plz explain chromosome/chromatid/chromatin with respect to ploidy and cell cycle

[rav4182]

so confused between the difference between chromosome, chromatid, and chromatin.

Which parts of the cycle do each exist?

Someone also please explain the ploidy concept. the starts off as 2n, then during the S phase becomes 4n, then after mitosis back to 2n right?


wikipedia doesnt help much, and the kaplan BB and cliffs ap bio barely scratch the subject.

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[Kahr]

Chromatin:
This is what makes up the chromosomes, it's a complex of the DNA and proteins (the proteins maintain the structure and help control the activities of the genes); it is termed euchromatin when it is all stringy and elongated, and heterochromatin when condensed.

Chromosome:
These are found in the nucleus(46 in somatic cells -- 2 sets of 23, one from each parent, and each member of a pair is a homologous chromosome; 23 in sex cells), and are made up of the chromatin as described above, which of course carry 100s to 1000s of genes. The chromosomes we think of are when the chromatin condenses. However, most of the time they are still there, just as long single strands of chromatin. It is after the DNA is duplicated (during S phase of Interphase, I believe) that it condenses. Now each duplicated chromosome has 2 sister chromatids.

Chromatid:
We think of this as sister chromatids formed as mentioned above. Each sister has an identical DNA molecule. They are attached at the waist by a centromere. When they separate during mitosis, each separated sister chromatid is now considered an individual chromosome, much as it was before duplication.

Your idea on ploidy sounds good for somatic cells, I'm sure someone more informed than I will chime in here with something to add, but for now I hope this helps.

Source: Campbell's 8th edition

 

[wall1two]

this may help as well or confuse the hell out of ya,

in S phase, 46 individual chromosomes (in the form of euchromatin, aka a stringy mess) are copied, creating 92 stringy mess chromosomes.

at prophase they are condensed into little rods called chromosomes and are connected to their copy at the hip. once the copy is connected at the hip to its "sister" (semi conservative twin) that unit is a duplicated chromosome. and a duplicated chromosome (x shaped) is formed from 2 chromatids that are "sisters" aka semiconservative copies of 1 original bar shaped chromosome.


so the things that line up in metaphase of mitosis can correctly be called chromosomes, sister chromatid pairs, sister chromatids or replicated chromosomes. and they are called all those things in campbells . meaning that 92 chromosomes form 46 sister chromosome pairs, also just called chromosomes at this point, and each new cell will contain 46 bar shapped chromosomes , each one was a sister chromatid that formed one of the x structures


campbells and wkikipediea and all the other texts ive come across call the x-shapped structures the line up in mitotic metaphase a a bunch of things.