VCE Biology Question Thread

[PhoenixxFire]

just wondering if anyone could explain to me when to talk when u should talk about dendritic cells rather than B cells


I just did a question which said how are antibodies are made and it said in the answers that dendritic cells engulf the antigen present it on its class 2 markers to t h cells which activate B cells which differentiate into b plasma cells. Could u just say the antigen binds to a specific naive B cell  causing the B cell to differentiate into b memory cells and  b plasma cells which produce antibodies
Thanks 

Here is a post I made earlier talking about the whole immune process. As other people have said I would definitly talk about both for humoral immunity.

Basically when a cell becomes viral infected both cell mediated and humoral immunity are activated. I'll go over everything tell me if something doesn't make sense.
General process always occurring in our bodies:
-B cells mature in the bone marrow. Here they are tested for self-reactivity, if they can bind to a self antigen they are normally destroyed (except for malfunctions ie autoimmune diseases.)
-Both Tc cells and Th cells mature in the thymus gland. Here they are also tested for self reactivity.

Throughout the body there are always B cells, Th cells, and Tc cells with a randomly generated antigen specificity.
B cells are found in lymph nodes (and throughout the lymph system)
Th cells are found in lymph nodes
Tc cells are found throughout the body tissues.

Normally:
B cells and Th cells just hang about waiting for their antigen to bind/be presented to them.
Tc cells travel throughout the body attempting to bind to peptide fragments presented on MHC1 markers.

If an intracellular pathogen (ie virus) enters the body, parts of it will inevitably end up in the blood and lymph. Some of it will enter cells and cause the creation of non-self peptide fragments which will be presented on MHC 1 markers at some point. The following will happen simultaneously.

Humoral Immunity will be activated:
-A naive B cell will bind to a free antigen (ie. not presented by an APC).
-A Th cell will be presented with its antigen on a MHC2 molecule by an APC.
-The 'selected' B cell and Th cell will then find each other and if they have bound the same antigen, the Th cell will release cytokines (the same cytokines that affect cell mediated immunity - see below.)
-These cytokines cause the B cell to divide (proliferate) and differentiate into B memory cells and B plasma cells.
-These cytokines also cause the Th cell to divide (proliferate) and differentiate into Th memory cells and Th active cells.
-The memory cells remain in the body to fight subsequent infection by a pathogen with the same antigen specificity and the B plasma and Th active cells fight off the current infection.

As the pathogen (virus) is intracellular, cell mediated immunity is also activated.
-Naive Tc cells are always travelling throughout the body, attempting to bind to peptide fragments presented on MHC1 markers.
-When they find one that they can bind to, the Tc cell is 'selected'.
-The Tc cell will release granzymes (including perforin) which causes the cell to undergo apoptosis.
-The Tc cell continues to travel throughout the body and kill cells presenting the same peptide but it will not divide and differentiate until cytokines are present.
-When cytokines have been released from Th cells (this could happen before or after the Tc cell is selected) the Tc cell will divide (proliferate) and differentiate into Tc memory cells and Tc active cells.
-The Tc memory cells will remain in the body to fight off subsequent infection by the same pathogen.
-The active Tc cells will travel throughout the body, inducing apoptosis in cells presenting the same peptide fragment.

[PhoenixxFire]

MHC 2 markers bind T helper cell, which will generate B cells

Other people have answered most of your questions. Just wanted to clarify that Th cells do not generate B cells. They secrete cytokines which cause already activated (have bound their antigen) B cells to divide and differentiate. Also the activated B cell and Th cell have to interact to cause the Th cell to release cytokines. See my post I quoted above for more on this.

[PhoenixxFire]

Hey,
1) I don't quite understand how the further the population out from Africa, the more similar their mitochondrial DNA is. (Africa would have most diverse mtDNA)
2) Also, a question from NEAP exam states that the further out from Africa, the more genetic diversity, so African population would be more genetically similar.
(sorry for the clumsy wording)

Aren't those two ideas contradicting each other ?
Is it because 1) is referring to mtDNA and 2) is just their nuclear genes

I would say that yes it is due to mtDna and nuclear DNA.
Not-African mtDna would be more similar than African mtDna as mtDna is passed down from mother to offspring with no input from father. As there were less mothers out of Africa than in Africa out of Africa DNA would be more similar.
(Like founder effect)

Non-African nuclear DNA would be more diverse as there are simply more individuals. More crossing over has resulted in more unique genotypes, more people also means more total mutations in the population.

[Melody_Zhou66]

Why is mtDNA more useful for tracing ancestry than nuclear DNA?
Also, do we need to know the roles of all the hominins?

[Apricot]

I normally mention it without any detail as they want to test your knowledge of cell mediated immunity not humoral. I would say something along the lines of .... the selected (or activated) Tc cell is stimulated to divide and differentiate by cytokines released from a selected (or activated) Th cell....

Isnt APC presenting the specific antigen linked to an MHC class 1 to a cytotoxic t cell also part of cell mediated immunity? So if i was asked to "outline the cell mediated immune response," would i mention role of APCs in activating Th cells, then APCs and Th both activating Ct cells and then discuss Ct cell role in destroying infected cells?

[HubHub]

Hi guys,
Can someone please explain to me the Out-Of-Africa theory, I don't understand. Is it that all homo Erectus left Africa to colonise the world, which then evolved into heidelbergensis which them evolved into Neanderthals and Homo sapiens, all of which died out except Homo sapiens and that the hominids in African eventually evolved into homosapiens too, which is why they have more genetic diversity and no neanderthal DNA??

[Shaqattack]

do u need to talk about APC and class 2mhc markers when describing how a vaccine work or can u say
- attenuated antigens are introduced into the body and bind to specific naive b cells
-b cells proliferate and differentiate into bplasma cells and b memory cells
-b plasma cells produce specific antibodies my which neutralizes the pathogens effect
-b memory cells provide a greater response if the pathogen appears again not allowing an infection to occur

[chrisjudd00]

I dont think it will hurt to at least mention that the helper T cells will release cytokines that activates the selected B cell to proliferate. However it might come down to the marks the question is out of.

[Darthdany]

During allopatric speciation, pretend we have species A: it gets split into two because of the geographical barrier. After generations, does one of the split off populations become a new species, species B, and the original population stay at A? Or do both populations become new species, species B and C for example?

Thanks

[Jackie Chan]

Why is mtDNA more useful for tracing ancestry than nuclear DNA?
Also, do we need to know the roles of all the hominins?

mtDNA is more useful than nuclear DNA for the following reasons:
- mtDNA does not under recombination unlike nuclear DNA
- mtDNA is easier to obtain in high yields as cells generally have many mitochondrions
- mtDNA doesn't undergo the same checks as nuclear DNA does so it has a greater mutation rate

[Domek]

Hi, are stromatolites trace fossils? Or do they actually contain fossilised bacteria? Different sources say different things so I'm a bit confused. Thanks!

[PhoenixxFire]

Isnt APC presenting the specific antigen linked to an MHC class 1 to a cytotoxic t cell also part of cell mediated immunity? So if i was asked to "outline the cell mediated immune response," would i mention role of APCs in activating Th cells, then APCs and Th both activating Ct cells and then discuss Ct cell role in destroying infected cells?

Yes. When I say activated (or selected) Tc cell I mean a Tc cell that has bound it's antigen but has not (yet) been stimulated by cytokines to divide and differentiate. Cytokines will not affect any Tc cell that hasn't bound its antigen and any Tc cells that have bound their antigen will not divide and differentiate unless cytokines are present.
I normally talk about Tc cells then when I get up to that part I say something along the lines of '.....the selected Tc cell will divide and differentiate into ...... when cytokines are released from an activated Th cell'
-The problem with talking about activation of Th cells is that you can't simply say 'An APC stimulated a Th cell to release cytokines' because it is wrong. You would have to also talk about an activated B cell binding to the activated Th cell etc.
There is not enough time (or marks) for this so save it for the humoral immunity questions.

[Domek]

During allopatric speciation, pretend we have species A: it gets split into two because of the geographical barrier. After generations, does one of the split off populations become a new species, species B, and the original population stay at A? Or do both populations become new species, species B and C for example?

Thanks

I think that both of them would become new species especially if they have been exposed to new selection pressures. Also, this is after many generations, i.e. thousands or even millions of years, so natural selection would probably have occurred in each of the populations over time anyway, as environments change over time and also mutations would have led them to be different from the parent population. But it really depends on the situation, and how long they are separated for.

[PhoenixxFire]

do u need to talk about APC and class 2mhc markers when describing how a vaccine work or can u say
- attenuated antigens are introduced into the body and bind to specific naive b cells
-b cells proliferate and differentiate into bplasma cells and b memory cells
-b plasma cells produce specific antibodies my which neutralizes the pathogens effect
-b memory cells provide a greater response if the pathogen appears again not allowing an infection to occur

Don't say attenuated antigens.
-The antigens are not attenuated, the pathogens are.
-Not all vaccines contain attenuated pathogens.

Neutralise the pathogens effect

Not very specific. It would be better to say 'causes the pathogen to agglutinate and assists in phagocytosis by attracting phagocytes to the area.
-This depends on how many marks the question is worth.

Vaccines do not stop infection
As soon as the pathogen is past the first line of defence (ie. as soon as it is in your internal environment) you are infected. Memory cells may result in the destruction of the pathogen fast enough that you do not show symptoms, but they do not stop you becoming infected.
A better way to say this would be 'B memory cells will cause a faster and larger immune response if a pathogen with the same specific antigen infects that person again.


NOTE: You may be required to talk about cell mediated immunity depending on the type of vaccine being talked about.

[PhoenixxFire]

During allopatric speciation, pretend we have species A: it gets split into two because of the geographical barrier. After generations, does one of the split off populations become a new species, species B, and the original population stay at A? Or do both populations become new species, species B and C for example?

Thanks

Domec is absolutely right, just wanted to add it is possible for only one to become a different species. - If either of them would be able to produce offspring with the ancestral population then they have not become a new species.

Think of the founder effect - a few individuals become geographically isolated, if they evolved into a new species it is unlikely that the (much) larger original population would change species unless they faced very dramatic changes in selection pressures. (having said that, with a population so small, it is unlikely that they would be able to cope with changing selection pressures well enough to become a new species).