Gene Expression

Humans have 30,000 to 70,000 genes, each consisting of a sequence of bases, the building blocks of the hereditary material DNA. Before they can carry out their function, genes are copied to make messenger RNA (mRNA), in a process called transcription. This molecule is in turn used as a template for the synthesis of a protein molecule (translation). This entire process, including transcription of RNA and translation of protein, is referred to as gene expression. Only a subset of the full set of genes is expressed in a given tissue at a given time. In fact, this differential pattern of gene expression is ultimately what distinguishes lung tissue from skin, liver, and muscle tissue.

Even within a given tissue type, different genes are expressed at different times. For example, there is a very tightly controlled sequence of gene expression during the course of embryonic development. Tissues also respond to metabolic and other challenges. The pattern of gene expression changes in the liver in response to the consumption of a large meal. Similarly, muscle gene expression changes in response to vigorous exercise or injury. Drugs can also affect gene expression. Researchers can use microarrays and other methods to measure these changes in gene expression, and from them learn about how cells respond to disease or to other challenges.

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