I don’t understand the division of the chromosomes. The math just doesn’t add up.
Mitosis: Start with 46 chromosomes (2n). Replicate to become 92 chromosomes. Then divide to produce 46 (2n) chromosomes. Okay, makes sense.
Meiosis: Start with 46 (2n). Replicate to become 92 chromosomes. Then divide to become 2 cells with (1n). How? They THEN divide to become 4 cells with (1n) as well. How? How can they divide when they’re just (1n). Am I missing something?
In: Biology
Okay so this is for meiosis, because you seem to have a good hold on mitosis. So you have 46 chromosomes and they come in pairs. Those pairs aren’t identical they’re just associated with each other. The n you’re talking about is the pairs.
Your cells replicate and you end up with 92 chromosomes. These are exact copies of your original 46 chromosomes. So you now have two copies of all of your 46 chromosomes.
Then your cell splits. In mitosis the identical copies would separate so each cell would have the same chromosomes. With meiosis it’s the pairs that separate. So each cell ends up with different chromosomes that don’t have pairs, but they do have identical copies. So before you had 46 chromosomes and a copy for each. Now each cell has only 23 chromosomes with identical copies.
Then both cells split again and separate the copies. Just like mitosis.
So the “n” count is a bit weird and seems to be what’s throwing you off. N is not the total # of copies of each chromatid (which is sometimes called “c”). “N” is more correctly equal to the number of centromeres present –so one “X” chromosome is 1n, but a chromosome once broken into its two “><” chromatids counts for 2 n’s.
Mitosis:
-Start with 46 chromosomes (2n/2c).
-Replicate to become 92 chromosomes. (2n/4c)
-Pulls apart replicated chromosomes and divides to produce 46 (2n/2c) chromosomes.
Meiosis:
-Start with 46 chromosomes (2n/2c) (just like Mitosis)
-Replicate to become 92 chromosomes. (2n/4c) (Just like Mitosis).
-Meiosis I divides not like Mitosis by pulling chromosomes apart into chromatids, but instead by shuffling the full chromosomes into each of two daughter cells. Which creates (1n/2c).
-Meiosis II then divides by pulling the chromosomes apart into chromatids. Which creates (1n/1c) in each daughter cell.
This Wikipedia image may help you visualize things:
https://upload.wikimedia.org/wikipedia/commons/thumb/7/74/Meiosis_Stages.svg/1920px-Meiosis_Stages.svg.png
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