Regulation of Gene Expression

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This process loosens chromatin structure, thereby promoting the initiation of transcription

The addition of methyl groups (methylation) can condense chromatin; the addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin

Animation: DNA Packing

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Fig. 18-7

Histone

tails

DNA

double helix

(a) Histone tails protrude outward from a

nucleosome

Acetylated histones

Amino

acids

available

for chemical

modification

(b) Acetylation of histone tails promotes loose

chromatin structure that permits transcription

Unacetylated histones

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The histone code hypothesis proposes that specific combinations of modifications help determine chromatin configuration and influence transcription

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DNA Methylation

DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some species

DNA methylation can cause long-term inactivation of genes in cellular differentiation

In genomic imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development

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Epigenetic Inheritance

Although the chromatin modifications just discussed do not alter DNA sequence, they may be passed to future generations of cells

The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance

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Regulation of Transcription Initiation

Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery

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Organization of a Typical Eukaryotic Gene

Associated with most eukaryotic genes are control elements, segments of noncoding DNA that help regulate transcription by binding certain proteins

Control elements and the proteins they bind are critical to the precise regulation of gene expression in different cell types

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Fig. 18-8-1

Enhancer

(distal control elements)

Proximal

control elements

Poly-A signal

sequence

Termination

region

Downstream

Promoter

Upstream

DNA

Exon

Exon

Exon

Intron

Intron

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