Basic Chemical Context of Life notes Flashcards
organisms are made up of ____
matter
matter
anything that takes up space and mass
electronegativity
ability of an atom to attract electrons
EN between two atoms directly affects what type of bond it forms
Ionic bond
the complete transfer of electrons from one atom to another
more electronegative atom pulls an electron from the less electronegative atom
results in a difference in charge between the atoms
covalent bond
sharing of electrons between atoms with similar electronegativities
single, double, or triple
nonpolar covalent bond
equal sharing of electrons
no atom has a greater pull of electrons than the other - identical electronegativities
polar covalent bond
unequal sharing of electrons
leads to formation of a dipole
hydrogen bond
a weak bond that can form between molecules that have a hydrogen atom attached to a highly electronegative atom (F, O, or N) that is attracted to a negative charge on another atom
- can be within a molecule (intramolecular) or between molecules (intermolecular)
- important bond in water and DNA
Van der Waals interactions
are a weak attraction rather than a true chemical bond
due to different distribution of electrons
- weaker and more transient than hydrogen bonds
- interaction gets stronger the larger the molecule is
- happens as electrons orbit the atom and there is a change is distribution
water
- polar molecule
- excellent solvent - the dipoles of H2O break up polar or charged ionic molecules
- high heat capacity - a lot of heat has to be added before the temperature changes (good thing! means water is temperature stable)
- more dense as a liquid than a solid
- ice floats - water expands as it freezes and becomes less dense than liquid form
- cohesion/surface tension - water is attracted to like substances due to its H bonds. this strong cohesion between water molecules produces a high surface tension
- adhesion - water is also attracted to unlike substances
Hydroxyl (OH)
polar and hydrophilic
carboxyl (COOH)
polar, hydrophilic, weak acid
amino (NH2)
polar, hydrophilic, weak base
phosphate
polar, hydrophilic, acid
carbonyl
- aldehyde
- ketone
polar and hydrophilic
methyl (CH3)
nonpolar and hydrophobic
monosaccharide
singular sugar molecule
(glucose or fructose)
alpha - OH down
beta - OH up
disaccharide
two sugar molecules joined by glycosidic linkage
(sucrose, lactose, maltose)
polysaccharide
series of connected monosaccharides; polymer
(glucose + fructose)
(glucose + galactose)
( glucose + glucose)
starch
a polymer of alpha-glucose molecules
- store energy in plant cells
glycogen
a polymer of alpha-glucose molecules
- store energy in animal cells
cellulose
a polymer of beta-glucose
- structural molecules for walls of plant cells and wood
chitin
a polymer similar to cellulose, except each beta-glucose group has a nitrogen containing group attached to the ring
- structural molecule in fungal cells as well as insect exoskeletons
lipids
hydrophobic molecules that function in insulation, energy storage, make up structural components like cholesterol and phospholipids in membranes, and participate in endocrine signaling
triglycerides
describe structures consisting of three fatty acid chains attached to a glycerol backbone. Can be saturated or unsaturated
saturated
contain no double bonds and have straight chains; are bad for health since the straight chains stack densely and form fat plaques
unsaturated
contain double bonds that cause kinks in chains; are better for health since chains stack less densely
can be cis or trans
phospholipids (diaclglycerols)
comprised of two fatty acids and a phosphate group (+R) attached to a glycerol backbone
- are amphipathic, which means has both hydrophobic and hydrophilic properties
steroids
comprised of three 6-membered rings and one 5-membered ring; include hormones and cholesterol
lipid derivatives/structures
- phospholipids
- waxes - esters of fatty acids and monohydroxylic alcohols, used as a protective coating on the skin
- steroids - sex hormones, cholesterol, corticosteroids - 4 ringed structures
-
carotenoids - fatty acid carbon chains with conjugated double bonds and five/six-membered carbon rings at each end. includes pigments which produce colors in plants and animals
* subgroups are carotenes and xanthophylls - porphyrins - 4 joined pyrrole rings that often complex with a metal (porphyrin heme complexes with fe in hemoglobin; chlorophyll with Mg)
-
adipocytes - specialized fat cells in two categories
* white fat cells - composed primarily of triglycerides with a thin layer of cytoplasm around it
* brown fat cells - have considerable cytoplasm, lipid droplets scattered throughout, and lots of mitochondria - glycolipids - similar to phospholipids but they have a carbohydrate group instead of a phosphate group
- lipoproteins - lipids are insoluble so they are transported in the blood via lipoproteins, which are lipids cores surrounded by phospholipids and apolipoproteins
cell membrane fluidity
cell membranes need to maintain a certain degree of fluidity and are capable of changing membrane fatty acid composition to do so
- in cold weather - cell membranes become more rigid. in order to avoid rigidity, cholesterol and mono and polyunsaturated fatty acids are incorporated into the membrane, which increases fluidity
- in warm weather - cell membranes become more fluid and flexible. in order to avoid cell membrane collapse, cholesterol is added to restrict movement. the fatty acid tails are saturated so they become straight and pack tightly, thus decreasing fluidity
amino acid structure
amino group
carbon side chain
carboxyl group
proteins
polymers of amino acids joined by peptide bonds.
protein functions
- storage proteins - casein in milk, ovalbumin in egg whites, and zein in corn seeds
- transport proteins - hemoglobin carries oxygen, cytochromes carry electrons
-
enzymes - catalyze reactions in both forward and reverse directions
* efficiency is determined by substrate and enzyme concentration, temperature, pH and presence/absence of any inhibitors
* amylase catalyzes reactions that break alpha-glycosidic bonds in starch -
cofactors - non-protein molecules that assist enzymes; the union of a cofactor + enzyme is a holoenzyme (when an enzyme is not combined with a cofactor, it is called and apoenzyme/apoprotein)
* if the cofactor is organic, it is a coenzyme
* if a cofactor is covalently bound to an enzyme, it is called a prosthetic group
protein classification
-
simple - formed entirely of amino acids
- albumins & globulins - functional proteins that act as carriers or enzymes
- scleroprotein - fibrous proteins, have structural function (e.g. collagen) -
conjugated - simple protein + non-protein
- lipoprotein - protein bound to lipid
- mucoprotein - protein bound to carbohydrate
- chromoprotein - protein bound to pigmented molecule
- metalloprotein - protein complexed around metal ion
- nucleoprotein - contains histone or protamine, bound to nucleic acid
primary protein structure
sequence of amino acids connected by peptide bonds
secondary protein structure
3D shape resulting from hydrogen bonding between amino acid and carboxyl groups of adjacent amino acids (alpha helix or beta sheet)
tertiary protein structure
3D structure that forms primarily due to non-covalent interactions between amino acid R groups
- non-covalent interactions include H-bonds, ionic bonds, hydrophobic effect (R groups push away from water), and Van der Waals forces
- includes disulfide bonds (strong type od covalent bond between cysteins) which contribute to tertiary structure
quaternary protein structure
3D shape of a protein that is a grouping of two or more separate peptide chains
globular proteins
- somewhat water soluble
- mostly tertiary
- diverse functions: enzymatic, hormonal, inter and intracellular storage, transport, osmotic regulation, immune response
fibrous/structural proteins
- not water soluble
- mostly secondary
- made of long polymers
- function: maintain and add strength to cellular and matrix structure (collagen or keratin)
membrane proteins
includes proteins that function as membrane pumps, channels, or receptors
protein denaturation
when proteins are taken out of their ideal temperature, pH range, or solvent, denaturation can occur
means the protein is reversed back to its primary structure
nucleic acids
overall term for DNA and RNA, essential to all forms of life
function: to encode, express, and store genetic information
nucleotides
monomers that make up nucleic acids and consist of a nitrogenous base, 5C sugar, and a phosphate group
Cell Theory
states that
- all living organisms are comprised of one or more cells
- the cell is the basic unit of structure, function, and organization in all organisms
- all cells come from preexisting, living cells
- cells carry hereditary information
RNA World Hypothesis
proposes that self-replicating RNA molecules were precursors to current life. also states that RNA stores genetic information like DNA and catalyzes chemical reactions, leads to the belief that RNA may have played a major role in evolution
central dogma of genetics
states that biological information cannot be transferred backwards from protein to protein or nucleic acid. Rather, information must travel from:
DNA –> RNA –> proteins
stereomicroscope
uses visible light to view the surface of a sample
pro: can view living samples
con: has low light resolution compared to a compound microscope
compound microscope
uses visible light to view a thin section of a sample
pro: can view some living samples (single cell layer)
con: may require staining
phase contrast microscope
uses light phases and contrast for a detailed observation of living organisms, including internal structures if thin
pro: has good resolution and contrast
con: not ideal for thick samples and produces a “halo effect” around perimeter samples
confocal laser scanning microscope and fluorescence
used to observe thin slices while keeping a sample in tact; common method for viewing chromosomes during mitosis
pro: can observe specific parts of cell using fluorescent tagging
con: can cause artifacts
can be used without fluorescence, dye specimen
scanning electron microscope (SEM)
pro: view surface of 3D objects with high resolution
con: can’t use on living samples, preparation is extensive as sample needs to be dried and coated; costly
cyro SEM
similar to SEM
pro: sample is not dehydrated so you can observe samples in their more ‘natural form’
con: can’t use on living samples, samples must be frozen, which can cause artifacts
transmission electron microscope (TEM)
pros: can observe very thin cross-sections in high detail, and can observe internal structures with very high resolution
cons: cannot be used on living samples, preparation of sample is expensive, and technique is costly
electron tomography
not a type of microscope, but a technique used to build up a 3D model of sample using TEM data
pro: can look at objects in 3D and see objects relative to one another
con: same as TEM, cannot be used on living samples, costly
centrifugation
common technique used to prepare a sample for observation or further experimentation. it spins and separates liquified cell homogenates into layers based on density
for cell based on highest density to lowest:
nuclei layer -> mitochondria/chloroplasts/lysosome -> microsomes/small vesicles -> ribosomes/viruses/larger macromolecules
differential centrifugation
forms continuous layers of sediment, where insoluble proteins are found in the pellet while soluble proteins remain in the supernatant, liquid above the pellet
enzyme functions
recall that enzymes are globular proteins that act as catalysts
role of ATP
ATP is a common source of activation energy, and the compound stores its potential energy in the form of chemical energy. new ATP is formed via phosphorylation, ADP and phosphate come together using energy from an energy rich molecule like glucose
regulation of enzymes
enzymes must be strictly regulated to ensure that they are only functional for specific use
Km
Michaelis constant, and represents the substrate concentration at which the rate of reaction is half of the max velocity of the enzyme, or Vmax
- a small Km indicates that an enzyme requires only a small amount of substrate to reach max velocity
- a higher Km means the enzyme needs more substrate to reach max velocity
allosteric enzymes
have both an active site for substrate binding and an allosteric site for the binding of an allosteric effector (can be an activator or inhibitor)
competitive inhibition
a substance that mimics the substrate and inhibits the enzyme by binding at the active site. the effect of competitive inhibition can be overcome by increasing substrate competition
Km raised but Vmax is not
noncompetitive inhibition
substance inhibits enzyme by binding elsewhere than the active sire, allowing the substrate to still bind, but the reaction is prevented from completing
Km is unchanged but Vmax is lowered
uncompetitive/anti-competitive inhibition
occurs when an enzyme inhibitor binds only to the formed enzyme-substrate complex, preventing formation of product
cooperativity
when an enzyme becomes more receptive to additional substrate molecules after one substrate molecule binds to the active site
- ex: hemoglobin is a quaternary protein with 4 subunits that each have an active site for oxygen. when oxygen binds to one site, the other sites are more likely to bind oxygen
peripheral membrane proteins
loosely attached to the surface of one side of the membrane
- generally hydrophilic
- held in place by H bonding and electrostatic interactions
- can disrupt/detach them by changing salt concentration or pH
integral membrane proteins
embedded in the cell membrane
- hydrophobic
- can be destroyed using detergent
transmembrane proteins
type of integral membrane that travels all the way through the cell membrane
channel proteins
provide a passageway through membrane for hydrophilic, polar, and charged substances
recognition proteins
type of glycoprotein that is used to distinguish between self and foreign
ion channels
used to pass ions across the membrane and referred to as gated channels in nerve and muscle cells
- voltage-gated (respond to difference in membrane potential)
- ligand-gated (chemically binds to open channel)
- mechanically gated (respond to pressure or vibration)
porins
allow the passage of certain ions and small polar molecules; increase the rate of water passing in kidney and plant root cells
- tend to be less specific
carrier proteins
specific to movement across the membrane via integral membrane protein
- changes shape after binding to specific molecule that enables it to be passed across
transport proteins
proteins that can use ATP to transport material across the membrane
- includes active transport (Na+ K pump)
- facilitated diffusion
adhesion proteins
attach cells to neighboring cells and provide anchors for stability via internal filaments and tubules
receptor proteins
serve as binding sites for hormones and other trigger molecules
phospholipid membrane permeability
- allows small, uncharged, non-polar, hydrophobic molecules to free pass the membrane (steroids)
- polar molecules may cross if they are small and uncharged
- every other substance needs a transporter
cholesterol
adds rigidity to animal cell membranes under normal conditions and maintains fluidity of the membrane at lower temperatures
sterols
adds rigidity to plant cell membranes
hopanoids
adds rigidity in prokaryotic cells
glycocalyx
a carbohydrate coat that covers the outer face of bacterial cell walls and animal cell plasma membranes
- consist of glycolipids attached to the plasma membrane, and glycoproteins that may serve as recognition proteins
- functions include adhesive capabilities, barrier to infection, or markers for cell-cell recognition
chromatin
- in nucleus
- general packaging structure of DNA around proteins in eukaryotes; tightness in packaging depends on cell stage
chromosomes
tightly condensed chromatin when the cell is ready to divide
histones
serve to organize DNA which coil around it into bundles called nucleosomes; these bundles are wrapped around 8 histone proteins
nucleolus
inside of the nucleus and serves as the site of ribosome synthesis
ribosomes
synthesized using rRNA and ribosomal proteins which are imported from the cytoplasm. exported to the cytoplasm for final assembly into completed ribosome
- function to make proteins*
- composed of two subunits: 60S + 40S = 80S in eukaryotes, and 50S + 30S = 70S in prokaryotes
- the two subunits are produced inside of the nucleolus and moved into the cytoplasm where they are assembles
- larger S value indicated a heavier molecule
nuclear lamina
a dense fibrillar network inside of the nucleus of eukaryotic cells that provides mechanical support; helps regulate DNA replication, cell division, and chromatin organization
nuceloid
the irregular shaped region within prokaryote cells that contains all or most of the cell’s genetic material
cytoplasm
most of the cell’s metabolic activity and transport occur here, and the area includes the cytosol and organelles
cytosol/cytoplasmic matrix
the difference between the cytosol and cytoplasm is that the cytosol doesn’t include the components suspended within the gel-like substance, it is JUST the gel-like substance
rough ER
studded with ribosomes and creates glycoproteins by attaching polysaccharides to polypeptides as they are assembled by ribosomes
- in eukaryotes, the rough ER is continuous with the outer nuclear membrane
smooth ER
ER without ribosomes that serves to synthesize lipids and steroid hormones for export. in liver cells, it breaks down toxins, drugs, and toxic by-products from cellular reactions
- smooth and striated muscle have smooth ER’s called sarcoplasmic reticulum that store and release ions like Ca2+
lysosomes
- vesicles produced from golgi
- contain digestive enzymes with low pH
- function in apoptosis
- break down nutrients, bacteria, and cell debris
Golgi
- transport various substances in vesicles
- plays important role in modifying and packaging proteins
- contains flattened sacs called cisternae
- cis face = incoming vesicles, trans face = secretory vesicles
peroxisomes
- common in liver and kidney
- breakdown substances, fatty acids, and amino acids
- in plant cells, they modify by-products of photorespiration
- in germinating seeds, they are called glyoxysomes which break down stores fatty acids
- peroxisomes produce H2O2, which they use to oxidize substrayes
microtubules
made up of the protein tubulin
* provide support and motility
* act as spindle apparatus which guides chromosomes during division
- can be found in flagella and cilia of all animal cells and lower plants like mosses and ferns in a 9+2 array - 9 pairs of microtubules with 2 singlets in the center
intermediate filaments
provide support for maintaining cell shape
- keratin
microfilament
- made of actin
- cell motility
- found in skeletal muscle, amoeba pseudopod, and cleavage furrows
microtubule organizing centers
- structures that include centrosomes and basal bodies
- found at base of each flagellum and cilium
- organize their development
- 9x3 array
- plants do have MTOCs
transport vacuoles
moves materials between organelles or between organelles and the plasma membrane
food vacuoles
temporary receptacles of nutrients that merge with the lysosomes in order to breakdown food
central vacuoles
- generally large
- can occupy most of the plant cell interior
- exert turgor when fully filled to maintain rigidity
- store nutrients
- carry out functions performed by lysosomes in animal cells
- specialized membrane called a tonoplast
storage vacuoles
location where plants store starch, pigments, and toxic substances such as nicotine
contractile vacuoles
found in single-celled Protista organisms like amoeba and paramecium
- function to collect and pump excess water out of the cell via active transport to prevent bursting
what makes up different cell walls
- cellulose - plants
- chitin - fungi
- peptidoglycan - bacteria
- polysaccharides - archaea
extracellular matrix
found in animals between adjacent cells; is occupied by fibrous structural proteins, adhesion proteins, and polysaccharides secreted by cells
- function to provide mechanical support and helps bind adjacent cells
- collagen is the most common protein, but we also see integrin and fibronectin
- is a network of collagen and proteoglycans connected to integrins in the cell membrane via fibronectin and laminin
- also function in transmitting mechanical and chemical signals between the inside and outside of the cell
focal adhesion
connection of the ECM to actin filaments in the cell
hemidesmosomes
connection of ECM to intermediate filaments like keratin
fibroblasts
cells that produce collagen and other connective tissue elements
plastids
organelles found in plant cells
mitochondria
double-layered organelles that make ATP, and serve as the site of fatty acid catabolism, or Beta-oxidation; have their own circular DNA and ribosomes
cytoskeleton
includes microtubules (flagella and cilia), microfilaments, intermediate filaments; found in eukaryotic cells
- aids in cell division, cell crawling, and the movement of cystoplasm and organelles
plant cells and water balance:
- in a hypotonic solution (their normal state), the vacuole swells and becomes turgid
- in an isotonic solution, the plant cell is flaccid
- in a hypertonic solution, the cell is plasmolyzed - the cytoplasm is pulled away from the cell wall
endomembrane system
the network of organelles and structures, either directly or indirectly connected, that function in the transport of proteins and other macromolecules into or out of the cell
- includes plasma membrane, ER, Golgi apparatus, nuclear envelope, lysosomes, vacuoles, vesicles, and endosomes, but NOT the mitochondria or chloroplasts
intracellular circulation
- brownian movement ( random particle movement due to kinetic energy, spreads small suspended particles throughout the cytoplasm)
- cyclosis/streaming: circular motion of cytoplasm within the cell to transport molecules
- endoplasmic reticulum: provides channel through cytoplasm, provides direct continuous passageway from plasma membrane to nuclear membrane
extracellular circulation
- diffusion: if cells are in close contact with the external environment, diffusion can suffice for food and respiration needs. this is also used for transport of materials between cells and interstitial fluid around cells in more complex animals
- circulatory system: required by complex animals with cells too far from the external environment
anchoring junctions
- includes desmosomes - keratin filaments attached to adhesion plaques which bind adjacent cells together via connecting adhesion proteins, providing mechanical stability by holding cellular structures together
- present in animal cells in tissues with mechanical stress
- including cells in the skin epithelium and cervix/uterus
tight junctions
- completely encircles each cell, producing a seal that prevents the passage of materials between cells
- is characteristic of cells lining the digestive tract where materials are required to pass through cells into the blood
- by doing this, tight junctions prevent the passage of molecules and ions through the space between cells
gap junctions
- narrow tunnels between animal cells (connexins) that prevent cytoplasms of each cell from mixing
- allow passage of ions and small molecules
- essentially channel proteins of two adjacent cells that are closely aligned
- tissues like the heart include these to quickly pass electrical signals
plasmodesmata
- narrow tunnels between plant cells
- narrow tube of endosplamic reticulum - desmotubule - that exchanges material through the cytoplasm surrounding the desmotubule
eukaryotes include all organisms except:
bacteria, cyanobacteria, and archaebacteria
prokaryotes
- no nucleus
- single, circular, naked, double-stranded DNA ( no chromatin level organization)
- ribosomes (50S + 30S = 70S)
- cell walls (peptidoglycan); archaea (polysaccharides) - many have sticky capsules on the cell wall
- flagella that are constructed from flagellin, not microtubules
bulk flow
collective movement of substances such as blood in response to a force or pressure
passive transport
- simple diffusion
- osmosis
- dialysis (diffusion of different solutes across a selectively permeable membrane)
- plasmolysis (movement of water out of a cell that results in its collapse)
- facilitated diffusion
- countercurrent exchange *diffusion by bulk flow in opposite directions such as blood and water in fish gills)
diffusion is net movement
active transport
movement of molecules against concentration gradient, requiring energy
- usually involves solutes like small ions, amino acids, monosaccharides
three types:
1. primary
2. secondary
3. group translocation
primary active transport
energy is directly used to move against concentration gradients
secondary active transport
- energy is indirectly used to move against concentration gradient (usually with an ion moving down its concentration gradient)
- can be antiport (exchange) or symport (cotransport)
group translocation
seen in prokaryotes when the substance being transported across the membrane is chemically altered in the process, which prevents it from diffusing back out
endocytosis
encompasses three different types of active transport
1. phagocytosis - undissolved material (solid) enters cell; white blood cell engulfs the material as the plasma membrane wraps outward around the substance
2. pinocytosis - the plasma membrane invaginated around dissolved material (liquid)
3. receptor-mediated endocytosis - a form of pinocytosis in which specific molecules called ligands bind to receptors
exocytosis
encompasses transportation out of the cell
Gibbs free energy
tell us whether a given chemical reaction can occur spontaneously
*ΔG = ΔH - TΔS
* H is enthalpy
* T is temperature
* S is entropy
- if ΔG is negative, the reaction can occur spontaneously
- an unfavorable reaction with a positive ΔG1 value can be driven be a second, highly favorable negative ΔG2 value
- in a spontaneous change, the Gibbs free energy goes down, stability goes up, and work capacity goes down
- Basal Metabolic Rate (BMR) increases as body size increases, but decreases per kg as size goes up
cellular respiration
overall oxidative, exergonic process (ΔG= -868kcal/mol) that breaks down glucose in order to derive energy in the form of ATP
mitosis
cell division in all body cells (somatic), except for germ/reproductive cells
5 steps:
1. prophase
2. metaphase
3. anaphase
4. telophase
5. cytokinesis
prophase
- nucleus disassembles
- nucleolus disappears
- chromatin condenses into chromosomes
- nuclear envelope breaks down
- mitotic spindle forms
- microtubules (composed of tubulin) begin connecting to kinetochores
metaphase
- chromosomes line up along the center of the cell and spindle fibers attach to the kinetochore of each chromatid
- karyotyping is performed in metaphase
anaphase
- microtubules shorten
- each chromosome is pulled apart into two chromatids
- once separated, it is a chromosome, and the chromosome number doubles
- the microtubules pull the chromosomes to opposite poles of the cell (disjunction)
- at the end of this phase, each pole has a complete set of chromosomes
note: cohesin proteins are what bind the sister chromatids together and break down
telophase
- chromosomes arrive at opposite poles and begin to decondense
- nuclear envelope material surrounds each set of chromosomes
- mitotic spindle breaks down
- nuclear division occurs in this step
cytokinesis
- cleavage furrow forms in animal cells - shortening of actin and myosin microfilaments and the plasma membrane is pulled into the center of cell
- a cell plate forms in plant cells, which involves vesicles from Golgi bodies migrating and fusing to form a cell plate. As the plate grows, it merges with the plasma membrane, eventually separating the two new cells
the cell cycle
G1, S, G2, and M (mitotic) phases
G1: cell increases in size and lots of proteins and ribosomes are synthesized. the G1 checkpoint ensures that everything is ready for DNA synthesis, and the phase as a whole is the most variable in length. if G1 checkpoint fails, cell enter G0, a non-dividing state
S: DNA synthesis occurs where a new DNA molecules is replicated from the first, providing for sister chromatids
G2: rapid cell growth occurs, organelles are replicated, and genetic material prepares for cell growth. G2 checkpoint checks for sufficient Mitosis Promoting Factor (MPF)
note: interphase occurs during G1, S, and G2. most time spent here
- growth occurs in all 3 interphase phases
- M checkpoint checks if all chromosomes are attached to kinetochores, triggers start of anaphase
cyclin-dependent kinases (CDK’s)
- a CDK enzyme activates proteins that regulate the cell cycle via phosphorylation
- activated by protein cyclins, which vary in type and concentration throughout each phase of the cell cycle
cyclin D - increases then decreases
cyclin E - increases in between G1 and S
cyclin A - gradually increases, high point in G2
cyclin B - gradually increases, high point at start of mitosis
growth factors
the plasma membrane contains receptors for growth factors that stimulate cell division, such as in a tumor cell
density-dependent inhibition
cells stop dividing when surrounding cells density reaches maximum
anchorage dependence
most cells only divide when they are attached to an external surface such as neighboring cells or placed on a culture dish
cancer cells
referred to as transformed cells. cancer drugs also inhibit mitosis by disrupting the ability of microtubules to separate chromosomes during anaphase, thus stopping replication
meiosis
occurs exclusively in germ cells
- meiosis I: reduction division, homologous chromosomes pair at the plate, migrate to opposite poles, yet there is no separation of sister chromatids
- meiosis II: analogous to mitosis
prophase I: meiosis
nucleus disassembles, nucleolus disappears, nuclear envelope breaks down, chromatin condenses, and mitotic spindle develops. the microtubules begin attaching to kinetochores, and crossing over occurs, which allows for genetic recombination
i . synapsis occurs, which involves homologous chromosomes pairing up, forming a tetrad (group of 4 chromatids) or bivalents
ii. chiasmata - region where crossing over of non-sister chromatids occurs
iii. synaptonemal complex - protein structure that temporarily forms between homologous chromosomes; gives rise to tetrad with chiasmata and crossing over
prophase I (meiosis): 5 steps
- leptotene - chromosomes start condensing
- zygotene - synapsis begins, synaptonemal complex forms
- pachytene - synapsis complete, crossing over
- diplotene - synaptonemal complex disappears, chiasmata still present
- diakinesis - nuclear envelope fragments, chromosomes complete condensing, and tetrads are ready for metaphase
metaphase I meiosis
homologous pairs line up along the metaphase plate, and microtubules attach to kinetochores of one member of each homologous pair
anaphase I meiosis
homologues within tetrads uncouple and are pulled to opposite sides (disjunction)
telophase I meiosis
- nuclear membrane develops and each pole forms a new nucleus that has half the number of chromosomes
- cell is now haploid
- interphase may occur after telophase I, depending on the species
meiosis II
involves chromosomes lining up on the metaphase plate and sister chromatids separating and migrating to opposite poles
prophase II meiosis
nuclear envelope disappears and spindle develops, but no chiasmata or crossing over occurs
metaphase II meiosis
chromosomes align on metaphase plate like in mitosis, but now with half the number of chromosomes (no extra copy)
anaphase II meiosis
each chromosome is pulled into 2 separate chromatids and migrate to opposite poles of the cell
telophase II meiosis
nuclear envelope reappears and cytokinesis -> 4 haploid cells form, with each chromosome consisting of one chromatid
fertilization/syngamy
fusion of two haploid gametes, results in a diploid zygote
three things that account for genetic variation:
- crossing over during prophase I
- independent assortment of homologues during metaphase I
- random joining of gametes, aka germ cells
cell division in plants
in plants, meiosis in sporangia produces spores (haploid), which undergo mitosis to become multicellular haploid gametophytes. their gametes fuse together to form a diploid zygote that grows via mitosis into a sporophyte. cells in the sporophyte (sporangia) undergo meiosis again to produce haploid spores that germinate and repeat the life cycle known as alteration of generations
two main ratios that dictate if a cell will divide:
- surface-to-volume ratio: as cells grow, the volume grows much larger compared to the surface area. when SA:V is large, exchange across the cell becomes much easier. when SA:V is small, exchange is harder, leading to either cell death or cell division to increase SA
- genome-to-volume ratio: genome size remains constant throughout life, as cell growth leads to only an increase in volume. G:V will be small and thus exceed the ability of its genome to produce sufficient amounts of regulation of activities. some large cells like paramecium and human skeletal muscle are multinucleated to deal with this
additional notes about cell division
- mitosis does not contribute to genetic variation
- if you are seeing chromosomes, that means the chromatin has condensed, so you’re observing mitosis - therefore, if you’re asked to determine the number of chromatids, assume they’ve already been doubled
