Exam Qs Biochem
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State the meaning of the term complementary when it is used to refer to DNA strands
(Complementary means the two strands must have base sequences) that match (all) A to T and C to G
State how the interactions in the adenine-thymine base pair differ from those you identified in part (b).
There are only two H-bonds in the adenine-thymine base pair.
Suggest why urea is effective at separating the complementary strands in DNA.
The amino/-NH2 groups in urea are able to substitute for the H–bonds in the double helix.
Cisplatin works by causing the death of rapidly dividing cells. Name the process that is prevented by cisplatin during cell division.
Dna replication
After cisplatin enters a cell, one of the chloride ligands is replaced by a water molecule to form a complex ion, B. (b)Give the equation for this reaction.
[Pt(NH3)2Cl2] + H2O → [Pt(NH3)2Cl(H2O)]+ + Cl−
An experiment is done to investigate the rate of reaction in part (b). (d)During the experiment the concentration of cisplatin is measured at one-minute intervals. Explain how graphical methods can be used to process the measured results, to confirm that the reaction is first order.
plot concentration (y-axis) against time (x-axis) and take tangents / (calculate the) gradients (to calculate rates) M2 Plot rate/gradients against conc M3 straight line through origin / directly proportional confirms first order
Explain how the base that you have chosen forms a base pair with guanine.
Base A Top N–H forms hydrogen bonds to lone pair on O of guanine The lone pair of electrons on N bonds to H–N of guanine A lone pair of electrons on O bonds to lower H–N of guanine
d) An adverse effect of cisplatin is that it also prevents normal healthy cells from replicating. Suggest one way in which cisplatin can be administered so that this side effect is minimised.
Use in very small amounts / target the application to the tumour
When cisplatin is absorbed into the human body, it undergoes a ligand substitution reaction and one chloride ligand is replaced by a water molecule forming a complex ion Q. (a) Write an equation for this substitution reaction to form the complex ion Q.
Pt(NH3)2Cl2 + H2O → [Pt(NH3)2Cl(H2O)]+ + Cl–
he first way involves a hydrogen atom, from one of the ammonia ligands on Q, bonding to an atom in a guanine molecule. State the type of bond formed to guanine and identify an atom in guanine that could form a bond to this hydrogen atom.
Hydrogen bond Oxygen or nitrogen
he second way involves a ligand substitution reaction in which an atom in a guanine molecule bonds to platinum by displacing the water molecule from Q. State the type of bond formed between guanine and platinum when a water molecule is displaced and identify an atom in guanine that could bond to platinum in this way
Coordinate Nitorgen or oxygen
State and explain one risk associated with the use of cisplatin as an anticancer drug
Killing them or causing damage (medical side effects) May attach to DNA in normal cells
The polypeptide chains are held together by hydrogen bonding. Explain how these form
Electron deficient h Which attracts lone paur of electorn on O
Explain why a polymer is non biodegradable
No polar r groups for attack by water/acids/alkalis/ nucleophiles or for hydrolysis. C-c bond too storng
Explain orign of interaction of hydrogen bonds between nh and oh
Nitorgen and oxygen are very electronegative. Therefore c=o and n-h are polar. This results in a hydrogen bonding between o and h In which lone pair of electrons for an oxygen atom is strongly attracted to the s+ h
Type of polymer for a polyester
Condensation polymer
Polyalkene co polymer. What type of polymerisation is it
Addition
Why cant a polyalkene coplymer be broken down by hydrolysis
C-c and c-f bonds too strong
Explain why strength of interactoion between two cytesine r groups differs from the strength of an interaction between a serine r group and an asparic acid r group
Two cys groups form a disulfide bridge Ser and asp r griups from bydropgen bonds Disulfide bridges are stronger than h bonds Because suilfide bridges are covalent bonds while h bonds arent
Consider amino acid serine. What structure will form when serene reacts with an excess of ch3 br.
So serine structure remains the same. However on the nh2. The n becomes surrounded by 3 ch3 groups and the br is just br- as a side product
Explain how a structural feature of this enzyme enables it to catalyse the hydrolysis of isomer G but not the hydrolysis of isomer F. [2 marks]
Enzyme has an active site The G-Enantiomer / Enzyme has the correct stereo chemistry / stereospecific Or For M2 allow opposite argument for F-Enantiomer 1 1 The G-Enantiomer / Enzyme has the complementary shape