The English alphabets range from A to Z and are used to form words and meaningful sentences. DNA also contains its own set of alphabets, known as nucleotides. In contrast to the English alphabets, DNA uses only A, G, T and C. Overall, these nucleotides are arranged to form “words”, which are collectively known as a gene.
Then, these genes form “sentences”, which are collectively known as a chromosome. Our DNA is stored in the nucleus, which is found in our cells. The DNA can be up to 2 meters long, and this poses an interesting question. How does a 2-meter material get compacted into the nucleus of our cell?
Compacting the DNA
Just like a thread, the strand of DNA is wound onto a spool and this gives rise to a complex known as the nucleosome. In terms of DNA packaging, this spool is made up of 4 pairs of core histone proteins (known as H2A, H2B, H3 and H4).
Each nucleosome is linked by a linker DNA, and this linkage is stabilised by the H1 histone protein. Think of H1 protein as a needle that holds the nucleosomes together. Several sets of nucleosomes form the chromatin. The chromatin is further compacted into a chromosome, which has an appearance of an “X”.
Epi-genetics – “in addition to” – genetics
The prefix “epi” means “above”, “over” or “in addition of”. Epigenetics is the study of changes in gene expression that do not involve alteration of the DNA sequence.
To make it easier, take the word “duck” for example. “Duck” can either be referred to as the aquatic bird, or the act of crouching. These words are expressed differently, depending on the context of use.
Similarly, expression of genes depends on many factors including epigenetics modifications. One of these modifications is known as methylation, which refers to the addition of a methyl group onto a DNA strand.
Just like punctuations, methylation can alter the expression of words in a sentence. Take this sentence for example: “Most of the time travellers worry about their luggage.” Now, add a comma and the sentence gives a whole new meaning: “Most of the time, travellers worry about their luggage.” Although these sentences are made up of the same words, they have an entirely different meaning from one another due to the addition of a punctuation mark. Epigenetics modifications can also occur on histones, resulting in changes in the chromatin.
Understanding DNA and its complexity
It is fascinating how there is so much going on with our DNA. There is even a National DNA Day that commemorates the discovery of the DNA structure. With the advent of various gene editing tools such as the CRISPR-Cas9 system, it is possible to develop DNA-based treatments that may help improve lives of people. To do so, it is important to study and understand the complexity of our DNA, and to examine the downstream effects that can occur when DNA is modified.
Source: Aguilar CA and Craighead HG. (2013) Micro- and nanoscale devices for investigation of epigenetics and chromatin dynamics. Nature Nanotechnology 8(10): 709-718.