why do we use methylene blue to stain cheek cells

Methylene blue is a common lab stain and is used for many different types of cells. Methylene blue is an oxidizing chemical with a lot of biological activity; it does a very good job of staining DNA and RNA. It is believed that methylene blue actually bonds with the base. Because of its affinity for DNA and RNA, methylene blue will produce a darker stain in areas where those components are present.

In the case of the human cheek cell, methylene blue causes the DNA in the nucleus to stand out so that the nucleus can clearly been seen in a light microscope. Since cheek cells have no pigmentation, without the dye there would be no contrast, which would make the cells hard to see and make it nearly impossible to view any internal details in them.
Methylene blue can be used in biology as a stain to indicate whether cells are dead or alive.

The stain cannot penetrate live cells however, in dead cells, it can penetrate the cell membrane and the cells will be stained blue.

It is also used to study RNA and DNA in gel or under the microscope. Iodine is a useful stain as well. Iodine is used when studying plant cells. It reacts with starch and turns blue-black, because it is a starch indicator. When studying plant cells, iodine can stain not only starch, but also enters the cell wall-cell membrane pores and can facilitate the staining of the nucleus, rendering it more visible under the microscope.

Iodine can work as a stain on animal cells as well, causing the cell membrane and nucleus to appear more visible.

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