jazz_xox_
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Please could someone check the following about meiosis, I had the idea of homologous chromosomes and homologous pairs mixed up so I've written it all out in my own words. Please tell me if there's any mistakes or any important info I've missed. Thank you


2 homologous chromosomes (1 maternal and 1 paternal) make up a homologous pair
Homologous pair of chromosomes chromosome is made up of a maternal chromosome and a paternal chromosome- homologous chromosomes are similar in length, gene position and centromere location

Replication of the homologous chromosomes give 2 double-armed chromosomes – one with the maternal sister chromatids, and one with paternal sister chromatids

Each double-armed chromosome is made of 2 sister chromatids that are joined at the centromere
  1. 1. In interphase, DNA unravels and replicates so there are now 2 copies of each chromosome (called chromatids)
  2. 2. DNA condenses to double-armed chromosomes made from sister chromatids that are joined in the middle by the centromere
  3. 3. Homologous pairs line up along the middle of the cell, and attach to the spindle by their centromere
  4. 4. Spindle fibres contract, homologous pairs are separated, halving the chromosome number
  5. 5. Homologous pairs are separated and move to opposite poles of the cell
  6. 6. There are now 2 daughter cells, which both contain half of the original DNA amount compared to the start of meiosis
  7. 7. Sister chromatids are then separated (by the spindle contracting and centromere dividing)
  8. 8. Sister chromatids move to opposite poles of the cell and cytokinesis occurs
  9. 9. Now 4 genetically different daughter cells, each with ¼ of the amount of DNA from the start of meiosis, but ½ of the DNA compared to a normal body cell- the cells are haploid
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OxFossil
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(Original post by jazz_xox_)
Please could someone check the following about meiosis, I had the idea of homologous chromosomes and homologous pairs mixed up so I've written it all out in my own words. Please tell me if there's any mistakes or any important info I've missed. Thank you


2 homologous chromosomes (1 maternal and 1 paternal) make up a homologous pair
Homologous pair of chromosomes chromosome is made up of a maternal chromosome and a paternal chromosome- homologous chromosomes are similar in length, gene position and centromere location

Replication of the homologous chromosomes give 2 double-armed chromosomes – one with the maternal sister chromatids, and one with paternal sister chromatids

Each double-armed chromosome is made of 2 sister chromatids that are joined at the centromere
  1. 1. In interphase, DNA unravels and replicates so there are now 2 copies of each chromosome (called chromatids)
  2. 2. DNA condenses to double-armed chromosomes made from sister chromatids that are joined in the middle by the centromere
  3. 3. Homologous pairs line up along the middle of the cell, and attach to the spindle by their centromere
  4. 4. Spindle fibres contract, homologous pairs are separated, halving the chromosome number
  5. 5. Homologous pairs are separated and move to opposite poles of the cell
  6. 6. There are now 2 daughter cells, which both contain half of the original DNA amount compared to the start of meiosis
  7. 7. Sister chromatids are then separated (by the spindle contracting and centromere dividing)
  8. 8. Sister chromatids move to opposite poles of the cell and cytokinesis occurs
  9. 9. Now 4 genetically different daughter cells, each with ¼ of the amount of DNA from the start of mitosis, but ½ of the DNA compared to a normal body cell- the cells are haploid
Well done. But don't forget the critical bit of business in prophase I after the homologous chromosomes pair up. After pairing up, homologous lengths of DNA are cut, exchanged and repaired in the process of genetic recombination. This is why the gametes end up being a unique combination of maternal and paternal genes
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jazz_xox_
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(Original post by OxFossil)
Well done. But don't forget the critical bit of business in prophase I after the homologous chromosomes pair up. After pairing up, homologous lengths of DNA are cut, exchanged and repaired in the process of genetic recombination. This is why the gametes end up being a unique combination of maternal and paternal genes
I just read one of your posts on an older thread to help me understand it all hahah!! And thank you, I'll add it in- this is the exchange of alleles in crossing over isn't it?

And also in 'independent segregation' is this the fact that either homologous chromosome could go in either daughter cell, in meiosis 1?

Thanks again
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OxFossil
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(Original post by jazz_xox_)
I just read one of your posts on an older thread to help me understand it all hahah!! And thank you, I'll add it in- this is the exchange of alleles in crossing over isn't it?

And also in 'independent segregation' is this the fact that either homologous chromosome could go in either daughter cell, in meiosis 1?

Thanks again

Yes, recombination splits up and swaps sections of maternal and paternal chromosomes/chromatids, which may separate genes/alleles that were carried together on the same chromosome. You can have multiple exchanges along the length of a chromosome/chromatid, so that this is also part of the process that leads to independent assortment of genes. (but bear in mind that it's not completely independent, as if two alleles are physically very close, they are less likely to get split up during recombination than genes which are very far apart. That's what is meant by genetic linkage.)

And as you say, the later separation of homologous chromosomes to form the haploid gametes is random. The result is that all of the maternal chromosomes will not be separated into one cell, while the all paternal chromosomes are separated into another. Instead, there is a random selection of either maternal or paternal from each homologue.

The overall result is that each haploid cell contains a unique mixture of genes from the organism's mother and father.
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jazz_xox_
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(Original post by OxFossil)
Yes, recombination splits up and swaps sections of maternal and paternal chromosomes/chromatids, which may separate genes/alleles that were carried together on the same chromosome. You can have multiple exchanges along the length of a chromosome/chromatid, so that this is also part of the process that leads to independent assortment of genes. (but bear in mind that it's not completely independent, as if two alleles are physically very close, they are less likely to get split up during recombination than genes which are very far apart. That's what is meant by genetic linkage.)

And as you say, the later separation of homologous chromosomes to form the haploid gametes is random. The result is that all of the maternal chromosomes will not be separated into one cell, while the all paternal chromosomes are separated into another. Instead, there is a random selection of either maternal or paternal from each homologue.

The overall result is that each haploid cell contains a unique mixture of genes from the organism's mother and father.
Thank you so much! When you are talking about genetic linkage, is this alleles that are close together on the same chromosome? What are the knock on effects of this?

Sorry for so many q's!
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OxFossil
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(Original post by jazz_xox_)
Thank you so much! When you are talking about genetic linkage, is this alleles that are close together on the same chromosome? What are the knock on effects of this?

Sorry for so many q's!
Yes, in principle, alleles are assorted entirely randomly during recombination. However, if two alleles are very close together on a chromosome, the chances that they will be separated by a crossing over is obviously lower than if one allele is at one end of the chromosome from the other. This means that some alleles tend to be inherited together more often than others (i.e they are linked). The calculations are not entirely straightforward, as of course there is usually more than one crossover along the length of the chromosome.

There are a couple of consequences. One that may be inferred is that this may not be accidental - for example, it allows two (or more) alleles that form a single functional grouping to stick together rather than risk getting disrupted. A second consequence is that it can be possible to construct a map of the sequence of genes on a chromosome based on how often they are separated during meoisis (this was how it was done before gene splicing technology was as advanced as it is now)
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karmacrunch
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(Original post by OxFossil)
Well done. But don't forget the critical bit of business in prophase I after the homologous chromosomes pair up. After pairing up, homologous lengths of DNA are cut, exchanged and repaired in the process of genetic recombination. This is why the gametes end up being a unique combination of maternal and paternal genes
Hi! Quick question, does crossing over occur in prophase II too?
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OxFossil
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(Original post by karmacrunch)
Hi! Quick question, does crossing over occur in prophase II too?
Nope. In prophase II, all that happens is the first stages of preparing to separate the sister chromatids
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karmacrunch
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(Original post by OxFossil)
Nope. In prophase II, all that happens is the first stages of preparing to separate the sister chromatids
Thank you!
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