A2 Biology | Post your doubts here

[hassam]

IN This thread i ll be trying to answer CIE'S typical A2 bio paper 4 questions in accordance wid markschemes bcos markschemes are hell of a job to study.......so any one can post the question with its anser below.....the ones u feel are typical of CIE....anser shud try to explain the ambiguous points in CIE's markscheme
EXAMPLE explain the role of -ve feedback in homeostasis in mammals? [4] o/n 07 p4 q.no 4
negative feedback mechanism helps to mantain constant internal environment.If there is a change in certain parameter,it is detected by receptor which brings about a response through the effector to suppress that change and bring level of parameter back to normal.

 

[hassam]

Q. Outline symptoms of cystic fibrosis? [4] o/n 07 q.no.4
Ans.symptoms of cystic fibrosis include collection of sticky mucus in airways of respiratory tract,vas deferens,pancreatic duct.lung tissue gets scarred and more prone to infection.also a person may become sterile due to collection of mucus in vas deferens

 

[hassam]

Q.explain what is meant by biodiversity? (3) o/n 07 p4
Ans.Biodiversity is the variation of life forms within a biome.the higher the genetic diversity within a species and between different species,higher 'll be the biodiversity.Also wide range of habitats in an ecosystem is indicator of high biodiversity

 

[XPFMember]

Assalamoalykum!!

Thanks a lottt for this thread.....I really need help with the pastpaper questions! That's true it's a big job to go on with the mark scheme! Jazak Allah Khairen!

Assalamoalaikum!!

In June:2010 Paper:41

Q:8 (b) (i) Outline, with reference to CF, what is meant by gene therapy.

Mark scheme says:

1 alters genotype ;
2 insert, dominant/normal, allele ; R gene
3 into, affected/appropriate, cells ;
4 use of vector/named vector ;
5 ref. recombinant DNA ;


In point 2: why does it say 'R gene'
er says: Common errors included references to gene rather than allele.


But the application booklet says: Since it is a recessive condition, such gene therapy does not need to remove or
replace the existing genes in the person’s cells – adding a working copy of the gene
to a cell and having it expressed would be sufficient to permit that cell to transport
chloride ions normally.

The Cambridge Revision guide also refers to inserting the gene. :%)


What's wrong...and whats right????

If you know this...can u help?

I have my exam soon...so i'll be needing much help...


Answer to the above problem:

AOA!

Here's how it's done:

In gene therapy of CF, the genotype of the cells affected by CF is altered by inserting normal alleles of genes into the target cells by using a genetically modified virus as a vector to infect the target cells. The normal form of the gene is transcribed and translated and in this way a normal functioning protein is produced in the target cells.

Hope this helps.

 

[OakMoon!]

You should've posted these in the designated Biology thread so that everyone could access it at all times.

 

[XPFMember]

You should've posted these in the designated Biology thread so that everyone could access it at all times.

Assalamoalaikum!

I know! But to be honest, in that biology thread, majority comes up with their doubts and sadly there's no one to he;p...so many queries remain unanswered!

This thread, is intended solely for the A2 pastpaper questions! All other questions, shall be posted in the other thread...
I will InshaAllah add the link to this thread in the first post of that thread!

I hope this makes sense! ardon:


Anyways, hassam I have problems with two questions at the moment! I hope you can help me with them!

Explain the roles of synapses in the nervous system.

Mark Scheme:
ensure one-way transmission;
receptor (proteins) only in postsynaptic, membrane / neurone ; ora
vesicles only in presynaptic neurone ; ora
adaptation / ACh amount reduces due to overuse of synapse ;
wide range of responses ;
due to interconnection of many nerve pathways ;
inhibitory synapses affect other synapses ;
involved in memory / learning ;
due to new synapses being formed ;
summation / discrimination ;

the points in bold...i dont know how to link them!

the other question about speciation:

I came across 3-4 questions, and usually mark scheme states these points...so if in general you cud tell me how to link these points...the ones in bold are the particular ones i have problem with!
(June:2010 Variant 1)

1 allopatric speciation ;
2 (lizard populations) separated by water ;
3 geographical/physical, barrier ;
4 no, breeding/gene flow, between populations ;
5 mutations occur ;
6 different selection pressures/different (environmental) conditions ;
7 genetic change ; e.g. different alleles selected for/change in allele frequency/
change in gene pool/advantageous alleles passed on ;
8 (can result in) different chromosome numbers ;
9 genetic drift ;
10 ultimately, reproductively isolated/cannot interbreed ;

 

[OakMoon!]

Mark Scheme:
ensure one-way transmission;
receptor (proteins) only in postsynaptic, membrane / neurone ; ora
vesicles only in presynaptic neurone ; ora
adaptation / ACh amount reduces due to overuse of synapse ;
wide range of responses ;
due to interconnection of many nerve pathways ;
inhibitory synapses affect other synapses ;
involved in memory / learning ;
due to new synapses being formed ;
summation / discrimination ;

Wide range of responses occur due to the inhibitory and stimulatory effects due to different types of neurotransmitters out there. Plus, it is possible that there are many synaptic terminals leading to one synapse and an interconnection is created between many pathways.
As for memory and learning, this takes place due to the new synapses formed for e.g. Mary Jones mentions that when the brain frequently recieves information about two or more things such as one of the voice of a person and second of the face of a person a new synapse is created that is a pathway for both these informations. So a linkage of two neurones using the synapse has been created. Apart from that summation of action potentials can take place at the synapsis leading to memory learning.

speciation:

I came across 3-4 questions, and usually mark scheme states these points...so if in general you cud tell me how to link these points...the ones in bold are the particular ones i have problem with!
(June:2010 Variant 1)

1 allopatric speciation ;
2 (lizard populations) separated by water ;
3 geographical/physical, barrier ;
4 no, breeding/gene flow, between populations ;
5 mutations occur ;
6 different selection pressures/different (environmental) conditions ;
7 genetic change ; e.g. different alleles selected for/change in allele frequency/
change in gene pool/advantageous alleles passed on ;
8 (can result in) different chromosome numbers ;
9 genetic drift ;
10 ultimately, reproductively isolated/cannot interbreed ;

The separation due to geographical differences can bring about huge mophologivcal, physiological and biochemical (genetic differences) changes in the separated specie which makes it unable to breed with the original population, hence, it is reproductively isolated.
Mutation can cause a different chromosome number, and also cause polyploidy which is part of sympatric speiation. Such polyploids are also reproductively isolated and result in a separate species. Different genes and chromosome number accounts for different genes.

 

[XPFMember]

Assalamoalaikum!

the second one is not clear is that all i will write under those points?

Q: Explain how natural selection can bring about evolution? [8]

Here's my answer: I need to add some more points...can someone help me with that, plz!

The individuals in population have great reproductive potential, however the population generally stays approximately the same size because most of the young do not survive long enough to reproduce. There will be variation in the members of a population. Those individuals with features that are best adapted to the environment are more likely to survive and reproduce passing on the advantageous alles onto their offsprings. Others are more likely to die.

Let me know if upto here, it's correct and how i do add the last points!
Here's the mark scheme points: (J'09 Q:9 b )


13 individuals in population have great reproductive potential / AW ;

14 numbers in population remain roughly constant ;

15 variation in members of population ;

16 environmental factors / named factor (biotic or abiotic) ; linked to 17 and 18

17 (cause) many, fail to survive / die / do not reproduce ;

18 those best adapted survive / survival of the fittest ;

19 (reproduce to) pass on alleles ; R genes

20 genetic variation leads to change in phenotype ;

21 ref: changes in, gene pool / allele frequency ;

22 over time produces evolutionary change ;

23 new species arise from existing ones / speciation ;

24 directional / stabilising, selection ;

 

[OakMoon!]

^You haven't talked about selection pressures. And how these pressures may stabilise or cause a directional selection. Talk about HOW environmental factors and predation can effect the allele frequency to alter (give an example or two). SURVIVAL OF THE FITTEST (These 4 words should definitely be part of an answer on natural selection, you just can't miss them). You haven't talked about how species can be evolve due to mutation or geographical separation.

 

[XPFMember]

Assalamoalaikum!

If possible,can u plz post the full answer....i need help..i have an exam ..plz

 

[XPFMember]

Assalamoalaikum!

If possible,can u plz post the full answer....i need help..i have an exam ..plz

As-salam-o-alaikum!

anyone...plz?? have an exam tomorrow

 

[XPFMember]

Assalamoalaikum!!

Awaiting for the previous answer...

and have another doubt:
In June:2010 variant three, for the question abt anaerobic respiration in yeast, the mark scheme says 'ethanal dehydrogenase'

Shudn't it be ethanol dehydrogenase?? because ethanal is changed to ethanol by adding hydrogen (it is reduced) not by removing hydrogen...that what's i understand!

moreover in the book and the guide it says ethanol!?

 

[Nibz]

Assalamoalaikum!

If possible,can u plz post the full answer....i need help..i have an exam ..plz

As-salam-o-alaikum!

anyone...plz?? have an exam tomorrow

WS!

Here's a copy-pasted answer that has many points from mark-scheme:
Individuals in population have great reproductive potential. Despite such reproductive potential, numbers in population remain roughly constant because many fail to survive or reproduce. Many environmental factors, whether biotic or abiotic, cause many to die or or not reproduce. Some fail to survive while others who survive do so because they are best adapted when environmental factors restrict the population size. Those survived are then able to reproduce and, by natural selection, pass on the advantageous alleles to the next generation. This genetic variation leads to a change in phenotype. Changes in allele frequency over a period of time produce evolutionary change and eventually a new specie is formed.

 

[XPFMember]

Thank you soooooooooooooooo much!!!!!!!!!!!!!!
JazakAllah Khair!! I thinkk this will work!!!!
many prayers...thanks again!

 

[Nibz]

Anytime
And thank you very much for your prayers!
Good Luck :]

 

[hassam]

describe how structure of mitochondria is adapted for oxidative phosphorylation?​

can any1 explain last point (highlighted one)​

 

[XPFMember]

Assalamoalaikum wr wb!
wanted to share this, I've written it myself with the help of ms. If you think there's any mistake, do let me know. Hope it helps. InshaAllah

Explain how a change in the nucleotide sequence in DNA may affect the amino acid sequence in a protein and hence the phenotype of the organism. (J’09 Q:9a)

1. code is three, bases / nucleotides ; A triplet code
2. (gene) mutation ; R chromosome mutation
3. base, substitution / addition / deletion ;
4. addition / deletion, large effect (on amino acid sequence) ;
5. frame shift ;
6. completely new code after mutation / alters every 3 base sequence which follows ;
7. (substitution) often has no effect / silent mutation ;
8. different triplet but same amino acid / new amino acid in non-functional part of protein ;
9. (substitution) may have big effect (on amino acid sequence) ;
10.could produce ‘stop’ codon ;
11.sickle cell anaemia / PKU / cystic fibrosis ;
12.reference to transcription or translation in correct context ; A description
13.AVP ; e.g. protein produced, is non-functional / not produced / incomplete

A triplet of bases is what which codes for an amino acid. Gene mutation may alter this code by a base addition or deletion and even by a base substitution. Base addition or deletions have large effects as they cause a frame shift, because they alter each 3 base sequence which follows and hence a completely new code after mutation. Substitution, on the other hand, often has no effect and it’s often called the silent mutation. This is because there are more than one triplets coding for the same amino acid, so even after substitution, if it codes for the same amino acid, it won’t have any effect. Though at times, it does produces big effects. For example, if it produces a stop codon, the polypeptide would never be completely formed. Similarly, it may result in a different amino acid, for example, in the case of sickle cell anaemia, a substitution results in the amino acid valine at the sixth position in the B chain instead of Glutamic acid.

(Didn't include point no.12...dunno how to add )

 

[XPFMember]

Assalamoalaikum wr wb!
Can anyone please confirm if the following answer is correct?
The last part abt melanin, I wrote it from what I remember as I cudnt find in the book/guide.


Explain, with examples, how the environment may affect the phenotype. (N’07 Q:11 b)

1. phenotypic variation results from interaction of genotype and environment / VP = VG
+ VE ;
2. environment may limit expression of gene(s) / AW ;
3. e.g. for size / mass / height ;
4. because, food / nutrients / ion, missing / malnutrition ;
5. named, nutrient / ion / mineral, missing ;
6. environment may, trigger / switch on, gene ;
7. ref. low temperature and change in animal colour ;
8. ref. high temperature and, curled wing in Drosophila / gender in crocodiles ;
9. ref. UV light and melanin production ;
10.ref. wavelength of light and, flowering / germination / fruit colour ;
11.other named trigger plus example ;
12.environment effect usually greater on polygenes / ora ;
13.environment may induce mutation affecting phenotype ;

Phenotypic variation results from the interaction of the genotype and the environment. Environment may limit the expression of the genes in an individual. Consider human height. If the individual has the number of alleles for tallness, he will have the potential to grow tall. However, if his diet is poor and the required proteins, for example, are missing in his diet, he will not be able to grow tall. Environment may trigger the genes, for example, hair colour in cats.They have many genes that influence their hair colour. They do this by coding for the production of enzymes. For example, in a particular type of cat, they have a gene that codes for the production of an enzyme which is sensitive to temperatures. It produces dark hair in regions where the temperature is lower eg: paws, ears and tail, and light hair in warmer parts of the body. Environment usually show greater effects on polygenes. Another example is the effect of UV light on melanin production. Area where people are exposed to more sunlight, and hence UV have darker complexion as the melanin production is higher in them.

 

[XPFMember]

Aoa!
need help with this one too...I tried coming up with an answer..is it correct? If u have a better one then plz do share.

Explain, with examples, how mutation may affect the phenotype.(J’06 Q:7 b)
1. gene) example ; (sickle cell / PKU )
2. change in gene / DNA / base change ;
3. different amino acid ;
4. different polypeptide / different protein / non-functional protein ;
5. AVP ; details
6. AVP ; details
7. (chromosome) example ; (Down’s, Turner’s syndromes)
8. structural changes in chromosomes ;
9. change in number of chromosomes ;
10.change in sets of chromosomes / ref. polyploidy ;
11.AVP ; details
12.AVP ; details

A change in the structure of a DNA molecule, producing a different allele of a gene, is a gene mutation. An example of this is Sickle Cell Anaemia. It is caused by a base substitution that has a significant effect on the phenotype. The sixth position in the B chain where the base sequence is CTT which codes for glutamic acid is replaced by CAT which codes for the amino acid Valine. Valine is hydrophobic in nature unlike glutamic acid which is hydrophilic. When the abnormal haemoglobin is in areas of low oxygen concentration, they stick together forming long fibres inside the red blood cells, pulling them into a sickle shape.
Mutations may also cause a change in the structure or number of whole chromosomes in the cell, known as chromosome mutation. An example of this is Down’s Syndrome, in which during the division process (meiosis), one of the ovumhas 24 chromosomes and the other 22. The one with 24 is viable and if fertilized will form 47 chromosomes instead of 46. These children usually have lowered life expectancy and mental retardation, etc.

 

[hassam]

ITS NOT REALLY A CIE QUESTION BT.....a doubt in my mind.....how FSH can cause superovulation when it normally leads to one oocyte per cycle only ,......any idea