A level Biology: Post your doubts here!

[Afza Munir]

u

It does! Thanks.

ur welkm...

 

[rehmanmunir]

on 7th nov my bio practical can anyone tell me how to make diluations any formula for finding the required concentration and volume?

 

[demon29]

paper5 M/J 2009 q3 (b) (iv) help please..

 

[Al-Anoud]

^^Can someone please post the file of the CD for this book?

 

[Gehad Mohamed]

Hello !
I need help in q 24

 

[Jinosupreme]

HIV-1 protease is an enzyme produced by the HIV virus.
Two identical chains of 99 amino acids form the enzyme. In each chain, amino acids 25, 26 and
27 in the sequence form part of the active site.
Which orders of protein structure control the shape of the active site?
A primary, secondary, tertiary and quaternary
B primary, secondary and tertiary only
C primary and quaternary only
D quaternary only

Answer is A, why isnt it B? :O

 

[Jinosupreme]

Cancer cells divide out of control, forming tumours.
Which statement describes the difference between a cancer cell and a normal cell?
A Cancer cells do not undergo cytokinesis.
B Cancer cells have a shorter interphase.
C Cancer cells do not have metaphase.
D Only cancer cells have mutated DNA

Why? Answer is B

 

[Jinosupreme]

Hello !
I need help in q 24

It is not C and D because the assemble of free DNA nucleotide is in opposite direction, from 5' to 3'.
It is not B because there is a URACIL instead of THYMINE. Uracil is for transcription and translation but NOT for DNA replication, should be thymine. Hence is A.

 

[Gehad Mohamed]

It is not C and D because the assemble of free DNA nucleotide is in opposite direction, from 5' to 3'.
It is not B because there is a URACIL instead of THYMINE. Uracil is for transcription and translation but NOT for DNA replication, should be thymine. Hence is A.

Thanks alot

 

[Gimmick]

HIV-1 protease is an enzyme produced by the HIV virus.
Two identical chains of 99 amino acids form the enzyme. In each chain, amino acids 25, 26 and
27 in the sequence form part of the active site.
Which orders of protein structure control the shape of the active site?
A primary, secondary, tertiary and quaternary
B primary, secondary and tertiary only
C primary and quaternary only
D quaternary only

Answer is A, why isnt it B? :O

The hint is in the question -- the HIV-1 protease is formed from two (identical) chains of 99 amino acids; for each chain (chain 1 and 2), amino acids 25, 26 and 27 form part of the active site.
The quarternary structure is due to the different chains that form an enzyme (for example, haemoglobin has a quarternary structure made up of four chains); in this case, since there are two chains, and since they both play a part in the shape of the active site, the quarternary structure controls the shape of the active site as well as the primary, secondary and tertiary structure (A)

 

[Jinosupreme]

Answer is D. Can Anyone explain?

Answer is D, why? :O

 

[Jinosupreme]

Some babies are born with a hole between the right and left atria. These newly born babies are
found to have an increased number of red blood cells.
What is the reason for this increase?
A More blood is needed because it is pumped faster.
B Newly born babies’ haemoglobin has a higher affinity for oxygen.
C Newly born babies’ haemoglobin molecules only have one haem group.
D There is less oxygen available to the newly born baby
Answer is D, can anyone explain?

 

[VelaneDeBeaute]

Some babies are born with a hole between the right and left atria. These newly born babies are
found to have an increased number of red blood cells.
What is the reason for this increase?
A More blood is needed because it is pumped faster.
B Newly born babies’ haemoglobin has a higher affinity for oxygen.
C Newly born babies’ haemoglobin molecules only have one haem group.
D There is less oxygen available to the newly born baby
Answer is D, can anyone explain?

Because of the 'hole' between the left and right atria, the oxygenated and de-oxygenated blood get mixed up. This means that not all of the de-oxygenated blood gets to attach to oxygen, hence less oxygen supply to the body. To counter for this, more RBCs are produced by the body as the more the RBCs, the more oxygen they can pick.
Clear?

 

[VelaneDeBeaute]

Jinosupreme
For the first question, you simply need to know what the three pathways are. The apoplastic pathway is the route taken because of the cohesive forces between the water molecules, i.e. the fact that there exists hydrogen bonding between the water molecules, hence they move in an uninterrupted stream. This movement is through cell walls, which are technically dead, so there can be no 'water potential difference' between adjacent cell walls., since their contents are same.
Now for the other two pathways. The vacuolar pathway is established when the water potential of the vacuoles of two adjacent plant cells differs, and this makes water contents to migrate from an area of higher potential towards the lower potential. Hence, the movement is accredited to difference in the water potentials. Similarly, the symplastic pathway is established when the cytoplasmic contents of adjacent cells are at varying water potentials, which makes the water molecules' net movement in the direction of lower water potential. Again, the difference in water potentials triggers the symplastic pathway.
Keeping all this under consideration, D is the right option.

 

[Jinosupreme]

Jinosupreme
For the first question, you simply need to know what the three pathways are. The apoplastic pathway is the route taken because of the cohesive forces between the water molecules, i.e. the fact that there exists hydrogen bonding between the water molecules, hence they move in an uninterrupted stream. This movement is through cell walls, which are technically dead, so there can be no 'water potential difference' between adjacent cell walls., since their contents are same.
Now for the other two pathways. The vacuolar pathway is established when the water potential of the vacuoles of two adjacent plant cells differs, and this makes water contents to migrate from an area of higher potential towards the lower potential. Hence, the movement is accredited to difference in the water potentials. Similarly, the symplastic pathway is established when the cytoplasmic contents of adjacent cells are at varying water potentials, which makes the water molecules' net movement in the direction of lower water potential. Again, the difference in water potentials triggers the symplastic pathway.
Keeping all this under consideration, D is the right option.

Omg, thank you SO MUCH!!!!! really appreciate your help

 

[Jinosupreme]

The following events occur during transcription.
1 Bonds break between complementary bases.
2 Bonds form between complementary bases.
3 Sugar-phosphate bonds form.
4 Free nucleotides pair with complementary nucleotides.
Before the mRNA leaves the nucleus, which events will have occurred twice?
A 1 and 2 only
B 1, 3 and 4 only
C 2, 3 and 4 only
D 1, 2, 3 and 4

Answer is A, anyone can explain pls

 

[Gehad Mohamed]

Hello ! Plz Help me in Q 2/9/8/17

 

[sam1234]

Can someone explain me q no 19 of mayjune 2013 p1 please, urgently. http://olevel.sourceforge.net/papers/9700/9700_s13_qp_12.pdf

 

[sam1234]

The following events occur during transcription.
1 Bonds break between complementary bases.
2 Bonds form between complementary bases.
3 Sugar-phosphate bonds form.
4 Free nucleotides pair with complementary nucleotides.
Before the mRNA leaves the nucleus, which events will have occurred twice?
A 1 and 2 only
B 1, 3 and 4 only
C 2, 3 and 4 only
D 1, 2, 3 and 4
Answer is A, anyone can explain pls

Since DNA moelcule unwinds and than mRNA binds to the DNA template, than DNA unwinds from the mRNA and the two DNA strands join again by hydrogen bonding, the process of winding and unwinding is repeated twice, hence A is the answer. Hope this helps

 

[sam1234]

Hello ! Plz Help me in Q 2/9/8/17

Ans 2 is C, because resolution is half the wavelength of the given wave, hence when wavelength increases resolution decreases, however increasing or decreasing resolutions doesn't affect magnification
Ans 8 is A, read the text book for that, it's a theory type question.
Ans 9 is C, because all the electrons are already bonded in that molecule, it has no capacity to bind further.
Ans 17 is B, mitosis only involves nucleus division, it's not responsible for cell growth.

Hope this helps,