Answer:
A) A corepressor must be present
Explanation:
The key to operon expression lies in the sequence of the operator and the presence or absence of a repressor. When the repressor binds the operator, it prevents RNA polymerase from initiating transcription. The capability of the repressor to bind the operator and inhibit transcription depends on the conformation of the repressor, which is regulated allosterically by a key compound in the metabolic pathway, such as lactose or tryptophan. In a repressible operon, such as the tryptophan (or trp) operon, the repressor is unable to bind to the operator DNA by itself. Instead, the repressor is active as a DNA-binding protein only when complexed with a specific factor, such as tryptophan , which functions as a corepressor.
When tryptophan is plentiful, tryptophan molecules act as corepressors by binding to the (1) inactive repressor and (2) change its shape, allowing it to bind to the operator, (3) preventing transcription of the structural genes
The structure of RNA differs fundamentally from that of DNA in three ways:<span>It forms a single strand sugar-phosphate chain.
The sugar in its nucleotides is ribose.
<span>The base thymine is replaced by the base uracil.
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DNA is made of chemical building blocks called nucleotides<span>. These building blocks are made of three parts: a </span>phosphate<span> group, a sugar group and one of four types of nitrogen bases. To form a strand of DNA, </span>nucleotides<span> are linked into chains, with the </span>phosphate<span> and sugar groups alternating.
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Each RNA nucleotide consists of three parts: a sugar, a phosphate<span> group, and a</span>nitrogen<span>-containing base. The four RNA bases are </span>adenine<span>, </span>uracil, guanine, andcytosine<span>—often referred to as A, U, G, and C. RNA shares three bases in common with </span>DNA<span>: </span>adenine<span>, guanine, and </span>cytosine<span>. Instead of </span>uracil<span>, </span>DNA<span> contains</span>thymine<span>.</span>
The air-filled spaces lined by mucous membranes found within flat and irregular bones in the body are called the sinuses.
<h3>What are bones?</h3>
The bones are the solid structures that we find in the body. The bones of a person are about two hundred and six in number. The primary function of the bones is to support the structure of the of the individual and to protect the vital and fragile structures such as the brain and the heart.
The air-filled spaces lined by mucous membranes found within flat and irregular bones in the body are called the sinuses.
Learn more about sinuses:brainly.com/question/28067493
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A homologous structure is an example of an organ or bone that appears in different animals, underlining anatomical commonalities demonstrating descent from a common ancestor. In other words, it's when very different animals have bones that appear very similar in form or function and seem to be related.