The control of gene expression is critical to all living organisms. the amino acid tryptophan is important for making many proteins. when there is plenty of tryptophan in a cell, a protein binds to the gene that codes for an enzyme that will make tryptophan. when there is no tryptophan present this protein falls off the dna allowing the gene to be expressed so more tryptophan can be made. a protein that binds to a site on dna next to a gene and blocks the transcription of that gene, thus preventing the synthesis of a protein that the gene prescribes is known as a:

Repressor

Explanation:

Repressors are transcription factors produced by regulatory genes during a process called repression.

During this process, a regulatory gene produces a repressor (a protein) that binds to an operator. Then, the promoter (site of the DNA where RNA polymerase binds to perform transcription) is blocked, thereby preventing transcription. In other words, the expression of structural genes does not occur.

b - the respond is innate and varies over time.

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when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.