A level Biology: Post your doubts here!

[megamind]

Not sure about this, but I'll try...

Base pairing helps to keep the DNA molecule stable, as if the bases were exposed, and not paired inwards in the way they are, they would react with cellular fluids, which would cause the loss of the DNA information. Also, base pairing occurs with hydrogen bonding, and individual hydrogen bonds are easy to break (mind that many hydrogen bonds are strong, but few or individual can be broken easily), this helps when the DNA must split for DNA replication or protein synthesis.

I asked my teacher told me this-

Purines with prymidines, ocuppy same space between the backbone strands,space constant more stable
complementary base-pairing(ATCG,hydrogen bond)is crucial to for replication and gene expression.,DNA is semi-conservative

 

[megamind]

I got more question from my teacher to anwser,

please help me with this-


What is refractory period?

Explain the difference
• absolute refractory period and relative refractory period
• resting potential and action potential
• polarised and depolarised

describe the processes that affect allele frequencies in populations with reference to the global distribution of malaria and sickle cell anaemia;

Use sickle cell anaemia as an example to illustrate and discuss how one genotype may be fitter than another when a particular environmental factor (infection with Plasmodium) is exerting a strong selection pressure

 

[HorsePower]

refractory period is the period of time for which the nerve can not response to a stimulus. During absolute refractory period, there is complete incapability of a response, but in relative refractory period there may be a response in case of a large stimulus.

 

[HorsePower]

depolarised is the state of plasma membrane when a positive potential is present across it due to opening of Na+ channels , causing Na+ ions to diffuse into the cell down the electrochemical gradient. The membrane is polarised when there is a negative potential present across the membrane.

 

[HorsePower]

srry this is all i can recall at the moment.

 

[aleezay]

Extremely stupid of me to ask such a question 10 days before the exam, but... why is glucose converted to sucrose for translocation? Just because its not as reactive?? :/

 

[HorsePower]

Extremely stupid of me to ask such a question 10 days before the exam, but... why is glucose converted to sucrose for translocation? Just because its not as reactive?? :/

also because it is soluble and may change the water potential of the tissue fluid

 

[PANDA-]

also because it is soluble and may change the water potential of the tissue fluid

Tissue fluid? He did say translocation=plants...

 

[HorsePower]

Tissue fluid? He did say translocation=plants...

oh srry my bad... i didnt read it properly

 

[PANDA-]

oh srry my bad... i didnt read it properly

NP but even in humans, glucose isn't converted to sucrose, it's carried as it is.

 

[ramnarace]

Does anyone have tips for paper 5 biology?

 

[HorsePower]

cud any one tell me what provides energy for H+ ions to be actively pumped into the inter membrane space of mitochondria during respiration ?

 

[ramnarace]

When electrons are moved along an electron transport chain,energy is lost, this energy is used to transport the H+ ion into the intermembrane space.

 

[nightmaressana]

My teacher gave these 4 marks question.
1. Why dna replication is important?
2. Why do b lymphocytes divide mitotically to produce a clone of cells.
3. How do does a tumor form? Describe the whole process.
4. What are the advantages of having sickle cell anaemia

For 4, i have absolutely no idea. But can these types of question come for four marks?
Can someone please answer them?

 

[HorsePower]

When electrons are moved along an electron transport chain,energy is lost, this energy is used to transport the H+ ion into the intermembrane space.

this is what i've learnt but my teacher say it is from the ATP already present in the cell...

 

[PANDA-]

My teacher gave these 4 marks question.
1. Why dna replication is important?
2. Why do b lymphocytes divide mitotically to produce a clone of cells.
3. How do does a tumor form? Describe the whole process.
4. What are the advantages of having sickle cell anaemia

For 4, i have absolutely no idea. But can these types of question come for four marks?
Can someone please answer them?

1. I don't know why DNA replication is important in general, but in mitosis, it produces genetically identical cells, so daughter cells can carry out the same function as parent cells, and no mutation can occur.
2. B lymphocytes are activated when receptors on it's plasma membrane bind to an antigen complementary to them, they then divide rapidly by mitosis, to form plasma cells and memory cells, plasma cells secrete antibodies which in one way or another destroy the bacteria, or make it easier for phagocytes to ingest them. Memory cells remain in blood for a long time and help to produce a faster, and greater secondary immune response, where more antibodies are secreted more quickly.
3. Carcinogens alter the DNA of a cell, particularly the genes that are responsible for the controlling of mitosis by under-expression of Tumor suppressor genes or by stimulating the over-expression of oncogenes, so the cell divides uncontrollably, forming a mass of cells... The tumour (if malignant) may break off and invade nearby tissues and organs.
4. Sickle cell anaemia causes the RBC's to reduce in volume by changing their shape to a crescent-like shape, so the Plasmodium Falciparum cannot live in these RBC's as they soon are destroyed due to their shape which quickly gets stuck into walls of vessels and ruptures as a result... Note that the advantage is having sickle cell trait HbAHbS alleles, because this way, the person is resistant to Malaria, and Sickle cell trait has no symptoms, but a person with Sickle cell anaemia, HbSHbS, although more resistant to Malaria, just dies of the sickle cell anaemia.

Also note that my knowledge is only limited, as I am an AS student still... Double check the info I've posted with a textbook or something, as I'm unsure of it

 

[HorsePower]

1. I don't know why DNA replication is important in general, but in mitosis, it produces genetically identical cells, so daughter cells can carry out the same function as parent cells, and no mutation can occur.
2. B lymphocytes are activated when receptors on it's plasma membrane bind to an antigen complementary to them, they then divide rapidly by mitosis, to form plasma cells and memory cells, plasma cells secrete antibodies which in one way or another destroy the bacteria, or make it easier for phagocytes to ingest them. Memory cells remain in blood for a long time and help to produce a faster, and greater secondary immune response, where more antibodies are secreted more quickly.
3. Carcinogens alter the DNA of a cell, particularly the genes that are responsible for the controlling of mitosis (Tumor suppressor genes) or by stimulating the over-expression of oncogenes, so the cell divides uncontrollably, forming a mass of cells... The tumour (if malignant) may break off and invade nearby tissues and organs.
4. Sickle cell anaemia causes the RBC's to reduce in volume by changing their shape to a crescent-like shape, so the Plasmodium Falciparum cannot live in these RBC's as they soon are destroyed due to their shape which quickly gets stuck into walls of vessels and ruptures as a result... Note that the advantage is having sickle cell trait HbAHbS alleles, because this way, the person is resistant to Malaria, and Sickle cell trait has no symptoms, but a person with Sickle cell anaemia, HbSHbS, just dies of the disease.

Also note that my knowledge is only limited, as I am an AS student still... Double check the info I've posted with a textbook or something, as I'm unsure of it

Nice .... if i may just add to the last answer... HbSHbS will be more resistant to malaria than HbNHbS but as mentioned will eventually die due to sickle cell anemia.

 

[HorsePower]

Plus, it is predicted that in the future people with HbNHbS will be more common since they have a selective advantage.

 

[nightmaressana]

1. I don't know why DNA replication is important in general, but in mitosis, it produces genetically identical cells, so daughter cells can carry out the same function as parent cells, and no mutation can occur.
2. B lymphocytes are activated when receptors on it's plasma membrane bind to an antigen complementary to them, they then divide rapidly by mitosis, to form plasma cells and memory cells, plasma cells secrete antibodies which in one way or another destroy the bacteria, or make it easier for phagocytes to ingest them. Memory cells remain in blood for a long time and help to produce a faster, and greater secondary immune response, where more antibodies are secreted more quickly.
3. Carcinogens alter the DNA of a cell, particularly the genes that are responsible for the controlling of mitosis by under-expression of Tumor suppressor genes or by stimulating the over-expression of oncogenes, so the cell divides uncontrollably, forming a mass of cells... The tumour (if malignant) may break off and invade nearby tissues and organs.
4. Sickle cell anaemia causes the RBC's to reduce in volume by changing their shape to a crescent-like shape, so the Plasmodium Falciparum cannot live in these RBC's as they soon are destroyed due to their shape which quickly gets stuck into walls of vessels and ruptures as a result... Note that the advantage is having sickle cell trait HbAHbS alleles, because this way, the person is resistant to Malaria, and Sickle cell trait has no symptoms, but a person with Sickle cell anaemia, HbSHbS, just dies of the disease.

Also note that my knowledge is only limited, as I am an AS student still... Double check the info I've posted with a textbook or something, as I'm unsure of it

Thank you so much, but im confused. Can these come for like 4 marks?
And is my answer for 2 correct? I mean since mitosis produces daughter cells with the the same dna which are able to synthesise the same antibodies, the same cell receptor which is need to be specific for one antigen and carry immune response. Then i will explain the function of memory cells and plasma cells. Also, they divide so that a quicker immune response is produced to prevent spreading of the disease.

My teacher has seriously confused me.

 

[PANDA-]

Plus, it is predicted that in the future people with HbNHbS will be more common since they have a selective advantage.

Oh yes, but I was forgetting that since this info was only mentioned in our IGCSE syllabus and is not present in the current AS syllabus. The only place where sickle cell is mentioned is in Biological Molecules... As how glutamic acid in the Haemoglobin molecule is replaced with valine... So I don't recall much of that topic.