that retains the properties of life. (remember the properties of life that we have seen in the introduction). That is, it either lives independently or has a built-in genetic capacity to do so. - New cells arise only from pre-existing cells; basis of reproduction and division of unicellular organisms and tissue repair in multicellular organisms **Cell Size:** - 1-100 µm (µm = 10-6 m) (with a few exceptions; for example, a chicken egg is 5 cm in diameter, and some nerve cells can be close to a meter long). - Observed with the microscope - Light: visible light optics (lenses); observe live specimens, limited resolution, and magnification - Fluorescence microscopy - Electronic: electron optics (magnets); observe fixed specimens, better resolution, and magnification. - Resolution: amount of detail visible; minimal distance at which 2 points are seen distinctly (ratio image size / object size) - Magnification: - Limits to cell size - Upper limit: lack of space. Surface area to volume ratio - Lower limit: - If a cell is very large (large volume), the surface of the plasma membrane may not be large enough to allow efficient exchange of the cell with its environment (absorption of nutrients and release of cellular wastes). - Solutions to upper limit: - Compartments which store and manage wastes and nutrients = organelles. - Adopt elongated shapes or structures (microvilli) - Larger organisms have more cells (not larger cells) **The Minimal Cell: Eukaryotes or Prokaryotes** - Cell membrane = plasma membrane - DNA (in space). - Ribosomes - Cytoplasm (H2O) - Protein synthesis **The Plasma Membrane** - 9 nm thick, phospholipid bilayer + proteins - Semipermeable membrane: - Impermeable to hydrophilic substances (ions, polar molecules such as water, sugars, ...) - Permeable to hydrophobic, non-polar substances (gases, lipid soluble molecules, cholesterol) - Transport proteins for passage of hydrophilic molecules **Prokaryotic cell** - Prokaryotes have no membrane bound cytoplasmic organelles, i.e. no nucleus, no mitochondria, no Golgi etc. Prokaryotes are also on average 10X - 100X smaller than eukaryotic cells. - Nucleoid: similar to nucleus, area for DNA; not membrane bound - DNA is a single circular chromosome - Cell wall: peptidoglycan (protein + carbohydrate) - Capsule: sticky polysaccharide for attachment; found in pathogenic bacteria - Pili: extensions for attachment; found in pathogenic bacteria - Flagella: locomotion ## Chapter 7 **Endomembrane System: Transport system for membrane proteins (transporters) and secreted proteins** - Ribosome on Rough ER - Protein synthesis - Secretory proteins: from RER → Golgi → secretory vesicle → PM - Synthesis of Cytoplasmic proteins, ex. insulin - Lipid synthesis enzymes - Cleavage enzymes - SER - Lysosomes: "stomach" for hydrolysis (macromolecules), ex. insulin - Vesicle - "Zip code" protein - Micro tubule mediated transport ## Chapter 8 **Nucleic Acids** - Important informational macromolecules; composition: CHNOP. - Monomers: nucleotides - Bond between nucleotides phosphodiester bonds - Two forms of nucleic acids: - DNA – Deoxyribonucleic acids made of 4 different deoxyribonucleotides, storage of genetic info - RNA - Ribonucleic acids made of 4 different ribonucleotides, protein synthesis - Some nucleotides, including ATP- adenosine triphosphate- have central roles in metabolism as energy carriers (see page 12-13) **DNA vs. RNA** ### Comparison Table | Criteria | DNA | RNA | |---|---|---| | Overall structure | ds double helix, antiparallel, complementary G = C, A = T | single stranded (ss) | | Monomers | deoxyribonucleotides | ribonucleotides | | Pentose sugar | deoxyribose | ribose (extra OH) | | Bases | G, A, T, C | G, A, U, C | | Function | Store genetic information | Carry genetic info for protein synthesis | | Location in cell | Euk. nucleus, cytoplasm | Euk. nucleus, cytoplasm | | Chromatin: | form of DNA that is packaged/complexed with proteins | | **Structure of nucleotide** - Phosphate groups are ionized (-O-) - Sugar - Nitrogenous bases - Purines (adenine (A), guanine (G)) - Pyrimidines (cytosine (C), thymine (T, in DNA) Uracil (U, in RNA) - One strand **Structure of DNA** - Phosphodiester linkage - Purine-pyrimidine equal spacing - 2 strands: antiparallel, complementary **ATP** - high energy bonds; unstable bonds, easily broken, linked to the release of free energy **ATP hydrolysis** - ATP hydrolysis is a favorable spontaneous reaction associated with the release of Gibbs (G) free energy useful to power cellular work - Chemical work: anabolic reactions - Mechanical work: cellular movement; muscle fiber contraction - Transport: molecular transport across the cell membrane - chapter 7 **The ATP cycle** - Dehydration synthesis (endergonic) → ATP → hydrolysis (exergonic) + H2O - Free energy from cellular respiration (glucose) → ADP + P - Cellular respiration is exergonic - Free energy to power cellular work (endergonic) ## Chapter 9 **Cellular Respiration** - Site of cellular respiration: Mitochondria ## Chapter 10 **Photosynthesis:** - Site of photosynthesis: Chloroplast - Thylakoid, green-chlorophyll-photosynthesis ## Chapter 11 **Peroximes** - Site of detoxification of oxygen radicals: O2 - Detoxify alcohol in liver cells - Fatty acid catabolism ## Chapter 12 **Cell Shape** - **Cell wall:** - Plants: cellulose (β-glucose polymer) - Fungi: chitin (N-acetylglucosamine polymer) - Bacteria: peptidoglycan: polymer made of sugars, and linked by short peptides. - Bacteria DO NOT have a CYTOSKELETON. **The Cytoskeleton** - Quaternary structure: cortex or gel - Maintain cell shape, anchoring of organelles in eukaryotic cells - Plays a dynamic role during cellular division, transport of proteins and organelles, and cell movement. - **Microfilaments**: also called actin filaments, 7 nm fibers, made of actin protein subunits; dynamic filaments (can grow and shrink) - Associated with PM → cortex - Shape: microvilli - Motility (pseudopod of amoeba or for phagocytosis) - Muscle contraction: actin-myosin network in sarcomere - Cell division: actin-myosin, during cytokinesis - **Intermediate filaments**: 10 nm fibers; ex: keratin in skin cells - Strength, shape of cells - Anchor organelles (like nucleus, and mitochondria - Form nuclear lamina - **Microtubules (MTs)**: 25 nm hollow filaments, made of tubulin protein dimers; dynamic. - Grow at centrosomes, microtubule organizing centers (MTOs – equivalent of centrosome in plant cells) and basal bodies (flagella, and cilia) - Cell shape: cilia, and flagella - Cell movement: 9+2 arrangement of MT - Cell division: chromosome movement (mitotic spindle) - Organelle movement: tracks for transport of vesicles, during protein secretion - ATP hydrolysis: energy requiring **Endomembrane system: Transport system for membrane proteins (transporters), and secreted proteins** - Ribosome on Rough ER - Protein synthesis - Secretory proteins: from RER → Golgi → secretory vesicle → PM - Synthesis of cytoplasmic proteins, ex. insulin - Lipid synthesis enzymes - Cleavage enzymes - SER - Lumen - Lysosomes: "stomach" for hydrolysis (macromolecules), ex. insulin - Vesicle - "Zip code" protein - Microtubule mediated transport ## Chapter 13 **Comparison of Prokaryotic Cells and Plant, and Animal Eukaryotic Cells** | Structure / Function | Prokaryotic Cells | Eukaryotic Cells | |---|---|---| | Plasma membrane | Present | Present | | Nuclear envelop and nucleus | Absent (no true nucleus) | Present (true nucleus) | | Chromosomes | Single circular DNA molecule | Usually several composed of chromatin (DNA + histones) | | Nucleolus | Present | Present | | Ribosomes | Present (smaller in Bacteria, like eukaryotes in Archaea) | Present (those in cytoplasm are larger, those in mito, and c'plasts like bacteria) | | Rough endoplasmic reticulum (rough ER) | Absent | Present | | Smooth endoplasmic reticulum (smooth ER) | Absent | Present | | Golgi bodies - folding, modifying and sorting of proteins produced by rough ER. Series of flattened sacs | Absent | Present | | Lysosomes - intracellular digestion | Absent | Present in animal cells mainly | | Mitochondria | Absent - same function may be performed by cell membrane | Present | | Chloroplasts | Absent - same function may be performed by cell membrane | Present in plant cells only | | Microtubules - MT | Absent | Present in all | | Centrioles - part of centrosome; mitotic spindle | Absent | Present except in (seed) plants | | Basal bodies - assembly of cilia, and flagella | Absent | Present except in (seed) plants. | | Cilia / flagella | Protein fibres not covered by plasma membrane | 9 + 2 array of microtubules enclosed by plasma membrane - absent in (seed) plants. | | Central vacuole, cell eating/flagella | Absent | Absent in animal cells = lysosome Present in plant cells | | Cell wall | Present (contains peptidoglycan in Bacteria but not in Archaea) | Absent in animal cells Present in plant cells - cellulose | | Capsule | Sticky polysaccharide layer surrounding cell wall | Absent | | Cell division | Amitotic by binary fission | By mitosis | - **Plant cells have a cell wall : therefor they cannot phagocytose. They cannot move with cilia and flagella. They have cell shapes with microvilli. However, plant cells have a cytoskeleton.** - **Animal cells have a cytoskeleton, proteins cables, quaternary structure. Microvilli. Cilia and flagella** **Organelle** - Compartment inside cell, generally surrounded by membrane (phospholipid bilayer) - Structure with specific function - ex. cytoskeleton. - Ribosomes - The endomembrane system is responsible for modifying, sorting, and transporting proteins to their destination (exterior, membrane, lysosomal/vacuolar or SER). - The nuclear envelope consists of pores which allow the passage of molecules in and out – from the cytoplasm into the nucleoplasm, and vice versa (ex: mRNA and ribosomes leave the nucleus) **Lysosomes (found only in animal cells)** - Hydrolytic enzymes - Digestion of macromolecules - Phagocytosis: digestion of large food particles from outside "stomach" of the cell - Autophagy: digestion of damaged organelles. - Example: macrophages (immune cells which combat bacterial infections), amoeba (protist) **Large central vacuole (found only in plant cells) ** - Storage of wastes, ions, pigments - Water homeostasis: turgor pressure and cell growth **Endomembrane system** - DNA → mRNA (nucleus) - Cytoplasm (protein: ex. cytoskeleton, enzymes) - RER (protein) - SER (lipid synthesis enzymes) - Golgi - Lysosomes (hydrolytic enzymes) - Secretory vesicle **Organelles that have a role in metabolism:** - **Mitochondria** (cellular metabolism and production of ATP) - **Chloroplasts** (photosynthesis) (only plant cells and some protists) - **Endosymbiont theory**: organelles originate from bacteria - aerobic bacteria → mitochondria - cyanobacteria → chloroplast - **Mitochondria** - cristae folds increase surface area for production of ATP - **Chloroplasts** - thylakoid, green chlorophyl - photosynthesis - **Peroximes** - Site of detoxification of oxygen radicals: O2 - Detoxify alcohol in liver cells - Fatty acid catabolism - **Cell wall** - Plants: cellulose (β-glucose polymer) - Fungi: chitin (N-acetylglucosamine polymer) - Bacteria: peptidoglycan: polymer made of sugars, and linked by short peptides. - Bacteria do not have a cytoskeleton. ## Chapter 14 **The Cell Cycle - Mitosis** - Asexual reproduction in eukaryotic unicellular organisms (and a few multicellular organisms.) - Growth, and development of multicellular organisms - Tissue renewal/repair of multicellular organisms - Two identical daughter cells are formed. - Each receives an identical set of chromosomes as well as cytoplasm. - The number of chromosomes is constant from one generation to the next. - 3 events: - DNA is replicated/copied, and condensed (prophase) - 2 sister chromatids per chromosome - Sister chromatids: identical DNA molecules attached by their centromere - Nuclear division or mitosis: chromosomes are divided. - Cytoplasmic division or cytokinesis. **4 phases of the cell cycle:** - **G0:** quiescence; cells do not grow or divide; metabolism; most cells. - **G1:** Gap phase 1 - Cells grow in size and metabolize - Organelle duplication - Genes are transcribed, and proteins are made in preparation for S phase - G1 checkpoint: point in time, checkpoint monitoring cell size and internal and external environments; commitment to divide - Commitment to complete cell division - **S:** synthesis - DNA synthesis; duplication of chromosomes. DNA polymerase - **G2:** Gap phase 2 - Duplication of centromeres or MTOCs (plant cells) - poles of the mitotic spindle. - Microtubule organizing centers - centrosome - Cells produce more proteins and a critical surface area / volume ratio is reached. **M Phase** - G1, S and G2 phases form the interphase: - Longest phase: nucleus visible, individual chromosomes not visible. - M phase: Mitotic: short chromosomes visible, and separating - Mitosis: nuclear division, or separation of sister chromatids. - Cytokinesis: physical separation of cell into 2 daughter cells (cytoplasmic division) **4 M- subphases** - **Prophase**: chromosomes condense (chromatin formation; DNA packaged with proteins), and appear X-shaped; condensation facilitates movement, and division of chromosomes; - Chromosomes are made of chromatin - DNA wrapped around histones - and further compacted, and attached to a protein scaffold. - **Prometaphase**: - nuclear envelope disappears - Centrosomes at opposite poles - Mitotic spindle complete (chromosomes become captured) - The distinction between prophase and prometaphase is not clear cut. - **Metaphase**: chromosomes align at metaphase plate (equator / mid-line). - Centromere - Polar MT - Kinetochore MT - Mitotic spindle at metaphase. MT = microtubule - **Anaphase**: sister chromatids separate to opposite poles. - Overlap of polar MTs reduces (spindle elongates) - Kinetochore MTs attached to centromeres of chromosomes become shorter - **Telophase**: - nuclear envelope reforms → two nuclei appear - Chromosomes decondense; spindle disappears; nucleolus reappears - **Cytokinesis**: - Animals: formation of a cleavage furrow - or pinching of the cell membrane by the contraction of the actomyosin ring (microfilaments). - Plant cells: formation of a cell plate - which is created by vesicles originating from the Golgi that align and fuse at the equator of the dividing cells. Eventually, one large vesicle fuses with the PM and separates the two cells. New cell wall is then deposited between the cells. **Cell division in bacteria (prokaryotes): also called binary fission.** - The single circular chromosome of bacteria is replicated at a single origin called the origin of replication. - The two origins separate at either end of the dividing bacterium. During replication the bacterium elongates. - The cells physically separate by inward growth of the cell membrane; new cell wall is deposited. - DNA replication is not synchronized with physical separation of the cells. - In rapidly dividing bacterial cells, the next round of DNA replication initiates before the previous cell separation is complete. This may lead to improper separation of chromosomes. **Checkpoints** - There are 3 basic checkpoints that monitor the progression of the cell cycle: - G1 (restriction) checkpoint in late G1 - G2 checkpoint - M checkpoint - These checkpoints "control" that the preceding phases are completed before cells proceed with the next phases. **The G1 checkpoint** - Ensures that cells have reached a critical size, and that environmental conditions, such as availability of growth factor, and nutrients, are adequate before the cells proceed to the S phase. - The following are examples of conditions in which the G1 checkpoint is involved: - Anchorage dependence: cells only divide if attached to a surface; if not the G1 checkpoint kicks in to stop cell division. - Density dependent inhibition: cells stop dividing once the entire surface is covered with cells - 1 layer. Once the surface is covered, the G1 checkpoint kicks in to stop further cell division. - DNA damage: cells stop dividing to repair the damage. The G1 checkpoint prevents cell division for as long as DNA remains damaged. **Apoptosis** - When DNA damage is beyond repair, cells may undergo programmed cell death - **apoptosis**. - A signal (internal or external) triggers mitochondria to activate a series of **caspases**. - Caspases are proteins that act as proteases, and initiate the apoptotic response: - Chromatin condenses, and nucleus fragments - DNA fragmentation **nucleases** - Apoptotic bodies form - the cell blebs - cellular fragmentation - Phagocytosis of apoptotic bodies - Protease: enzyme that breaks down proteins - Nuclease: enzyme that breaks down nucleic acids (DNA) - **Cell cycle regulation**: when the G1 checkpoint fails to work, cells may divide when conditions are not optimal. This may result in transformation of cells through accumulation of DNA mutations and formation of cancer cells. - Anchorage dependence - Density-dependent inhibition - Most cells are in interphase, but some cells are in prophase, anaphase, or telophase ## Chapter 15 **Meiosis and Sexual Life Cycles** - Advantages of sexual reproduction: - Increases genetic variability (variation in traits) of offspring. - Increases potential of survival of a population in ever changing environment - Homologous chromosomes - One copy is inherited from mother, the other from father - Have the same length and shape - Have the same banding (staining) patterns - Have the same genes and order of genes - Have similar, but not identical DNA sequences - have different versions of the same genes. - Karyotype: human diploid number is 46; display of metaphase chromosome showing banding patterns, genes, number of chromosomes and ploidy. - 23 pairs; 22 somatic pairs (autosomes) + 1 pair sex chromosomes; - XX = female, maternal, ovum always carries 22 + X; - XY = male, paternal, sperm carries 22 + Y or X. - The human haploid genome is the sum of all genes (30000) found in one set of 23 chromosomes. - Example of homologous pairs (metaphase) - Human sexual reproduction proceeds through three events: - Meiosis (formation of gametes) - Fertilization (unites sperm and egg to form a zygote) - Mitosis and development (creates a multicellular functional organism from one starting cell, the zygote) - Meiosis that produces sperm and egg (oocyte) is called spermatogenesis and oogenesis respectively. - Somatic cells: diploid, 2n: 2 copies of every chromosome - Gametes: haploid, n: 1 copy of every chromosome - Human diploid number: 2n= 46 - Human haploid number: n=23 - **Meiosis:** - Meiosis divides the chromosomes number by half for each forthcoming gamete. - Thus, if both parents are diploid, (2n) the gametes that are formed are haploid (n). - Later, the union of the two gametes at fertilization restores the diploid number in the new individual (n + n = 2n). - **Meiosis** consists of 2 nuclear divisions; meiosis I and meiosis II, but DNA replication occurs only once in S of interphase. - **GERM CELL (in ovary or testis)** - **Meiosis I**: - Prophase I: - Chromosomes condense and pair up - Synapsis homologous chromosomes - synaptonemal complex (glue) - Crossing over: homologous chromosomes exchange chromatid segments. - Metaphase I: alignment of tetrads (pair) alone equator - Homologous chromosomes are held by chiasmata, former crossing over sites - Independent assortment: homologous chromosome pairs (tetrads) align independently of one another - Anaphase I: separation of homologous chromosomes, but not of sister chromatids. - Telophase I: chromosomes decondense (spindle disappears, nuclear envelope reforms) - Cytokinesis does not necessarily occur. - Resulting nuclei are haploid. - **Meiosis II**: - Prophase II: chromosomes recondense; no prior DNA synthesis. - Metaphase II: chromosomes align at equator. - Anaphase II: "sister" chromatids separate - at this point the chromatids are no longer truly identical sisters because of crossing over. - Telophase II: chromosomes decondense, and cytokinesis occurs. - Resulting in 4 different haploid cells = gametes, sperm, and egg - 4 different haploid nuclei reform. - Variation in traits among offsprings: - Production of unique gametes by meiosis - Homologous but not identical parental chromosomes in germ cell - Crossing over during prophase I - Independent assortment during metaphase I - random fusion of egg (female) and sperm (male) cells - *crossing over:* during meiosis, each pair of chromosomes may exchange segments, a process called crossing over. Each time they do, they exchange hereditary instructions or genes. - **Early prophase I** - crossing over - "synaptonemal complex" - **Late prophase I** - chiasmata - Sisters - Former points of crossing over - Kinetochore - Centromere - **Independent assortment:** meiosis assigns one of every pair of chromosomes to a forthcoming gamete - but which gamete is its destination is matter of chance. This random assignment of chromosomes during meiosis I is called independent assortment. - **Comparison of mitosis and meiosis** - Mitosis - somatic, embryonic - Meiosis - germ cells - # divisions: 1 vs. 2 - Crossing over (prophase I): no vs. yes - Independent assortment (metaphase I): no vs. yes - DNA synthesis: 1 vs. 1 - Number of cells produced: 2 vs. 4 - Ploidy of cells: 2n, identical vs. n, genetically different - **Mitosis and meiosis I are similar because** - **Sister chromatids are separated.** - **2n → 2^23 different gametes per individual, possible (based on independent assortment) alone** - **Meiosis:** - Diploid 46 - No chromatids - 2n = 46 - 2n = 46 - 2n=46 - 4n=92 - 2n = 46 - 2n = 46 - n=23 - n=23 - n=23 - n=23 - Haploid gametes: no chromatids - **Mitosis:** - 2n= 46 - no chromatids - 2n=46 - 4n = 92 - 2n = 46 - 4n=92 - 2n = 46 - 2n = 46