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    Ok. Meiosis is the production of gametes. So at the start of the proccess there is a diploid cell. It has 23 pairs of chromosomes each with 4 chromatids.

    Then they mix up. So you have some of your mothers in one cell and some of you fathers in another, but they divide into two groups each of which has one 4, one 17, one 20 etc. -- A haploid cell.

    Then these divide to create four daughter cells (of the original cell -- two from each cell). The chromosomes are pulled apart in this process to leave sets of 2 chromatids (so half a chromosome) in each of the 4 cells.

    These are all genetically different. Thus allowing for genetic variation.

    Ok. Then in fertilisation (the meeting of the gametes) a sperm and an egg come together. To form a zygote. The sperm has chromosomes with 2 chromatids in, and so does the egg. Thus upon fertilisation creating one complete set of chromosomes.

    However I have always been taught that after fertilisation the zygote has 2 sets of the 23 chromosomes.

    Do the 2 chromatids in the gametes replicate themselves to form full chromosomes? How does this happen?
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    I'm not sure what you are trying to say here. Can you rephrase the question?
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    (Original post by 2776)
    I'm not sure what you are trying to say here. Can you rephrase the question?
    The gametes are made up of chromosomes with 2 chromatids -- a half set of chromosomes. Yet after fertilisation a zygote has 2 full sets of chromosomes.

    How is this possible?
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    (Original post by 2776)
    I'm not sure what you are trying to say here. Can you rephrase the question?
    it's all the scientific words that are confsing me........even though i should know them by monday!
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    (Original post by samdavyson)
    The gametes are made up of chromosomes with 2 chromatids -- a half set of chromosomes. Yet after fertilisation a zygote has 2 full sets of chromosomes.

    How is this possible?
    an egg and a sperm have 23 chromosomes each, becase of mieosis. when fertilised, the zygote has 2 sets of chromosomes; one set from the spermand one from the egg
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    (Original post by samdavyson)
    Ok. Meiosis is the production of gametes. So at the start of the proccess there is a diploid cell. It has 23 pairs of chromosomes each with 4 chromatids.
    A chromosome is made up of 2 chromatids and only human diploid cells have 23 pairs chromosomes

    but i doubt GCSE requires details of how 2 haploid cells fuse to form a diploid cells then it's mitosis ie zygote->adult
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    I sort of see what you are saying. I'm doing double award and I know you definitely do not have to learn most of the top part as it is far to complicated for GCSE. However I'm not sure whether you'd need it for GCSE Biology, but you seem to know it all indepthly.
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    (Original post by happysunshine)
    I sort of see what you are saying. I'm doing double award and I know you definitely do not have to learn most of the top part as it is far to complicated for GCSE. However I'm not sure whether you'd need it for GCSE Biology, but you seem to know it all indepthly.
    i thought he was asking about it out of curiosity
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    samdavyson: I've been wondering exactly the same thing...and I can't work it out...I understand meiosis and chromosomes and all that, but I don't get where the extra chromosomes come from to make up two pairs of 23 chromosomes. At stage 1, the pairs split to form two separate cells, each containing 23 chromosomes. Then the chromosomes themselves split at the centromere to form 4 new cells (gametes). But then surely you've only got half the number of chromosomes you're supposed to have, even with two gametes fusing? What you've got is 23 half chromosomes fusing with 23 half chromosomes. Do these "half" chromosomes replicate themselves to make them whole chromosomes again? Hard to explain...

    Hang on...breakthrough coming...aha! Don't you...actually no. Sorry, false alarm.

    I'm guessing they must replicate themselves. But I'm not sure at all.

    Hmm.

    (BTW I'm doing GCSE Biology separate sciences, but all this gamete stuff does seem to be on the syllabus for GCSE Double Award as well)
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    "they must replicate themselves. " they do.
    "Do these "half" chromosomes replicate themselves to make them whole chromosomes again? " - According to the CGP book, the chromosomes themselves split again "mitosis-style" to form 2 identical cells - which is why i think after fertilisation the zygte has 2 full sets of chromosomes, one set from mother (chromosome splitting) and second set from father(chromosome splitting).

    (Original post by glance)
    At stage 1, the pairs split to form two separate cells, each containing 23 chromosomes. Then the chromosomes themselves split at the centromere to form 4 new cells (gametes).

    But then surely you've only got half the number of chromosomes you're supposed to have, even with two gametes fusing? What you've got is 23 half chromosomes fusing with 23 half chromosomes. Do these "half" chromosomes replicate themselves to make them whole chromosomes again? Hard to explain...
    No, these "half chromosomes" are not halves at all. They are full chromosomes.

    When you consider the "X-shaped" chromosomes, these consist of 2 copies of the actual chromosome in terms of genetic information.

    It's not that in a zygote you have "half chromosomes"; rather in a cell about to divide and form gametes, you have "double chromosomes" !

    As far as I understand, you only get these "X-shapes" formed when the cell is about to start dividing i.e. after the genetic material has been duplicated during interphase and the chromosomes condensed.

    Does this help at all?

    It's not very well taught - the distinction between the "rod shaped" chromosome and the "X" "double chromosomes" - them both being referred to just as "chromosomes" is very confusing. But that's biology for you.

    Rosie
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    question about biology?????? seeing that we are on the subject..........

    do you get marks for drawing diagrams.......say i didnt know how to explain the question fully ..could i still get marks for doing a diagram ..of mitosis or wateva the question was about?????? :confused:
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    I doubt it. The mark scheme probably won't allow. Anyway, if you can draw a diagram of it, surely you'd be able to explain it. The big marks normally go for use of "technical" words.

    (Original post by chimacold)
    question about biology?????? seeing that we are on the subject..........

    do you get marks for drawing diagrams.......say i didnt know how to explain the question fully ..could i still get marks for doing a diagram ..of mitosis or wateva the question was about?????? :confused:
    Sure, diagrams can help you to explain a point often more easily than in words.

    Just make sure you label your diagram well and include *some* written explanation.. also drawing relevant-but-not-really diagrams that relate to the topic but don't answer the question will not score highly, if at all.

    I always found best technique if you had enough time was to write AND draw diagrams AND include equations and basically chuck in as much stuff as you can that relates to the question.

    Rosie
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    Let the examiner sort through it all. It's his job anyway.

    (Original post by glance)
    I doubt it. The mark scheme probably won't allow. Anyway, if you can draw a diagram of it, surely you'd be able to explain it. The big marks normally go for use of "technical" words.
    Sorry, but this isn't really right, unless my experience of exam boards is completely different to yours. Markschemes often state that the marks may be gained from a diagram, and even if they don't, that's the kind of thing that is discussed in examiners' meetings to decide how to award marks for it.

    By the way, if you think "the big marks normally go for use of "technical" words" - a diagram is a good place to get lots of these words in by use of appropriate labels!

    Often a note next to /under the label explaining something helps too, e.g.

    "Spindle fibres - these help to pull the chromosomes apart"

    Rosie

    (Original post by glance)
    Let the examiner sort through it all. It's his job anyway.
    Or her.
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    "his" as in collective...like in french...lol

    sorry, by "big" marks, I meant to get the full marks for a question, you can't just draw a diagram. You have to explain it or label it using words such as "spindle fibres"... i.e. technical words. Just flicking through some of the mark schemes this seems to be the case. Actually sometimes in the question it does say you can draw a diagram to help you. Sorry, my bad.

    (Original post by glance)
    "his" as in collective...like in french...lol

    sorry, by "big" marks, I meant to get the full marks for a question, you can't just draw a diagram. You have to explain it or label it using words such as "spindle fibres"... i.e. technical words. Just flicking through some of the mark schemes this seems to be the case. Actually sometimes in the question it does say you can draw a diagram to help you. Sorry, my bad.
    Yeah.........in exams etc, a diagram without labels is just a collection of lines.

    Rosie
 
 
 

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