Spermatogenesis and Oogenesis Question Help

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IGCSETANK
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#1
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As I was working my way through Gametogenesis, I became confused when looking at the diagrams depicting the meiosis divisions.
In the textbook, the Secondary oocytes have a chromosome set of 1n yet try divide to produce 2 other substances that also have 1n.
In order for this to happen doesn't the secondary oocyte have to replicate its own chromosomes first?
Many thanks,
Tank.
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Chanaradh
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Yes it does, the secondary oocytes will replicate its 1n chromosomes before the division to produce the second polar body.
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Kallisto
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What Chanaradh said is true: in the secondary oocyte, 1n replicates to get a (second) polar body in the one hand, but an ovum in the other hand which is produced to be fertilized.
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Jpw1097
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(Original post by IGCSETANK)
As I was working my way through Gametogenesis, I became confused when looking at the diagrams depicting the meiosis divisions.
In the textbook, the Secondary oocytes have a chromosome set of 1n yet try divide to produce 2 other substances that also have 1n.
In order for this to happen doesn't the secondary oocyte have to replicate its own chromosomes first?
Many thanks,
Tank.
The previous responses are incorrect. DNA replication only occurs in the 'S' phase during interphase. Oogonia (stem cells) proliferate and differentiate into primary oocytes. The cell cycle is arrested in prophase I until puberty when, with each menstrual cycle, a primary oocyte completes meiosis I, producing a secondary oocyte and a polar body. The secondary oocyte is arrested in metaphase II until fertilisation. If the secondary oocyte is fertilised, the secondary oocyte completes meiosis II (producing a second polar body) and an ovum is transiently formed.

Oogonia have 2n chromosomes - i.e. they are diploid. When these chromosomes are replicated during interphase - forming primary oocytes - there are still only 2n chromosomes, it's just that each chromosome is now made up of two chromatids. The chromosome number is 2n, while the chromatid number is 4c. With each menstrual cycle, a primary oocyte completes meiosis I; the homologous chromosomes are separated to opposite poles of the cell forming two haploid cells (each with 1n chromosomes but 2c chromatids). One of these haploid cells will be the polar body which degenerates while the other will go onto be the secondary oocyte. However, each chromosome is still made up of two sister chromatids. Each cell has a chromosome number of 1n, and a chromatid number of 2c. After fertilisation, the secondary oocyte completes meiosis II; the chromosomes are now split and the sister chromatids which are now pulled to opposite poles of the cell forming two haploid cells (each with a chromsome number of 1n and a chromatid number of 1c). One of these cells will be the polar body which degenerates while the other forms the ovum.

As you can see, there is no DNA replication in the secondary oocyte. Yes, the secondary oocyte is haploid (i.e. has 1n chromosomes) but each of those chromosomes is made up of two sister chromatids (2c chromatids).

Hopefully that makes sense.
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Jpw1097
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#5
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(Original post by Chanaradh)
Yes it does, the secondary oocytes will replicate its 1n chromosomes before the division to produce the second polar body.
(Original post by Kallisto)
What Chanaradh said is true: in the secondary oocyte, 1n replicates to get a (second) polar body in the one hand, but an ovum in the other hand which is produced to be fertilized.
Not true, the 1n chromosomes each have two chromatids. The chromosomes simply split into two chromatids. There is no replication.
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Kallisto
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#6
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(Original post by Jpw1097)
Not true, the 1n chromosomes each have two chromatids. The chromosomes simply split into two chromatids. There is no replication.
Okay, that was my bad then. I was just wrong. Thanks for make it clear.
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