VCE Biology Question Thread

[paper-back]

Do cytotoxic T cells have antibodies present on their surface? or do they just recognise abnormalities in the MHC 1 markers of infected cells through their own MHC 1 marker proteins?

[lach3087]

Hey was just need some help with this question. Explain how two organisms with the same genotype can have different phenotypes. Thanks.

[paper-back]

Hey was just need some help with this question. Explain how two organisms with the same genotype can have different phenotypes. Thanks.

Phenotype is subject to both one's genotype AND their environment, hence two organisms may have the same genotype but exhibit very different phenotypes
E.g. Identical twins, though having equivalent genotypes, may have distinguishing physical features due to the different lifestyles either one has adopted

[cosine]

Can someone delight me and explain the concept of alleles? Are each chromosome number 1 in every human code for the same proteins?

[warya]

Can someone explain DNA probes and DNA sequencing for my sac tomorrow 

[cosine]

Can someone explain DNA probes and DNA sequencing for my sac tomorrow 

A DNA probe is typically a single stranded DNA molecule of a couple bases, which binds to another single stranded DNA molecule with complementary bases, and then this double stranded molecule of DNA will carry out it's general function. However, the DNA probe is marked/flagged with a marker, so it can be seen by genetic engineers, i.e it can be exerted into a dye and hence display a glow etc.. Because this is so, we can see where the probe will end up, henceforth it allows genetic engineers to identify which cell/tissues are responsible for the production of a certain protein or whatever DNA is used for..

[lach3087]

My class is completing a SAC at GTAC this Monday and I was just wondering if anyone could give me some insight into what takes place there, as my teacher hasnt been to clear. Thanks

[BakedDwarf]

Hey guys, what exactly do we need to know about gene regulation? The study design states: "the concept of gene regulation (the switching on and off of genes by factors expressed by regulator genes and environmental factors)"

The information below is all I honestly know about gene regulation; if I got anything wrong, or if I'm missing info, please tell me.

Gene regulation is the regulation of the extent to which a gene is expressed in a cell; genes may be switched on/off:

- depending upon its function and the specialised function of the cell (we don’t want to produce
 enzymes where or when the substrate is not present)

- to conserve energy as protein synthesis (gene expression) requires energy

- for the differentiation of cells with specialised functions

Regulator gene is a gene that switches structural genes ‘on’ and ‘off’ by coding for a repressor protein that may bind to the operator and hence block transcription; the operator is a region of DNA situated around a promoter that interacts with a specific repressor

[Biology24123]

Hi guys. How long does it take to see improvement in practice exams

[heids]

Can someone delight me and explain the concept of alleles? Are each chromosome number 1 in every human code for the same proteins?

Yes, all chromosome 1s in all humans (except those with mutations) will have the same genes that code for the same proteins in the same loci.  However, different humans may have different variations/forms/DNA sequences of each gene (different alleles).

Can someone explain DNA probes and DNA sequencing for my sac tomorrow 

Copying straight from my notes

DNA sequencing identifies the exact order of bases in a DNA molecule. 
It uses a single-stranded DNA template to make lots and lots of copies of the DNA, working like PCR except with an extra step, termination.
You could memorise the 4 main steps with the acronym SEAT:
•   Separate DNA strands
•   Anneal primers
•   Extend primers
•   Terminate strand
How is the strand terminated?  You add dideoxynucleotides to the mixture.  They’re DNA nucleotides missing a hydroxyl group on their 3’ end, so they can’t bond with the next nucleotide.  Each of the 4 different dideoxynucleotides is labelled with a different dye.  Occasionally, the taq polymerase builds a dideoxynucleotide into the growing strand, stopping the strand.  You organise how soon the dideoxynucleotide is added by carefully controlling the concentration of dideoxynucleotides to normal nucleotides.

The aim of this is to get lots of copies of the DNA strand, each copy one nucleotide longer than the last.  So, the first one with a primer plus one nucleotide, the second a primer plus 2 nucleotides, the third a primer plus 3 nucleotides, and so on.  And the last nucleotide in each chain is always a dideoxynucleotide with a dye marker attached.

Then, you send your copies through gel electrophoresis which orders them by length; in order, they pass a laser which records the dye colour attached to each copy and thus records which base is the last base in the strand.

So, the first scanned strand is the primer + 1 nucleotide, and that one nucleotide is recorded.  The second is the primer + 2 nucleotides, and the second of those nucleotides is recorded.  And so on, until the sequence of the whole segment is recorded!
So, with DNA sequencing, you use PCR with termination by dye-labelled dideoxynucleotides to produce lots of DNA strands each one longer than the last.  You then sort them by gel electrophoresis, and by detecting the dye colour in order, you can tell the whole sequence!

@BakedDwarf - too busy to answer, will do so later if someone else hasn't.

@Bio12415125331256 - can take a while, it depends on how much time you spend revising and learning the content in between!  Don't despair, but remember you MUST learn the theory before you can improve.

[cosine]

Yes, all chromosome 1s in all humans (except those with mutations) will have the same genes that code for the same proteins in the same loci.  However, different humans may have different variations/forms/DNA sequences of each gene (different alleles).

Copying straight from my notes

DNA sequencing identifies the exact order of bases in a DNA molecule. 
It uses a single-stranded DNA template to make lots and lots of copies of the DNA, working like PCR except with an extra step, termination.
You could memorise the 4 main steps with the acronym SEAT:
•   Separate DNA strands
•   Anneal primers
•   Extend primers
•   Terminate strand
How is the strand terminated?  You add dideoxynucleotides to the mixture.  They’re DNA nucleotides missing a hydroxyl group on their 3’ end, so they can’t bond with the next nucleotide.  Each of the 4 different dideoxynucleotides is labelled with a different dye.  Occasionally, the taq polymerase builds a dideoxynucleotide into the growing strand, stopping the strand.  You organise how soon the dideoxynucleotide is added by carefully controlling the concentration of dideoxynucleotides to normal nucleotides.

The aim of this is to get lots of copies of the DNA strand, each copy one nucleotide longer than the last.  So, the first one with a primer plus one nucleotide, the second a primer plus 2 nucleotides, the third a primer plus 3 nucleotides, and so on.  And the last nucleotide in each chain is always a dideoxynucleotide with a dye marker attached.

Then, you send your copies through gel electrophoresis which orders them by length; in order, they pass a laser which records the dye colour attached to each copy and thus records which base is the last base in the strand.

So, the first scanned strand is the primer + 1 nucleotide, and that one nucleotide is recorded.  The second is the primer + 2 nucleotides, and the second of those nucleotides is recorded.  And so on, until the sequence of the whole segment is recorded!
So, with DNA sequencing, you use PCR with termination by dye-labelled dideoxynucleotides to produce lots of DNA strands each one longer than the last.  You then sort them by gel electrophoresis, and by detecting the dye colour in order, you can tell the whole sequence!

@BakedDwarf - too busy to answer, will do so later if someone else hasn't.

@Bio12415125331256 - can take a while, it depends on how much time you spend revising and learning the content in between!  Don't despair, but remember you MUST learn the theory before you can improve.

Sorry but that makes absolutely no sense to me

[warya]

Is an intron the same as the non-coding section of a gene/DNA molecule?

[heids]

Sorry but that makes absolutely no sense to me

OK.  So, at a certain place on chromosome 1, ALL of us have the same gene (hypothetically, the gene that decides whether we have pink or blue hair, because it codes for pigment proteins).  All of us have this hair colour gene on chromosome 1, that decides our hair colour, but we don't necessarily have exactly the same DNA sequence at this gene; some of us have the sequence that codes for a blue-pigment protein, while some of us have the sequence that codes for a pink-pigment protein.  So, some of us have a blue allele and some a pink allele, because we have different VARIATIONS/forms of the gene/trait.

Then, we all have two copies of the chromosome - if we have two blues, we have blue hair, two pinks, we have pink hair, (homozygous), and one of each?  Well obviously blue is the better colour , so blue must be dominant, and the heterozygote will have blue hair.

Naturally it doesn't actually work like this, but hope dat makes some sense...

Is an intron the same as the non-coding section of a gene/DNA molecule?

Yep, because it gets cut out of the mRNA sequence, it's never translated, so it doesn't code for amino acids, so it's a non-coding section

[HighTide]

Do cytotoxic T cells have antibodies present on their surface? or do they just recognise abnormalities in the MHC 1 markers of infected cells through their own MHC 1 marker proteins?

Cytotoxic T cells have no antibodies present on surface. Only B cells have antibodies present on surface. The antibodies are basically receptors to which the cell can attach to a specific antigenic epitope and act against it. On T cells, these are called T cell receptors.
So in summary: B cells: antibodies, T cells: T cell receptors.
And no, they don't recognize abnormalities. When an antigen is ingested by a cell, it is digested and its epitope is presented on the MHC markers. When the specific cytotoxic cell locates a complementary epitope presented on the MHC markers, they will secrete perforins which lyse the membrane, thus killing the infected cell. This is very different from B cells which just use antibodies.

Is an intron the same as the non-coding section of a gene/DNA molecule?

bangali_lok was right in stating that they are non-coding segments of DNA and that its not translated. It may look as though they serve no purpose, however, introns are used to create functional RNA molecules which don't code, but play an important role in the regulation of genes..So in other words they're not 100% useless.

[StupidProdigy]

Is there any type of variation that can occur in meiosis 2? Independent assortment and crossing both happen in meiosis 1 yeah? Thanks