Difference between chromatid and chromatin?

[llamafarmer]

Hi, I'm confused on the differences between chromatid, chromatin, chromatid pairs and chromosomes. My main confusion is at which points during mitosis, is the genetic material in each of these forms. If that makes sense?

Also, where do histones and proteins come in to the process? Still staying along the lines of what form the genetic material is in.

I'm doing A Level Biology, so please try and keep explanations to the level of a 17-18 year old. Thank you!

[Daniel01]

http://answers.yahoo.com/question/index ... 011AAEl5SU and this may help http://en.wikipedia.org/wiki/Histone

[kk]

Eukaryotic cells (e.g. a human cell) have cell nucleus, containing nuclear DNA. This DNA is always in form of chromosomes. Chromosomes are made of DNA and associated proteins (e.g. histones, scaffolding proteins, telomere binding proteins). In each of our cells nuclear DNA is fragmented to long pieces of double stranded (double helix) DNA, these fragments are wrapped around histone proteins and are associated with other structural proteins (scaffold proteins), and are called chromosomes.

The fine structure describing how exactly DNA is wrapped onto histone complexes is called chromatine structure. Millions of tiny nucleosomes (146 base pair long DNA wrapped around a histone complex) are linked together to form the chromatine structure. If you zoom out, you see larger protein complexes (scaffold proteins) used as an internal framework for the chromatin to build up chromosomes.

Before mitosis (or meiosis) DNA (chromosomes) is doubled. This process is called DNA replication. As a result of DNA replication, each chromosome appear as two chromosomes next to each other, being exact copies of each other. It this state, instead of calling them chromosomes, we call them chromatids. Thus, two sister chromatids are linked together at their specific DNA sequences called centromeres.

Interesting, but chromosomes are usually invisible under light microscopes. That is, DNA is loosely associated with histones, the density of chromosome is low. In this so called euchromatin stage, information coded in DNA is available for transcription. In mitosis (and meiosis), general transcription is shut down, DNA associates with proteins tightly, becomes a very dense structure called heterochromatin. Chromosomes are now visible under light microsope. Note, that chromosomes are now visible as duplicates, so we actually call each chromosome a chromatid, a member of a duplex structure. Once these sister chromatids separate, the single "chromatids" can again be called chromosomes.

--- as far as I'm concerned.

[kk]

except chromatine is spelled chromatin