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Genetic Changes 101: How Researchers Scout Them Out.

What is DNA?
DNA–or deoxyribonucleic acid–contains the information cells need to make proteins. In many organisms, DNA is contained in a compartment of the cell called a nucleus, though some organisms like bacteria don’t have a nucleus to compartmentalize their DNA. DNA is composed of four building blocks–A, C, G, T– that can be imagined as similar to beads on a string, only arranged in countless configurations. The specific order the building blocks are arranged one after another is called the DNA sequence.

What is a gene?
A gene is a unit of DNA that contains the information to make a protein. Generally, one gene contains the information to make one type of protein. Humans have about 25,000 genes with the information, or codes, to make proteins. However, human DNA is many times longer than the total length of the 25,000 genes. That’s because human DNA contains long stretches of so called “non-coding” DNA, in other words, DNA that does not contain codes to make proteins.

What is a genome?
A genome is an organism’s complete set of DNA, that is all its genes and non-coding DNA. Genome sequencing refers to determining the order in which DNA building blocks are arranged for an organism’s entire DNA. In humans, this amounts to ordering three billion building blocks. In recent years, DNA sequencing is performed in a machine called a DNA sequencer.

How can researchers tell genes and non-coding DNA apart?
There are specific DNA sequences signaling the start and stop of a gene. For example, the sequence ATG signal the start of a gene and TAG signals a stop. Below is a representation of a DNA sequence. If you look closely, you might find two hidden genes.

GAC TTC AAT GAT TAC GGT ATA TGC CGT TAT GAG GGG TAT ATG GAC TTC AAT GAT TAC GGT ATA TGC CGT TAT GAG GGG TAT TAG TAC GGT ATA TGC CGT TAT GAG TAC GGT ATA TGC CGT TAT GAG ATG TAC GGT ATA TGC CGT TAT TAG AAT GAT TAC GGT ATC TGC

Here’s the DNA sequence again. The underlined areas show the start and stop signals for “blue” and “green” genes; the grey areas represent non-coding DNA:

GAC TTC AAT GAT TAC GGT ATA TGC CGT TAT GAG GGG TAT ATG GAC TTC AAT GAT TAC GGT ATA TGC CGT TAT GAG GGG TAT TAG TAC GGT ATA TGC CGT TAT GAG TAC GGT ATA TGC CGT TAT GAG ATG TAC GGT ATA TGC CGT TAT TAG AAT GAT TAC GGT ATG TGC

How can researchers determine genetic changes, like between different types of cells?
As mentioned earlier, DNA is located inside cells, and there are simple techniques to extract it. If researchers have two sets of cells–like chemotherapy resistant and non-resistant white blood cells–they can extract DNA from each type of cell and put it in a DNA sequencer. After that, they will have two DNA sequences. Let’s borrow the DNA sequence from the previous question. Let’s say it’s from non-resistant cells:

GAC TTC AAT GAT TAC GGT ATA TGC CGT TAT GAG GGG TAT ATG GAC TTC AAT GAT TAC GGT ATA TGC CGT TAT GAG GGG TAT TAG TAC GGT ATA TGC CGT TAT GAG TAC GGT ATA TGC CGT TAT GAG ATG TAC GGT ATA TGC CGT TAT TAG AAT GAT TAC GGT ATC TGC

Here’s the DNA sequence from chemotherapy resistant cells. If researchers compare the DNA sequences from the non-resistant and resistant cells side-by-side, three genetic changes in the blue gene become apparent, as highlighted below:

GAC TTC AAT GAT TAC GGT ATA TGC CGT TAT GAG GGG TAT ATG GAC TTC GAT GAT TAC GGT ATA TCC CGT TAT GAG GGA TAT TAG TAC GGT ATA TGC CGT TAT GAG TAC GGT ATA TGC CGT TAT GAG ATG TAC GGT ATA TGC CGT TAT TAG AAT GAT TAC GGT ATC TGC