Studied by 2 people
what does the ‘p’ in the hardy-weinberg equation mean?
the frequency of the dominant allele
what does the ‘q’q mean in the hardy-weinberg equation?
the frequency of the recessive allele
what does p2 mean in the hardy-weinberg equation?
the frequency of the homozygous dominant individuals
what does 2pq mean in the hardy-weinberg equation?
the frequency of the heterozygous individuals
what does q2 mean in the hardy-weinberg equation?
the frequency of the homozygous recessive individuals
what are the seven assumptions of the hardy-weinberg principle?
organisms are diploid
organisms reproduce only by sexual reproduction
the generations are discrete, and do not overlap
mating is random
the population size is infinitely large
allele frequencies are equal in both sexes
no migration, mutation, or selection
what does the hardy-weinberg principle show?
that if certain assumptions are made, the frequency of alleles of a gene will remain constant from generation to generation in any population
what are the disturbing factors that change a gene pool?
gene mutation
migration
genetic drift
natural selection
genetic drift
random changes in the allele frequency of a particular gene resulting from ‘sampling error’ during sexual reproduction in a small population
genetic bottleneck
a sudden hostile physical condition could sharply reduce a natural population to a very small number of survivors
the number of the affected population might quickly return to normal
the new population would be built from a very small sample of the original population
small sample of the original gene pool, possibly with some alleles lost altogether
founder effect
a small number of organisms may become isolated in a new environment
genotypes of these organisms are likely to form a small sample from the original gene pool
when these individuals reproduce to form a larger population, the allele frequencies will remain a small sample of the gene pool of the species
autonomic nervous system
the network of motor neurones carrying impulses to smooth muscle and glands, controlling involuntary (unconscious) actions
somatic nervous system
the system of motor neurones carrying impulses to muscles controlling voluntary (conscious) actions
where is the cerebrum located?
above and around the remainder of the brain
extension of the forebrain
what does the cerebrum do?
voluntary actions are coordinated
many of the involuntary actions coordinated
what is the cerebral cortex and what does it do?
covered in grey matter to a depth of 3mm, and is densely packed with non-myelinated neurones
divided into left and right halves, each of which is responsible for the opposite half of the body
each side of the cerebral cortex is divided into four lobes
frontal, parietal, temporal, and occipital
receives a large amount of sensory information and initiates action potentials in motor neurones to bring about complex behaviour
what does the cerebellum do?
control of involuntary muscle movements of posture and balance
precise, voluntary movements involved in hand manipulations, speech, and writing are coordinated
ensures actions are carefully coordinated - does not initiate motor activity
what would happen if the cerebellum was damaged?
damage to the cerebellum does not cause paralysis
still able to direct movement but such movements are clumsy
fine control needed is absent
what does the medulla oblongata do?
regulatory centres concerned with maintaining the rate and force of the heartbeat and the diameter of blood vessels
respiratory centre adjusts the basic rate of breathing
ascending and descending pathways of nerve fibres connecting the spinal cord column and brain cross over
neuroglia cells
cells found in the brain that are responsible for the support and protection of the neurones of the brain and peripheral nervous system
what does the hypothalamus do?
control centre for the autonomic nervous system
monitors and controls body temperature
monitors and controls the levels of sugars, amino acids, and ions in osmoregulation
feeding and drinking reflexes, aggressive behaviour, and reproductive behaviour, are controlled
works with the pituitary gland to monitor hormones in the blood and control the release of hormones
link between the nervous and endocrine systems
what is the brain and what does it do?
highly organised mass of interneurones connected with the rest of the nervous system by numerous motor and sensory neurones
responsible for complex patterns of behaviour, in addition to many reflex actions
how does the brain control all body functions?
receiving impulses from sensory receptors
integrating and correlating incoming information in association centres
sending impulses to effector organs (muscles and glands) causing bodily responses
storing information and building up an accessible memory bank
initiating impulses from its own self-containing activities
personality, emotions, imagination, creation, planning, calculation, prediction, and abstract reasoning
how does the brain develop in the embryo?
develops from the anterior end of the neural tube
tube enlarges to form three primary structures: forebrain, midbrain, and hindbrain
various parts of the mature brain develop from these by selective thickening and folding processes of their walls and roof
interneurones
short neurones that connect individual neurones together
what does the sympathetic nervous system do?
more active in times of stress to produce ‘fight or flight’ responses
at their junctions with effector tissues (muscles or glands) the neurones release noradrenaline
increase ventilation rate
causes dilation of the tissues
has no effect on the tear glands
has no effect of the salivary glands
slows peristalsis
constricts bladder sphincter muscles
what does the parasympathetic nervous system do?
concerned with conservation of energy and the replacement of body reserves
at their junctions with effector muscles the neurones release acetylcholine
decreases ventilation rate
causes constriction of pupils
causes the secretion of tears
causes the secretion of saliva
accelerates peristalsis
causes the relaxation of the sphincter muscles of the bladder and contraction of the muscular wall of the bladder (under overall conscious control)
dihybrid inheritance
the transfer from generation to generation of two different genes
the two genes might control two different features of the phenotype (non-interacting genes) or might both contribute to the control of a single feature of the phenotype (interacting genes)
might occur on the same chromosome (linked genes) or on different chromosomes (unlinked genes)
sex linkage
a condition in which the locus of a gene is located on a sex chromosome; the gene itself is described as sex-linked
sex linkage in mammals almost always refers to a gene with its locus on an X chromosome - very few genes on a mammalian Y chromosome
wild type
the natural phenotype of any particular, characteristic
a phenotype which is unaffected by any gene mutation
recombinant
an organism with a phenotype that results from the crossing over of linked genes during gamete production in one, or both of its parents
resulting phenotype has a combination of characterises not shown by either parent
what is the expected phenotype ratio of a cross with two, unlinked, non-interacting genes with one parent homozygous dominant and one parent homozygous recessive?
9 : 3 : 3 : 1
how can the chi-squared test be used?
to compare the expected numbers of each phenotype with the observed numbers of each phenotype
why would the chi-squared test be used?
random variation is common
to interpret whether the results you obtain from a genetics experiment reflect theoretical knowledge
schwann cells
cells found wrapped around dendrons and axons, which produce the myelin sheath
myelin sheath
a layer of fatty tissue secreted by schwann cells surrounding an axon
node of ranvier
a small gap in the myelin sheath surrounding an axon
impulse
a momentary reversal in electrical potential difference in the membrane
a change in the amounts of positively and negatively charged ions between the inside and outside of the membrane of a nerve fibre
resting potential
the potential difference (approx -70mV)
how is the resting potential produced?
there is active transport of potassium ions in across the membrane, and of sodium ions out across the membrane
ions are transported by a Na+-K+ pump with transfer of energy from ATP
ions concentrate on opposite sides of the membrane
makes no change to potential difference by itself
facilitated diffusion of potassium ions out across the membrane and sodium ions back in
membrane is more permeable to potassium ions flowing out
causes tissue fluid outside neurone to contain many more positive ions that are present in the cytoplasm inside
inside becomes more negatively charges, resting neurone is polarised
difference in charge is known as the potential difference
what triggers an action potential?
a stimulus received at a receptor cell or sensitive nerve ending
energy transferred by a stimulus causes a temporary and local reversal of the resting potential
how do ion channels cause an action potential?
globular proteins that span the entire width of the membrane
some are permeable to potassium, some are permeable to sodium
central pore with a gate that can open and close
all channels are closed during a resting potential
what do sodium ions do in an action potential?
energy of the stimulus opens the gates of the sodium channels in the cell surface membrane
sodium ions diffuse in down their electrochemical gradient
cytoplasm inside the neurone fibre quickly becomes progressively more positive with respect to the outside
charge reversal continues until the potential difference has altered from -70mV to +40mV
an action potential has been created in the neurone fibre
how is action potential transmission an example of positive feedback?
as the action potential travels the length of the neurone fibre, the membrane quickly starts to re-establish the resting potential
how is a potential difference of -70mV established again after an action potential?
sodium channels close and potassium channels open almost immediately after an action potential has passed
potassium ions can exit the cell down an electrochemical gradient into the tissue fluid outside
causes interior of the neurone to start to become less positive again
potassium channels also close
resting potential re-established by the sodium-potassium pump and the process of facilitated diffusion
what is the refractory period of a neurone?
period in which the neurone fibre is not excitable for 5-10 milliseconds after the passing of an action potential
large excess of sodium ions inside the neurone fibre and further influx is impossible
as the resting potential is progressively restored, it becomes increasingly possible for an action potential to be generated again
maximum frequency of impulses is between 500 and 1000 per second
what is the all-or-nothing principle?
either a stimulus depolarises the membrane sufficiently to reverse the potential difference, or it not does not and does not produce an action potential
with all sub-threshold stimuli, the influx of sodium ions is quickly reversed and the resting potential is re-established
what happens when stimuli are above the threshold?
as the intensity of the stimuli increases, the frequency at which the action potentials pass long the fibre increases
with a very intense stimulus, action potentials pass along the fibre at an accelerated rate, up to the maximum possible permitted by the refractory period
means that the effector (or the brain) realises the intensity of a stimulus from the frequency of action potentials
saltatory conduction
conduction of an action potential along a nerve fibre by jumping from node to node, thus increasing the speed
acetylcholine
a commonly occurring transmitter substance at synapses
what does a synapse consist of?
synaptic knob (swollen tip) of the axon of one neurone (the pre-synaptic neurone)
the dendrite or cell body of another neurone (post-synaptic neurone)
synaptic cleft, a gap of around 20mm
how are transmitter substances produced?
produced in the golgi apparatus in the synaptic knob, and held in tiny vesicles prior to use
what is the first step in synapse transmission?
the arrival of an action potential at the synaptic knob opens calcium ions channels in the pre-synaptic membrane. calcium ions flow in from the synaptic cleft
what is the second step in synapse transmission?
the calcium ions cause vesicles of transmitter substance to fuse with the pre-synaptic membrane and they release a transmitter substance into the synaptic cleft. the transmitter substance diffuses across the synaptic cleft
what is the third step in synapse transmission?
the transmitter substance binds with a receptor protein on the post-synaptic membrane
in the post-synaptic membrane, there are specific receptor sites for each transmitter substance
each receptor acts as a channel in the membrane that allows a specific ion to pass
the attachment of a transmitter molecule to its receptor instantly opens the ion channel
when a molecule of ACh attaches to its receptor site, a sodium channel opens
as the sodium ions rush into the cytoplasm of the post-synaptic membrane, depolarisation of the post-synaptic membrane occurs
as more and more ACh molecules bind, it becomes increasingly likely that depolarisation will reach the threshold level
this process of build-up to an action potential in the post-synaptic membranes is called facilitation
what is the fourth step in synapse transmission?
the transmitter substance on the receptors is quickly inactivated
enzyme cholinesterase hydrolyses ACh to choline and ethanoic acid
molecules are inactive as transmitters
causes ion channel to close, and so allows the resting potential in the post-synaptic neurone to be re-established
what is the fifth step of synapse transmission?
the inactivated products of the transmitter re-enter the pre-synaptic neurone, are re-synthesised into transmitter substance, and packaged for re-use
excitatory post-synaptic potentials (EPSPs)
small currents (tiny areas of depolarisation) making the post-synaptic membrane less negative, which makes it more likely to trigger a new action potential as it becomes more likely to reach the threshold level
inhibitory post-synaptic potentials (IPSPs)
small currents making the post-synaptic membrane more negative, which makes it less likely to trigger a new action potential as it makes it less likely to reach the threshold level
how is an IPSP caused?
GABA (a transmitter substance commonly found in the brain) attaches to receptor sites and causes the opening of Cl- ion channels, which sets up tiny areas of further polarisation
how does lidocaine cause numbness?
lidocaine blocks voltage-gated Na+ ion channels
post-synaptic membrane is not able to depolarise, so no action potentials can travel to the brain to record pain
pain receptors need to depolarise to initiate pain signals, so they never respond
motor neurones in the region need to depolarise, so the lips feel ‘droopy’ and they cannot be controlled for a while
lidocaine is quickly metabolised in the liver so the effect begins to wear off after an hour or so
how can nicotine cause alertness?
nicotine binds with acetylcholine receptor sites in synapses
blockage causes more ACh to be produced and a feeling of greater alertness as synapses are excited
how can nicotine cause relaxed pleasure?
high levels of dopamine and ACh stimulate the release of endorphins in the brain, which are the chemicals that produce a feeling of relaxed pressure
how does nicotine lead to addiction?
endorphin release is a very powerful response; the brain quickly associates this pleasurable experience with the action preceding it and this naturally leads to addiction
effect of nicotine is very enjoyable but very short-lived: classic formula leading for a craving for more
what is the effect of cobra venom?
one of the proteins in cobra venom binds irreversibly to ACh receptors on the post-synaptic membrane
acetylcholinesterase has no effect on the venom protein
Na+ ion channels remain permanently open and after an initial acton potential the membrane is unable to depolarise and no further action potentials can be generated
this causes general paralysis, including respiratory muscles, which results in death due to suffocation
what do auxins do?
auxins increase the plasticity of the cell walls of the growing meristem cells, allowing them to expand further
tropism
a plant growth response where the direction of movement is determined by the direction of the stimulus
how does auxin travel through a plant?
auxins are manufactured by cells undergoing repeated cell division, such as those found at the stem and root tips
concentration of auxin is highest in the stem and root tips
auxin is transported to the region of growth behind the tip, where it causes cells to elongate
auxin is used up and inactivate
how can coleoptiles be used to investigate phototropism?
light stimulus is perceived by stem tissue at the stem apex
growth-promoting substance is formed at the apex and passes down the stem to where the growth response occurs
auxin can pass through gelatine or agar blocks
an asymmetrically replaced source of auxin has the same effect on growth as unilateral life
explanation of positive pho tropic response of stems
what do cytokinins do?
stimulate cell division by attaching to receptor sites on cell surface membranes and triggering the formation of transcription factors inside the cell
what effects do auxin have?
promote cell elongation in stem growth
promotes root formation in cuttings
promotes apical dominance
no effect on bud dormancy
inhibits leaf fall
what effect do gibberellins have?
promote cell elongation only with auxin
inhibits root formation
enhances auxin effect on apical dominance
breaks bud dormancy
no effect on leaf fail
what effect do cytokinins have?
promotes cell division in stem growth
no effect on root growth
promotes lateral bud growth (antagonistic to apical dominance)
breaks bud dormancy
no effect on leaf fail
micropropagation
a form of tissue culture used to grow many copies of a single plant using just a few cells
carefully breaking down the meristems of young plants into individual cells, and then growing the cells on sterile media
what is the main function of gibberellins?
control of internode (the distance along a plant between one side branch and the next) length and seed development
involved in other growth substances in complex interactions
how do gibberellins bring about their effect?
combine with DELLA proteins in the cytoplasm
DELLA proteins normally prevent transcription from activating several important genes involved in seed germination
when combined with GA, DELLA proteins no longer prevent these transcription factors form operating and hence the process of germination can begin
what is the role of gibberellins in germination of a seed?
gibberellin formed in embryo as germination begins
gibberellin diffuses to protein store
gibberellin triggers synthesis of hydrolytic enzymes
soluble food store translocated to embryo where it is respires, or used to build new cells
what does the classical model of interactions between plant growth substances suggest?
the inhibition of side shoots is a result of the action of auxin produced in the growing tip, which is transported downwards
auxin is transported downwards, possibly via phloem, and prevents the activation of the genes leading to the production of cytokinins, which would stimulate the axillary buds to begin cell division and growth of new stems, so the bud remains dormant
side shoots will normally grow from axillary buds found in the angle between the branches and the main stem
what evidence is there to support the classical model?
cytokinin levels rise when terminal buds are removed and that the presence of auxin inhibits the biosynthesis of cytokinin
application of cytokinin to dormant axillary buds can stimulate them into growth
what evidence is there which is lacking or contradictory to the classical model?
levels of auxin found in axillary buds are not always sufficient to account for inhibition
predicted effects do not always take place
other scientists have suggested different models concerned with the transport of auxin, which is partially contradictory to the classical model
models can be suggested and used to make prediction
phyochrome
a conjugated protein in plant cells thought to be responsible for detecting changes in red and far-red light illumination
Pr
blue pigment which absorbs mainly red light of wavelength 660nm
Pfr
blue-green pigment that absorbs mainly far-red light of wavelength 730nm
how are the two forms of phytochrome converted to one another?
when Pr is exposed to light (or red light on its own) it is converted to Pfr
in the dark (or if exposed to far-red light alone) Pfr is converted to Pr
photomorphogenesis
the influence of light on plant growth and development
phytochrome is the pigment system involved in photomorphogenesis
red/far-red absorption spectrum of phytochrome corresponds to the action spectrum of some specific effects of light on development
what is the active form of phytochrome in photomorphogenesis?
Pfr
stimulates some effects in plant development and inhibits others
how is flowering stimulated in plants?
FT gene is activated in the leaves of photoperiodically-induced plants
FT mRNA travels from induced leaves to stem apex
FT mRNA is translated into FT protein
FT protein, bonded to a transcription factor, activates several flowering genes and switches off the genes for vegetative growth
which responses to light use phytochrome as their receptor?
the synthesis of chlorophyll - plants kept in the dark have little chlorophyll but when exposed to light they quickly become green
the germination of some seeds is heavily influenced by exposure to red and far-red light
plant leaves of the same species often have different size and shape when grown in low or bright light conditions
islets of langerhans
distinct groups of cells found within the pancreas that produce insulin and glucagon
what do peptide/amine hormones do?
interact with specific receptors on the outside of the cell surface membrane
attach to receptors on the cell and trigger the release of a second messenger, often the messenger cAMP
cAMP activates exiting proteins in the cytoplasm to form enzymes that bring about specific changes
formation of cAMP continues as long as the hormone binds to the receptor site
what do steroid hormones do?
pass through the cell surface membrane and interact with specific receptors within the cytoplasm
combine with receptors to form transcription factors
transcription factors bind to specific genes and as a result protein synthesis is initiated
produce the enzymes required to bring about specific changes
what are the multiple effects of adrenaline?
heart: increases heart rate
lungs: increases breathing rate
circulatory: vasoconstriction
liver: increases breakdown of glycogen in liver to increase blood sugar
muscle: increases readiness to contract (causing shivering in extreme fear or excitement)
plant growth substances
produced in a region of plant structure, e.g. stem or root tips, in unspecialised cells
not necessarily transported widely or at all, and some are active at sites of production
not particularly specific - tend to influence different tissues and organs, sometimes in contrasting ways
animal hormones
produced in specific glands in specialised cells
transported to all parts of the body by the bloodstream
effects are most highly specific to a particular tissue or organ, and without effects in other parts or on different processes
how are hormones broken down?
hormones only circulate in the bloodstream briefly
broken down in the liver, and the breakdown products are excreted by the kidney
long-acting hormones must be secreted continuously to be effective
what do hormones do?
help control and coordinate body activities
cause changes to specific metabolic actions of their target organs
target organ
an organ that will respond to a specific hormone
fovea
where vision is most accurate - greatest density of photoreceptors
cones concentrated at and around fovea
convergence
a single optic nerve fibre has synapses connecting it to several rod cells
in bright light, cone cells can provide a coloured, much more accurate image
light can be gathered from a larger area to produce an action potential in dim light
visual acuity
ability of the eye to distinguish between two points close together
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