Chromosome, chromatid confusion

[Astra]

So, when are chromosomes found in their liner form vs. The X shaped form?

When in the linear form, each chromosome has 1 chromatid and when in X form, each has 2 chromatids?

My understanding is the following

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[aldol16]

Chromosomes are replicated during the S phase of interphase - that's when you get two sister chromatids attached at the centromere. The chromatids separate during anaphase-telophase and then you end up with one chromatid in each daughter cell, which goes through G1 and then on to S again.

 

[Astra]

Chromosomes are replicated during the S phase of interphase - that's when you get two sister chromatids attached at the centromere. The chromatids separate during anaphase-telophase and then you end up with one chromatid in each daughter cell, which goes through G1 and then on to S again.

This is true for mitosis,

but in meiosis 1, there is only 1 spindle fiber attached to the kinetochore and this leads the cells getting a chromosome with 2 chromatids. So in this case, the 2 chromatids attached together are called a chromosome correct?

and this type of chromosome is only found during cell division and is not the actual resting shape of a chromosome correct?

 

[eeyevon]

Before DNA replication, each chromosome has 1 chromatid (2n)
After DNA replication, each chromosome has 2 chromatids (4n)

Mitosis: at prophase: 1 chromosome has 2 chromatids (4n)

metaphase: 1 chromosome has 2 chromatids (4n)
anaphase: sister chromatids separate: now 1 chromosome has 1 chromatid (2n)
telophase: 1 chromosome has 1 chromatids (2n)​

Meiosis: At the start of meiosis, DNA has just replicated, so each chromosome has 2 chromatids (4n)
Meiosis I:

Prophase I: Tetrads form: 1 Tetrad = 2 chromosomes and 4 Chromatids. (a pair of sister chromosomes)
Metaphase I: Tetrads line up (1 tetrad = 2 chromosomes and 4 chromatids).
AnaphaseI: Tetrads Split. 1 chromosomes has 2 chromatids (sister chromosomes split)
Telophase I: 1 chromosomes has 2 chromatids​

Meiosis II:

Prophase II: 1 chromosome has 2 chromatids
Metaphase II: 1 chromosomes has 2 chromatids
Anaphase II: sister chromatids within each chromosome split. Result is 1 chromosome has 1 chromatid
TelophaseII: 1 chromosome has 1 chromatid
​

Lets follow humans as an example: we have 46 chromosomes
Before DNA Replication:
46 chromosomes, 46 chromatids
After DNA replication
46 chromosomes, 92 chromatids (because each chromosome has 2 chromatids)

Mitosis:
we start with 46 chromosomes and 96 chromatids
Prophase: 46 Chromosomes 96 chromatids
Metaphase: 46 Chromosomes 96 chromatids
Anaphase: sister chromatids split: 46 chromosomes 46 chromatids
Telophase: 46 chromosomes 46 chromatids

Meiosis: start 46 chromosomes 92 chromatids
MeiosisI

Prophase I: 46 chromosomes 92 chromatids 23 tetrads (1 tetrad = 2 chromosomes and 4 chromatids)
MetaphaseI: 46 chhromosomes 92 chromatids 23 tetrads
Anaphase I: Tetrads split 23 chromosomes 46 chromatids 0 tetrads
Telophase I: each cell has 23 chromosomes and 46 chromatids​

Meiosis II:

PII: 23 chromosomes 46 chromatids
MII: 23 chromosomes 46 chromatids
AII: sister chromatids split: 23 chromosomes 23 chromatids
end of telophase II: 23 chromosomes and 23 chromatids


Hope this isnt confusing, message me if you want me to draw it out

​

 

[Astra]

Before DNA replication, each chromosome has 1 chromatid (2n)
After DNA replication, each chromosome has 2 chromatids (4n)

Mitosis: at prophase: 1 chromosome has 2 chromatids (4n)

metaphase: 1 chromosome has 2 chromatids (4n)
anaphase: sister chromatids separate: now 1 chromosome has 1 chromatid (2n)
telophase: 1 chromosome has 1 chromatids (2n)​

Meiosis: At the start of meiosis, DNA has just replicated, so each chromosome has 2 chromatids (4n)
Meiosis I:

Prophase I: Tetrads form: 1 Tetrad = 2 chromosomes and 4 Chromatids. (a pair of sister chromosomes)
Metaphase I: Tetrads line up (1 tetrad = 2 chromosomes and 4 chromatids).
AnaphaseI: Tetrads Split. 1 chromosomes has 2 chromatids (sister chromosomes split)
Telophase I: 1 chromosomes has 2 chromatids​

Meiosis II:

Prophase II: 1 chromosome has 2 chromatids
Metaphase II: 1 chromosomes has 2 chromatids
Anaphase II: sister chromatids within each chromosome split. Result is 1 chromosome has 1 chromatid
TelophaseII: 1 chromosome has 1 chromatid
​

Lets follow humans as an example: we have 46 chromosomes
Before DNA Replication:
46 chromosomes, 46 chromatids
After DNA replication
46 chromosomes, 92 chromatids (because each chromosome has 2 chromatids)

Mitosis:
we start with 46 chromosomes and 96 chromatids
Prophase: 46 Chromosomes 96 chromatids
Metaphase: 46 Chromosomes 96 chromatids
Anaphase: sister chromatids split: 46 chromosomes 46 chromatids
Telophase: 46 chromosomes 46 chromatids

Meiosis: start 46 chromosomes 92 chromatids
MeiosisI

Prophase I: 46 chromosomes 92 chromatids 23 tetrads (1 tetrad = 2 chromosomes and 4 chromatids)
MetaphaseI: 46 chhromosomes 92 chromatids 23 tetrads
Anaphase I: Tetrads split 23 chromosomes 46 chromatids 0 tetrads
Telophase I: each cell has 23 chromosomes and 46 chromatids​

Meiosis II:

PII: 23 chromosomes 46 chromatids
MII: 23 chromosomes 46 chromatids
AII: sister chromatids split: 23 chromosomes 23 chromatids
end of telophase II: 23 chromosomes and 23 chromatids


Hope this isnt confusing, message me if you want me to draw it out

​

Perfect explanation. Thank you very much for taking the time to type all of that up! Really appreciate it and am grateful!

 

[eeyevon]

Sure thing, just focus on the areas where the numbers are changing. good luck

 

[aldol16]

This is true for mitosis,

but in meiosis 1, there is only 1 spindle fiber attached to the kinetochore and this leads the cells getting a chromosome with 2 chromatids. So in this case, the 2 chromatids attached together are called a chromosome correct?

and this type of chromosome is only found during cell division and is not the actual resting shape of a chromosome correct?

This is also true for meiosis. The splitting of the sister chromatids occurs in meiosis 2, but the sister chromatids must be present before meiosis can begin - again, in the S phase.

 

[Giovanotto]

https://www.khanacademy.org/science...vision/v/chromosomes-chromatids-chromatin-etc

This helped me straighten out these three confusing and almost interchangeable terms. He starts explaining it at around 9 minutes.

 

[7331poas]

Before DNA replication, each chromosome has 1 chromatid (2n)
After DNA replication, each chromosome has 2 chromatids (4n)

​

Hey thanks for this outline. I have a question though because I cannot wrap my head around the amount of chromosomes for some reason. its literally the most confusing thing on the entire MCAT for me.

So what you are saying is that before the S phase, we have 46 monovalent DNA? So after replication we have what, 92 chromatid ----> 92 chromosomes ---> 46 pairs of chromosomes ?

I am really confused about this because my understanding has always been that humans have 23 pairs of homologous chromosomes and 46 chromatid.

Furthermore, you say that after DNA replication each chromosome has 2 chromatid? In that case wouldnt that be when we are diploid? I thought the human cell was *never* 4n?

 

[theonlytycrane]

A normal human cell contains 23 pairs of homologous chromosomes = 46 total chromosomes. After DNA replication, the cell still has 46 total chromosomes, but now each chromosome contains 2 sister chromatids for a total of 92 chromatids.

"2n" and "n" notation refer to the cell being diploid or haploid, respectively. Diploid cells have 2 copies of each chromosome and haploid cells have only 1 copy. A cell with a replicated chromosome with 2 sister chromatids is still considered as having only 1 copy of the chromosome.

 

[7331poas]

A normal human cell contains 23 pairs of homologous chromosomes = 46 total chromosomes. After DNA replication, the cell still has 46 total chromosomes, but now each chromosome contains 2 sister chromatids for a total of 92 chromatids.

"2n" and "n" notation refer to the cell being diploid or haploid, respectively. Diploid cells have 2 copies of each chromosome and haploid cells have only 1 copy. A cell with a replicated chromosome with 2 sister chromatids is still considered as having only 1 copy of the chromosome.

ah ha. I might be starting to get at least the nomenclature. Ive been power watching some khan academy too which is helping.

So those 2 "sister chromatids" are only counting as 1 chromosome because they are connected at the centromere right? Whereas before, the pairs of homologous chromosomes count as 2 chromosomes because they are not connected.

So in G1: 46 chromatid.
After S: 92 chromatid.

But in both scenarios you still have 46 chromosomes because those newly replicated chromatids are now in the "X" form?

 

[theonlytycrane]

@7331poas I think you got it And the chromatids will eventually separate from one another either in mitosis or meiosis.

 

[7331poas]

@7331poas I think you got it And the chromatids will eventually separate from one another either in mitosis or meiosis.

So now that I look at it more. We actually have 4n during a very brief period of Anaphase? We would have 96 "chromosomes" then?

For instance if an organism has a diploid number of 2.

G1: 2 chromatid, 2 chromosomes
S: 4 chromatid, 2 chromosomes (because they are sister and linked)
Pro: Same
Meta: Same
Ana: 4 chromatid, 4 chromosomes (because the sister chromatid split). So actually we have double the chromosomes as when we are diploid here?

 

[theonlytycrane]

@7331poas If it's mitotic anaphase, and before you proceed onto telophase and cytokinesis, yes.