AQA BIOL2 Biology Unit 2 Exam - 26th May 2011

Please can anyone help me with question 4a on this paper http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL2-W-QP-JUN09.PDF

the mark scheme is here http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL2-W-MS-JUN09.PDF but I don't understand the answer so if anyone could explain I'd be veeeeery grateful!

At the end of telophase in mitosis, the number of chromosomes is the same as it was at the beginning, it's only in meiosis that the number of chromosomes halves. But, before mitosis begins, the DNA replicates, so at prophase, there is twice as much DNA as you would have in a cell, so at the end of telephase, it halves. In the formation of gametes, the DNA at the beginning of prophase is split amongst four daughter cells, so would have to be divided by four.

Reply 541

LifeIsGood

12

Original post by Master.K

Yeah but atleast you'd know how to answer 'similar' sorts of questions.... :smile:

Yup. How many past papers you done for this exam?

Reply 542

LifeIsGood

12

Original post by oHellno

At the end of telophase in mitosis, the number of chromosomes is the same as it was at the beginning, it's only in meiosis that the number of chromosomes halves. But, before mitosis begins, the DNA replicates, so at prophase, there is twice as much DNA as you would have in a cell, so at the end of telephase, it halves. In the formation of gametes, the DNA at the beginning of prophase is split amongst four daughter cells, so would have to be divided by four.

That's so confusing as it seems you're mixing up Mitosis & Meiosis, I don't understand what you're saying except the first bit. Can you explain please?

Reply 543

parallal

Original post by lolo-x

Please can anyone help me with question 4a on this paper http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL2-W-QP-JUN09.PDF

the mark scheme is here http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL2-W-MS-JUN09.PDF but I don't understand the answer so if anyone could explain I'd be veeeeery grateful!

Prophase is when the chromosomes coil up/condense and telophase is when they uncoil, so there will still be the same number of chromosomes. But the mass of DNA in telophase will be half that in prophase because at prophase the amount of DNA will have doubled (DNA replication) and after telophase the cell will have already have divided and the mass of DNA would have been halved. 26 chromosomes and 30 mass of DNA will be the "normal" amount.

Because telophase and prophase are apart of mitosis they are involved in diploid cells and because a sperm cell is a haploid cell it will have half the number of chromosomes and therefore half the mass of DNA.

Hope that helps.

(edited 12 years ago)

Reply 544

SpecialApple

1

I've been doing past papers and been getting High C's / Low B's which is ace for me but I'm scared i'll get in the exam and panic like crazy and ruin everything! Eeek :frown:

Does anyone have any advice cos I really want to do well!!

Reply 545

kingsmod1

Original post by SpecialApple

I've been doing past papers and been getting High C's / Low B's which is ace for me but I'm scared i'll get in the exam and panic like crazy and ruin everything! Eeek :frown:

Does anyone have any advice cos I really want to do well!!

try and answer the Questions right LOL

:aetsch:

Reply 546

lolo-x

Original post by LifeIsGood

Ok, Mitosis produces identical cells with the SAME number of CHROMOSOMES. That's the key priniciple you need to be able to apply to this question.

At prophase you've got 26 chromosomes so at telophase you have 26. Why? ^Apply that principle. [If it was meiosis, it'd be halved.]

However, the DNA mass changes as mitosis divides into TWO cells so the mass of DNA divides by two.

And with the sperm cell, it's just halved as it's haploid

Original post by oHellno

At the end of telophase in mitosis, the number of chromosomes is the same as it was at the beginning, it's only in meiosis that the number of chromosomes halves. But, before mitosis begins, the DNA replicates, so at prophase, there is twice as much DNA as you would have in a cell, so at the end of telephase, it halves. In the formation of gametes, the DNA at the beginning of prophase is split amongst four daughter cells, so would have to be divided by four.

Original post by parallal

Prophase is when the chromosomes coil up/condense and telophase is when they uncoil, so there will still be the same number of chromosomes. But the mass of DNA in telophase will be half that in prophase because at prophase the amount of DNA will have doubled (DNA replication) and after telophase the cell will have already have divided and the mass of DNA would have been halved. 26 chromosomes and 30 mass of DNA will be the "normal" amount.

Because telophase and prophase are apart of mitosis they are involved in diploid cells and because a sperm cell is a haploid cell it will have half the number of chromosomes and therefore half the mass of DNA.

Hope that helps.

thank you! i understand now, i think i was overcomplicating the question. and i hadn't thought that the mass of dna at prophase is double what it normally is and at telophase is restored to normal.

so is there always the same number of chromosomes throughout mitosis of one cell? thats all thats confusing me now.

Reply 547

LifeIsGood

12

Original post by lolo-x

thank you! i understand now, i think i was overcomplicating the question. and i hadn't thought that the mass of dna at prophase is double what it normally is and at telophase is restored to normal.

so is there always the same number of chromosomes throughout mitosis of one cell? thats all thats confusing me now.

I think so yes as mitosis divides to produce 2 identical cells with the same number of chromosomes

Reply 548

Master.K

18

Original post by LifeIsGood

Yup. How many past papers you done for this exam?

Four.

Reply 549

f.chowdhury

pleeeeeeease can someone explain to me tissue fluid?? I dont get it :frown:

Reply 550

LifeIsGood

12

Original post by f.chowdhury

pleeeeeeease can someone explain to me tissue fluid?? I dont get it :frown:

I'll do my best :smile:

There is high hydrostatic pressure at the arteriole end
So soluble substances are forced out
The large proteins remain
The water potential at the arteriole end is lowered [as all the water has been forced out]
Water returns by osmosis

That's one thing I don't understand is the water potential as it's not clear. The markschemes don't make it clear when they go that "water potential is lowered so water moves back into the venule end by osmosis"?

Reply 551

parallal

Original post by f.chowdhury

pleeeeeeease can someone explain to me tissue fluid?? I dont get it :frown:

Tissue fluid is basically the liquid part of the blood that drains out of the capillaries into the spaces between cells. It allows important substances to be transferred to the cells and provides cells with a constant environment (constant pH, temperature and water potential).

At the arteriole end of a capillary there is a higher pressure, called the hydrostatic pressure, which forces fluid out of the capillary and into the spaces between cells. The fluid moves by osmosis down a concentration gradient because there is more water in the capillary than near the cells. At the venous end of the capillary there is a greater concentration of plasma proteins (less fluid) in the capillary so there is a negative water potential and the tissue fluid moves from the cells back into the capillaries. Any fluid that isn't reabsorbed is drained into the lymph capillaries

Reply 552

parallal

Original post by LifeIsGood

That's one thing I don't understand is the water potential as it's not clear. The markschemes don't make it clear when they go that "water potential is lowered so water moves back into the venule end by osmosis"?

That's because in the venous end there are more plasma proteins than tissue fluid (because the tissue fluid exited at the arteriole end) and this creates a negative water potential (W.P. is lowered) so fluid moves down the conc. gradient from where there is more fluid (between the cells) to where there is less fluid (at the venule end of the capillaries).

Reply 553

F1's Finest

21

im hoping for a high A on this paper just to secure my overall A

Reply 554

ewoo3148

im so bored of revision now! i just want the exam to be now!! any good revision techniques guys? :\

Reply 555

TlanTlan

6

Original post by LifeIsGood

I'll do my best :smile:

There is high hydrostatic pressure at the arteriole end
So soluble substances are forced out
The large proteins remain
The water potential at the arteriole end is lowered [as all the water has been forced out]
Water returns by osmosis

That's one thing I don't understand is the water potential as it's not clear. The markschemes don't make it clear when they go that "water potential is lowered so water moves back into the venule end by osmosis"?

Can also return by the lymphatic system too.

Reply 556

oHellno

12

Original post by LifeIsGood

That's so confusing as it seems you're mixing up Mitosis & Meiosis, I don't understand what you're saying except the first bit. Can you explain please?

Okay, I'll try again.

During the cell cycle, the DNA replicates in interphase, so by the time it gets to the stage of Prophase, there is twice as much genetic information as there is in a normal cell. So when the cells divide in mitosis, the chromatids are pulled apart, and one sister chromatid from each chromosome goes into each cell. So essentially, the the mass of DNA at the start of prophase is split between the two daughter cells. The number of chromosomes stays the same though.

In meiosis 1, the homologous chromosomes are separated, so you've got 23 chromosomes in each of the two cells. But during meiosis 2, they divide in a similar way to mitosis, so the chromatids are separated. So you've got four daughter cells, and the mass of DNA is split between those four.

Reply 557

liviaaa

Please can someone help with any of the following? :smile:

-Difficulties in defining a species?

-Importance of taxonomy?

-Ethics for antibiotic resistance?

Thanks!

Reply 558

jessplease

Original post by LifeIsGood

I think so yes as mitosis divides to produce 2 identical cells with the same number of chromosomes

i dont understand, wouldnt the number of chromosomes have to double at some point to have now two cells with the same number of chromosomes, so when you replicate dna arnt you replicating chromosomes? ahhh confused