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■ Figure 3.16 The structure of chromatin. Part of the DNA is wound around complexes of histone proteins, forming particles known as nucleosomes.

Condensed chromatin, where nucleosomes are compacted

Acetylation r

Acetylation

Acetylation of chromatin produces a more open structure

Transcription factors attach to chromatin, activate genes (producing RNA)

Acetylation of chromatin produces a more open structure

Transcription factors attach to chromatin, activate genes (producing RNA)

Chromatin Gene Expression

Deacetylation causes compaction of chromatin, silencing genetic transcription

■ Figure 3.17 Chromatin structure affects gene expression. The ability of DNA to be transcribed into messenger RNA is affected by the structure of the chromatin. The genes are silenced when the chromatin is condensed. Acetylation (addition of two-carbon groups) produces a more open chromatin structure that can be activated by transcription factors, producing mRNA. Deacetylation (removal of the acetyl groups) silences genetic transcription.

Bft It is estimated that only about 300 genes out of a total of 30,000 are active in any given cell. This is because ^ each cell becomes specialized for particular functions, in a process called differentiation. The differentiated cells of an adult are derived, or "stem from," those of the embryo. Early embryonic stem cells can become any cell in the body—they are said to be totipotent. As development proceeds, most genes are silenced as cells become more differentiated. Adult stem cells can differentiate into a range of specific cell types, but are not normally totipotent. For example, the bone marrow of an adult contains such stem cells (also described in chapter 13, p. 371). These include hematopoietic stem cells, which can form the blood cells, and mesenchymal stem cells, which can differentiate into osteo-cytes (bone cells), chondrocytes (cartilage cells), adipocytes (fat cells), and others. Neural stem cells (also described in chapter 8, p. 203) have been identified in the adult nervous system. These can migrate to particular locations and differentiate into specific neuron and glial cell types in these locations. Many scientists hope that stem cells grown in tissue culture might someday be used to grow trans-plantable tissues and organs.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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