Eukaryotic cells regulate gene expression at several levels. One important early step is controlling whether the transcription machinery can reach a gene. This lesson explains how chromatin structure and chemical modifications to histones and DNA help open or silence genes without changing the DNA sequence.
The human genome contains more than 20,000 genes distributed among 23 pairs of chromosomes. To fit inside the nucleus, DNA is compacted and organized. DNA wraps around proteins called histones, forming units called nucleosomes. Under an electron microscope, nucleosomes can look like beads along a string of DNA.
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What does epigenetic regulation mean?
It is regulation of gene activity through changes that do not alter the DNA base sequence. Some epigenetic marks can persist through cell divisions.
What are nucleosomes, and why do they matter?
Nucleosomes are DNA wrapped around histone proteins. Their position and packing help determine whether transcription machinery can access nearby genes.
How can histone acetylation affect chromatin?
Histone acetylation generally reduces histones’ positive charge, weakening their attraction to negatively charged DNA and often making chromatin more accessible.
What is DNA methylation, and where is it often found?
DNA methylation is the addition of a methyl group to DNA, commonly to cytosine at CpG sites. CpG islands occur in many gene promoter regions, where methylation is often associated with reduced transcription.
What is genomic imprinting?
Genomic imprinting is parent-of-origin-specific gene expression: a gene silenced in the gametes of one parent can remain silenced in offspring depending on which parent transmitted it.
Quiz questions
What is a nucleosome?Answer: A structural unit formed when DNA wraps around histone proteins.
How can chromatin remodeling regulate gene expression?Answer: It changes nucleosome position or DNA-histone association, influencing access to genes.
Why can histone acetylation often make DNA more accessible?Answer: It reduces histones’ positive charge, weakening their attraction to negatively charged DNA.
What is a common effect of methylation in a gene promoter?Answer: It is commonly associated with reduced transcription or gene silencing, depending on context.
A chromatin region has high DNA methylation and deacetylated histones. What is a likely outcome?Answer: Transcription is likely reduced because compact chromatin limits access to transcription machinery.
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.