Chromatin

Chromatin and DNA Compaction
This is an illustration of a chromatin fiber unraveling progressively. It unravels to reveal nucleosomes (8 histones with DNA wrapped round them) and the various bases in the DNA molecule. Credit: BSIP/UIG/Getty Images

What Is Chromatin?

Chromatin is a mass of genetic material composed of DNA and proteins that condenses to form chromosomes during eukaryotic cell division. Chromatin is located in the nucleus of our cells.

The primary function of chromatin is to compress the DNA into a compact unit that will be less voluminous and can fit within the nucleus. Chromatin consists of complexes of small proteins known as histones and DNA.

Histones help to organize DNA into structures called nucleosomes by providing a base on which the DNA can be wrapped around. A nucleosome consists of a DNA sequence of about 150 base pairs that is wrapped around a set of eight histones called an octomer. The nucleosome is further folded to produce a chromatin fiber. Chromatin fibers are coiled and condensed to form chromosomes. Chromatin makes it possible for a number of cell processes to occur including DNA replication, transcription, DNA repair, genetic recombination, and cell division.

Euchromatin and Heterochromatin

Chromatin within a cell may be compacted to varying degrees depending on a cell's stage in the cell cycle. Chromatin in the nucleus exists as euchromatin or heterochromatin. During interphase of the cycle, the cell is not dividing but undergoing a period of growth. Most of the chromatin is in a less compact form known as euchromatin.

More of the DNA is exposed in euchromatin allowing replication and DNA transcription to take place. During transcription, the DNA double helix unwinds and opens to allow the genes coding for proteins to be copied. DNA replication and transcription are needed for the cell to synthesize DNA, proteins, and organelles in preparation for cell division (mitosis or meiosis).

A small percentage of chromatin exists as heterochromatin during interphase. This chromatin is tightly packed, not allowing gene transcription to take place. Heterochromatin stains more darkly with dyes than does euchromatin.

Chromatin in Mitosis

Prophase

During prophase of mitosis, chromatin fibers become coiled into chromosomes. Each replicated chromosome consists of two chromatids joined at a centromere.

Metaphase

During metaphase, the chromatin becomes extremely condensed. The chromosomes align at the metaphase plate.

Anaphase

During anaphase, the paired chromosomes (sister chromatids) separate and are pulled by spindle microtubules to opposite ends of the cell.

Telophase

In telophase, each new daughter chromosome is separated into its own nucleus. Chromatin fibers uncoil and become less condensed. Following cytokinesis, two genetically identical daughter cells are produced. Each cell has the same number of chromosomes. The chromosomes continue to uncoil and elongate forming chromatin.

Chromatin, Chromosome, and Chromatid

People often have trouble distinguishing the difference between the terms chromatin, chromosome, and chromatid. While all three structures are composed of DNA and found within the nucleus, each is uniquely defined.

Chromatin is composed of DNA and histones that are packaged into thin, stringy fibers. These chromatin fibers are not condensed, but can exist in either a compact form (heterochromatin) or less compact form (euchromatin). Processes including DNA replication, transcription, and recombination occur in euchromatin. During cell division, chromatin condenses to form chromosomes.

Chromosomes are single-stranded groupings of condensed chromatin. During the cell division processes of mitosis and meiosis, chromosomes replicate to ensure that each new daughter cell receives the correct number of chromosomes. A duplicated chromosome is double-stranded and has the familiar X shape. The two strands are identical and connected at a central region called the centromere.

A chromatid is either of the two strands of a replicated chromosome.

Chromatids connected by a centromere are called sister chromatids. At the end of cell division, sister chromatids separate becoming daughter chromosomes in the newly formed daughter cells.

Sources:

  • Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Chromosomes and Chromatin. Available from: http://www.ncbi.nlm.nih.gov/books/NBK9863/
  • "Chromatin." Genetics Home Reference. Updated February 15, 2016. http://ghr.nlm.nih.gov/glossary=chromatin.