VCE Biology Question Thread

[HighTide]

A particular protein has 100 amino acids.
a. How many codons would there be in the mRNA strand that codes for this?
I wrote 102, 100 for the 100 amino acids, 1 for the start codon and 1 for the stop codon. But the answer is 100?

In technical terms its one codon per amino acid.

[HasibA]

A particular protein has 100 amino acids.
a. How many codons would there be in the mRNA strand that codes for this?
I wrote 102, 100 for the 100 amino acids, 1 for the start codon and 1 for the stop codon. But the answer is 100?

idk if im misleading you but a similar question was on the 2015 bio exam and i think they didnt count the start and stop codons? like i said, im not sure, so take this info with a grain of salt. Question referred to was something 'how codons/amino acids long is ..' or something like that

[blacksanta62]

Hey guys, I was fine with meiosis until I asked my teacher a question and now I'm confused. I'll try to ask my question(s) in a way that you guys can understand because it seemed to have confused my teacher. Basically, how many chromosomes do we start with in meiosis? Do we start with 23 chromosomes, a haploid cell which will duplicate it's DNA (but this won't be enough to give us 4 haploid cells at the end) or a diploid cell (where we have 23 chromosomes from the mother and 23 from the father) which will then duplicate and we end up with 92 chromosomes which will be enough for 2 genetically different daughter cells and then 4 genetically different haploid cells? What are the cells which undergo meiosis called (germline???)? In meiosis, are the parents chromosomes already together or is it purely the fathers chromosomes which will duplicate and undergo meiosis and crossover and whatnot (same for the mother)? I thought that the parents chromosomes were already together and they crossed over with each other etc but apparently that's not the case.

If someone could answer these questions which have me bogged down and help me better understand meiosis I would be so grateful. Thank you

[solution]

What are the cells which undergo meiosis called (germline???)?

Yes, germline cells.

In meiosis, are the parents chromosomes already together or is it purely the fathers chromosomes which will duplicate and undergo meiosis and crossover and whatnot (same for the mother)? I thought that the parents chromosomes were already together and they crossed over with each other etc but apparently that's not the case.

Crossing over occurs in prophase I. Not sure what else you wrote though.

a diploid cell (where we have 23 chromosomes from the mother and 23 from the father) which will then duplicate and we end up with 92 chromosomes

During DNA replication (before meiosis begins), the DNA replicates, but the chromosome number does not change. 46 single stranded chromosomes become 46 double stranded chromosomes.

[blacksanta62]

1) Ok
2) Crossing over and recombination occurs at prophase 1 while at anaphase 1 the homologous pairs are pulled to opposite poles of the cells because of the retracting spindle fibres
3) Sorry, I don't know how to word the questions any better than that
4)Does that mean we have 92 chromatids but 46 pairs? This last point has me a bit confused 

Thanks
Edit: homologous pairs

[solution]

1) Ok
2) Crossing over and recombination occurs at prophase 1 while at anaphase 1 the homologous pairs are pulled to opposite poles of the cells because of the retracting spindle fibres
3) Sorry, I don't know how to word the questions any better than that
4)Does that mean we have 92 chromatids but 46 pairs? This last point has me a bit confused 

Thanks
Edit: homologous pairs

Oh yes, I don't know why I wrote anaphase :\

No, you have 23 "pairs" (well, 22 autosome pairs and 2 sex chromosomes). Before DNA replication, there are 23 pairs of single stranded. After DNA replication, each chromosome becomes double stranded (before replication they look like |, but after replication they look like the familar "X" structure). The number of chromosomes or pairs does not change. Yes, after replication there will be 92 chromotids (2 for each replicated chromosome).

[vox nihili]

Hey guys, I was fine with meiosis until I asked my teacher a question and now I'm confused. I'll try to ask my question(s) in a way that you guys can understand because it seemed to have confused my teacher. Basically, how many chromosomes do we start with in meiosis? Do we start with 23 chromosomes, a haploid cell which will duplicate it's DNA (but this won't be enough to give us 4 haploid cells at the end) or a diploid cell (where we have 23 chromosomes from the mother and 23 from the father) which will then duplicate and we end up with 92 chromosomes which will be enough for 2 genetically different daughter cells and then 4 genetically different haploid cells? What are the cells which undergo meiosis called (germline???)? In meiosis, are the parents chromosomes already together or is it purely the fathers chromosomes which will duplicate and undergo meiosis and crossover and whatnot (same for the mother)? I thought that the parents chromosomes were already together and they crossed over with each other etc but apparently that's not the case.

If someone could answer these questions which have me bogged down and help me better understand meiosis I would be so grateful. Thank you

We start off with 46 chromosomes at the start of meiosis, though each of these chromosomes has two chromatids. A chromosome with two chromatids can be pulled apart to make two separate chromosome, because each chromatid is basically just one molecule of DNA (we count chromosomes by counting centromeres).
So basically what happens is that during the S phase of the cell cycle, each chromosome is duplicated, such that it has two chromatids (google this if you don't understand; a picture will help!). Then the two chromatid chromosomes enter meiosis. During the first stage of meiosis, the chromosomes are independently assorted, giving rise to two haploid daughter cells. Then those haploid daughter cells undergo another round of division; this time, where the chromosomes are broken in half, liberating each of the chromatids (just like mitosis).

Cells that produce gametes are actually called gametocytes. Cells of the germ line include the progeny of the gametocytes (i.e. sperm and eggs).



As far as the the molecular biology of meiosis is concerned, there is no difference between those chromosomes derived from the mother and those from the father. In fact, even once they're in the zygote, where those chromosomes came from (i.e. mother or father) hardly matters

[solution]

When drawing Punnett squares, do we need to label each gamete with 1/2 (monohybrid cross)? What about about labelling each genotype as 1/4?

When a question asks for genotypic ratio, and there is only one option. what do we write? For example, if a cross results in all Ww, would the ratio be: 1 Ww?

[Yacoubb]

When drawing Punnett squares, do we need to label each gamete with 1/2 (monohybrid cross)? What about about labelling each genotype as 1/4?

When a question asks for genotypic ratio, and there is only one option. what do we write? For example, if a cross results in all Ww, would the ratio be: 1 Ww?

You don't have to label proportions on the punnet square unless it specifically tells you to do so.

In that situation, just write 100% Ww. Typically, though, you wouldn't be asked to show the ratio if you're only dealing with one genotype.

[plsbegentle]

Can someone explain to me about sticky and blunt ends?

[HighTide]

Can someone explain to me about sticky and blunt ends?

Different restriction enzymes cut DNA into sticky or blunt ends. In sticky ends,  a part stands out and the two pieces kinda tesselate. In blunt, it's just like two rectangular pieces being cut.
Sticky ends are more advantageous for a variety of reasons like cost and etc. Mainly though, they ligate more easily since there's two overhanging areas in the cut pieces of DNA, so they can fit together and form hydrogen bonds in complementary base pairing.

[Mapleflame]

Hi there,
I am currently enrolled in Thushan's tutorial video thing, and I was just wondering what time he does the livestreams?
Thanks,
~A.H

[blacksanta62]

Hey guys, still a little unclear on linked genes. So if genes are linked we get an unequal amount of variation in the gametes and if they assort independently we get an equal amount of variation in the gametes because the line up at the equator of the cell at metaphase I (and II) independently, right?

If someone could correct anything I've misunderstood I would really appreciate it since this is still a misconception for me.

Thanks
Edit: Or add anything I've missed

[plsbegentle]

Regarding gel electrophoresis, what does standard and brand mean?
Thanks

[Sine]

Regarding gel electrophoresis, what does standard and brand mean?
Thanks

Not sure what brand means?

Standard is just DNA strands of known length for comparison with experimental results.