Sister Chromatids and Homologous Pairs?

[MedPR]

Sister chromatids are the two chromosomes (original plus its copy) attached by the kinetochore. I thought sister chromatids were homologous pairs as well?

Here is the picture that's confusing me.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html#centromere

Is the sister chromatid of chromosome 1 a copy of chromosome 1? If yes, then what is the chromosome that makes a homologous pair with chromosome 1?

All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number.
These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined.

So each individual has two copies of every chromosome. Using chromosome 1 as an example, You and I and everyone else have 2 copies of chromosome 1, which is equivalent to saying one homologous pair of chromosome 1?

In mitosis, every cell replicates chromosome 1 to make a sister chromatid of chromosome 1? So, in a cell undergoing mitosis, there are actually four copies of chromosome 1 in the cell? Since in a non-mitotic cell there are two homologous chromosomes, so two copies, then in mitosis the cell makes a copy of each of those homologous chromosomes forming four total sister chromatids?

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[milski]

Sister chromatids are the two chromosomes (original plus its copy) attached by the kinetochore. I thought sister chromatids were homologous pairs as well?

Syster chromatids are the two copies of the same chromosome attached together. They still make only one chromosome.

Here is the picture that's confusing me.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html#centromere

Is the sister chromatid of chromosome 1 a copy of chromosome 1? If yes, then what is the chromosome that makes a homologous pair with chromosome 1?

You cannot really say a sister chromatid of a chromosome. Depending on cell phase, each chromosome is going to consist of one or two chromatids. When there are two chromatids, they are called sister chromatids. Chromosome 1 will have another chromosome 1 that makes a homologous pair with it. Chromosome 2 will have another chromosome 2 and so on.

So each individual has two copies of every chromosome. Using chromosome 1 as an example, You and I and everyone else have 2 copies of chromosome 1, which is equivalent to saying one homologous pair of chromosome 1?

Yes, that's correct, ignoring any genetical anomalies.

In mitosis, every cell replicates chromosome 1 to make a sister chromatid of chromosome 1? So, in a cell undergoing mitosis, there are actually four copies of chromosome 1 in the cell? Since in a non-mitotic cell there are two homologous chromosomes, so two copies, then in mitosis the cell makes a copy of each of those homologous chromosomes forming four total sister chromatids?

In mitosis, the chromatid in chromosome one is replicated and becomes two sister chromatids. They are still considered a single chromosome one. Then the two chromatids are separated mechanically to become two chromosomes. Same process happens for the other homologous chromosome one, so you end up a total of four chromosomes. Each new cells gets two of these chromosome, one coming from each of the homologous chromosomes of the parent.

 

[MedPR]

Ok thanks. Yes, I understand that a sister chromatid isn't really considered a chromosome until the two sister chromatids separate during Anaphase. However, while connected, the sister chromatids have already completed replication and have all of the parts indicative of a complete chromsome, right?

Secondly, the picture shown at this link: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html#centromere

So in that picture there are a total of 2 chromosomes and 2 sister chromatids. The "homologous pair" is the 2 original chromosomes that got replicated to make 2 sister chromatids, right?

 

[milski]

Ok thanks. Yes, I understand that a sister chromatid isn't really considered a chromosome until the two sister chromatids separate during Anaphase. However, while connected, the sister chromatids have already completed replication and have all of the parts indicative of a complete chromsome, right?

Yes, they have all the features. But they still count as a single chromosome until separated.

Secondly, the picture shown at this link: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html#centromere

So in that picture there are a total of 2 chromosomes and 2 sister chromatids. The "homologous pair" is the 2 original chromosomes that got replicated to make 2 sister chromatids, right?

There are two chromosomes and two pairs of sister chromatids. The two original chromosome were a homologous pair, correct. Each of the chromosomes made 2 sister chromatids.

 

[MedPR]

Yes, they have all the features. But they still count as a single chromosome until separated.



There are two chromosomes and two pairs of sister chromatids. The two original chromosome were a homologous pair, correct. Each of the chromosomes made 2 sister chromatids.

Ok thanks! Just want to clarify one more thing, the sister chromatids, while still attached at the centromere, are not considered homologous, correct?

 

[milski]

Ok thanks! Just want to clarify one more thing, the sister chromatids, while still attached at the centromere, are not considered homologous, correct?

Correct.
Chromosomes can be homologous or not.
Chromatids can be sister or not.

 

[MedPR]

Correct.
Chromosomes can be homologous or not.
Chromatids can be sister or not.

Perfect, thank you. I'll probably have a bunch of meiosis related questions up later tonight 🙂 Going to review that while I'm here at work.

 

[milski]

Perfect, thank you. I'll probably have a bunch of meiosis related questions up later tonight 🙂

Good luck!

Going to review that while I'm here at work.

👍😉