Overview: A Mystery at the Cellular Level
Genes within the nucleus of every cell contain all the information to control cellular growth, life, and reproduction. The DNA regulating those processes is embedded within special proteins called chromatin. For hundreds of years, scientists have wondered why some of the chromatin is found in clumps at the edge of the cell nucleus. They knew that it must be there for a reason, but weren’t sure what it was.
What Is Chromatin?
Chromatin is the name of a group of proteins and nucleic acids, called that name because they stain easily with dyes. It is chromatin that forms chromosomes when cells divide, so a lot of it is found loosely wrapped around individual strands of DNA that are active. This is the substance that makes DNA look like beads curved into a double helix when viewed with powerful electron microscopes. Some chromatin is found more tightly wrapped around genes that are not actively participating in cell division, and these bodies of heterochromatin are found at the edge of the nucleus.
Barr Bodies and Chromatin
Scientists knew some of the ways that chromatin works at the cellular level. In mammalian females, cells contain a Barr body of tightly wrapped chromatin around one of the X chromosomes. Since chromatin stains easily, the Barr bodies are relatively easy to see. They are the basis of the “sex test” used on some athletes, when scrapings from the inside of the cheek are examined.
Chromatin Is Strong
Chromatin is a much stronger substance than the DNA it protects. It is loosely wrapped around strands of DNA so that they can stay together when they are stretched out when the cell divides, and so the daughter chromosomes can stay together without damage. Otherwise, cells would not be able to divide and form copies when they reproduce. Valuable genetic information would be lost in transfer.
What Happens When Chromatin Is Tightly Wrapped?
One of the effects of chromatin’s tight wrapping around the second X chromosome in the Barr body is to deactivate it. Therefore, females do not get a “double dose” of genetic material from the second X chromsome. Recently, scientists in Germany and Singapore have discovered another important way that heterochromatin works. This is a key to the mystery of the chromatin in the bodies around the edge of the nucleus. Some genes within the cell are silent, held in their positions by tightly wrapped heterochromatin. Scientists in Singapore and Germany recently discovered that if the heterochromatin proteins are removed from the cell, so that the “silent genes” fall further into the nucleus, then cell function was disrupted. In mouse models, removal of the proteins caused genes to misfire, resulting in muscular failure, blood vessel disease, and heart failure. This process may hold important clues to many diseases.
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