Processes

Question 1

Meiosis 1

Answer 1

The chromosomes that replicated prior to meiosis 1, EACH consist of 2 genetically identical chromatids attached at the centromere.

The chromosomes condense and the homologous chromosomes pair up.

Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged.

This crossing over of DNA is random and produces genetically different recombinant chromosomes.

Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle.

The orientation of the pairs of homologous chromosomes at the equator is random.

The chromosomes of each pair are separated and move towards OPPOSITE poles.

Cytokinesis occurs and 2 daughter cells are formed.

Question 2

Meiosis 2

Answer 2

Each of the 2 cells produced in Meiosis 1 undergoes further division, during which the sister chromatids of each chromosome are separated.

Question 3

Sodium Potassium pump

Answer 3

The pump has its ion binding sites exposed to the cytoplasm and there is a high affinity for Na+. So binding occurs.

This causes phosphorylation by ATP, resulting in the pump changing conformation.

The pump has its ion binding sites exposed to outside to the cell. The affinity for Na+ is low and so the Na+ are released out of the cell.

There is a high affinity for K+, so binding occurs from out of the cell.
This triggers de phosphorylation of the pump.

The pump returns to the original conformation with its binding sites exposed to the cytoplasm. There is a low affinity for K+, and so they are taken into the cell.

Now the affinity returns to the start.

Question 4

Hydrophobic signalling

Answer 4

Hydrophobic signalling molecules diffuse through the phospholipid bilayer.

The molecule binds to transcription factors in the cytosol.

The hormone-receptor complex moves to the nucleus.

The complex binds with specific sequences of DNA.

This affects gene expression.

Question 5

Hydrophilic Signalling

Answer 5

The hydrophilic signalling molecule binds to transmembrane receptors. They DO NOT enter the cytosol.

The transmembrane receptors change conformation when the ligand binds to the extra cellular surface.

The signal molecule does not enter the cell.

The signal is transduced across the plasma membrane.

Transmembrane receptors act as signal transducers, converting the extra cellular ligand binding event into intracellular signals.

This alters cell behaviour.

Question 6

Nerve Impulse Transmission

Answer 6

Neurotransmitters bind to receptors (ligand gated channels) at a synapse.

This causes the ligand gated channel to open, so Na+ diffuses into the neuron.

This Na+ movement causes depolarisation of the plasma membrane.

This depolarisation reaches a critical threshold level.

Voltage gated channel opens, so the Na+ diffuse into the neuron down the electrochemical gradient.

This causes a wave of electrical excitation along the neurons plasma membrane.

The voltage builds up and so the Na+ channel is inactivated and the K+ channel opens .

The K+ diffuse out of the neuron, resulting in repolarisation.

Resting potential is restored, so the K+ channels close and the Na+ channels are ready.

Electrochemical gradient is reset.

Question 7

Rod cells

Answer 7

Retinal absorbs a photon of light which causes it to change shape.

In turn rhodopsin changes conformation to become photoexcited rhodopsin.

A cascade of proteins amplifies the signal.

Photo-excited rhodopsin activates a G protein called transducin which activates the enzyme phosphodiesterase.

Phosphodiesterase catalyses the hydrolysis of a molecule called cyclic GMP.

This causes closure of the ion channels in the membrane of Rod cells.

This triggers nerve impulses in the neurons in the retina.

A very high degree of amplification results in rod cells being able to respond to low intensities of light.

Question 8

Natural Selection

Answer 8

Populations produce more offspring than the environment can support.

Individuals with mutations that best fit their environment are more likely to survive longer and produce more offspring.

Breeding occurs and alleles that confer an advantage are passed on to the next generation.

Question 9

X chromosome inactivation

Answer 9

In homogametic females (XX) one of the 2X chromosomes present in each cell is inactivated RANDOMLY at an early stage of development.

X chromosome inactivation prevents a double dose of gene products which could be harmful to cells.

Carriers are less likely to be affected by any delirious mutations on these X chromosomes.

As the X chromosome inactivated in each cell is random, 1/2 the cells in any tissue will have a working copy of the gene in question.

Question 10

Viral Life Cycle Stages

Answer 10

Viral antigens attach to the host cell
Viral DNA is injected into the host cell
Viral DNA is replicated by host cell enzymes
Viral genes are transcribed to RNA which is translated to make viral proteins
New viral particles are assembled and released

Question 11

Non-specific defence against parasitic attack

Answer 11

First line defences are PHYSICAL BARRIERS and CHEMICAL SECRETIONS, that work together to prevent parasites from entering the bodily fluids.

Second line defences ( INFLAMMATORY RESPONSE, PHAGOCYTES and NATURAL KILLER CELLS) happen as a response after a parasite has entered the bodily fluids.

Question 12

Inflammatory response

Answer 12

Histamine is released
Triggers dilation of blood vessels (enhancing blood flow to area)
Increased permeability of blood vessels
Causes swelling
Stimulates phagocytes to migrate to the area

Question 13

Phagocytosis

Answer 13

Phagocytes engulf and digest the foreign object using powerful enzymes contained in the lysosomes

Question 14

Natural killer cells

Answer 14

Type of white blood cell
Detect abnormal cell-surface proteins found on virus-infected cells / cancerous cells
Attach to the stricken cell & release chemicals into it
The chemicals induce cell death
Phagocytes engulf and digest the resulting cell debris

Question 15

Specific defences against parasitic attack

Answer 15

A range of white blood cells constantly circulates, monitoring the tissues.

If the tissues become damaged or invaded, cells release CYTOKINES that increase blood flow

This results in non-specific and specific white blood cells accumulating at the site of infection or tissue damage.

Question 16

Lymphocytes

Answer 16

Mammals contain many different lymphocytes, each possessing a receptor on its surface, which can potentially recognise a parasite antigen.

Binding of an antigen to a lymphocyte’s receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte.

Some selected lymphocytes will produce antibodies, others can induce apoptosis in parasite infected cells.

Question 17

Antibodies

Answer 17

Antibodies posses regions where the amino acid sequence varies greatly between different antibodies.

This variable region gives the antibody its specific type for the binding antigen.

When the antigen binds to this binding site, the antigen-antibody complex formed can result in INACTIVATION of the parasite, rendering it susceptible to a phagocyte, or can stimulate a response that results in cell lysis.

Memory lymphocytes are also formed.

Question 18

The Scientific Cycle

Answer 18

Observation

Construction of a testable hypothesis : a potential explanation for an observed effect

Experimental design : the design of the experiment must provide data to confirm or refute the hypothesis.

Gathering, recording and analysis of data : accuracy, precision when getting data measurements

Evaluation of results and conclusions : after data analysis, it should be decided whether the data supports the hypothesis

Formation of a revised hypothesis ( where necessary )

Question 19

Structure of Primary Papers

Answer 19

Title
Abstract (summary of aims and findings)
Introduction (purpose and context of study)
Method (contains information to allow other scientists to repeat)

Results (raw + processed data)
Conclusion (also discuses validity and reliability of design)
Reference

Question 20

Apoptosis

Answer 20

Apoptosis is triggered by cell death signals that can be internal or external.

External death signal molecules bind to a SURFACE receptor protein, and trigger a protein cascade within the cytoplasm.

An internal death signal resulting from DNA damage, causes activation of the p53 tumour suppressor protein.

Both types of death signal result is the ACTIVATION of a caspases that cause cell DESTRUCTION.

Apoptosis is essential during DEVELOPMENT of an organism to remove cells no longer required as development progresses or during metamorphosis.

Cells may initiate apoptosis in the absence of growth factors.

Question 21

Synthesis

Answer 21

Lipids and proteins are synthesised in the endoplasmic reticulum.

Lipids are synthesised in the smooth endoplasmic reticulum and inserted into its membrane.

The synthesis of all proteins begins in the cytosolic ribosome.

The synthesis of cytosolic proteins is completed here and these proteins remain in the cytosol.

Transmembrane proteins carry a signal sequence that halts translation and directs the ribosome synthesising the protein to dock with the ER to form RER.

Translation continues after docking and the protein is inserted into the membrane of the ER.

Question 22

Movement of proteins

Answer 22

Once the proteins leave the ER, they are transported by vesicles that bud off the ER and fuse with the Golgi Apparatus.

As the proteins move through the Golgi Apparatus, they undergo post-translational modifications.

The addition of a carbohydrate group is a major modification.

Vesicles that leave the Golgi Apparatus, take proteins to the plasma membrane and the lysosomes.

Vesicles move along microtubules to other membranes and fuse with them within the cell.

Question 23

The secretory pathway

Answer 23

Secretory proteins are translated by ribosomes on the RER, and enter its lumen.

The proteins move through the Golgi apparatus and are packaged into secretory vesicles.

These vesicles move to and fuse with the plasma membrane, releasing the proteins out of the cell.

Many secreted proteins are synthesised as inactive precursors and require proteolytic cleavage to produce active proteins.

Question 24

Amino acids

Answer 24

The sequence of amino acids determines the structure of the protein.
Proteins are polymers of amino acid monomers.
Amino acids have the same basic structure differing only in the R group present.
Amino acids are classified according to R group : acidic, basic, polar and hydrophobic
Proteins have a wide range of functions due to the diversity of R groups
Amino acids are linked by peptide bonds to form polypeptides.

Question 25

4 levels of folding

Answer 25

Polypeptides go through up to 4 levels of folding to become a protein.

The primary structure :

The sequence of amino acids are synthesised into the polypeptide.
This determines the proteins function

The secondary structure :

The protein can be folded in 2 ways : alpha helix or beta pleated sheets .
This structure is stabilised by hydrogen bonding along the backbone of the polypeptide strand.

Tertiary structure :

This can be the final folded 3D shape
The conformation is stabilised by interactions between R groups :
H Bonding, ionic bonding, disulphide bridge, LDFs, hydrophobic interactions

Quaternary structure :

Exists in proteins with 2+ connected polypeptide subunits

Question 26

The cell membrane

Answer 26

The cell membrane is composed of the phospholipid bilayer with proteins embedded in the layers.

There are 2 types of proteins found in the phospholipid bilayer :

Integral proteins :

Integral proteins penetrate the hydrophobic interior.
Regions of hydrophobic R groups allow strong hydrophobic interactions that hold the integral proteins within the bilayer.

Peripheral proteins :

Not embedded in the membrane.
They have hydrophilic R groups on their surface and are bound to the surface of membranes via ionic and hydrogen bond interactions.

Question 27

Channel and Transporter proteins ( type of integral protein )

Answer 27

In order to perform more specialised functions, different cell types have different channel and transporter proteins.

Channel proteins :

Have a hydrophilic channel, and are highly selective .

Some channels are gated and CHANGE CONFORMATION to allow / prevent diffusion.
The 2 types of channels are ligand gated and voltage gated.

Ligand gated channels are controlled by binding of signal molecules to the channel protein.
Voltage gated channels are controlled by changes in ion concentration.

Transporter proteins :

Bind to the specific substance to be transported and undergo a conformational change to transfer the solute across the membrane against their concentration gradient.

Question 28

Glucose Symport

Answer 28

The Sodium Potassium pump, pumps Na+ out of the cell.
The pump generates a lower concentration of Na+ within the cell.
The Glucose symporter transports Na+ down its concentration gradient alongside glucose which is transported against its concentration gradient.

The Sodium gradient created by the pump drives the active transport of glucose.

Question 29

Insulin

Answer 29

When blood glucose levels are high, insulin is produced.
Since it is hydroPHILIC it binds to transmembrane receptors on the EXTRAcellular surface.
This results in the signal being transduced.

This causes an intracellular signalling cascade, triggering recruitment of GLUT4 glucose transporter proteins, to the cell membrane of fat and muscle cells.

Question 30

Immune Evasion

Answer 30

Parasites have evolved ways of evading the immune system.

1. Mimic host antigens
   Endoparasites mimic host antigens to evade detection and modify host immune response to reduce their chances of destruction.
2. Antigenic Variation
   Antigenic variation in some parasites allows them to change between different antigens during the course of infection of a host.
   It may also allow re infection of the same host with the new variant
3. Integrate its genome into the host genome
   Some viruses escape immune surveillance by integrating their genome into the host genome, existing in an inactive state called latency.
   The virus becomes active again when favourable conditions arise.

Question 31

Challenges in treatment and control

Answer 31

Epidemiology is the study of out break and infectious disease.

The herd immunity threshold is the density of resistant hosts in the population required to prevent an epidemic.

Vaccines contain antigens that will elicit an immune response.

The similarities between host and parasite metabolism makes it difficult to find drug compounds that ONLY target the parasite.

Antigenic variation has to be reflected in the design of vaccines.
Some parasites are difficult to culture in the laboratory making it difficult to design vaccines.

Challenges arise where parasites spread most rapidly as a result of OVERCROWDING OR TROPICAL CLIMATES.
These conditions make coordinated treatment and control programmes difficult to achieve.

Civil engineering projects to improve sanitation combined with coordinated vector control may often be the only practical control strategies.

Improvements in parasite control reduce child morality and result in population wide improvements in child development and intelligence, as individuals have more resources for growth and development.