Chromatin is basically a DNA in the nucleus which is the uncondensed form of chromosomes. When a DNA replicates itself, it produces two chromatids which are joined together at centromere. Both chromatids are genetically identical.
What is Chromatin?
Chromatin is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. Nuclear DNA does not appear in free linear strands; it is highly condensed and wrapped around nuclear proteins in order to fit inside the nucleus. Chromatin exists in two forms. One form, called euchromatin, is less condensed and can be transcribed. The second form, called heterochromatin, is highly condensed and is typically not transcribed. Under the microscope in its extended form, chromatin looks like beads on a string. The beads are called nucleosomes. Each nucleosome is composed of DNA wrapped around eight proteins called histones. The nucleosomes are then wrapped into a 30 nm spiral called a solenoid, where additional histone proteins support the chromatin structure. During cell division, the structure of the chromatin and chromosomes are visible under a light microscope, and they change in shape as the DNA is duplicated and separated into two cells.
What is Chromatid?
A chromatid is one copy of a newly copied chromosome which is still joined to the other copy by a single centromere. Before replication, one chromosome is composed of one DNA molecule. Following replication, each chromosome is composed of two DNA molecules; in other words, DNA replication itself increases the amount of DNA but does not increase the number of chromosomes. The two identical copies—each forming one half of the replicated chromosome—are called chromatids. During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes. Chromatid pairs are normally genetically identical, and said to be homozygous; however, if mutation(s) occur, they will present slight differences, in which case they are heterozygous. The pairing of chromatids should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome. Chromonema is the fibre-like structure in prophase in the primary stage of DNA condensation. In metaphase, they are called chromatids. Chromatids may be sister or non-sister chromatids. A sister chromatids is either one of the two chromatid of the same chromosome joined together by a common centromere. Once sister chromatids have separated (during the anaphase of mitosis or the anaphase II of meiosis during sexual reproduction), they are again called chromosomes. Although having the same genetic mass as the individual chromatids that made up its parent, the daughter “molecules” are called chromosomes in a similar way that one child of a pair of twins is not referred to as a single twin. The DNA sequence of two sister chromatids is completely identical (apart from very rare DNA copying errors). A non-sister chromatid, on the other hand, refers to either of the two chromatids of paired homologous chromosomes, that is, the pairing of a paternal chromosome and a maternal chromosome. In Chromosomal_crossovers, non-sister (homologous) chromatids form chiasmata to exchange genetic material during the prophase I of meiosis.
Key Differences between Chromatin and Chromatid
- When the cell is not dividing, the strands of DNA are called as chromatin and in mitosis after replication, the chromosomes have two chromatids.
- Chromatin is the indistinguishable mass of DNA molecules whereas chromatids are a part of a chromosome attached to it with a centromere.