VCE Biology Question Thread

[vox nihili]

Thanks grannysmith!

1. Would we lose marks if we called chromatin chromosomes? For example, the S phase's DNA structure is chromatin, but if I say that during the S phase each chromosome is replicated to produce sister chromatids, would this be incorrect?

2. Is there a checkpoint between the S phase and G2 phase?

3. Describe a difference between meiosis and mitosis: Mitosis results in two identical diploid daughter cells, whereas meiosis results in four genetically varying haploid daughter cells. Is this correct guys?

Many thanks

1. I think you're confusing chromatin and chromosomes. Chromosomes describes the discrete chunks of DNA and protein that get reproduced in S-phase and get divided in mitosis/meiosis. Chromatin is the substance—i.e. the mixture of DNA and proteins (incl. histones)—of which chromosomes are made. The relationship between chromatin and chromosomes is exactly the same as the relationship between phospholipids and membranes.

2. Yes.

3. No, each gamete isn't genetically distinct at the end of meiosis. There are two distinct sets of genetically identical gametes.

[cosine]

1. I think you're confusing chromatin and chromosomes. Chromosomes describes the discrete chunks of DNA and protein that get reproduced in S-phase and get divided in mitosis/meiosis. Chromatin is the substance—i.e. the mixture of DNA and proteins (incl. histones)—of which chromosomes are made. The relationship between chromatin and chromosomes is exactly the same as the relationship between phospholipids and membranes. But can you answer the question? Would it be correct or not?

2. Yes. So is there a checkpoint between G1 and S phase, S and G2 phase, and G2 and M phase? My book doesnt have one between S and G2

3. No, each gamete isn't genetically distinct at the end of meiosis. There are two distinct sets of genetically identical gametes. Can you please elaborate on this one?

Cheers

[vox nihili]

Thanks grannysmith!

1. Would we lose marks if we called chromatin chromosomes? For example, the S phase's DNA structure is chromatin, but if I say that during the S phase each chromosome is replicated to produce sister chromatids, would this be incorrect?

2. Is there a checkpoint between the S phase and G2 phase?

3. Describe a difference between meiosis and mitosis: Mitosis results in two identical diploid daughter cells, whereas meiosis results in four genetically varying haploid daughter cells. Is this correct guys?

Many thanks

1. Yes, you would lose a mark if you called chromatin chromosomes, as they are not the same thing. Much in the same way you'd lose a mark if you called a horse "cells". Cells do indeed make up a horse, just in the same way chromatin makes up a chromosome. If you did say that each chromosome is replicated in S-phase you'd be correct. The chromosomes are replicated, the chromatin is replicated. Chromosomes are replicated because you're building new chromatin.

2. There's one in G1 and there's one in G2. I believe this is the way that the VCE course approaches it.

3. Meiosis I involves the separation of homologous chromosomes, giving rise to two genetically distinct daughter cells. Each of these daughter cells is haploid, with each chromosome containing two chromatids. In meiosis II, the haploid daughter cells from meiosis I divide. This division involves the separation of sister chromatids, not homologous chromosomes; therefore, as in mitosis, the daughter cells from meiosis II are genetically identical to their progenitor. Therefore at the end of meiosis you have two distinct sets of identical gametes.

[kimmytaaa]

hey guys
can someone please explain about meiosis?
the function of it

[cosine]

hey guys
can someone please explain about meiosis?
the function of it

Meiosis is composed of two stages known and meiosis 1 and meiosis 2. Just to make the explanation easier, I will refer to meiosis in humans. Basically meiosis occurs in germ line cells, otherwise known as sex cells. These sex cells are diploid, so they contain 46 chromosomes or in other words 22 homologous pairs (the other two are X or Y chromosomes). These homologous chromosomes come from our parents, half from the father and half from the mother. Meiosis 1 involves the germ line cell in only one of your parents, this cell will undergo DNA replication before hand and so the 46 chromosomes will all have sister chromatids that are held together by a centromere.



So as you can see in the image above, there are 46 chromosomes, but look at chromosome number 1, there are two of those, the one on the left came from the father and the one on the right came from the mother. These chromosomes are both the same, hence they are called homologous chromosomes. Before meiosis occurs, every single one of these chromosomes replicates and hence they appear as sister chromatids.




Meiosis 1:
Prophase 1: The nuclear membrane disintegrates, centrosomes appear and the spindle fibre grows and chromatin condenses into chromosomes.
Metaphase 1: The homologous chromosomes all line up in the centre of the cell, along the equator. The spindle fibres attach to each homologous chromosome.
Anaphase 1: The homologous chromosomes are separated from each other, each to an opposite pole of the cell as the spindle fibres pull them apart. (for example, looking at chromosome number 1 above, there are two and because they are the same, what is the point of having them in the same new cell? Exactly, there is no point. So this helps me remember that in anaphase 1, the homologous chromosomes are separated from each other, because the cell will not benefit if it contains both of them, so one chromosome goes into cell 1 and the other goes into cell 2).
Telophase 1: Two new nuclei form around the set of chromosomes and the spindle fibres break down. The chromosomes turn into chromatin
Cytokinesis 1: The cleavage furrow moves inwards forming a region of abscission, which divides the two cells from each other.

The final result of meiosis 1 is the production of two daughter cells that both contain 23 chromosomes, where the chromosomes have sister chromatids on them.

Meiosis 2:
Prophase 2: The nuclear membranes disintegrates and spindle fibre forms as centrosomes appear. The chromatin condenses into chromosomes.
Metaphase 2: The chromosomes line up in the centre of the cell along the equator as the spindle fibres attach to the centromeres that connects the two sister chromatids.
Anaphase 2: The sister chromatids are pulled apart to opposite poles of the cell as the spindle fibres shorten. (once again, these sister chromatids are identical to each other. So why would the new cells want two chromosomes that code/instruct for the same things? Exactly, they don't need them both, so each chromatid is pulled apart to opposite cells).
Telophase 2: Two new nuclei form for each cell as the chromosomes appear as chromatin. The spindle also breaks down.
Cytokinesis 2: The cleavage furrow moves inwards which forms an abscission region that divides the two cells into four cells.

The result of meiosis 2 is four daughter cells that all have 23 chromosomes that are not composed of sister chromatids.

Now there will be four gametes formed each with only 23 chromosomes. Say for example the above was the production of four sperm cells. One of these cells will fuse with an egg cell of a female (fertilisation) and they will combine their chromosomes to a total of 46 chromosomes. This new fused cell is known as a zygote and it will undergo nuclear division (mitosis) for further development.

[StupidProdigy]

With the attached picture, does it go like this (in terms of chromosomes and chromatids):
nucleus 1-there is two chromosomes
nucleus 2-there is four chromatids
nucleus 3-four chromatids
nucleus 4-four chromosomes

Just trying to make sure I get the terminology! 1 chromosome does not equal 2 chromatids yeah?

[TheAspiringDoc]

With the attached picture, does it go like this (in terms of chromosomes and chromatids):
nucleus 1-there is two chromosomes
nucleus 2-there is four chromatids
nucleus 3-four chromatids
nucleus 4-four chromosomes

Just trying to make sure I get the terminology! 1 chromosome does not equal 2 chromatids yeah?

I'm new to this too so I might not be right so hopefully someone can pick me up if I'm wrong..
Anyway, here goes.
I'm pretty sure that 1 chromosome does equal 2 chromatids (perhaps with the exception of the X/Y chromosome bits..?
In reality there are 46 chromosomes in the stage depicted by picture 1.
They they then double to become 92. (pic 2)
Every one of the 92 chromosomes split in anaphase (pic 3)
The cell then splits in two, (this is called cytokinesis) leaving 46 chromosmes in each of the two progeny (offspring) cells as shown in picture 4.
Does this all make sense?
Please correct me if I'm wrong someone

P.S. HMS? Her Majesty's Ship?

[cosine]

With the attached picture, does it go like this (in terms of chromosomes and chromatids):
nucleus 1-there is two chromosomes
nucleus 2-there is four chromatids
nucleus 3-four chromatids
nucleus 4-four chromosomes

Just trying to make sure I get the terminology! 1 chromosome does not equal 2 chromatids yeah?

I'm new to this too so I might not be right so hopefully someone can pick me up if I'm wrong..
Anyway, here goes.
I'm pretty sure that 1 chromosome does equal 2 chromatids (perhaps with the exception of the X/Y chromosome bits..?
In reality there are 46 chromosomes in the stage depicted by picture 1.
They they then double to become 92. (pic 2)
Every one of the 92 chromosomes split in anaphase (pic 3)
The cell then splits in two, (this is called cytokinesis) leaving 46 chromosmes in each of the two progeny (offspring) cells as shown in picture 4.
Does this all make sense?
Please correct me if I'm wrong someone

P.S. HMS? Her Majesty's Ship?

When a chromosome has no sister chromatids, it is only called a chromosome. If the chromosome has sister chromatids, you can call each 'strand' a chromatid, but the whole structure with both chromatids held together by the centromere, that's a chromosome.

SmartProdigy, you are correct, but ill add a few more specifics:
Nucleus 1: homologous pair of chromosomes (2 chromosomes in total)
Nucleus 2: two chromosomes in total, 4 sister chromatids in total
Nucleus 3: four chromosomes, not chromatids. Chromatids only exist when they are held together by a centromere, if they are pulled apart (in anaphase), they are now the 'new' chromosomes of each daughter cell.
Nucleus 4: four chromosomes.

[heids]

With the attached picture, does it go like this (in terms of chromosomes and chromatids):
nucleus 1-there is two chromosomes
nucleus 2-there is four chromatids
nucleus 3-four chromatids
nucleus 4-four chromosomes

Just trying to make sure I get the terminology! 1 chromosome does not equal 2 chromatids yeah?

1 - 2 chromosomes, 2 chromatids
2 - 2 chromosomes, 4 chromatids
3 - 2 chromosomes, 4 chromatids
4 - each cell has 2 chromosomes, 2 chromatids
EDIT with 3 and 4 - I suppose cosine's probably right about chromatids being only when there are two together, I honestly don't know and always called them chromatids.  Forgive my ignorance.

Between the S phase in interphase and cytokinesis, a human cell has 92 chromatids and 46 'double-stranded' chromosomes, because there are two identical copies of each chromosome attached at the centromere - cytokinesis splits the cell into two, each with one chromatid or one copy of each chromosome.  So, the rest of the time, all body cells have 46 chromatids, or 46 single-stranded chromosomes.  Think of single-stranded chromosomes, only one copy of each chromosome, as the 'normal' state in cells.

But, make sure that you know the difference between sister chromatids (which are two identical copies of the same chromosome) and homologous chromosomes (two copies of the same chromosome, i.e. have the same genes in the same locations and are the same size and shape.  BUT they're not identical, because one's from mum and one's from dad and they may have different alleles at the gene loci).

So, in almost all cells (all but gametes), most of the time (all but S - cytokinesis) cells have 46 chromosomes, 2 members in each homologous pair (so non-identical two copies of each chromosome, one from mum and one from dad), and each of these chromosomes is single-stranded (so only one copy).

Hope that makes sense, it's actually quite confusing!

Please correct me if I'm wrong someone

OK, done

[StupidProdigy]

Awesome clarity in all three responses. I get it 100% now, feels so relieving. Thankyou thankyou thankyou

[TheAspiringDoc]

Whilst we're all on the topic of cell division, could someone please explain to me the concept of diploid and haploid cells?
also, what does the '2n' notation mean with reference to cells and mitosis..?
Thanks!

[heids]

Whilst we're all on the topic of cell division, could someone please explain to me the concept of diploid and haploid cells?
also, what does the '2n' notation mean with reference to cells and mitosis..?
Thanks!

So, homologous chromosomes are two chromosomes with the same size and shape and location of the centromere, and they have the same genes that code for the same traits in the same places.  As humans, we have 23 pairs of homologous chromosomes (well not exactly in males, because X and Y are a pair but aren't homologous coz they're different shapes with different genes) - one in each pair comes from our mum and one from our dad.  The two chromosomes in each pair AREN'T identical, because they may (not necessarily, remember if they're the same then you're homozygous at that locus?) have different alleles at each gene locus, so they have a different DNA sequence.

Diploid means that a cell has both members of a homologous pair, e.g. 46 chromosomes.
Haploid means that a cell only has one member of each pair, e.g. 23 chromosomes.
(that is, hope that's what they actually mean, that's always how I thought of it but might not be the correct definition)

n is the number of chromosomes we have (23 in humans) - all cells except our gametes are 2n (diploid) because they have two copies of each chromosome (both members of each pair), they're 2n and thus have 2 x 23 = 46 chromosomes.

[cosine]

Whilst we're all on the topic of cell division, could someone please explain to me the concept of diploid and haploid cells?
also, what does the '2n' notation mean with reference to cells and mitosis..?
Thanks!

#InBeforeBangali_Lok

You need to understand that diploid cells have 'di' meaning two, sets of chromosomes in each cell. So If i said that the diploid number in human cells is 46, that means each somatic (body) human cell has 46 chromosomes. This also means that the 'ha'ploid (try remembering di=2, ha=half) contain half the number of chromosomes in each cell.

Mitosis is the nuclear division of somatic cells, hence the cells that undergo the division will always have 46 chromosomes, so they are_______________.

Meiosis is the nuclear division of Germ line cells that result in the production of four haploid cells, meaning that once meiosis is complete, the resulting gametes will all have only 23 chromosomes. The reason behind this is because this gamete will fuse with the gamete of another gender, and they both contain 23 + 23 chromosome = 46 chromosomes.

Haploid cells = gametes = n = 23 chromosomes
Diploid cells = somatic(excluding germ line cells) 2n = 46

The 2n notation is like the only math you will probably see in biology (unfortunately xD). If:




EDIT: Scratch that hashtag.. Too quick for us..

[heids]

the only math you will probably see in biology (unfortunately thankfully xD).

EDIT: Scratch that hashtag.. Too quick for us..

I win, a soda for me!!
But seriously, except for that bit I had to cross out in the quote, your explanation was better.

[TheAspiringDoc]

Mitosis is the nuclear division of somatic cells, hence the cells that undergo the division will always have 46 chromosomes, so they are haploid?

Is it haploid?
Thanks for the explanations guys