What process converts facultative heterochromatin into euchromatin?
a) Histone acetylation
b) DNA methylation
c) Histone deacetylation
d) DNA acetylation
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Recall that eukaryotic chromosomes are long strands of DNA molecules that contain the genetic information of an organism. They are located within the nucleus of the cell, and are composed of several structures. Once such structure is chromatin, which is a complex of DNA and protein packaged into a compact form that makes up the overall chromosome molecule. Remember that chromatin forms from DNA wrapped around histone proteins.
There are two primary varieties of chromatin that are important to understand for the MCAT exam: heterochromatin and euchromatin. Figure 1 shows a comparison of heterochromatin and euchromatin. As you can see, the main difference between these two structures is how densely packed the chromatin fibers are. In euchromatin, the fibers are loosely packed. In this way, the DNA of euchromatin is accessible by RNA polymerase, and it can be actively transcribed. Heterochromatin, on the other hand, is densely packed chromatin. Unlike euchromatin, RNA polymerase cannot access the DNA in heterochromatin, and it cannot be transcribed. A useful mnemonic to differentiate between euchromatin and heterochromatin is that euchromatin (U-chromatin) is unwound. Because “euchromatin” begins with the “U” sound, it is easy to remember that euchromatin is the one that starts with U: unwound.
Heterochromatin exists in two distinct forms. One is called constitutive heterochromatin, which is always in a heterochromatin state. In this way, constitutive heterochromatin is never transcribed and generally consists of repetitive DNA sequences with structural roles. Telomeres, the repetitive DNA sequences at the end of chromosomes, are a form of constitutive heterochromatin; they protect the ends of DNA from degradation and also prevent DNA from shortening during DNA replication. Likewise, centromeres, another region of repetitive DNA, also consist of constitutive heterochromatin. Centromeres link sister chromatids together during mitosis and serve as assembly sites for kinetochores, the protein complexes to which the mitotic spindle binds.
The other form of heterochromatin is called facultative heterochromatin. Facultative heterochromatin can switch between a heterochromatin and euchromatin state, depending on the conditions. This type of heterochromatin may include coding regions in which the gene expression has been turned off. There are several ways facultative heterochromatin can switch between the inactive and active forms.
One method for how facultative heterochromatin can become euchromatin is through histone acetylation. Recall that DNA has a negative charge and is attracted to the positive charge of histone proteins. Histone acetylation will add an acetyl group to the side chains of the positively charged basic amino acids on the histone proteins. This action removes the positive charge, weakening the attractive electrostatic interaction between DNA and histone proteins. This process makes the chromatin less densely packed and converts the heterochromatin into euchromatin, and the gene can now be expressed.
Similarly, the opposite process, histone deacetylation, can occur. Instead of adding acetyl groups, histone deacetylation will remove acetyl groups. This process will reintroduce the positive charge onto the histone protein and turn euchromatin into facultative heterochromatin.
As a side note, gene expression can also be altered through a process called DNA methylation. Generally, DNA methylation, or the addition of methyl groups on DNA molecules, results in silencing gene expression. An important example of DNA methylation is the inactivation of one of the two X chromosomes in female egg cells.
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