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Jimmy20002012
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#701
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(Original post by PickwickianGeek)
Can anyone explain to me the evidence for the semi conservative model of DNA replication on pages 166/167? I've read it but it doesn't make a lot of sense to me...
I struggled with this, but there is an amazing youtube video which explained it really well:

http://m.youtube.com/watch?feature=plpp&v=JcUQ_TZCG0w


Hope this helped


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PickwickianGeek
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#702
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(Original post by Jimmy20002012)
I struggled with this, but there is an amazing youtube video which explained it really well:

http://m.youtube.com/watch?feature=plpp&v=JcUQ_TZCG0w


Hope this helped


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Yeah that really helped thanks! Only a week left, hope I'll be able to get all my revision done in time!
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ImAz
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#703
Finally gotten my head around oxygen dissociation today, thank god! Only another 179125 topics to go..


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Neon-Soldier32
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(Original post by Nisa091)
I need some advice..
I just started revising for Biology Unit 2 now and I don't know anything! I have 2 exam which one of it is Psychology on the 5th. I'm doing Biology privately so I'm currently stressed out.. I had a tutor but he told me to focus on Unit 1 so he made me spend 7 months on Unit 1 which was a waste of time! I could have done Unit 2 as well
I have this booklet which summarize everything I need to know, I wanted to know if it is possible to know everything before the exam on the 3rd June?
So you want to learn the whole unit 2 spec, something which has took a lot of other candidates months to do, in a mere 8 days? Of course you can't do it.

The best thing for you to do would be to simply do past papers, all of them and get your head around the main concepts - mitosis / meiosis, haemoglobin, antibiotics and energy stores.

(Original post by PickwickianGeek)
Can anyone explain to me the evidence for the semi conservative model of DNA replication on pages 166/167? I've read it but it doesn't make a lot of sense to me...
I don't think it's particularly important that you understand that, but:

DNA is made of nucleotides - basic, but necessary knowledge - now, nucleotides can be either purines (Adenine / guanine) which have 2 nitrogen / carbon rings or pyrimidines (cytosine / thymine) which have 1 carbon / nitrogen ring.

DNA if normally 14N - don't know why, it just is! But Watson and Crick got some 15N (heavy) DNA - DNA where both strands were 15N. Then, they centrifuged that strand and it was near the bottom of the test tube because, as you remember from unit 1 the heaviest stuff goes to the bottom. Then, W & C but the 15N DNA in a test tube and left it to replicate. Now, the normally stuff happened here and the DNA replicated semi-conservatively.

But how do you know that, I hear you ask. Well, when 2 double stranded DNA mols had formed W & C centrifuged them and found that they were in a higher position than the 15N DNA. This is because in each of the 2 DNA mols one of the 2 strands was 15N and the other was 14N. This occured on both DNA mols and so they were lighter than the original 15N DNA molecule, this shows that DNA is semiconservative - each new strand of DNA has a strand of parental DNA in it.

Want more evidence?

When W & C repeated the experiment they now how some DNA which was 14/15N and the rest of the DNA was all all 14N as only 2 strands were 15N and so when they centrifuged the !$N only stands also formed and were in a higher position than the 14/15N strand.

As I said, you don't need to know that, I don't think.

(Original post by sandrabondaruk)
Could someone explain the difference between crossing over and independent assortment to me? My revision notes and revision guide seem to contradict themselves


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Crossing over is where homologus chromosomes form chiasmata between one another and swap blocks of DNA which then rejoin by genetic recombination. Independant assortment is where one of the homologus chromosomes is pulled to one pole of the cell. Which chromosome is pulled is a random process and occurs in both meiosis1 and 2.
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JackTheNerd
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(Original post by cheesypuff)
45% on knowledge and understanding
50% HSW related

Why AQA want to stuff the Biology papers with HSW, I have no Idea
But what I do know is that sometimes it is underrated as easy
How do you get better at HSW questions?
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chelley2
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(Original post by cheesypuff)
45% on knowledge and understanding
50% HSW related

Why AQA want to stuff the Biology papers with HSW, I have no Idea
But what I do know is that sometimes it is underrated as easy
What's the other 5% on ?
There's nothing I hate more than HSW.my ideal paper ;100%knowledge and understanding and no bull on HSW!


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cheesypuff
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yeah, most of unit 2 is hsw but they do throw in some easy knowledge questions for mercy
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JackTheNerd
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Here's a link of a thread that has loads of questions for a variety of topics for bio2 (maybe all of them, I haven't checked)

http://www.thestudentroom.co.uk/show....php?t=2159981
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Scienceisgood
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#709
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#709
(Original post by PickwickianGeek)
Can anyone explain to me the evidence for the semi conservative model of DNA replication on pages 166/167? I've read it but it doesn't make a lot of sense to me...
Might help if you told us which book you are referring to. =l
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cheesypuff
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(Original post by JackTheNerd)
How do you get better at HSW questions?
How do you get better at something which makes no sense??#nonsense
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kiyubi
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#711
(Original post by Paulineuh)
The endodermal cell actively transports mineral ions into the xylem and this in turn, lowers the water potential of the xylem and since the endodermal cells have a higher water potential this results in a water potential gradient so water moves by osmosis into the xylem. The actively transporting of mineral ions cause a force called root pressure which helps the movement of water up the plant
thank you so much!=P
Sorry fr wasting yr tym=]
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UnknownGuy
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What is the 6 marker predicted on?

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VigneshSB
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Hey guys, could someone please help me with this question 'How do insects exchange gases? ' I know its through the tracheal system, but how would you describe that? Thanks in advance!

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Magenta96
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(Original post by VigneshSB)
Hey guys, could someone please help me with this question 'How do insects exchange gases? ' I know its through the tracheal system, but how would you describe that? Thanks in advance!

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basically the exchange surface is the tracheoles. Oxygen will enter the tracheae of the insect, and move down its concentration gradient into the respiring cells. CO2 diffuses down its concentration gradient also, to the spiracles ready to be released into the atmosphere. Oxygen isn't transported in an insects body by the circulatory system (haemoglobin etc) because tracheoles branch off the tracheae and so air can diffuse straight into the respiring cells. The insects also use rhythmic abdominal movements to control air moving in and out the spiracles.
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starfish232
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(Original post by VigneshSB)
Hey guys, could someone please help me with this question 'How do insects exchange gases? ' I know its through the tracheal system, but how would you describe that? Thanks in advance!

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So insects have spiracles present on their body which have valves allowing control over gases entering and leaving. So say for example you have oxygen and it's going to diffuse into the insect through the spiracles due to there being a higher concentration of oxygen outside than inside the insect. Oxygen then diffuses along the trachea which then divides into the tracheoles. The tracheoles are in contact with the cells so oxygen the diffuses into cells (have lower conc of oxygen compared to tracheoles) to be used for respiration. The same is for co2 but it's the opposite. Ventilation also occurs to help increase rate of gas exhange as difference in concentration gradient is maintained.


I hope this makes sense.
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starfish232
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Could someone please explain root pressure and cohesion tension theory. I don't quite understand it.

Thanks
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homefind
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Insects never have really come up in the detail in the past papers so maybe there will be set or a 5 marker about insects and how they carry out gaseous exchange


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VigneshSB
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Hey man, thanks for answering my question!

I'll do your question now! Root pressure occurs at the xylem of roots. You know how water moves from the root hairs by the symplast and apoplast up to the endodermis, where ALL the water is taken up by endodermis cells(casparian strip forces apoplast into endodermis cells also). Now, the endodermis cells actively move ions into the xylem vessel. This lower the xylem's water potential and the water from the endodermis cells moves by osmosis into the xylem. Th pressure caused by this is root pressure

Cohesion tension - When water evaporates from the cells of the leaf, it lowers their water potential. This causes a osmotic gradient, whereby water moves across cells to replace the evaporated water. Now water molecules form h bonds between each other, so they form long chains that extend down the xylem; this is cohesion. When the water molecules move to replace the lost water, the whole chain moves up to xylem too. The movement of these chains up the xylem causes an inward pressure; this is the tension. Hence cohesion tension theory

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VigneshSB
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Thanks magenta That cleared it up for me a lot

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Suzanna5678
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#720
Can someone explain immunological comparisons of proteins? Really don't understand
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