By studing the dieses or problems with the patients
...the DNA molecule that packaged into thread-like structures. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.
Definite article is used before a noun (person place or thing) to indicate that the identity of the noun is known to the reader.
indefinite article is used before a noun that is general or when its identity is not known.
Autonomic nervous system innervates the entirety of the gastrointestinal tract
