DNA and chromatin regulation | Biomolecules | MCAT | Khan Academy
Gene Expression Regulation: An Overview
Introduction to Gene Expression Regulation
- The regulation of gene expression can occur at multiple stages, from transcription initiation to post-translational modifications of proteins.
- This regulatory ability allows cells to efficiently manage energy by expressing proteins only when necessary, likening the cell's behavior to that of a "lazy couch potato."
Structure and Function of DNA and Chromatin
- DNA is organized into chromosomes as chromatin, which consists of DNA, histone proteins, and non-histone proteins. Nucleosomes are the repeating units in chromatin made up of 146 base pairs wrapped around a core of eight histones (H2A, H2B, H3, H4).
- Histone acetylation occurs at the amino terminal tails by histone acetyltransferase (HAT), leading to chromatin uncoiling for gene accessibility. Conversely, histone deacetylation by HDAC results in a condensed structure that reduces transcription.
Epigenetic Regulation and Chromatin States
- Modifications that affect gene expression heritably are termed epigenetic regulation. Two forms of chromatin exist: heterochromatin (densely packed and inactive) and euchromatin (less dense and active). The analogy compares heterochromatin to hibernating bears versus euchromatin welcoming transcriptional machinery.
DNA Methylation Mechanism
- DNA methylation involves adding a methyl group to cytosine nucleotides via methyltransferase, primarily occurring in CpG islands. This process is crucial for normal development and is linked with genomic imprinting and X-chromosome inactivation. Abnormal methylation can lead to cancer development.
- Methylated DNA can hinder transcriptional protein binding or be bound by methyl-CpG-binding domain proteins (MBDs), which recruit additional regulatory proteins affecting gene expression stability.