paper 2 revision

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?

1. 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?

2. 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?

3. the transmitter substance binds with a receptor protein on the post-synaptic membrane


1. in the post-synaptic membrane, there are specific receptor sites for each transmitter substance


1. each receptor acts as a channel in the membrane that allows a specific ion to pass
2. the attachment of a transmitter molecule to its receptor instantly opens the ion channel


1. when a molecule of ACh attaches to its receptor site, a sodium channel opens
2. as the sodium ions rush into the cytoplasm of the post-synaptic membrane, depolarisation of the post-synaptic membrane occurs
3. as more and more ACh molecules bind, it becomes increasingly likely that depolarisation will reach the threshold level


1. 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?

4. the transmitter substance on the receptors is quickly inactivated


1. enzyme cholinesterase hydrolyses ACh to choline and ethanoic acid
2. molecules are inactive as transmitters
3. 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?

5. 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?

1. light stimulus is perceived by stem tissue at the stem apex

2. growth-promoting substance is formed at the apex and passes down the stem to where the growth response occurs

3. auxin can pass through gelatine or agar blocks

4. an asymmetrically replaced source of auxin has the same effect on growth as unilateral life

5. 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?

1. gibberellin formed in embryo as germination begins

2. gibberellin diffuses to protein store

3. gibberellin triggers synthesis of hydrolytic enzymes

4. 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