Throughout the series on the Simple Cell we have seen how everything is organized and in its expected place and function. Nothing in the cell is left to chance. The nucleus is no exception. In fact, in some ways, the nucleus is more organized and complex than the rest of the cell. One aspect of the complexity and organization of the nucleus is the chromatin.
The key to all of life as we know it is contained in the genetic material contained in ever cell. The heart and brains of this genetic material is the DNA (Deoxyribonucleic Acid). The DNA contains the blueprints for life down the smallest molecular detail; therefore it must be well protected from outside influences. To carry out that protection, the cell has an elaborate system in place to protect, read, translate and utilize this valuable information in order to ensure its integrity and ability to be passed on from one generation to the next. Chromatin plays an important role in all of this.
Ever since Watson and Crick first discovered the double helix structure of DNA, we’ve all been lead to believe that the DNA in our cells are these long twisted strands of genetic material. However, have you ever wondered how those long strands of DNA fit into the nucleus of the cell? It does so with the help of chromatin.
Chromatin is a complex matrix consisting of DNA, histones and other proteins that is neatly packaged together to form chromosomes. Individual loops of coiled DNA wrap around a histone protein to form a nucleosome. The nucleosomes are then coiled together in 30 nanometer chromatin fibers. As the chromatin fibers form, the nucleosomes look like beads-on-a-string. The chromatin fibers again are coiled even tighter than before as they form into chromosomes.
Packing and compacting DNA strands. If human DNA was stretched out, it would measure nearly 80 inches long. The primary function of chromatin is to take that 80 inches of DNA and coil and compact it enough to fit into an area no more that 0.0000004 inches in diameter in the cell nucleus. Not only does the chromatin have to coil and compact the DNA to fit into such a small area, but it also has to keep it all accessible and usable.
Nuclear organization. Certain sections of the genome are generally found in specific regions with the nucleus. The chromatin helps organize and compartmentalize the genome into their respective regions of the nucleus.
Strengthen DNA strands. During the metaphase stage of cell division, the structure of chromatin changes into a form whose function is geared primarily for strength in preventing damage to the DNA as the daughter chromosomes separate.
Replication and transcription during cell division. During the interphase stage of cell division, chromatin takes on two different forms: euchromatin and heterochromatin.
Euchromatin. A lightly packed form of chromatin that is often classified as containing active DNA. It contains a high concentration of genes and often associated with the transcription. That part of the euchromatin that is not transcribed usually is converted into heterochromatin.
Heterochromatin. A tightly packed form of chromatin that is often classified as containing inactive DNA and is involved in maintaining the integrity of chromosomes along with gene regulation. Heterochromatin is further divided into two types: constitutive and facultative. Constitutive heterochromatin contains regions of DNA that are not well expressed. They often are found near the centromere and telomere areas of the chromosome. Facultative heterochromatin is less consistent than constitutive heterochromatin in that it may exist in one region in one cell and not in the next. Facultative heterochromatin has been associated with such processes as morphogenesis or differentiation and may result in the silencing on an entire chromosome.
Chromatin is a complex molecular structure with multiple functions that seems perfectly designed to pack, strengthen and control DNA. It seems obvious that chromatin is another example of an infinitely wise Creator God who designed the living cell and left nothing to chance. It reminds me of one of my favorite passages of Scripture, Job 12:7-10:
“But ask the beasts, and they will teach you;
the birds of the heavens, and they will tell you;
or the bushes of the earth, and they will teach you;
and the fish of the sea will declare to you.
Who among all these does not know
that the hand of the LORD has done this?
In his hand is the life of every living thing
and the breath of all mankind.
Anaphase The stage in mitosis or meiosis following metaphase in which the daughter chromosomes move away from each other to opposite ends of the cell.
Interphase the period of the cell cycle during which the nucleus is not undergoing division, typically occurring between mitotic or meiotic divisions.
Meiosis part of the process of gamete formation, consisting of chromosome conjugation and two cell divisions, in the course of which the diploid chromosome number becomes reduced to the haploid.
Metaphase the stage in mitosis or meiosis in which the duplicated chromosomes line up along the equatorial plate of the spindle.
Mitosis the usual method of cell division, characterized typically by the resolving of the chromatin of the nucleus into a threadlike form, which condenses into chromosomes, each of which separates longitudinally into two parts, one part of each chromosome being retained in each of two new cells resulting from the original cell.
Prophase the first stage of mitosis or meiosis in eukaryotic cell division, during which the nuclear envelope breaks down and strands of chromatin form into chromosomes.
Replication the process by which double-stranded DNA makes copies of itself, each strand, as it separates, synthesizing a complementary strand.
Transcription the process by which genetic information on a strand of DNA is used to synthesize a strand of complementary RNA.