Cell Biology: Cell Cycle and Division

• Cell biology includes the cell cycle
• The cell cycle is a series of event that results in DNA replication and division of the cytoplasm and organelles
• Division results in two daughter cells
• The stages are interphase, mitosis and cytokinesis

Cell Division

• Necessary for growth and development in multi-cellular organisms
• Repairs and maintains by replacing old, damaged or dead cells
• Essential for reproduction to produce gametes like sperm and eggs

Phases of the Cell Cycle

• There are four phases in eukaryotic cells: G1, S, G2, and M phase
• Interphase is the longest stage (90% of the cycle)
• G1 phase includes the growth phase and prep for DNA synthesis of the S phase
• RNA and protein synthesis occurs, and the cell doubles in size
• If a cell does not need to divide, it enters the G0 "resting" phase
• Most human cells are is G0 phase as metabolically active but no longer multiply unless called back according to the organism's requirement
• Some cells once formed remain in G0 phase and can’t be replaced if damaged, such as the cells in the heart and the brain
• Other cells enter the G0 phase, but can re-enter the cell cycle and replaced over time, e.g., skin cells
• S phase includes the synthesis of nuclear DNA where the 46 chromosome forms the sister chromatid for a total of 92 chromosomes
• After DNA synthesis, chromosome strands condense into heterochromatin
• G2 phase ensures the DNA synthesis is complete before mitosis
• The cell continues to produce proteins and organelles needed for cell division
• G2 contains a checkpoint to make sure the mitosis promoting factor (MPF) is high enough for division

Mitosis

• Involves 5 phases based on physical state of chromosomes and spindle
• The final physical cell division is cytokinesis
• The phases include prophase, prometaphase, metaphase, anaphase, and telophase

Mitosis Phases

• Prophase: chromatin condenses into visible chromosomes; mitotic spindle begins to form; the nucleolus disappears
• Prometaphase: nuclear membrane breaks down; centrosomes migrate to opposite ends of the cell; spindle fibers attach to chromosomes' centromeres; chromosomes move toward center
• Metaphase: chromosomes align in the center of the cell at the metaphase plate
• Anaphase: each centromere and kinetochore splits into two; sister chromatids are pulled apart by spindle fibers, moving toward opposite poles
• Telophase: chromosomes arrive at poles and de-condense back into chromatin; mitotic spindles dissociate; nuclear envelope forms around each set of chromosomes

Cytokinesis

• The cytoplasm is divided equally creating two new daughter cells
• A ring of actin is responsible for creating the "cleavage furrow" which contracts
• The process results in two distinct daughter cells

Molecular Composition of Cells

• 70% Water
• 29% Organic Molecules
• 1% Inorganic Ions
• Inorganic ions such as Sodium, Bicarbonate, Calcium, Potassium, Phosphate, Magnesium and Chloride
• These are involved in the metabolism for cell function

Four Classes of Molecules

• The macromolecules formed by polymerization of hundreds or thousands of low-weight precursors
• Molecules include carbohydrates, lipids, proteins and nucleic acids
• Amino acids link to form proteins
• Nucleotides link to form nucleic acids
• Simple sugar link to form carbohydrates

Carbohydrates

• Formula is (CH20)n and gets its name from "carbo" and "hydrate"
• Provides cellular energy
• Polysaccharides are formed of monosaccharides, where the storage form is sugars
• The number of carbons in monosaccharides include 3 (triose), 4 (tetrose), 5 (pentose), 6 (hexose) and 7 (heptose)
• Trioses, pentoses and hexoses are more common
• Glucose(C6H12O6) is the principal source of cellular energy
• Sugars containing 5 or more carbons cyclize to be the predominant molecule within cells
• The cyclized sugars can be in 2 forms a or b
• Monosaccharides can also join by glycosidic bonds via dehydration reactions
• A disaccharide is 2 joined sugars
• An oligosaccharide is between 3 to 9 sugars joined
• Polysaccharides contain a large amount of sugars joined
• The storage can be animal glycogen, or plant starch
• Glycogen and starch are from a-glucose monomers
• C1 of one glucose links to C4 of the next glucose to form an unbranched chain
• If C1 of one glucose links to C6 of the next glucose to form a branched chain
• Branched chains are in glycogen and one form of starch (amylopectin)
• A form of starch is amylose which is an unbranched molecule
• Branch points have multiple ends that cleave off glucose molecules for rapid release of glucose

Lipids

• Macromolecules soluble in non-polar solvents (hydrophobic)
• Roles include important energy storage, components of cell membranes, cholesterol which is part of steroid hormones and vitamin D
• The simplest lipid are fatty acids
• Fatty acids form hydrocarbon chains with a carboxyl group (COO) at one end
• Fatty acids include the saturated and unsaturated forms
• Saturated fatty acids include palmitic acid (16 carbons) and stearic acid (18 carbons)
• Unsaturated fatty acids include oleic acid (18 carbons)
• Double bonds position is specified by numbering carbons starting from the methyl end, is also called the omega (w) labeling system
• Oleic acid omega is 9; Linolenic acid omega is 6 and causes inflammation
• Eicosapentaeinoic acid is omega 3; both w3 and w9 are beneficial
• Fatty acids are stored in the form of "Triacylglycerols" (triglycerides), where consisted of linked to 3 fatty acids and a glycerol
• Triacylglycerols are non-polar forming accumulation into fat droplets
• Broken down to produce energy when needed
• Yielding more than twice as much energy than carbohydrates and proteins
• Cholesterol consists of 4 hydrocarbon rings
• But is hydrophobic with a hydroxyl group attached on one end for it to be weakly hydrophilic as well - allowing it to be an amphipathic

Cell Organelles

• Subcellular structures where each has specific job to perform within the cell
• Cytoplasm is a thick solution that fills cell with water, salts and proteins
• Nucleus is the genetic information and control center
• Chromatin is made of coiled DNA bound to histones
• Nucleolus synthesizes ribosomal RNA (rRNA) and the ribosome
• The double membrane is made of the nuclear envelope (NE_ that separates the contents from the cytoplasm
• Nuclear pores permit transfer of molecules, which is linked to the RER
• Ribosomes makes protein (protein factories that is ribonucleic acid (RNA))
• Made of ribosomal RNA and protein
• Free floating ribosomes keep proteins within in cell but the attached ribosomes to the ER, manufacture proteins inserted into the cell to be secreted
• The difference between the ribosomes and other cells is there is no membrane
• The ER (endoplasmic reticulum) holds the two membranes - either rough or smooth
• Smooth endoplasmic reticulum helps biosynthesis the cholesterol and steroids
• Rough endoplasmic reticulum is responsible for protein modification

Golgi Apparatus (Golgi Body)

• The cells protein packaging
• Proteins from RER transport via vesicles into membrane where proteins are packed
• The vesicles diffuse through membrane for post-translational modifications such as glycosylation
• The membranes form to increase area for function
• Mitochondria generate the cell energy (the cell POWER house)
• Surrounded by two membranes where the inner membrane has many folds called cristae to help increase surface area for reactions
• Number of mitochondria dictates speed of metabolic rate
• Have DNA and ribosomes to synthesis membrane proteins
• Lysosomes help with protein digestion
• Composed of hydrolitic enzymes
• Centrosome serves as the engine of cell division
• The microtubule is organized and composed of a barrel shape called centrioles
• Radiate from centrosome to both poles to help chromosomes help in cell division

Cellular Diversity

• Measured in size via micrometers
• Unicellular example is bacteria, whereas multicellular are the plants and animals
• The diversity in shape and function relates to the function and environment

Cell Classification

• Depends on having a nucleus or not
• Eukaryotes means that the have a nucleus
• Prokaryotes means that they lack a nucleus

Bio-membranes

• The selective-permeable that separates from other parts that functions with difference in composition
• Their structures compose carbohydrates, lipids and protein
• Carbohydrates are very small leaflet
• Proteins to lipid differs in different membrane function
• Plasma membrane includes a mixture of the two
• Inner Mitochondria membrane has much more protein vs lipid for biological function in space
• Myelin sheath have much more lipid than protein for a thick permeability
• Membrane lipids include: phosphoglycerides, Sphingomyelins , and they are made of fatty acids
• Have hydrophilic or hydrophobic properties that relate to PO4, alcohol or chains
• Carbohydrates on the lipids include cerebrosides and gangliosides

Cell Membrane

• Cholesterol intercalates among the phospholipids on the membrane
• Includes sugar containing properties, sphingosine or double bounds
• Plasma membrane is known to form as long sheets where hydrophobic regions are protected and hydrophilic regions are immersed in water
• Membrane fluidity is determined in relative movement of lipids vs another

Fluidity

• Movement depends on the strength of interactions
• Stronger implies more rigid; weaker implies more fluid
• Length of hydrocarbon chain changes fluidity, shorter more fluid; longer less fluid
• Increasing compactness will decrease fluidity
• Cholesterol creates a buffering action
• Cooling down temperature means clustering up, and more space due to disruptions increases
• Rising temperature increases too much freedom and loses compactness

Membrane Proteins

• Attached to the lipid bilayer and transport selectively
• The cell adhesion process helps cells attach
• Cadheren is needed for cell contact
• Integrin is needed for cell-matrix adhesion

Proteins

• Variable macromolecules containing the thousands of different proteins
• The protein roles are structures, transport, information, defense, enzyme
• Monomers that comprise proteins are called amino acids
• Amino acids are linked by polypeptides and dehydration synthesis
• Four broad categories include the non polar, polar, acid or charge
• Basic groups terminate in the amino
• Each side chain will have 3 letters and 1 letters.

Nucleic Acids

• Includes DNA & RNA
• Polymerase in cells, DNA is in the Nucleus and RNA in the Ribosomes
• Types include mRNA (messenger), rRNA (ribosomal), and tRNA (transport)
• DNA & RNA are made from nulceotides and include the purine bases (adenine A, guanine G)
• Pyrimidine basis (cytosine C, thymine T, uracil U)
• Sugars include deoxyribose and ribose in DNA formation.
• The monomers are the bases + sugars = nucleocides, along with the phosphate group
• There are phosphodiester bonds between the 5' phosphate of one nucleotide and the 3' hydroxyl of the next via dehydration synthesis
• small polymers of nucleotides are is called oligonucleotides
• large chains of monomers with sense of direction terminate with 5 prime phosphate group and on hydroxyl at the 3 group
• The backbone of these bases allow for writing and function