VCE Biology Question Thread

[Biology24123]

Although B cells have specific antibodies on their surfaces, which will bind to the specific antigens presented on the pathogen, this does not yet activate the B cell. The B cell must first engulf the antibody-antigen complex. Once engulfed, the B cells degrades the pathogen into fragments and presents antigenic fragments on it's surface via MHC Class II proteins. These proteins sit on the membrane until a specific T Helper Cell will come along, and form a MHC II - TCR complex, that is, when the T helper cell binds with the B cell's MHC II marker that holds the antigen fragment, the helper cell releases chemicals which stimulate/activate the B cell to proliferate into plasma B cells and memory B cells.

Yes, but the T helper cell must also be activated

[cosine]

Can DNA-DNA hybridisation be done with fossils? In other words, do fossils still have DNA in them?

[Bruzzix]

Can DNA-DNA hybridisation be done with fossils? In other words, do fossils still have DNA in them?

In short, it depends on how old the fossils are. DNA degrades over time so the younger the DNA is, the higher quality it will be. If you obtain a very old specimen such that the nuclear DNA is of such a low quality that it's not very useful then mitochondrial DNA (mtDNA) will be used instead. It is used instead because there are many copies of mtDNA in a cell whereas there are only 1-2 copies of nuclear DNA present so there is a higher chance of getting better quality DNA from the mtDNA. Scientists then analyse this DNA to determine the organisms ancestry.

[cosine]

So does DNA-DNA hybridisation (and mtDNA in that case) only compare the relatedness of two fossils/organisms or can this set a date to fossils too, for example like how old they are?

Also, how is mtDNA different between members of a species? I know that mtDNA is a single circular chromosome, but how does it work in terms of evolution?

[BakedDwarf]

Also, how is mtDNA different between members of a species? I know that mtDNA is a single circular chromosome, but how does it work in terms of evolution?

mtDNA undergoes mutation over time and, as a result, populations that have been separated for long periods accumulate different mutations in their mtDNA. Basically, the more mutations a population has accumulated over time, the longer they have been separated and so the more unrelated they are.

This can be seen in the South African population of humans. They have the most mtDNA mutations than any other human population which suggests a long evolutionary history. This is why the out-of-africa hypothesis for the origin of humans is suggested to be correct

[Bruzzix]

I said C but the solutions say D (like always no explanation is given -_-)
I analysed the pedigree and it could be both autosomal recessive and X-linked recessive... Or have I made a mistake?

[Biology24123]

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I said C but the solutions say D (like always no explanation is given -_-)
I analysed the pedigree and it could be both autosomal recessive and X-linked recessive... Or have I made a mistake?

Only males are affected, it's unlikely that only males would be affected by an autosomal trait

[Bruzzix]

Only males are affected, it's unlikely that only males would be affected by an autosomal trait

Ohhh ty. It's kind of annoying though how they give two options that are both technically correct

[Biology24123]

Do we need to know the difference between animal and plant hormones?
Do we need to know specific examples of homeostatic systems that aren't included in the question
The difference of function between Grana and Thylakoids?
Are T helper cells involved in the allergic response?

[Bruzzix]

I said D but the answer is B? I thought independent assortment could not occur in mitosis?

[heids]

Ohhh ty. It's kind of annoying though how they give two options that are both technically correct

Yeah, that shouldn't ever happen in a VCAA exam.  You're totally right, stupid question.  Though it did say the 'best' rather than 'only' answer... and bio123124's answer is right in showing it as the 'best' answer.

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I said D but the answer is B? I thought independent assortment could not occur in mitosis?

A, B and D are definitely wrong.  Assuming I haven't misunderstood their poor phrasing, D can't work because homologous pairs are disjoined in meiosis, so daughter cells could be TE, Te, tE or te (i.e. one allele each from 7 and 9), not Tt or Ee (i.e. 7's alleles go into one daughter cell and 9's into another).  As you say, independent assortment doesn't happen in mitosis.

I think C is right, though; because when one of the two daughter cells from the first division goes through the second division, identical sister chromatids are pulled apart like in mitosis, and the two daughter cells of this division should be identical.  I think.

[cosine]

Yeah, that shouldn't ever happen in a VCAA exam.  You're totally right, stupid question.  Though it did say the 'best' rather than 'only' answer... and bio123124's answer is right in showing it as the 'best' answer.

A, B and D are definitely wrong.  Assuming I haven't misunderstood their poor phrasing, D can't work because homologous pairs are disjoined in meiosis, so daughter cells could be TE, Te, tE or te (i.e. one allele each from 7 and 9), not Tt or Ee (i.e. 7's alleles go into one daughter cell and 9's into another).  As you say, independent assortment doesn't happen in mitosis.

I think C is right, though; because when one of the two daughter cells from the first division goes through the second division, identical sister chromatids are pulled apart like in mitosis, and the two daughter cells of this division should be identical.  I think.

Why can't independent assortment occur in mitosis?

[heids]

Why can't independent assortment occur in mitosis?

Independent assortment is about which member of each homologous pair goes into which daughter cell - that it's random and the outcome for one chromosome doesn't impact on the outcome for another chromosome.  Since mitosis doesn't involve the disjunction of homologous chromosomes (both members of each pair go into each daughter cell), independent assortment isn't relevant.

Poorly explained as I find this a tricky concept to put into words.  All it means is that it's totally random which chromosome in each pair goes into which cell, and that's nothing to do with mitosis as all daughter cells are identical.

Edit: lol, I still find myself posting here despite trying to stop.  I compulsively answer questions even when I don't trust my correctness.

[cosine]

Independent assortment is about which member of each homologous pair goes into which daughter cell - that it's random and the outcome for one chromosome doesn't impact on the outcome for another chromosome.  Since mitosis doesn't involve the disjunction of homologous chromosomes (both members of each pair go into each daughter cell), independent assortment isn't relevant.

Poorly explained as I find this a tricky concept to put into words.  All it means is that it's totally random which chromosome in each pair goes into which cell, and that's nothing to do with mitosis as all daughter cells are identical.

Edit: lol, I still find myself posting here despite trying to stop.  I compulsively answer questions even when I don't trust my correctness.

Thanks bangali_lok

Is it as simple as that in mitosis, each daughter cell will have the identical genome as the parent cell, whereas the gametes produced via meiosis will have differing chromosomes because the alignment of these homologous chromosomes in meiosis 1 will end up with 2 cells with varying chromosomes, and then during meiosis 2, the chromosomes are separated?

For example if we have the genotype: TtBb

In meiosis, the homologous chromosomes split up and it does not matter whether the T and B end up in one cell, or the T and b in one, it is random, that is, their assortment into gametes is not dependent on other chromosomes?

[heids]

Thanks bangali_lok

Is it as simple as that in mitosis, each daughter cell will have the identical genome as the parent cell, whereas the gametes produced via meiosis will have differing chromosomes because the alignment of these homologous chromosomes in meiosis 1 will end up with 2 cells with varying chromosomes, and then during meiosis 2, the chromosomes are separated?

For example if we have the genotype: TtBb

In meiosis, the homologous chromosomes split up and it does not matter whether the T and B end up in one cell, or the T and b in one, it is random, that is, their assortment into gametes is not dependent on other chromosomes?

Yes, totally.  Imagine you're producing gametes through meiosis - you have two (non-identical) copies of each chromosome, one from your mum and one from your dad - and you'll pass down one of these chromosomes.  So one of your gametes could look like this:
chromosome 1 - your mum's
2 - dad's
3 - dad's
4 - dad's
5 - mum's

and so on; or it could be 1 - mum's, 2 - mum's, 3 - dad's, 4 - mum's, 5 - mum's and so on.  There are millions of possible combinations of chromosomes, and once you add crossing over...

But in mitosis, daughter cells are identical and this doesn't happen as homologous chromosomes don't disjoin, rather, all cells will have both your mum's and your dad's chromosomes.