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AQA BIOL2 Biology Unit 2 Exam - 26th May 2011 watch

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    Hello everyone!

    I was revising today, and realised basically I understand why there is the first cell division in meiosis. But why have a 2nd?

    The first stage created 2 haploid cells as far as I can make out, and then by dividing again, doesn't the 4 cells just get one chromatid from each chromosome? How does this work, and surely when they fuse in fertilisation there would just be two lots of 23 chromatids fusing, whereas what is wanted is two lots of 23 chromosomes fusing in fertilisation to get 23 homologous pairs?

    What am I missing?

    Any help appreciated, thanks!
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    (Original post by aleema1992)
    Going good for bio not other subjects wa bou u?
    u prepared for this thursdaii
    not fully prepared on these topics :- variation, antibiotic resistance and biodiversity.i have read and understand but i feel am not confident to answer HSW que. do you have any exam questions related to this topics to help me out...
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    (Original post by george424)
    Hello everyone!

    I was revising today, and realised basically I understand why there is the first cell division in meiosis. But why have a 2nd?

    The first stage created 2 haploid cells as far as I can make out, and then by dividing again, doesn't the 4 cells just get one chromatid from each chromosome? How does this work, and surely when they fuse in fertilisation there would just be two lots of 23 chromatids fusing, whereas what is wanted is two lots of 23 chromosomes fusing in fertilisation to get 23 homologous pairs?

    What am I missing?

    Any help appreciated, thanks!
    The first division will form two intermediate haploid cells in which both of them will contain 23 chromosomes, with each chromosome having 2 chromatids and so a total of 46 chromatids in each of these 2 cells.

    Now the second division occurs, in this the 23 chromosomes with 2 chromatids split to form 2 separate chromatids. These chromatids are now referred to as chromosomes hence forming 4 cells each containing 23 chromosomes.

    Main Point: When Chromatids split from the chromosome they become 2 separate chromosomes.

    I hope you understand now.
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    Anyone got the Nelson Thornes textbook,go to page 192

    Is this topic even in the spec? I've never seen a question on this ever, not even on the old specification papers
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    (Original post by therapist_1)
    The first division will form two intermediate haploid cells in which both of them will contain 23 chromosomes, with each chromosome having 2 chromatids and so a total of 46 chromatids in each of these 2 cells.

    Now the second division occurs, in this the 23 chromosomes with 2 chromatids split to form 2 separate chromatids. These chromatids are now referred to as chromosomes hence forming 4 cells each containing 23 chromosomes.

    Main Point: When Chromatids split from the chromosome they become 2 separate chromosomes.

    I hope you understand now.
    very well done ive always been wondering this because the officl book was so confusing but it doesnt usually come up, maybe concept is gd tohugh
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    (Original post by Nuss)
    Anyone got the Nelson Thornes textbook,go to page 192

    Is this topic even in the spec? I've never seen a question on this ever, not even on the old specification papers
    yes it is
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    (Original post by kingsmod1)
    yes it is
    Could anyone direct me to some questions on it? Can't find any
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    (Original post by Nuss)
    Could anyone direct me to some questions on it? Can't find any
    how does water enter xylem and how does actice transport of salt help ????

    Explain and describe lol
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    Oh crap, wrong page, Nelson Thornes textbook,go to page 134

    Never seen a question on it
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    (Original post by kingsmod1)
    how does water enter xylem and how does actice transport of salt help ????

    Explain and describe lol

    Once water has entered the protoplast of the endodermal cells, they will actively pump ions and salts into the xylem by active transport. This will create a water potential gradient, because the water potential of the xylem will decrease and there will be a higher water potential in the endodermal cells. Water will then move down a water potentail gradient by osmosis into the xylem.

    HOPE THIS HELPS!
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    (Original post by Kandy_Kain_94)
    Once water has entered the protoplast of the endodermal cells, they will actively pump ions and salts into the xylem by active transport. This will create a water potential gradient, because the water potential of the xylem will decrease and there will be a higher water potential in the endodermal cells. Water will then move down a water potentail gradient by osmosis into the xylem.

    HOPE THIS HELPS!
    nice answer but Nuss was meant to answer that LOL
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    (Original post by kingsmod1)
    nice answer but Nuss was meant to answer that LOL
    LOL, oh well sorry, it helped me revise because I had to explain it from memory
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    (Original post by therapist_1)
    The first division will form two intermediate haploid cells in which both of them will contain 23 chromosomes, with each chromosome having 2 chromatids and so a total of 46 chromatids in each of these 2 cells.

    Now the second division occurs, in this the 23 chromosomes with 2 chromatids split to form 2 separate chromatids. These chromatids are now referred to as chromosomes hence forming 4 cells each containing 23 chromosomes.

    Main Point: When Chromatids split from the chromosome they become 2 separate chromosomes.

    I hope you understand now.
    Thanks so much, that really helps! I understand it now.

    And to everyone who says it won't come up, it probably won't, I was just interested in the subject I'm studying and want to understand it fully!
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    (Original post by kingsmod1)
    how does water enter xylem and how does actice transport of salt help ????

    Explain and describe lol
    salts reduces wp in root creating wp gradient so water enters by osmosis
    apoplast/symplast used to transport water through cortex to xylem

    but my real point was, is the stuff on page 134 of the nelson even in the spec?
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    So has everyone revised cerebral functions???
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    (Original post by James A)
    lower affinity for oxygen means that when it unloads it, it unloads it easier. am i correct ??
    I believe lower affinity for oxygen means when it is shifted to the right.
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    (Original post by TlanTlan)
    I believe lower affinity for oxygen means when it is shifted to the right.
    correct because a high affinity means it binds to it more easily ( moves left ), so yes, your right
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    (Original post by therapist_1)
    :ff2:
    In metaphase chromosomes do not line up at the centromere of the cell they line up at the equator of the cell. Then the centromere of the chromosome is attached to a spindle fibre.

    In anaphase the spindle fibres contract and not shorten, you could have lost marks for incorrect terminology.

    Its just that, that made me think alot, you were quite right after all.:rolleyes:
    no, it is correct in aqa mark schemes to refer to the spindles as shortening.
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    (Original post by affleming)
    no, it is correct in aqa mark schemes to refer to the spindles as shortening.
    Oh I'm sorry, I answered that just refering to my notes and the text book. Just to stay on the safe side its better to say contracts :iiam:
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    Or both :awesome:
 
 
 
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