How is DNA Organized in the Human Cell?

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Our DNA contains the genetic code for creating every single protein in our bodies. All cells contain an almost identical copy of our DNA in their nuclei. However, since DNA molecules can be over 6 feet long if stretched out, they are packed and organized in a very specific manner in order to fit in the very small space of the nucleus. Here are the different levels of DNA organization:

  • Human DNA GraphicDouble helix DNA: This is the unpacked form of our DNA, in which two complementary strands are chemically linked together and are spiraling around counterclockwise, forming a “ladder-like” double-stranded molecule. Each “step” of the ladder consists of a pair of nucleotides, which are the basis of DNA sequences and are commonly known by their initials (A and T, G and C). In every cell, genetic information exists in two copies, with one coming from each of the two parents.

  • Nucleosome: This is the first step of “packaging” inside the nucleus. In a nucleosome, a segment of double helix DNA is wrapped around a set of proteins called histones. Usually, the length of DNA in each nucleosome is fixed to about 150 nucleotide pairs. When many nucleosomes are seen together in series, they have the appearance of “beads on a string”, which is a common term used for this level of organization.

  • Chromatin: In this next level, nucleosomes are tightly grouped together forming a fiber that is about 30 nm in diameter. There are two types of chromatin, namely euchromatin and heterochromatin, that differ in how compact they are and whether they allow unpacking for gene expression. These two types can be located on a microscope in different parts of the nucleus.

  • Chromosome: This is the final form of DNA organization inside the nucleus. In the chromosome form, chromatin is even more tightly packed. In the end, the entirety of the double helix DNA is packaged in 23 pairs of chromosomes, that fit in the 6μm wide cell nucleus.

It is worth mentioning here that DNA does not only exist in the cell nucleus. There is some amount of DNA in other organelles inside our cells called the mitochondria. Mitochondrial DNA is also organized in small chromosomes and contains genetic information for mitochondrial function. Unlike nuclear DNA, which is inherited from both parents, mitochondrial DNA is only inherited from the mother via the egg.

More DNA organization sources:

— Vicky Kanta