Attracting Abundance

- 97 -

The Law

Belief - IX

Our Cellular Biology - VIII

Cells package their DNA not only to protect it, but also to regulate which genes are accessed and when. Consider the genes like files in your office. In you filing cabinet, one tray consists of many files. These multiple files are analogous to your DNA. The filing trays are continuously being opened and closed to access files. You take out a file (gene), copy it on separate papers (mRNA), and return the file to the tray (DNA) in the filing cabinet in its original location. As the filing cabinet saves space in your office, so does the DNA packaging conserves space in cells. Packaging is the reason why the approximately two meters long human DNA can fit into a cell that is only a few micrometers wide. Moreover it is estimated that the human bods contains about 50 trillion cells - which works out to 100 trillion meters of DNA in every human. Now consider the fact that the Sun is 150 billion meters from earth. This means that each of us has enough DNA to go from here to the Sun and back more than 300 times, or around the Earth's equator 2.5 million times! How does all this become possible?

The answer to this question lies in the fact that certain proteins compact DNA into the microscopic space of the nucleus of the cell. These proteins are called histones. The resulting DNA-histone complex is called chromatin. The mass of DNA and histone in chromatin are almost equal. Imagine that you want to store your garden hose in a compact place. You do so by coiling the hose. Coiling requires work. Thus within the nucleus, histones provide the energy, iin the form of electrostatic interaction, to fold the DNA. Histones are positively charged proteins and DNA is negatively charged, so histones bind with DNA very tightly. As a result the resultant chromatin can be packaged into a much smaller volume than DNA alone. The chromatin undergoes further condensation to form chromosomes. So while chromatin is a lower order of DNA organization, chromosomes are the higher order of DNA organization.

Note: The mechanisam is further elaborated in the figure below and accompanying explanatory notes.

Chromosomes are composed of DNA tightly-wound around histones.

Chromosomal DNA is packaged inside microscopic nuclei with the help of histones. These are positively-charged proteins that strongly adhere to negatively-charged DNA and form complexes called nucleosomes. Each nuclesome is composed of DNA wound 1.65 times around eight histone proteins. Nucleosomes fold up to form a 30-nanometer chromatin fiber, which forms loops averaging 300 nanometers in length. The 300 nm fibers are compressed and folded to produce a 250 nm-wide fiber, which is tightly coiled into the chromatid of a chromosome

The basic repeating structural (and functional) unit of chromatin is the nucleosome which contains eight histones and about 146 base pairs of DNA. The packaging of DNA into nucleosomes shortens the fiber length about seven fold. In other words, a piece of DNA that 1 meter long will become a "string of beads" chromatin fiber just 14 cms long. Despite this shortening, a half-foot chromatin still much too long to fit into a nucleus, which is typically only 10 to 20 microns in diameter. Therefore chromatin is further coiled into an even shorter, thicker fiber, termed the "30-nanometer fiber" because it is approximately 30 nanometers in diameter.

A chromatid is one-half of two identical copies of a replicated chromosome. During cell division , the identical copies are joined together at the region of the chromosome called the centromere. Joined chromatids are known as sister chromatids. Once the paired sister chromatids separate from one another, each is known as a daughter chromosome.

Processes such as transcription require the two strands of DNA to come apart temporarily, thus allowing the polymerases to access the DNA template. However the presence of nucleosomes and the folding of chromatin into 30 nanometer fibers pose barriers to the enzyme that copy DNA. It is therefore important for the cells to have means of opening up chromatin fibers and/or removing histones temporarilty to permit transcription. Such a process is reversible as the chromatin is restored to its compact state after transcription is complete.

Eukaryote cells possess, typically, multiple pairs of linear chromosomes, all of which are contained within the cellular nucleus. When the cells are not dividing, the chromatin is less tightly packed.. The loose configuration is important because it facilitates transcription. As the cell progresses towards dividing itself, the chromatin starts to condense into tighter packages and become 10,000 times shorter than the linear DNA strand if it was pulled taut.

See below a short animation video on DNA packaging 

Namaste

आपण लवकरच भेटू

Āpaṇa lavakaraca bhēṭū

Prabir

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