VCE Biology Question Thread

[vox nihili]

What is the difference between a selection pressure and a selective agent?

Are retroviruses always made of RNA rather than DNA?

Retroviruses by definition are RNA viruses.

[Nomi16]

What is the difference between a selection pressure and a selective agent?

Are retroviruses always made of RNA rather than DNA?

Selective agent is the pressure or a factor acting on the species, it can be natural and artificial.
Selection pressure is eventually the same thing however it is a newer and more relative term to the current study design. Rather stick with Selection pressure than selective agent. And yes retroviruses are always composed of RNA because that is the only distinct difference between a virus and a retrovirus. And good luck for the big day.

[vox nihili]

Selective agent is the pressure or a factor acting on the species, it can be natural and artificial.
Selection pressure is eventually the same thing however it is a newer and more relative term to the current study design. Rather stick with Selection pressure than selective agent. And yes retroviruses are always composed of RNA because that is the only distinct difference between a virus and a retrovirus. And good luck for the big day.

Just to clarify, retroviruses are a TYPE of virus. They’re a special kind of virus the details of which are extraneous to the VCE course.

[omxrk]

Hey guys,

I have mostly completed my studies for Biology. The only thing that I keep avoid studying are the evolution chapters... How well do I have to know the time periods and also do I have to be familiar with all the different Homo species?

Thanks.

[Nomi16]

Hey guys,

I have mostly completed my studies for Biology. The only thing that I keep avoid studying are the evolution chapters... How well do I have to know the time periods and also do I have to be familiar with all the different Homo species?

Thanks.

Believe me that i have spent the least time on these chapters, however you should be confident with the "causes of evolution". Otherwise do trial exams and you will find out that which part vcaa focuses more on.

[PhoenixxFire]

Believe me that i have spent the least time on these chapters, however you should be confident with the "causes of evolution". Otherwise do trial exams and you will find out that which part vcaa focuses more on.

Details about evolution are new to the study design. No one can seem to agree but generally:
-Know the dates of major extinctions events
-Know the names of key species/classifications (eg. Tetrapods, that-species-that-became-mammals, etc)
-Know key features and be able to define hominin/hominoid/primate
-Know that date of first life on Earth/eukaryotic life/multicellular life/terrestrial life

[PhoenixxFire]

In the cell-mediated response
-Antigen presenting cells (dendrites and macrophages) travel to the lymph nodes where specific T helper cells are present.
- The T helper cells bind to their specific antigen (MHC II markers), and cause a cascade of reactions. Cytokines are released which activate and cause Cytotoxic T cells to differentiate.
- The Cytotoxic T cells then look for and bind self infected cells (MHC I markers)- binding to the cells and secreting proteins destroying them.

A few minor points:
-APC's don't necessarily travel to the lymph node - They could find their Th cell anywhere in the lymph system it is just more likely at lymph nodes because there is a higher concentration of them there at any one time.
-Th cell and Tc cell activation happens simultaneously, Tc cells are always travelling throughout the body attempting to bind to the peptide fragments presented on MHC 1, they just will not fully divide and differentiate until cytokines are present (as a result of Th cells being activated).
-The last point about binding self infected cells is slightly innacurate. If they can bind to the MHC 1 and the peptide being presented they will kill it but they are also responsible for killing transplanted tissue - They know the cell is non-self (or cancerous) because they cannot bind to the MHC 1 molecule, this means they kill the cell regardless of whether they can bind to the peptide fragment being presented.
-The proteins Tc cells secrete are called cytotoxic enzymes (one type you should know the name of is perforin but there are others)

- In the humoral response, native B cells bind to their specific antigen and await activation. When cytokines are released from T helper cells- activating B cells, they differentiate and form B memory and plasma cells. The plasma cells release antibodies- that bind to antigens- causing agglutination and engulfment by phagocytosis.
The B memory cells remain in the lymph nodes, and allow quick production of plasma cells --> antibodies, in subsequent exposures.

-Native B cells is the wrong terminology the correct word is naive B cells. Remember the B cells are not naive if they are memory cells from a previous immune reaction.
-After binding their antigen they do not await activation, they travel throughout the immune system looking for a Th cell which has bound the same antigen (they release cytokines to find each other). When they have found each other they undergo contact dependant signalling - The B cell presents the antigen to the Th cell much the same way macrophages/dendritic cells do. This is what causes the Th cells to release cytokines.
-Very minor points - Call them B plasma cells not Plasma cells. - Antibodies do not cause phagocytosis they assist it.
-B memory cells do not only remain in the lymph nodes they travel throughout all of the lymph. same as other immune cells.

[zenith101]

Hey, first of all, thanks heaps.

-After binding their antigen they do not await activation, they travel throughout the immune system looking for a Th cell which has bound the same antigen (they release cytokines to find each other). When they have found each other they undergo contact dependant signalling - The B cell presents the antigen to the Th cell much the same way macrophages/dendritic cells do. This is what causes the Th cells to release cytokines.

Regarding this point, is this naive B cells or differentiated B plasma cells that look for Th cells?

Thanks

[PhoenixxFire]

This is naive B cells - They do not differentiate (they do a little but for VCE's sake they don't) until they have been activated by cytokines.

[vox nihili]

A few minor points:
-APC's don't necessarily travel to the lymph node - They could find their Th cell anywhere in the lymph system it is just more likely at lymph nodes because there is a higher concentration of them there at any one time.
-The last point about binding self infected cells is slightly innacurate. If they can bind to the MHC 1 and the peptide being presented they will kill it but they are also responsible for killing transplanted tissue - They know the cell is non-self (or cancerous) because they cannot bind to the MHC 1 molecule, this means they kill the cell regardless of whether they can bind to the peptide fragment being presented.
-The proteins Tc cells secrete are called cytotoxic enzymes (one type you should know the name of is perforin but there are others)
-After binding their antigen they do not await activation, they travel throughout the immune system looking for a Th cell which has bound the same antigen (they release cytokines to find each other). When they have found each other they undergo contact dependant signalling - The B cell presents the antigen to the Th cell much the same way macrophages/dendritic cells do. This is what causes the Th cells to release cytokines.

Be careful about the level of detail you provide in your answers. Most of what you’ve said here is well and truly beyond the course. Your examiners simply won’t have the expertise (they’re VCE biology teachers) to mark your responses if they include content beyond the course.

To address the points I’ve highlighted:

-you’re safe to assume the contact happens at the lymph nodes. Naive b-cells and t-cells are confined to lymph nodes...the APCs contact T-cells there because they won’t be found anywhere else.
-fairly sure that your point about cancer cells is beyond the course. It’s also slightly inaccurate. It takes more than just the absence of an MHC molecule to trigger death of that cell.
-traditionally we’ve referred to granzymes and perforin in VCE. I’ve never seen them referred to as cytotoxic enzymes in VCE, though this is accurate
-this is not strictly speaking true. The details of this are beyond the course, so I’m only telling you because I suspect you’ll be interested, but B-cells that have been activated actually do actively look for T-cell help. They do so by migrating to the part of the lymph node where T-cells hang out, in the hope that they will encounter an activated TH Cell (which migrates towards the B-cells) that can provide he help.

This is naive B cells - They do not differentiate (they do a little but for VCE's sake they don't) until they have been activated by cytokines.

They divide a little bit, they don’t really differentiate. Differentiation is contingent on T-cell help. This is 2nd-3rd year uni stuff though so don’t worry about it

[ezferns]

Can someone pls explain to me Q 25 2007 E2?

[ezferns]

Details about evolution are new to the study design. No one can seem to agree but generally:
-Know the dates of major extinctions events
-Know the names of key species/classifications (eg. Tetrapods, that-species-that-became-mammals, etc)

Can you please tell us which specific ones to know? I have no idea what to memorise for this.

[vceme]

What's the difference between a promoter and a primer?
I've had a look on google but I still don't understand it unfortunately :/
Thank you in advance!!! 

[PhoenixxFire]

A promoter is a sequence of bases found on a chromosome, upstream of a coding region of a gene.
This is where RNA polymerase binds during transcription

A primer is a sequence of DNA that is complementary to the upstream region of a sequence that is being amplified.
During PCR the primer binds upstream of the sequence of DNA of interest, then taq polymerase extends the primer (making a new strand of DNA).

Also Ezferns i'm figuring that out myself (lots of scribbling on paper its hard to find all the information I want in one source) Once i've got it all organised i'll upload it for everyone (probably tomorrow).

[smamsmo22]

Just wondering; say a (new; no existing memory cells against it) virus enters the body and infects/enters cells. An adaptive immune response against infected cells will not occur unless the particular associated viral antigen has already been presented to  the specific naive T cell corresponding to the particular antigen/pathogen and activated it, producing specific cytotoxic t cells. Otherwise, no cytotoxic t cells would exist and there would be no means of destroying the infected cell with adaptive immune system mechanisms?
This isn't much of a question but I just wanted to check I had this around the right way in my head (: