VCE Biology Question Thread

[cosine]

For this question, i just want to grasp a conceptual understanding of it.

It says that the blood samples were taken from the victim in which the blood had been deposited on the victim during the crime scene. DNA was extracted from white blood cells in each of the blood samples and electrophoresis of the samples was carried out.

You know how in each well there are multiple fragments, are these fragments meant to be of the same gene locus? For example, you see well 2 where it is the crime scene samples, those fragments correspond mostly to the suspect 2, but when the geneticists took the blood sample from the suspect 2, would they have taken the DNA sample from the same gene loci that they took from the crime scene sample to see whether or not the suspect would produce similar sized fragments of the same gene? Because the restriction enzyme that cuts the crime scene suspects samples would also cut the DNA of the actual suspect with the same number of fragments if it were the same loci? Also if this is the case too, do the other suspects have different sized lengths because they all have different alleles for the gene loci?

This whole theory could be incorrect because it is just my understanding of it, if it is incorrect, mind enlightening me with not the process of electrophoresis, but what samples are used from suspects? Because a question earlier on in the year asked what geneticists would do further to ensure that the suspect was indeed present during the crime scene, and the answer said that they should test another gene loci and see if the sample at the crime scene would match with the suspect's gene loci fragments. Any ideas?

Many thanks in advance.

[cosine]

For this question, it says which enzyme(s) would cut this piece of DNA, but all three of EcoRI, AluI and HindIII have recognition sites for the DNA molecule, so how can C be the correct answer if those two enzymes are not the only ones that can cut the DNA ?

[Biology24123]

Can someone explain to me why the answer is not B, C or D?

Here is my thoughts:
B. Mast cells are leucocytes, and we know that blood cells are produced in the bone marrow - mast cells are not lymphocytes, so they're not involved in the lymphatic system?
C. Vessels have thick, muscular walls, unsure about this one, I just know that lymph vessels only flow fluids one way
D. Lymph is pumped by the heart, I am pretty sure lymph is pumped through muscular contractions?

You explained all of them correctly

[Biology24123]

For this question, it says which enzyme(s) would cut this piece of DNA, but all three of EcoRI, AluI and HindIII have recognition sites for the DNA molecule, so how can C be the correct answer if those two enzymes are not the only ones that can cut the DNA ?

Don't think you attached the whole question

[LoadedWithPotatoes]

For this question, it says which enzyme(s) would cut this piece of DNA, but all three of EcoRI, AluI and HindIII have recognition sites for the DNA molecule, so how can C be the correct answer if those two enzymes are not the only ones that can cut the DNA ?

It says 'read in the 5' to 3' direction', not the 3' to 5' direction. There is no restriction sequence in which the ECoRI can cut if you read the sequence of DNA in the 5' to 3' direction.

[cosine]

It says 'read in the 5' to 3' direction', not the 3' to 5' direction. There is no restriction sequence in which the ECoRI can cut if you read the sequence of DNA in the 5' to 3' direction.

Ah, i see. So is this a rule with restriction enzymes that they only recognise their recognition sites from the 5'-3' direction?

[grindr]

The second question.

I thought one chromatid =  one molecule of DNA
and that one single stranded chromosome is made of one chromatid?
So two chromosomes comprising of two chromatids would equal 2 DNA molecules?

The answer is 4.

[cosine]

The second question.

I thought one chromatid =  one molecule of DNA
and that one single stranded chromosome is made of one chromatid?
So two chromosomes comprising of two chromatids would equal 2 DNA molecules?

The answer is 4.

The fact that chromosomes are only visible under conditions of mitosis, and when they have condensed from chromatin into chromosomes during prophase of mitosis, that means they have already all undergone the DNA replication process, so they are all present as sister chromatids. I know it's hard to see, but usually in karyoptypes the chromosomes are present as homologous pairs, with their corresponding sister chromatid, too. So because pair 1 has two chromosomes, and each of them has 1 sister chromatid, there are a total of 4 chromatids, and hence 4 DNA molecules too.

[cosine]

Homologous pairs are not shown in karyotypes as the alleles of each chromosome have not been replicated. Each chromosome is made of 2 chromatids = 2 DNA molecules

What do you mean?
Look at the above karyotype, the chromosomes are present as homologous pairs, and each chromosome has a sister chromatid, hence the pair 1 has 4 chromatids, hence 4 DNA molecules in total?

[Biology24123]

What do you mean?
Look at the above karyotype, the chromosomes are present as homologous pairs, and each chromosome has a sister chromatid, hence the pair 1 has 4 chromatids, hence 4 DNA molecules in total?

Oh ok. I didn't see that image

[grindr]

What do you mean?
Look at the above karyotype, the chromosomes are present as homologous pairs, and each chromosome has a sister chromatid, hence the pair 1 has 4 chromatids, hence 4 DNA molecules in total?

OHH Gottit!! Didn't see that they were double stranded chromosomes. Thanks Cosine 

[THEBEAST]

So this circumstance only occurs when :
- Rh -ve mother
- Rh +ve child
Remember Rh antigens are proteins on the surface of RBC.
So once the mother gives birth the child's blood may pass through the placenta and into her body. Because she is Rh negative her immune cells will detect it as foreign, leading to the production of antibodies and what is most important here - memory B cells.
The real problem , however, occurs at the second child (given that he/she too is Rh +ve)
During pregnancy some cells will pass through to the mother. Antibodies are produced rapidly and greatly. They will attack the childs RBC leading to damage and causing haemolytic disease ,where child is yellowy because RBC lyse and haemoglobin is released.
Now at each subsequent pregnancy response will be stronger and greater, due to the presence of memory B cells. 
How can we prevent this? At the first child birth the  mother is injected with antibodies for the Rh factor (this is passive artificial immunity) so that she doesn't produce her own , and no memory cells are produced. This is repeated at each child birth .

Thank you i understand it much better now!

[cosine]

Can someone explain to me the concept of the Out of Africa Theory?

[THEBEAST]

Can someone explain to me the concept of the Out of Africa Theory?

The Hypothesis that Homo sapiens first evolved in Africa and then migrated out replacing other populations they came in contact with.
Evidence
- Greatest mtDNA variation exist in African populations
- Oldest and transitional hominin fossils are only found in Africa

[THEBEAST]

How does a "population bottleneck" contribute to a lack of variation if the numbers subsequently increase?
I don't know how to put this in a 2 mark answer that VCAA will accept