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Thread: Mitosis versus Meoisis have I got this correct please?

  1. #1 Mitosis versus Meoisis have I got this correct please? 
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    I have spent the last week or so studying these two mechanisms. Its been confusing, below is a summary of my notes.

    These notes are just the very essential details, I have gone in deep at places, but is the following the classical statement of the two processes - at least in summary?

    or do I have some errors still?

    Below I focus on the product DNA sequences rather than on the mechanisms of achieving this product...

    thanks for reading feedback appreciated.



    1] In interphase, preceding either mitosis or meiosis, in S sub phase , 2n chromosomes duplicate and form sister chromatids tied together at the centromeres.Because the duplicate chromosomes are still bonded together, they are still considered to be one chromosome each. Therefore, 2n chromosomes duplicated, equals 2n chromosomes(!)




    2] MITOSIS (key points):

    At prophase: no crossover

    At Metaphase chromosomes line up ALONG the metaphase plate.

    At Anaphase the sister chromatids are pulled apart 2n chomosomes (now without sister chromatids) which then migrate to cell ends.

    At telophase a new nucleus forms around each cell end,

    cytokinesis occurs

    End product = two cells with 2n chromosomes and no sister chromatids.






    3] MEIOSIS (key points)

    meoisis 1:

    At prophase 1 crossover occurs (first source of variation)

    At Metaphase 1 chromosomes line up on EITHER SIDE of the metaphase plate (paternal and maternal homologous chromosomes are randomly allocated to each side - a second important source of variation) .

    At anaphase 1 the chromosome pairs are pulled to each side of the cell

    After telophase 1 and cytokinesis 1 this leads to TWO HAPLOID cells with two sister chromatids for each chromosome.




    No duplication of sister chromatids in interphase 2

    Meiosis 2:

    Meoisis 2 commences with the end product of meoisis one. this is two cells with 1n chromosomes in each nucleus (these chromosomes are each sister chromatids pairs - considered as one chromosome per pair)

    In Anaphase 2 the sister chromatids are pulled apart. As the sister chromatids now have an independent existence, they are then considered independent chromosomes and therefore counted indepenmdently

    (i.e in each of the two haploid cells produced by meoisis 1, the 1n chromosomes present at the start of the Meoisis 2 now (after telophase 2) become two cells both still with 1n chromosomes

    end result = four haploid cells with 1n chromosomes per cell.

    Note: at the end of meoisis one in human females one cell degeneerates and the process continues only with one cell; therefore the end product of meiosis 2 in this case is 2 ova with 1n chromosomes each.


    phew!
    is this a correct classical view?

    thanks

    Zero


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  3. #2  
    Forum Professor Zwirko's Avatar
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    I'll tackle the first part for now since it was shorter to go through.

    Genetics terminology is very confusing at times, frustratingly so at times. It has been my observation that it is the terminology used in describing mitosis and meiosis that trips students up, more than the mechanism itself. Failure to get to grips with the terminology usually prevents one from getting to grips with the process. If you can manage to understand and correctly use the terms unduplicated chromosome, duplicated chromosome, chromatid, sister chromatid and homologous chromosomes then the rest should pretty much fall into place.

    1) Most of your description of mitosis seems accurate enough to me, although your terminology may be an issue.
    A regular human cell has a 46 DNA molecules in the nucleus (22 pairs of autosomes and a pair of sex chromosomes), hence it is diploid (2n). As it replicates its genetic material during S phase, the cell can be described as being transiently tetraploid (4n); much of mitosis is thus about getting back to the diploid state. To put it another way, mitosis is all about getting the duplicated chromosomes separated.

    So, unduplicated chromosomes are replicated to produce duplicated chromosomes; the two DNA molecules in that duplicated chromosome are known as sister chromatids. Note that an unduplicated chromosome has a totally different structure to that of a duplicated chromosome. The difference between these two things with very similar names seems to be a point of confusion for many people. The former is like an unwound ball of string, the latter like two neatly wound balls of string connected to one another, yet both are termed "chromosome".

    A cell doing nothing much is diplod (2n). A cell that has a full set of duplicated chromosomes is actually tetraploid (4n). Its daughter cells are diploid (2n). That is: unduplicated chromosomes in parent cell (2n) -> duplicated chromosomes (4n) during part of cell cycle -> unduplicated chromosomes (2n) in daughter cells.

    Excuse my artwork :-D

    unreplicated chromosome:


    (one DNA molecule)


    replicated chromosome:



    (two DNA molecules)


    Edited to add the following:

    It would be incorrect to state that there is no cross-over at all in mitosis (despite what most books say). Depending upon how deep you want to go into genetics you could either ignore the important exceptions or make note of the fact that it does happen (but that it is not particularly important in the context of understanding the differences between mitosis and meiosis). Mitotic recombinations (such as reciprocal crossovers and non-reciprocal crossovers) are important for DNA repair mechanisms and have implications in disease. Technically, it occurs pre-mitosis; but is usually referred to as mitotic recombination.


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