VCE Biology Question Thread

[cosine]

Interesting question. Probably not relevant to the current study design but I'll try explain why it's B.

Firstly, what would happen if the sun sets at point P? Decreased sunlight would mean a decrease in photosynthesis, until the light dependent reactions cease to occur. Oxygen production would therefore decrease until it is no longer produced. I think cosine misinterpreted the graph - oxygen is still being produced between P and Q - it's just that the rate at which it increases is zero.

Option B makes sense. The rate of photosynthesis, as indicated by the production of oxygen, plateaus at a point where the rate of water transported by the xylem reaches a maximum. Because water is a necessary reactant of the light dependent stage, the rate at which it is transported towards the photosynthetic cells becomes a limiting factor.

You're right, my bad.
However, would what I said be true if oxygen production would cease?

[Biology24123]

Can someone please explain why chromosomal translocations don't affect the phenotype

[grannysmith]

Can someone please explain why chromosomal translocations don't affect the phenotype

Translocations, unlike deletions (generally), do not cause the loss of genes and their alleles - they're just moved to another chromosome and this doesn't prevent their expression.

[vox nihili]

Can someone please explain why chromosomal translocations don't affect the phenotype

grannysmith's answer explains perfectly when they don't, but I just want to highlight the fact that translocations do quite often affect the phenotype. If the breakpoints of the chromosome are in a gene, for instance, then this would disrupt the gene. However, this is actually relatively rare and most translocations don't affect phenotype.

[kimmytaaa]

Has anyone done or is having biology sac on mitosis/meiosis?

[StupidProdigy]

Can someone provide an example and basic rundown with a genetic cross. I understand that it requires a heterozygous recessive genotype to be crossed with another genotype, but I don't get why. Does it determine the genotype of the thing being crossed with the homozygous recessive? and if so, why is that? Sorry for the simplish question, but this inheritance stuff does my head in...Thankyou in advance

or a link to a site with a clear example is good too thanks

[cosine]

I have a few questions about the cell cycle:

1. What actually happens during the G0, G1 and G2 stages? I know the cell 'grows', but is that all we need to know for VCE?

2. During the S phase, DNA is replicated. I understand why there is a leading strand and why there is a lagging strand. I understand how DNA ligase only works from the 5'-3' direction, and that it works on both strands. However, does DNA Ligase also seal the phosphodiester bonds between nucleotides of the leading strand? Or is it just the lagging strand. Why?

3. When does chromatin condense into chromosomes? Is it just before DNA replication (S phase) or is it after the chromosomes replicate?

4. I get how we can only see sister chromatids attached by a centromere after DNA replication, but is the 'double stranded' chromosome just called chromosomes, or do we have to say the sister chromatids?

5. Do we need to know anything about animal-cell replication? If so, how much do we need to know and what is the major difference between the two? I know that plant cells all have cell walls, so obviously something about cytokinesis will be different

6. Is interphase part of mitosis? And what about cytokinesis?

7. What exactly happens during prophase? Is it sufficient enough to say that the nuclear membrane disintegrates (hence the nucleus also disappears), and two centrioles appear that will migrate to opposite poles of the cell? Do the spindle fibres form/grow during prophase?

8. During the metaphase of mitosis, I understand that the sister chromatids line up in the centre of the cell along the equator, is the process of the spindle fibres attaching to the centromeres part of metaphase?

Cheers, missed you all xD

[heids]

Can someone provide an example and basic rundown with a genetic cross. I understand that it requires a heterozygous recessive genotype to be crossed with another genotype, but I don't get why. Does it determine the genotype of the thing being crossed with the homozygous recessive? and if so, why is that? Sorry for the simplish question, but this inheritance stuff does my head in...Thankyou in advance

or a link to a site with a clear example is good too thanks

Test cross, you mean?  Let's imaginatively label the dominant allele as 'A' and recessive as 'a'.
Basically, you have one organism that shows the dominant phenotype, but you don't know its genotype.  It could be either heterozygous (Aa) or homozygous dominant (AA).  So, we cross it with a homozygous recessive (aa).

If the genotype was homozygous dominant:
        A          A
a       Aa       Aa

a       Aa        Aa
you'll get all heterozygous offspring, so ALL offspring will show the dominant phenotype.

Whereas, if it's heterozygous, 1/2 of the offspring will show the recessive phenotype (draw the Punnett square).

Essentially, if a cross between a dominant phenotype and recessive produce EVEN ONE offspring with a recessive phenotype, you know that the dominant parent was heterozygous.  (Note, even if all the offspring are consistently dominant, that doesn't actually prove the parent was homozygous dominant, because it could just be by chance that they had no recessive offspring.  However, the more offspring there are, the larger the probability that this is the case.)

[StupidProdigy]

Test cross, you mean?  Let's imaginatively label the dominant allele as 'A' and recessive as 'a'.
Basically, you have one organism that shows the dominant phenotype, but you don't know its genotype.  It could be either heterozygous (Aa) or homozygous dominant (AA).  So, we cross it with a homozygous recessive (aa).

If the genotype was homozygous dominant:
        A          A
a       Aa       Aa

a       Aa        Aa
you'll get all heterozygous offspring, so ALL offspring will show the dominant phenotype.

Whereas, if it's heterozygous, 1/2 of the offspring will show the recessive phenotype (draw the Punnett square).

Essentially, if a cross between a dominant phenotype and recessive produce EVEN ONE offspring with a recessive phenotype, you know that the dominant parent was heterozygous.  (Note, even if all the offspring are consistently dominant, that doesn't actually prove the parent was homozygous dominant, because it could just be by chance that they had no recessive offspring.  However, the more offspring there are, the larger the probability that this is the case.)

You deliver everytime bangali_lok, thankyou!! (and yes I did mean to say test cross...why'd I write genetic cross?  )

[heids]

I have a few questions about the cell cycle:

1. What actually happens during the G0, G1 and G2 stages? I know the cell 'grows', but is that all we need to know for VCE? Yeah, grows and produces enzymes needed for the S and M phase, don't need more.  NB, G0 phase is a 'resting' phase when the cell isn't dividing or even really preparing to divide.

2. During the S phase, DNA is replicated. I understand why there is a leading strand and why there is a lagging strand. I understand how DNA ligase only works from the 5'-3' direction, and that it works on both strands. However, does DNA Ligase also seal the phosphodiester bonds between nucleotides of the leading strand? Or is it just the lagging strand. Why? BEYOND THE COURSE (my typical answer when I'm clueless ), nah, someone else answer this pls.

3. When does chromatin condense into chromosomes? Is it just before DNA replication (S phase) or is it after the chromosomes replicate? During prophase, so after S.

4. I get how we can only see sister chromatids attached by a centromere after DNA replication, but is the 'double stranded' chromosome just called chromosomes, or do we have to say the sister chromatids? Don't know quite what you mean.  I think you just mean 'from the S phase to metaphase, can we refer to the chromosomes just as "chromosomes"?', if so, yes you can.

5. Do we need to know anything about animal-cell replication? If so, how much do we need to know and what is the major difference between the two? I know that plant cells all have cell walls, so obviously something about cytokinesis will be different Yeah, don't think you really have to know anything different except about the cell wall as you say.

6. Is interphase part of mitosis? And what about cytokinesis? Technically, no, mitosis is the division of the nucleus; like interphase and cytokinesis, it's just a part of cell division.  Interphase prepares for mitosis and cytokinesis, and cytokinesis (division of cytoplasm) occurs at the same time as the telophase stage of mitosis.

7. What exactly happens during prophase? Is it sufficient enough to say that the nuclear membrane disintegrates (hence the nucleus also disappears), and two centrioles appear that will migrate to opposite poles of the cell? Do the spindle fibres form/grow during prophase? Plus, the chromatin bundles up short and tight into compact chromosomes.  And yes, spindle fibres start forming in prophase.

8. During the metaphase of mitosis, I understand that the sister chromatids line up in the centre of the cell along the equator, is the process of the spindle fibres attaching to the centromeres part of metaphase? Yes.

Cheers, missed you all xD I can't believe how much I rely on AN stuff for English tutoring (and even Texts and Traditions, the stuff I wrote myself that I want to refer to but don't have any copies except on AN!)

You deliver everytime bangali_lok, thankyou!! (and yes I did mean to say test cross...why'd I write genetic cross?  )

Hope it was helpful and makes sense, let me know if not
P.S. Forgot that the test cross is also used in dihybrid crosses to tell the linkage of genes (whether they're linked or not, and if they are, how close they are to each other on the chromosome.

[StupidProdigy]

Is there a difference between dna profiling and pcr? Thankyou

[heids]

Is there a difference between dna profiling and pcr? Thankyou

Yes.  PCR = copying, DNA profiling = identification.

PCR is making lots and lots of copies of a segment of DNA (through the cycling heat-cool-heat process which involves separating the DNA strands, annealing primers, and copying the strand using taq polymerase).

DNA profiling is looking at people's genotypes at specific loci, like to identify if a suspect's DNA matches up with a sample found at the crime scene.  It gets certain hypervariable regions of the DNA where there are heaps of different possible alleles.  The sections are called short tandem repeats, or STRs, which are sections of DNA that have a short sequence of 3-5 bases repeated over and over again.  Pretty much, different number of repeats = different allele, like I might have 11 repeats and you might have 17, so you have a different allele.

So, we use the PCR reaction to get copies of a person's DNA at each STR locus (you have to pick specific primers with specific sequences to get copies of different loci in the DNA), then we send them through gel electrophoresis - the further the allele travels, the shorter it is, and thus the fewer the repeats it has.  So you can work out which allele each individual has at each STR locus, and once you have like 9 to 13 loci, you have their DNA profile which you can compare with other samples for whatever reason.

[cosine]

Hope it was helpful and makes sense, let me know if not
P.S. Forgot that the test cross is also used in dihybrid crosses to tell the linkage of genes (whether they're linked or not, and if they are, how close they are to each other on the chromosome.

Cheers bangali_lok, much appreciated.

I am having a bit of trouble with the big picture of meiosis, however. May you enlighten me, please? xD

[heids]

Cheers bangali_lok, much appreciated.

I am having a bit of trouble with the big picture of meiosis, however. May you enlighten me, please? xD

Will do tomorrow morning.

[cosine]

Few questions about meiosis:

1. To produce sperm cells or egg cells, they are produced from a cell that is a diploid cell. I know this diploid cell is a germ cell, but is it a somatic cell?

2. When you define diploid cells, would you say that only body cells are diploid?

3. When does DNA replication occur during meiosis? Is it in meiosis 1 or 2?

4. Are there X and Y or X and X chromosomes in ALL of our cells? Somatic AND gametes?

5. How much do we need to know about crossing over?

6. Is there DNA replication during Meiosis 2?

7. When the gametes are produced, they only have 23 chromosomes, right? If so, what happens to the sperm/egg cells now?