Potential AOB templates Flashcards
What is the effect on the trp operon when tryptophan (an effector molecule) is absent?
When tryptophan is absent, the repressor is in its inactive conformation and is unable to recognise and bind to the operator. The trp operon is turned on and RNA polymerase is able to recognise and bind to the promoter and able to transcribe the structural genes of the trp operon. mRNA is synthesised and translated into 5 polypeptides.
Enzymes that synthesises tryptophan are produced and thus tryptophan is synthesised.
What is the effect on the lac operon when lactose is absent?
In the absence of lactose, the lac repressor is synthesised in its active conformation and it recognises and binds to the operator, thus operon is turned off. RNA polymerase is unable to recognise and bind to promoter and is unable to transcribe the structural genes of the lac operon. mRNA is not synthesised.
ß-galactosidase, lac permease and lactose transacetylase are not produced.
What is the effect on the lac operon when lactose is present at low level of glucose?
When lactose is present at low level of glucose, the concentration of cyclic AMP (cAMP) increases. cAMP binds to the allosteric site of CAP, CAP assumes active conformation and can recognise and bind to the CAP binding site. The attachment of the CAP bends the DNA and recruits RNA polymerase to the promoter. RNA polymerase recognises and bind to the promoter at high affinity. Transcription of the lac operon proceeds at a high level.
Explain why cancer is a multi-step process.
1) The development of cancer is multi-step process as it requires the accumulation of mutation in the genes which controls regulatory checkpoints of the cell cycle in a single cell.
2) This results in the rate of cell division to be greater than the rate of cell death, leading to uncontrolled cell division.
3) When cells of the tumour invade surrounding tissues, and metastasise to other sites, the tumour is said to be malignant and cancer has developed.
Describe the advantages of using nucleotide sequences in reconstructing phylogenetic relationships.
1) It is unambiguous and objective as it is not dependent on subjective observations involving qualitative differences.
2) It is quantifiable and can be converted to numerical form and open to statistical analysis
3) It has many points of comparison as homologous regions of DNA from different species provides many points of comparison as each nucleotide position is a point of comparison.
Explain how does an influenza virus enter a host cell.
Haemagglutinin recognises and bind to sialic acid-containing receptors on the host cell’s plasma membrane. The influenza virus then enters the host cell via receptor-mediated endocytosis, in which the plasma membrane invaginates and pinches off, placing the influenza virus in an endocytic vesicle
Acidification and uncoating occurs, in which the viral envelope fuses with the endocytic membrane, releasing the capsid. The capsid is then degraded by the host cell cellular enzymes, releasing the viral genome into the cytoplasm.
Explain how HIV virus enters the host cell.
gp120 recognises and binds to the CD4 molecule on the host cell plasmamembrane.
The interaction between the gp120 and CD4 induces a conformational change in gp120, enabling the binding of gp120 on the host cell’s chemokine receptor.
The interaction between the gp120 and chemokine receptor brings about a conformational change in gp41 on the HIV virion, allowing fusion of HIV envelope and the target cell’s plasma membrane
The fusion of the HIV envelope and the target cell’s plasma membrane allows the genetic material and capsid to enter the target cell. The capsid is then removed by cellular enzymes, releasing the viral RNA and reverse transcriptase and integrase into the host cell.
Explain how do new strains of virus arise?
Antigenic Shift occurs, during which two or more different viruses combine to form a new subtype by a mixture of the surface antigens of the two or more original strains.
Two or more existing strains of a virus infect the same host cell simultaneously, random assembly of RNA segments from these different strains of viruses produce novel combinations of viral genes from different strains that is capable of infecting humans
Explain the result of a virus undergoing antigenic drift.
Point mutations in the genes that code for haemagglutinin occured. The accumulation of mutation results in slight changes in the shape of haemagglutinin.
The host immune system no longer recognise the antigenic sites on the haemagglutinin on the new strain of the influenza virus.
The change in shape of the haemagglutinin enable it to be complementary in shape to other membrane receptors found on new types of host cells, allowing the virus to infect other cell types.
Describe the replication process of HIV virus
The viral reverse transcriptase catalyses the synthesis of a cDNA strand complementary to the viral RNA.
The RNA strand is then degraded
The synthesis of a second DNA strand complementary to the cDNA occurs and a double-stranded DNA is formed
The double-stranded DNA is transported to the host cell nucleus and incorporated into the host cell’s DNA as a provirus, catalysed by integrase
During transcription, the host’s RNA polymerase transcribes the provirus genes into viral RNA molecules which functions as mRNA for the translation of viral proteins or genetic material for new virus particles.
Describe the replication process of influenza.
The viral RNA of influenza virus act as a template for the synthesis of cRNA, catalysed by RNA-dependent RNA polymerase, using the RNA nucleotides found in the host cell nucleus.
The cRNA act as the template for the synthesis of new copies of viral RNA or it act as a viral mRNA which is transcribed to produce viral proteins such as glycoproteins.
Why are viruses described as obligate parasites?
Viruses only contain one type of nucleic acid.
Viruses are dependent on the host cell’s metabolic machinery for reproduction and cannot reproduce outside the cell
Describe the stages in the Calvin cycle.
1) Carbon fixation
1 molecule of carbon dioxide combines with one molecule of ribulose 1,5-bisphosphate, catalysed by rubisco, to form an unstable 6C intermediate. The unstable 6C intermediate immediately breaks down to form 2 molecules of glycerate phosphate.
2) PGA reduction
Glycerate phosphate is phosphorylated by 1 molecule of ATP to form glycerate bisphosphate. Each molecule of glycerate phosphate is reduced by 1 molecule of NADPH to form triose phosphate. NADPH is oxidases to form NADP.
3) RuBP regeneration
Some of the triose phosphate has to be used to regenerate the RuBP used during carbon fixation. 1 molecule of ATP is need to regenerate one molecule of RuBP
Describe and explain how do new species arise.
Environment:
The sub-populations were exposed to different environments and were thus subjected to different selection pressures such as__________.
Since there was variation in these sub-populations, individuals with favourable characteristics were at selective advantage. Hence evolutionary changes occured independently in each sub-population.
Isolation:
The sub-populations were geographically isolated, leading to allopatric speciation and were physiologically isolated, leading to sympatric speciation.The sub-populations are incapable of interbreeding and thus gene flow was distrupted.
Evolution:
Over successive generations, evolution of different species on each island has occured, leading to adaptive radiation
Explain how gene mutation result in sickle cell anaemia
A single base pair mutation whereby adenine replaces thymine in the gene that coded for the ß chain has occured.
In the mRNA formed, GUG is coded for instead of GAG, resulting in missense mutation.
Thus the hydrophobic valine is replaces hydrophilic glutamic acid. The 3D conformation of haemoglobin changed.
This substitution decreases solubility of the deoxygenated HbS and thus at low oxygen concentration, hydrophobic areas of different HbS molecules will stick together. HbS molecules will polymerise and precipitate out of solution to form rigid fibres.
This causes red blood cell to change from a circular biconcaved- shape to a sickle shape.
