1
Q
Draw a diagram with 6 cellular membranes in a plant cell
A
check the notes
2
Q
Properties of cell membranes
A
partially permeable -> controls the passage of fat-soluble organic molecule in and out
act as barriers which control what passed thru them
many enzyme reactions happen within them
fluid —> allow changes in shape (phagocytosis/ osmotic)
chemical secretions are contained within vesicles which fuse with cell membrane to be released
allows cellular compartments to have different conditions
3
Q
Cell membranes made of?
A
Phospholipid bilayer
4
Q
Draw a phospholipid
A
check the diagram
Polar hydrophilic head (- charge) = water soluble
Hydrophobic tail = water insoluble
5
Q
When PLBL submerged in water form what?
A
Micelles => used for drug delivery
6
Q
Properties of PL
A
membrane has to be fluid BUT fairly rigid -> can burst if too much water diffuses in or if penetrated with thick needle
after a careful penetration, the membrane can self-seal
saturated acids compress when decreasing the temp —> press on each other making a dense membrane
unsaturated acids compress, the kinks created distortion, pushing the tails away, keeping the membrane fluid
ratio of saturated : unsaturated determines the fluidity of membrane in cold
7
Q
Cholesterol in PL
A
found between phospholipids
regulates membrane fluidity (strength and support)
make it harder for ions to pass
—> stops the PL from packing too closely, increasing fluidity at low temp, CM isn’t too rigid
—> binds to hydrophobic tails, stabilizing PL, causing them to pack more closely, stabilising the cell membrane from becoming too fluid
8
Q
Proteins in PL
A
help transporting vital ionic substances
some are free-floating, some are fixed
proportion of PL containing US fatty acids affects how freely proteins move within the membrane
Many proteins have hydrophobic part —> buried in lipid bilayer,
and a hydrophilic part —> involved in a variety of activities
9
Q
Internal vs Peripheral Proteins in PLBL
A
Integral - embedded within the hydrophobic core, are often transporters and receptor, make up the channel protein
Peripheral - outer or inner surface of the membrane,
10
Q
Glycoproteins and Lipoproteins
A
Glycoprotein/ glycolipids (protein/lipid with carbohydrate) - on the surface cell - bind with substances at the cell’s surface (form hydrogen bonds) to stabilize the membrane - cells recognising/ attachment each other (like ‘antigen’)
Lipoproteins - lipids + protein
11
Q
Enzymes and specific receptors: PLBL
A
Specific receptors - sensitive to hormones
Enzymes on the inner cell membrane - control reactions
12
Q
Simple gaps in PLBL
A
Simple gaps - allow the movement of ionic substances in both directions
13
Q
Active vs passive proteins in PLBL
A
Active carrier system - protein pore that use ATP for transportation, binding site for specific molecule, changes shape to transport substances (e.g. Na/K pump)
Passive carrier system - protein pore that use facilitated diffusion for transportation, binding site for specific molecule, changes shape to transport substances (e.g. glucose carrier protein)
14
Q
Channel Protein vs Gated channel
A
Channel Protein - protein channels thru the lipid bilayer of a membrane that are open or closed, depending on conditions (hydrophilic channel), uses facilitated diffusion, only lets 1 type of ion thru (Mg channel = Mg ions pass)
Gated channel - passive transport ONLY (diffusion), opens only to specific stimuli = more control
15
Q
How are small vs large molecules transported thru PLBL?
A
Small, non-polar molecules - gaps between the phospholipids
Large, polar molecules - channel proteins and carrier proteins
16
Q
Fluid Mosaic Model
A
floating proteins in lipid sea
fat soluble molecules can diffuse thru, polar substances require proteins
17
Q
Exam Question: why phospholipids form a bilayer in plasma membranes?
A
Plasma is aqueous
Polar hydrophilic head of phospholipid containing phosphate and glycerol faces the aqueous solution
Non-polar hydrophobic tail of phospholipid containing fatty acids and glycerol are “tucked in” the hydrophobic core
heads face the aqueous solution on both sides forming a hydrophobic core - phosphate groups interact with aqueous environment
18
Q
Cell Transport
A
Passive = diffusion, facilitated diffusion, osmosis
Active = active transport, endocytosis, exocytosis
19
Q
Diffusion
A
SMALL + NON-POLAR
movement of molecules down the conc. gradient
lipid molecules and very small substances diffuse thru the membrane
no metabolical energy from ATP required
depends on the KE of molecules (random motion)
due to conc, pressure or electrochemical gradient
eventually spread and reach uniform distribution
BUT hydrophilic molecules and ions which are larger than CO2 cannot move across!
20
Q
Facilitated Diffusion
A
LARGE + POLAR + IONS
diffusion takes place thru carrier proteins or channel protein (protein lined pores make it possible)
substances with strong charge (polar) or large molecules
move down the conc. gradient
Proteins in the membrane that allow specific substances to move passively down their conc. gradient
21
Q
Channel Protein in Facilitated Diffusion
A
Channel Protein -> each type allows one particular type of molecule to pass = depending on molecule’s shape and charge
Gated Channel -> some open when a specific molecule is present or electrical charge across the membrane (e.g. passage of nerve impulse along neurons)
22
Q
Carrier Protein in facilitated diffusion
A
Carrier Protein -> they float on the surface of membrane - proteins on outside are used to move molecules inside and vice versa = specific shape for particular protein depending on shape of molecule
once the protein binds to the molecule, it passes thru the membrane to the other side and once passed, it’s released + protein reverts to its original shape => the carrier protein changes shape while it’s carrying smth
once the surface protein binds to the molecule, it rotates thru the membrane and once the molecule is released, it rotates back to the surface + reverts to it original shape
e.g. RBC have a carrier protein to help moving glucose into cells rapidly
23
Q
Fick’s Law
A
Rate of diffusion is proportional (SA * difference in conc)/length or thickness paths
24
Q
Osmosis
A
The net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane
-> the conc of free water molecules (not attached with solute)
-> pure water = highest water potential = 0
specialised form of diffusion
partially permeable membrane
movement of FREE water molecules
solutes can be accumulated on the either side => conc gradient allows the movement of water
water potential => not associated with solute molecules
25
Osmotic Conc
Osmotic Conc - a measure of the conc of the solute in solution that have osmotic effect (only those dissolved substances)
-> large insoluble molecules have NO effect
-> soluble particles have an effect (big plasma protein)
26
Modelling osmosis
Artificial membrane that is permeable to water but impermeable to sucrose => presence/absence of sucrose in certain regions of the model can show can be shown by Benedict’s Test for non-reducing sugars
27
Isotopic vs Hypotonic vs Hypertonic
Isotonic - osmotic conc in the cell is the same as outside the cell
Hypotonic - osmotic conc in the cell is higher as outside the cell
Hypertonic - osmotic conc in the cell is lower as outside the cell
28
Animal Cell: osmosis
Want to keep the water movement to the minimum
-> too much = bursts => haemolysed
-> too little = shrink
Surrounding solution is slightly hypertonic - water moves out by osmosis - cell contents shrivel == cell wrinkels
29
Plant cell: osmosis (turgor)
cellulose cell wall prevents it from bursting => turgid
Surrounding solution is hypotonic - water enters by osmosis - as the cytoplasm swells and presses on the cell walls, it generates hydrostatic pressure - inward pressure of the cell wall on the cytoplasm increases until it cancels the tendency for water molecules to move in == pressure potential - when the osmotic force moving water into plant cell is balanced by pressure potential forcing it out, the plant cell becomes TURGOR
-> cells are turgor most of the time
30
Plant cells: osmosis (cell sap)
Surrounding solution is slightly hypertonic - water moves out by osmosis (turgor is lost) - cell membrane begins to pull away from the cell wall as the protoplasm shrinks == incipient plasmalysis
-> we measure incipient plasmolysis using serial dilution, looking for the point at which 50% of cells are plasmolysed = equivalent to the solute potential of CELL SAP
31
Plant cells: osmosis (plasmolysed)
Surrounding solution is very hypertonic - most water moves out by osmosis - vacuole is reduced and the protoplasm will shrink away from the cell wall completely == PLASMOLYSIS
-> size of cell doesn’t change due to cell wall
32
Incipient Plasmolysis
Incipient Plasmolysis - the point at which so much water has moved out of the cell by osmosis that turgor is lost and the cell membrane begins to pull away from the cell wall as the protoplasm shrinks
33
Plasmolysis
Plasmolysis - the situation at when a plant cell is placed in hypertonic solution when so much water leaves the cell by osmosis that the vacuole is reduced and the protoplasm is concentrated and shrinks away from the cell wall
34
Plasmolysis
Plasmolysis - the situation at when a plant cell is placed in hypertonic solution when so much water leaves the cell by osmosis that the vacuole is reduced and the protoplasm is concentrated and shrinks away from the cell wall
35
Active Transort
always involves a carrier protein, which often spans the whole membrane
molecules or ions
by breaking down of ATP
may be specific
one way system: only in the direction required by the cell (sometimes, those can move back by diffusion thru open channels)
faster than diffusion
36
What is ATP and draw its structure?
ATP - phosphorylated nucleotide that is formed of molecule of adenine, ribose sugar and 3 phosphate groups
*check the diagram*
37
ADP to ATP
During respiration phosphate is added to ADP molecule to make ATP. More than 1 ATP is made every time respiration happens → if energy is needed, it’s converted back to ADP (the high energy bond is broken so energy is released) and then goes back to mitochondrion to be converted back into ATP
ATP —> ADP + phosphate + energy
ATP carries energy from the mitochondria to the parts of cells that need it
38
How does active carrier systems involving ATPase work?
Active Carrier system involves ATPase -> this enzyme catalyzes the hydrolysis of ATP by breaking 1 bond and forming 2 new ones => provides energy for the carrier protein to change shape for transportation/release of molecule to return to normal state
Molecule/ion binds to carrier protein in the plasma membrane at a receptor site
ATP binds to the protein on the other side which causes it to split by hydrolysis, into ADP and phosphate, releasing energy
Protein changes shape and opens on the opposite side of the membrane
Molecule/ion is released on the other side of the membrane
ATP is reformed and protein reverts to its original shape
39
Na-K pump
Sodium pump - actively moves K+ ions into the cell and Na+ I ions out => vital for nervous system - each neuron impulse depending on the influx of sodium ions thru axon membrane = these ions must be actively pumped out of the neuron again so another impulse can pass
1. 3 sodium ions bind to the pump n the inside of the cell
2. ATP hydrolysed -> ADP + Pi, energy released
3. Pump changes shape and moves 3 sodium ions out of the cell
4. 2 K+ ions bind from the outside of the cell
5. Pumps reset to their original shape and release K+ ions
40
Endocytosis + name 3 types
Endocytosis - vesicle surrounding/taking in the material thru vesicle formation, INTO the cell
1. Phagocytosis
2. Pinocytosis
3. Receptor-mediated endocytosis
41
Phagocytosis
-> Phagocytosis (cell eating) - pseudopods surround the material - join together to form the vesicle = ingestion of bacteria (large scale)
42
Pinocytosis
-> Pinocytosis (cell drinking) - tiny amounts of surrounding fluid are taken in minute vacuoles (microscopic scale) = take in of minerals/nutrients
43
Receptor-mediated endocytosis
-> Receptor-mediated endocytosis - molecules to be transported into the cell so they bind to the receptor on the surface cell membrane, once they’re all bound, membrane folds inwards, forming a protein-coated pit
44
Exocytosis
Exocytosis - emptying of secretory vesicle at the surface of the cell via fusion with surface cell membrane, OUT of the cell
-> Hormone delivery - vesicles contacting hormones fuse to release them into the outside
45
Factors affecting in the rate of exo/endocytosis
number of carrier proteins
speed of individual carrier protein
ATP availability: rate of respiration => factors that affect respiration (cyanide inhibits respiration)
46
Q: Suggest how endocytosis/exocytosis provide evidence for FMM of membranes?
The membrane is able to stretch and surround the material, leading to vesicle formation which proves the fluidity of the membrane, rather than it being rigid
47
Gas Exchange
Single celled organisms=> very little cells - large SA:V ratio - small diffusion distance - O2/CO2 can directly diffuse from the outside to the cells
As the SA:V ratio increases, the speed and efficiency of diffusion decreases hence there’s a need for specialised tissue for GE
48
7 adaptations of alveoli
All the alveoli (as the structure) have a large SA to volume ratio -> greater rate of diffusion of gases
The grape-like shape of alveolus increases its SA -> greater rate of diffusion of gases
Rich blood supply surrounding the alveoli -> maintain high concentration gradient + decreases the diffusion distance + frequent replacement of alveolar air in the alveoli-> greater rate of diffusion of gases
Alveoli has a single layer of flattened epithelial cells + capillaries walls are 1 cell thick to decrease the diffusion distance -> greater rate of diffusion of gases
The alveoli have a moist layer inside them to allow gases to dissolve into -> greater rate of diffusion of gases
Ventilation increases the concentration gradient -> greater rate of diffusion of gases
Permeable surfaces to allow the free passage of gases Macrophages - white blood cells carrying out phagocytosis
49
Elastic recoil of lungs
Between capillaries and alveolar wall = elastic connecting tissue holding everything together - helps to force air out of the lungs, which are stretched when you breathe in
50
Phospholipid Lung Surfactant
coats the alveoli + makes breathing easier, prevents alveoli’s natural tendency to collapse
51
Inhalation
Contraction of diaphragm and
External intercostal muscles (internal IC muscles relax)
Diaphragm moves down (flattens) and
rib cage moves out
This increases the volume of
thorax cavity
The pressure inside thorax and
lungs decreases
The pressure in the outside air is
greater than inside the lungs
Air moves into the lungs
52
Exhalation
Relaxation of diaphragm and
External intercostal muscles (internal IC muscles contract)
Diaphragm moves up and
rib cage moves in
This decreases the volume of
thorax cavity
The pressure inside thorax and
lungs increases
The pressure in the outside air is
lower than inside the lungs
Air moves out of the lungs
53
Mucus: cilia and goblet cells
Goblet cells -> produce mucus (snot) which traps bacteria and dust preventing them from getting into lungs and damaging the cells there
Cilia -> move in wave like motion to push the mucus out of the respiratory system so you can swallow it (then all the bacteria is going to be killed in stomach by acid)
54
Biological catalyst
BIOLOGICAL catalyst - a protein (catalyst) that speeds up the reaction within living things
55
Metabolic/ anabolic/ catabolic
ANABOLIC reaction = building up of new chemicals
CATABOLIC reaction = breaking of chemicals
-> Metabolic reaction - any reaction that includes breaking apart/building them up
56
How do enzymes work?
- by providing an alternative reaction pathway with lower Ea
57
Enzymes: AS and substrate analogy
Every substrate has a specific shape that’s unique to this particular molecule. For example, the shape of a protein molecule isn’t the same as the shape of a starch polymer. Therefore, the active sites (of the enzymes to break these polymers apart) aren’t the same. It’s similar to the lock and the key model -> the key has the specific shape and will only fit in the specific lock, just like the active site and the substrate. The enzyme-substrate complex is like the lock and key model. The catalyst for breaking the starch won’t react with a protein polymer because the shape of the protein polymer is NOT the same as the shape of the catalyst’s active site.
58
Induced-fit hypothesis
=> active site still has a distinct shape and arrangement, but it’s flexible - after the substrate enters, the shape of it modified around to form an active complex (conformational changes) to ensure the ideal binding arrangement to maximise the ability to catalyze the reaction, once the product is released, enzyme returns to its inactive, relaxed form
59
Temp coefficient formula + ranges
*check the formula*
Between 0 to 40 C, Q10 for any reaction is 2 = RoR doubles for every 10C in temp rise
Outside that range, the enzyme-catalyzed reactions in humans decreases markedly, while for other reactions, it changes slowly due to denaturing (AS changes shape)
60
Factors affecting enzyme activity
1) Temperature (too cold -> little KE, too hot -> enzymes denature)
2) The concentration of enzymes (more enzymes -> works faster)
3) PH (acidity) - (too acidic -> enzymes denature)
4) The concentration of substrate (more substrate -> increase, then works slower)
-> increases for a period of time, then the enzyme becomes saturated (all AS are occupied by substrates) and a further increase in substrate conc wont increase the RoR
5) Inhibitors - a substance that slows down or prevents the particular chemical reaction/ reduces the activity of the reactant, catalyst or enzyme -> poison (if there’s poison in your body, it’ll prevent enzymes from doing their job)
61
Molecular activity + draw maximum rate of reaction diagram
Molecular Activity/ Turnover Number - the number of substrate molecules transformed by 1 enzyme per minute
62
How does temp affect enzyme activity?
Low temperature -> the enzymes and substrates do not have enough kinetic energy to collide with enough force for the reaction to happen ∴ there are less enzyme-substrate complexes formed per unit of time (the rate of reaction decreases)
At the optimum* temperature -> enough kinetic energy for the enzymes and substrates to collide with enough force for them to form the enzyme - substrate complex ∴ there are more enzyme - substrate complexes formed per unit of time (the rate of reaction increased)
Above the optimum -> the kinetic energy inside the enzyme is too high -> breaks the bonds between the amino acids -> loss of tertiary and quaternary structures, causing the active site to change its shape, preventing the substrate from binding -> the enzyme denatures ∴ there are less E-S complexes formed (the rate of the reaction decreases)
63
How does pH affect enzyme activity?
Optimum pH -> most enzymes work the best at one pH -> where the most successful collisions happen and the most E-S complexes are formed
Below or above the optimum -> changes the formation of hydrogen and disulfate bonds holding together the enzymes break because they interact with the solution around it -> the shape of the active site changes ->the enzymes denature
Both acidic and basic pHs can cause the enzyme to denature -> the presence of the extra H+ ions (in acidic) or OH- ions (in basic) affects the hydrogen bonds which disturbs the secondary/tertiary structure -> the change of the active site in form. The enzyme denatures ∴ the substrate cannot fit in
64
Initial RoR and measuring RoR
Initial Rate of Reaction - measure taken to compare the rates of enzyme controlled reactions under diff conditions
-> tangent to the curve at the beginning of the reaction
-> excess substrate - rapid development of E-S complexes - reaches steady rate = measure the amount of product produced over period of time
65
Nuclei acid Structure
Nucleic Acid = polynucleotides ⇒ mononucleotides joined in condensation reaction by phosphodiester bonds (2 ester bonds and a phosphate) between sugar on one nucleotide and a phosphate group of the next nucleotide ⇒ hydroxyl group exposed on one end and a phosphate group exposed on another side, with elimination of water
66
Nucleotides
Nucleotides = Pentose sugar + nitrogenous base + phosphate group
-> phosphate group (PO4 -3) => makes nucleotides acidic and carry negative charge
67
Nitrogenous Bases
Purine - has 2 nitrogen containing rings => A and G
Pyrimidine - has 1 nitrogen containing ring => U, T and C
-> a purine base always bonds with pyrimidine base
68
DNA Structure
DNA structure -> double helix - 10 base pairs for each complete twist (double stranded - anti parallel - run in opposite direction)
-> repeating deoxyribose sugar - phosphate parts = backbone
-> in the middle, complementary nitrogenous bases
-> thymine joins with adenine (A-T) = 2 hydrogen bonds
-> guanine joins with cytosine (C-G) = 3 hydrogen bonds
-> 2 strands are held together by hydrogen bonds between the complementary base pairs (formed between amino and carbonyl groups of purine and pyrimidine bases on opposite sides)
69
RNA Structure
RNA structure -> single stranded (mRNA)
-> repeating ribose sugar - phosphate parts = covalent backbone
-> uracil joins with adenine (A-U)
-> guanine joins with cytosine (C-G)
-> can either be folded into complex shapes, held by hydrogen bonds or remain as a long-thread like molecule (tRNA)
70
RNA vs DNA
Deoxyribose vs Ribose
Single stranded vs Double stranded
Uracil vs Thymine
Presence of 2’ hydroxyl group on RNA makes it more susceptible to hydrolysis => DNA is used for storage, RNA is used for transport
71
Theories about DNA Replication
Conservative Replication - original double helix remains intact and in some way instructs the formation of new, identical double helix made up of entirely new material
Semiconservative Replication - DNA unzips along hydrogen bonds and new nucleotides align along each strand = each new double helix contains 1 strand of original DNA and 1 strand made up of new material
72
Evidence for semi-conservative method
They grew several generations of gut bacteria in medium with nitrogen isotope (N15) ONLY = those are denser than usual N14
Bacteria growing there grew up taking in that radioactive isotope to make cell chemicals, including proteins and DNA using N15 ONLY
They moved bacteria in another medium with N14 isotope ONLY
-> conservative: some DNA would have expected density for N15 ONLY, whilst some will have a density expected for N14 ONLY
-> semiconservative: after 1 replication cycle all DNA would have same density, half-way between N15 and N14 (as half of nitrogen would come from original N15 containing DNA and half would come from new medium containing N14)
DNAs did end up having the same density => reproduces by Semiconservative method
73
Enzymes involved in DNA Replacation
DNA helicase = unzips the 2 strands of the DNA
DNA polymerase = lines the new nucleotides along the DNA template strands
DNA ligase = catalyses the formation of phosphodiester bonds between new nucleotides
74
DNA Replication: Process
DNA helicase unzips the DNA by breaking the hydrogen bonds between base pairs on two antiparallel polynucleotide DNA strands to form 2 single strands => each will act as a template
In nucleus, there are free nucleotides - these contain 3 phosphate groups => nucleoside (base + sugar) triphosphate or “activated nucleotides” - brought to DNA and align with their complementary bases on each of the template DNA strand
Once in place, activated nucleotides are joined together by DNA polymerase => synthesises the new DNA strand + catalyses the condensation reaction between deoxyribose sugar and phosphate group of adjacent nucleotides with new strands, creating a sugar-phosphate backbone of the new DNA strand
DNA polymerase cleaves 2 extra phosphates and uses the energy released to create to phosphodiester bonds => hydrogen bonds form between the complementary base pairs of the template and the new DNA strand
DNA is then rewound by another enzyme
75
DNA Polymerase: leading strand
DNA polymerase can only build the new strand in one direction continuously (5’ to 3’ direction = leading strand) => as DNA unzips from 3’ to 5’, DNA polymerase will attach to the 3’ end of the original strand moving towards the replication fork
76
DNA Polymerase: lagging strand
Lagging strand - DNA polymerase moves away from the replication fork (from the 5’ to 3’ end) => DNA moves away from the replication fork (from 5’ to 3’) hence can only synthesise the lagging strand in small segments (Okazaki fragments)
-> DNA ligase joins these lagging strands segments together to form a continuous complementary strand
-> DNA ligase catalyses the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone
77
Triplet Code (codons)
Each sequence of 3 bases in a gene codes for 1 AA
Some of triplet of bases code for start (TAC - DNA, AUG - mRNA = methionine)
Stop signals (ATT -> UAA, ACT -> UGA, ATC -> UAG) - these tell the cell where individual genes start and stop
78
Gene Definitions
Gene - sequence of nucleotide bases in DNA that codes for the production of a specific sequence of AA, that make up polypeptide
79
What does it mean that the genetic code if degenerate/ non-overlapping/ universal?
Code is degenerate -> multiple codons can code for the same AA -> limits effect of mutation
Code is non-overlapping -> only read once, the adjacent codons don’t overlap, the same letter is not used for 2 diff codons; in other words no single base can take part in the formation of more than 1 codon
Code is universal -> almost every organisms use this code, same triplet code codes for the same AA in all living things (transferrable between species - genetic engineering)
80
Introns vs Exons
Coding sequences - exons (2%)
Non-coding sequences - introns (98%)
81
Steps in protein synthesis
Transcription (making mRNA)
Translation (using tRNA to assemble the protein)
82
Transcription: what happens?
This happens inside the nucleus
The RNA\\ polymerase unzips the DNA by breaking the hydrogen bonds between nitrogenous bases
This exposes the gene to be transcribed
Complementary copy of the gene is made from the RNA nucleotides called mRNA(free activated RNA nucleotides pair up via hydrogen bonds with complementary bases on the DNA, sugar-phosphate groups are then bonded together in reaction catalysed by the enzyme RNA polymerase to form a sugar-phosphate backbone of mRNA ) - the enzyme RNA polymerase speeds up that process (moves along template strand in 3’ to 5’ direction -> mRNA grows in 5’ to 3’ direction)
When transcription is completed, the hydrogen bonds between mRNA and DNA break and the double stranded DNA reforms
mRNA leaves the nucleus through the nuclear pore into the cytoplasm
83
Translation: what happens?
It occurs in the cytoplasm
Freely floating ribosomes bind to mRNA and start reading mRNA (one codon at a time)
Free floating tRNA molecules with the complementary anti-codons are bringing the corresponding amino acids to the codons in the correct order and form a hydrogen bond between
Two tRNA molecules fit onto the rRNA at any given time, bringing the AA they each carry side by side
Amino acids are joined together by peptide bond via condensation reaction to form a polypeptide
The process continues until a ”stop codon” on mRNA strand is reached - acts as signal for translation to stop
Polypeptide is then formed into certain shape to form a protein
84
Sense vs antisense strands
Antisense (template or transcribed strand) - the strand of DNA with which the RNA nucleotides pair
Sense (coding, non-template, non-transcribed) - the 2nd strand
85
Structure of tRNA
The tRNA - single stand molecule that folds in clover-like structure, has a triplet of unpaired bases at one end = anticodon, and a region at the other end where a specific AA can attach (20 diff tRNA molecules with specific anticodon and AA binding site)
it binds with specific AA (in cytoplasm from digestion of protein) and bring them to the mRNA molecule on the ribosome
near the beginning of the mRNA to the starting codon (AUG) -> signal to start off translation
86
Gene mutations
when a single codon is changed which leads to it coding for a diff AA hence forming a diff polypeptide chain
can happen in gametes -> most devastating as it’s passed to offsprings or during division of body cells
can happen to any cell at any time, usually occur during copying of DNA for cell division
Mutagen (X-ray, ionising radiation, chemicals) - exposure to them leads to increase of rate at which mutations occur
our body has its own DNA repair system => specific enzyme cut or repair any parts of DNA that become broken or damaged
87
Mutations provide variation?
If the new codon codes for the same AA = point mutation has no effect=> most mutations are neutral (neither improve nor worsen chances of survival) ⇒ mutations occur in the non-coding DNA which doesn’t affect protein synthesis; code is degenerate hence the codon for AA is not altered
Mutations can produce superior protein => gain reproductive advantage
Mutations can be catastrophic if in the protein which function is imp for cells (changes the polypeptide chain which changes the AS on the enzyme hence the substrate no longer binds which affects the activity if this reaction)
88
Point Mutations + 3 types of point mutations
Point Mutations - a change in single base of DNA code, caused by error occurring during the DNA replication
Substitution - where one base substitutes for another
Deletion - where one base is completely lost from the sequence
Insertion - where an extra base is added
89
Chromosomal Mutations and 3 types of it
Chromosomal Mutations - change in position of whole gene within the chromosomes, caused by error occurring during cell division
Duplication - where genetic material is duplicated within the chromosome
Deletion - where the genetic material is completely removed from the chromosome
Inversion - where genetic material is flipped around within the chromosome
90
Whole chromosome mutation
Whole-chromosome mutation - entire chromosome is either lost during meiosis or duplicated in one cell by errors in the process of
91
Sickle cell mutations
genetic disease that affects the protein chains of haemoglobin in RBC
result of point mutation ⇒ change in 1 base in one codon changes a single AA in a chain = alters the nature of protein
haemoglobin molecules stick together to form rigid rods that give RBC a sickle shape
they don’t carry oxygen efficiently and prevent blood flowing in the capillaries
lead to severe pain and even death
92
Mutation definition
Mutation - rare, random change in the base sequence of DNA that can be inherited
93
Protein synthesis if the mutation occurs
mRNA will have an incorrect base as it is complementary to DNA
mRNA will go into the cytoplasm
tRNA that is complementary to mRNA might have an incorrect amino acid
Amino acid order would be incorrect
Protein might be wrong shape
If the active site will end up the wrong shape, the substrate might no longer fit
94
Locus definition
Locus – The position where a gene is found on a chromosome
95
Homologous pairs of chromosome
Every individual has 2 copies of each allele; one on each chromosome in a homologous pair -> a pair of chromosomes that match in size and shape, and that contain the same genes at the same loci
96
Co-dominance?
Co-dominance - both alleles contributing to phenotype (gene is represented by capital letter and the allele is by diff superscript letter)
Blood Types:
A and B are co-dominant (Antigen A or Antigen B)
O is recessive to both A and B (no antigen on RBC)
97
Monogenic Cross
Monogenic Cross - inheritance of a single pair of alleles
only 1 allele from each pair passes into a single gamete due to meiosis = separation of homologous chromosomes during first division of meiosis
98
Test Cross
breeders need to know the genotypes of organisms so they know if they’ll breed true
all offsprings will have the same phenotype (homozygous genotype) (assuming there’s no mutation)
both homozygous dominant and heterozygous are indistinguishable => test cross with homozygous recessive
99
Why is it unethical/inconvenient to use people in genetic experiments?
usually only have 1 baby per pregnancy
9 month gestation
small families
some characteristics don’t show until much later in life
100
Ideal organisms for genetic experiments
Cheap and easy to grow (minimise costs)
Short life cycles (results seen quickly)
Produce large numbers of offspring (statistically relevant results)
Clear, easily distinguishable traits (Clear observations)
E.g. Drosophila, pea plants, Aspergillus, E.coli.
101
Sampling Errors in genetic experiments
Combination of alleles in gametes is completely random
Some offspring die before sampling
Inefficient sampling techniques
Offspring failed to be accounted for
-> Having large sample sizes reduces the potential sampling error
102
Sex Linkage
Humans have 22 sets of autosomes (everything in the body) and 1 pair of sex chromosomes (sex linked genes located there - any sex linked disorder is here)
if the gene is on the X chromosomes, males will have 1 copy of this gene whilst females will have 2 copies of this gene
X chromosome has many more genes than Y chromosome so sex-linkage that involves Y chromosome is very rare
103
X/Y chromosomes
Y Chromosome - 23 million bases but only has 78 coding genes => SRY gene starts the development of the testes which makes the male hormones and produce a male foetus
X Chromosome - 250 million bases which possibly has 1200 protein coding genes => female characteristics, and others like blood clotting factor and the ability to differentiate colours
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Colour Blindness
Genes carried on the X chromosome are sex linked (mainly seen in males)
recessive or mutant allele on the X chromosome passed to a son by a mother are always expressed
eg Colour Blindness = mutated recessive allele on the X chromosome
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Haemophilia A
caused by mutations with blood clotting genes on the X-chromosome
gene for clotting factor VIII on the X chromosome missing
homozygous form of haemophilia is rare and female babies with this condition are unlikely to survive birth (XaXa - double bleeding problem)
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Cystic Fibrosis
Genetic disease
Because of faulty recessive allele (1000 mutations discovered) => CFTR is an enormous protein (large gene found on chromosome 7 - mutation in anywhere can affect CFTR)
Cystic fibrosis transmembrane regulatory (CFTR) channel protein lines the channels thru which the chloride ions are actively pumped out onto the epithelial cells and move into the mucus outside the cells in the lungs = water potential is lowered in mucus and water moves out front he cells into mucus by osmosis
Many people are carriers but don’t know about it => when 2 heterozygous ppl have a baby, there’s ¼ chance to have a baby with the disease
Most common = DF508 - lack effective CFTR proteins => chloride ions build up in their cells instead of moving out through channels as protein’s tertiary structure is distorted = water doesn't move out their cells to dilute the mucus on the surface of their membranes (water moves into the cells by osmosis from the tissue fluid making the mucus more sticky) => blockage of the airways in the lungs
GE is much slower as there’s a thick layer of mucus between the blood and air
Reduced air flow in and out of the lungs, making the diffusion gradient less steep
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Respiratory Problems in CF
Thick, sticky mucus builds up in tiny airways of the lungs are reduces the air flow into alveoli, obstructing the smaller bronchioles
There's a smaller concentration gradient between the air and the blood in the lungs which reduces the gas exchange + mucus reduces the surface area available in lungs for gas exchange
Results in severe coughing as the body is trying to expel the mucus ; breathless, tired and lacking energy due to oxygen deficit; mucus is so sticky that cilia can’t move it out so the lungs gradually fill up with mucus => less efficient for gas exchange
Bacteria and pathogens are trapped in mucus which is NOT removed hence pathogen-leaden mucus build up results in ideal condition for bacteria to multiply
The body normally secretes antibodies into the mucus which inactivates the pathogens. In cystic fibrosis, the normal chemical balance of the mucus is changed as water is moved out of the mucus into the cells = solutes become more concentrated ⇒ dehydrated surfaces lose their natural antibacterial properties because the WBC and their antibodies cannot function effectively in the thickened mucus
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Digestive Problems in CF
Pancreatic duct carries the digestive enzymes from the pancreas to the duodenum is lined with thin mucus layer => in CF, thick mucus builds up and blocks it so the enzymes don’t reach duodenum ⇒ ppl cannot digest food properly hence they don’t get enough nutrient from food
Digestive enzymes trapped in the pancreas may start to digest and damage the cells of the pancreas. If they affect the cells which make insulin, then the person may also develop diabetes
It also makes it more difficult for any digested food to be absorbed into the blood. The gut secretes mucus to prevent the digestive enzymes damaging the delicate lining of the gut and to act as a lubricant. But, when this mucus is very thick and sticky,it forms a barrier between the contents of the gut and the lining of the intestine and clogs up the villi, reducing the surface area for absorption
⇒ severe risk of malnutrition and they often struggle to maintain body mass
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Normal vs CF
NORMAL (picture 1)
Cl- ions are pumped in
Cl- ions are pumped out through CFTR channel protein
Na+ absorption is inhibited
Outer cell surface becomes very concentrated, so water moves out of the cells by osmosis due to lowered water potential and into the surface liquid and mucus
Watery mucus and medium is easily moved by cilia
CF (picture 2)
Cl- ions are pumped in
Cl- ions are NOT pumped out through CFTR channel protein
Na+ absorption occurs quickly
Inside cell surface becomes hyperconcentrated, so water moves into of the cells by osmosis due to lowered water potential and from the surface liquid and mucus
Thick, sticky mucus and medium traps cilia => blockages
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Reproductive problems in CF
In women, the mucus in the reproductive system normally changes through the menstrual cycle => when the woman is fertile it becomes thinner to help the sperm get through the cervix and along the oviducts
Women with cystic fibrosis usually produce fertile eggs, but the thick mucus can block the cervix so sperm cannot reach them + blocks the oviducts, making fertilisation even less likely
Men with cystic fibrosis are often infertile. They may lack the tube that carries sperm out from the testis into the semen (the vas deferens). If the vas deferens is present, it may be partly or completely blocked by thick, sticky mucus so that only a reduced number of sperm (or no sperm at all) can leave the testis
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Sweat Glands in CF
The faulty CFTR protein means that people with CF usually have sweat that is more concentrated and salty than normal -> sweat is mainly salty water that is produced in your sweat glands =>
Normally, as the sweat passes along the duct of a sweat gland, salt (sodium chloride) is reabsorbed, largely as a result of the CFTR protein moving chloride ions into the cells. Sodium ions follow along a concentration gradient. This reabsorption of salt prevents you from losing too much salt in the sweat (in sweat glands the chloride pump works in the opposite direction to that in the mucus-producing glands, where chloride ions are moved out of the epithelial cells).
Without functioning CFTR proteins, the chloride ions remain in the sweat, and so do the sodium ions => the sweat is very salty
Loss of sodium and chloride ions causes health problems linked to the balance of ions in the body (levels of sodium and chloride ions are important nervous system and the heart)
If too much salt is lost in the sweat, the concentration of the body fluids changes, which can affect the heart
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Genetic Screening
For babies with a genetic disease, it is imp to diagnose the condition as early as possible => improve their chances of survival and health
Carried out during pregnancy to find out whether the fetus is affected by a severe genetic disorder/ on newborn babies to identify problems and give treatment as early as possible
Identification whether any of the parent is a carrier before conception
It is possible to detect CF allele in a carrier who has no symptoms -> sample of blood, or some cells from the inside of the mouth, can be used to carry out a simple test to identify the allele
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PKU
recessive autosomal disease (carrier parents)
The body lacks the enzyme that is needed to digest the AA phenylalanine=> AA builds up in the blood = irreversible damage to the brain and nervous system -> if diagnosed, the baby can be given a special diet free of phenylalanine
Phenylketonuria is diagnosed using a simple blood test => screening is much cheaper than caring for severely affected
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Prenatal Screening
They can go ahead and have a family as usual, hoping that they are lucky and that their children inherit healthy genes
They can be prepared to support and take care of them if they don't
They may decide not to have children at all, to prevent passing on a faulty gene even in a carrier
Go ahead with pregnancies but to have screening during each pregnancy
-> used to try and discover if a fetus is affected by a serious condition early in the pregnancy (may choose to do abortion)
-> used to prepare parents for the fact that their child has a genetic disorder which is not compatible with life, so their child will die before or at birth
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Amniocentesis
Removing about 20cm3 of the amniotic fluid which surrounds the fetus using a needle and syringe
Done at about the 16th week of pregnancy
Fetal epithelial cells and blood cells can be recovered from the fluid after spinning it in a centrifuge
Cells are cultured for 2-3 weeks and then a number of genetic defects and the sex of the baby can be determined from examination of the chromosomes
CONS
relatively late in the pregnancy making it very difficult for the parents if termination of the pregnancy is necessary
results are not available until 2-3 weeks after the test
carries a 0.5-1% risk of spontaneous abortion after the procedure, regardless of the genetic status of the fetus
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Chorionic villus sampling
Small sample of embryonic tissue is taken from the developing placenta => bigger sample of fetal tissue available for examination
Cells can be tested for a wide range of genetic abnormalities
Carried out much earlier in the pregnancy, so if a termination is necessary it is physically less traumatic for the mother
Results are also available more rapidly than for amniocentesis.
CONS
0.5-1% risk that the embryo may spontaneously abort after the tissue sample is taken, though the risk of miscarriage at this stage of pregnancy is high anyway
All paternal X chromosomes are inactivated in fetal placental cells so any problems in the genes on that chromosome cannot be detected by this technique
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Pre-implanted genetic diagnosis
screening of an early embryo before it is implanted in the uterus have been introduced
Evidence suggests that this causes no harm to the development of the embryo. The genetic make-up is checked and only those embryos free of the problem alleles are placed in the mother's uterus to implant and grow
=> removes the faulty allele from the gene pool (genetic conditions found only in boys (e.g. haemophilia) only female embryos would be implanted)
Hormone treatment stimulates many Graaflan follicles to develop in the ovary at the same time
During an operation secondary oocytes are removed using syringe
Sample of semen is produced by male (or extracted if spermduct is blocked)
Sperm and oocytes are mixed in suitable solution => fertilisation occurs
Zygotes divide by mitosis in culture solution to produce embryos => a cell is removed from the embryos for preimplantation genetic diagnosis
Two or three healthy embryos are inserted into the uterus
Remaining embryos early embryos are frozen and stored for liquid nitrogen for later use
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GENETIC COUNSELLING
Finding out about genetic diseases in their family is very traumatic -> Genetic counselors are trained to help people to understand and accept having a genetic disease -> assess statistical risk of a couple producing an affected child and help couples recognise the options they have
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Ethical Issues
depend on the personal beliefs of the parents
accepted moral codes and laws of a society
don’t accept genetic screening because their religious beliefs mean they accept whatever happens to them and their children
others have religious beliefs which mean they see scientific advances as a blessing which may help them and their unborn children.
possibility of false positives (when a healthy child is wrongly shown to have a genetic defect) and false negatives (when a child with damaged chromosomes appears normal in the tests) => no test is 100% reliable and parents have to decide what their attitude is to the possibility that a test may be wrong
tests such as amniocentesis and chorionic villus sampling can cause a miscarriage, whether or not the fetus has genetic problems
difficult for parents to accept that the child they are expecting is sick => abortion?
worries about the quality of life the child would have and/or their ability to care for the child
social pressure towards the view that the only acceptable baby is a perfect baby
-> allows time to grieve for the loss of the baby or accept the child they have
-> gives time to put emotional and practical support in place for when the baby arrives.
Biology AS
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