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AQA BIOL 2 - A Few Questions

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    Hi,

    I've been revising by reading through my notes, and there are a few things I'm unsure about, if anybody would like to help.

    1. Where do the free nucleotides come from in the semi-conservative replication of DNA.

    2. Homologous pairs confuse me. Are there homologous pairs in normal cells as well as pre-meiosis cells? Are they in every cell? What are the numbers involved (in a human).

    3. How does water move into the xylem when it all converges at the Casparian strip in the symplastic pathway? Does it move via osmosis or actively transport in? How does the Casparian strip block the water in the first place?

    4. In comparison of amino acids across different species, why does a precipitate form when Species C (the species being compared) is injected with the serum from the rabbit, (or whatever the intermediate creature is)

    Thanks! :rolleyes:
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    i'm guessing AQA Biol2? haha. Erm, in 4. a precipitate forms as species C's antibodies combine with species A antigens? sorry - definitely dropping Biology haha!
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    (Original post by FuLLuPMepOrtION)
    i'm guessing AQA Biol2? haha. Erm, in 4. a precipitate forms as species C's antibodies combine with species A antigens? sorry - definitely dropping Biology haha!
    Yeah AQA Biol 2. But why does that produce a precipitate? Hmm...
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    (Original post by George_)
    Hi,

    I've been revising by reading through my notes, and there are a few things I'm unsure about, if anybody would like to help.

    1. Where do the free nucleotides come from in the semi-conservative replication of DNA.

    2. Homologous pairs confuse me. Are there homologous pairs in normal cells as well as pre-meiosis cells? Are they in every cell? What are the numbers involved (in a human).

    3. How does water move into the xylem when it all converges at the Casparian strip in the symplastic pathway? Does it move via osmosis or actively transport in? How does the Casparian strip block the water in the first place?

    4. In comparison of amino acids across different species, why does a precipitate form when Species C (the species being compared) is injected with the serum from the rabbit, (or whatever the intermediate creature is)

    Thanks! :rolleyes:

    Well I can answer 3 for you... The Casparian strip has 1 entry point that water MUST go through, this is because it is coated in a waxy substance and there is only one access point, this is because the Casparian strip controls WHAT is going into the Xylem, if the plant had no control of water intake it can quite easily flood all the cells causing them to burst, or something along those lines...

    Hope that helped... revising for this myself
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    (Original post by Booyah)
    Well I can answer 3 for you... The Casparian strip has 1 entry point that water MUST go through, this is because it is coated in a waxy substance and there is only one access point, this is because the Casparian strip controls WHAT is going into the Xylem, if the plant had no control of water intake it can quite easily flood all the cells causing them to burst, or something along those lines...

    Hope that helped... revising for this myself
    Ahh Ok. And then water move via osmosis. It must be because it's a slower process, as like you said, not to flood it?

    Hmm :P

    How are you revising? At the moment I am just reading through stuff I am not too sure about. Then I'll do past papers all tonight and tomorrow up until the exam.

    What about you?
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    (Original post by George_)
    Ahh Ok. And then water move via osmosis. It must be because it's a slower process, as like you said, not to flood it?

    Hmm :P

    How are you revising? At the moment I am just reading through stuff I am not too sure about. Then I'll do past papers all tonight and tomorrow up until the exam.

    What about you?

    Past papers + copying from my text book... Reading around topics I do not fully understand such as Conjugation which I have just found out is on the syllabus today... I recommend you study with friends when it comes to reading the syllabus, there's a bit more banter and I tend to remember it. Always do syllabus text book copying and then past paper questions, just works for me...
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    (Original post by Booyah)
    Past papers + copying from my text book... Reading around topics I do not fully understand such as Conjugation which I have just found out is on the syllabus today... I recommend you study with friends when it comes to reading the syllabus, there's a bit more banter and I tend to remember it. Always do syllabus text book copying and then past paper questions, just works for me...
    Hmm yeah. I just started today for Unit 2 (in the last week, I mean. I've done some in the past as well), which I know sounds bad but I've got all these other exams as well. Do you think a day and a half is enough? I feel like it's not, but I am trying hard at the moment to get as much done.
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    (Original post by George_)
    Hi,

    I've been revising by reading through my notes, and there are a few things I'm unsure about, if anybody would like to help.

    1. Where do the free nucleotides come from in the semi-conservative replication of DNA.

    2. Homologous pairs confuse me. Are there homologous pairs in normal cells as well as pre-meiosis cells? Are they in every cell? What are the numbers involved (in a human).

    3. How does water move into the xylem when it all converges at the Casparian strip in the symplastic pathway? Does it move via osmosis or actively transport in? How does the Casparian strip block the water in the first place?

    4. In comparison of amino acids across different species, why does a precipitate form when Species C (the species being compared) is injected with the serum from the rabbit, (or whatever the intermediate creature is)

    Thanks! :rolleyes:
    3 and 4 have been answered, I'll answer 1 and 2.

    1. The free nucleotides are floating in the nucleoplasm.

    2. In a gamete, there are 23 chromosomes (haploid number). When this gamete fuses with another, the number of chromosomes doubles (46, diploid number). Any normal body cell (one which is not a gamete) has 46 chromosomes: 23 homologous pairs of chromosomes; 1 chromosome from the mother and 1 from the father.
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    (Original post by thegodofgod)
    3 and 4 have been answered, I'll answer 1 and 2.

    1. The free nucleotides are floating in the nucleoplasm.

    2. In a gamete, there are 23 chromosomes (haploid number). When this gamete fuses with another, the number of chromosomes doubles (46, diploid number). Any normal body cell (one which is not a gamete) has 46 chromosomes: 23 homologous pairs of chromosomes; 1 chromosome from the mother and 1 from the father.
    1. So there are just free nucleotides floating anyway? But in the diagram it kind of shows them falling out of the previous strands. Is there not any replication involved?

    2. Thanks, so in a normal body cell, there's 23 pairs, but 46 chromosomes altogether? Right. Thanks
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    (Original post by George_)
    Hi,

    I've been revising by reading through my notes, and there are a few things I'm unsure about, if anybody would like to help.

    1. Where do the free nucleotides come from in the semi-conservative replication of DNA.

    2. Homologous pairs confuse me. Are there homologous pairs in normal cells as well as pre-meiosis cells? Are they in every cell? What are the numbers involved (in a human).

    3. How does water move into the xylem when it all converges at the Casparian strip in the symplastic pathway? Does it move via osmosis or actively transport in? How does the Casparian strip block the water in the first place?

    4. In comparison of amino acids across different species, why does a precipitate form when Species C (the species being compared) is injected with the serum from the rabbit, (or whatever the intermediate creature is)

    Thanks! :rolleyes:
    1) well for the first bit, the nucleotides are free in the nucleus and when the DNA Helicase breaks the hydrogen bonds, the complementary nucleotides are activated by energy and assembled by the DNA polymerase.

    2) Not sure about No 2 because homologous pairs confuse me as well. But, if you think of them to be kind of like alleles, it makes it slightly easier to understand (for me anyway). I think they are only found in sex cells. They might be in normal cells but they will probably be recessive or inactive. Sorry if i couldn't be of more help for number 2

    3) already been answered

    4) The precipitate forms because the antibodies of the rabbit will respond to specific antigens on proteins from species C. There are usually 3 species involved. proteins are collected from Species A (let's say humans) and injected into the rabbit. The rabbit produces anti-human antibodies (because it's immune system recognises the antigens from the human as foreign so there is a humoral response to it). A serum is then collected from the rabbit and injected into species C (species A and species C are what's being compared) the antibodies of the rabbit respond to the antigens in species C and forms a precipitate (The more precipitate formed, the greater the number of similar antigens between species A and Species C). really hope this helps.
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    (Original post by George_)
    1. So there are just free nucleotides floating anyway? But in the diagram it kind of shows them falling out of the previous strands. Is there not any replication involved?

    2. Thanks, so in a normal body cell, there's 23 pairs, but 46 chromosomes altogether? Right. Thanks
    1. :yes: Just like amino acids / glucose, which exist as free molecules in the cytoplasm / nucleoplasm. Bear in mind that in DNA replication, you're making 4 strands of DNA in comparison to 2 strands before replication, so 2 strands' worth of nucleotides have to come from somewhere. These additional nucleotides are the ones which I was alluding to earlier

    2. :yes:
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    (Original post by blackraven)
    ...really hope this helps.
    Yeah, it helps a bit. Especially for the last one. But I still don't really get where the free nucleotides are coming from.
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    (Original post by thegodofgod)
    1. :yes: Just like amino acids / glucose, which exist as free molecules in the cytoplasm / nucleoplasm. Bear in mind that in DNA replication, you're making 4 strands of DNA in comparison to 2 strands before replication, so 2 strands' worth of nucleotides have to come from somewhere. These additional nucleotides are the ones which I was alluding to earlier

    2. :yes:
    Ahh OK! So they are just like free floating chemicals already produced, ready for the time of replication, or whatever. Ahh I get it now. Thanks to all of you!

    I'll carry on revising, and if any more questions come up, I'll post here again :P
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    (Original post by George_)
    Hi,

    I've been revising by reading through my notes, and there are a few things I'm unsure about, if anybody would like to help.

    2. Homologous pairs confuse me. Are there homologous pairs in normal cells as well as pre-meiosis cells? Are they in every cell? What are the numbers involved (in a human).

    Thanks! :rolleyes:
    All cells have homologous pairs, normally as linear DNA but after replication they become homologous chromosomes. During meiosis the homologous pairs are separated, so one of each pair goes into the new cell (independant segregation). Then the sister chromatids divide in the second division.
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    hi sorry can i post some questions in this thread, no one's anwering them on the other one
    i hope everyone's revision's going good

    can someone please help me:

    1) let's say we got a 6 marker on courtship behaviour; what do we meant to write

    2) on genetic diversity:
    - selection for high-yielding breeds of domesticated animals and strains of plants
    - ethical issues involved in the selection of domesticated animals

    3)haemoglobin:
    - the role of haemoglobin in the transport of oxygen
    - the loading, transport and unloading of oxygen in relation to the oxygen dissoxiation curve = was this the Jan/12 6 marker question?
    - the effects of carbon dioxide concentration

    4) Gas exchange:
    - across the body surface of a single-celled organism
    - by leaves of dicotyledonous plants (mesophyll and stomata)
    - accross the gills of a fish (gill lamallae and filaments)
    - and the countercurrent principle - textbook confused me !

    5) Mass transport:
    - over large distances, efficient supply of materials is provided by mass transport

    6) the blood system:
    - the structure of capillaries and their importance in metabolic exchange

    7) the passage of water through a plant:
    - the structure of a dicotyledonous root in relation to the pathway pf water from root hairs through the cortex and endodermis to the xylem

    8) the genetic comparisons

    9) antiobiotics

    10) genetic variation in bacteria
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    (Original post by George_)
    Hmm yeah. I just started today for Unit 2 (in the last week, I mean. I've done some in the past as well), which I know sounds bad but I've got all these other exams as well. Do you think a day and a half is enough? I feel like it's not, but I am trying hard at the moment to get as much done.
    What on Earth do you think I am doing... . I find that it is nice to have learnt a lot before now however, you never learn it all, and so it is nice to understand it and just look over reasoning etc. and how to answer exam questions
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    (Original post by Booyah)
    What on Earth do you think I am doing... . I find that it is nice to have learnt a lot before now however, you never learn it all, and so it is nice to understand it and just look over reasoning etc. and how to answer exam questions
    Yeah, I suppose you're right :P

    Also, could somebody summarise how to answer diversity questions. Any ideas how to answer them? You know, e.g. some many animals are dying in this area etc. How does this reduce diversity etc. etc.
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    (Original post by George_)
    Yeah, I suppose you're right :P

    Also, could somebody summarise how to answer diversity questions. Any ideas how to answer them? You know, e.g. some many animals are dying in this area etc. How does this reduce diversity etc. etc.
    Well the reduction in the number of species lowers the Index of diversity, in addition the death of plants reduces habitats and producers of food chains, things like that, you gotta be more specific about the question...
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    (Original post by Booyah)
    Well the reduction in the number of species lowers the Index of diversity, in addition the death of plants reduces habitats and producers of food chains, things like that, you gotta be more specific about the question...
    The spotted owl is a bird. Numbers of spotted owls have decreased over the past 50 years. Explain how this decrease may affect genetic diversity...

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    (Original post by George_)
    The spotted owl is a bird. Numbers of spotted owls have decreased over the past 50 years. Explain how this decrease may affect genetic diversity...

    The spotted owl was a member of a food chain as prey, this reduced the number of predators in the population as their food source decreased. Something like that when you look at species diversity whenever it is plants there are habitats and food to consider, in the place of animals however it is important to consider possible predators i.e. the owl may be hunted by pissed off mice with nuclear warheads...

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