Question

Genes that are expressed at all times at relatively constant levels are known as ______ genes. a. inducible b. repressible c. positive d. constitutive e. negative

Short answer

d. constitutive

Step-by-step solution

Understand the different gene expression categories

Inducible genes are usually 'off' but can be turned 'on'. Repressible genes are usually 'on' but can be turned 'off'. Positive genes promote gene transcription. Constitutive genes are expressed all the time at fairly constant levels. Negative genes inhibit gene transcription.

Relate the expressions to the statement in the question

The question refers to genes that are expressed at all times at relatively constant levels. From the explanation of gene expression categories, this statement refers to constitutive genes.

Key concepts

Gene Expression

Gene expression is the process by which the information from a gene is used to synthesize functional gene products, such as proteins. This process is crucial because it determines how cells function and how they respond to changes in their environment.

Gene expression can be complex, involving several steps from transcription, where RNA is synthesized from DNA, to translation, where proteins are synthesized from RNA. The levels of expression can vary widely. Some genes, like constitutive genes, are expressed continually. Others, such as inducible or repressible genes, have expression levels that are tightly regulated based on the cell's needs.

• Transcription: DNA is converted into RNA.
• Translation: RNA is used to make proteins.
• Regulation: Determines when and how much gene product is made.

Inducible Genes

Inducible genes are a fascinating category of genes. These genes typically remain inactive or "off" until an external signal or environmental change triggers their expression. Imagine them as being in a standby mode, awaiting a specific "go" signal, such as the presence of a substrate that needs processing.

One classic example is the lac operon found in bacteria. The lac operon is activated in the presence of lactose, which serves as an indicator to synthesize the necessary enzymes for its metabolism.

Characteristics of inducible genes include:

• Typically off or low expression-state.
• Switched on in response to environmental stimuli.
• Useful for adaptive responses.

Repressible Genes

Repressible genes, in contrast to inducible genes, are normally "on" or actively expressed under regular conditions. These genes are commonly involved in pathways or processes necessary for cell function or survival. However, their expression is inhibited in response to specific signals when their products are no longer required or need to be conserved.

For instance, the trp operon in bacteria is repressed in the abundant presence of tryptophan. When enough tryptophan is available, the bacteria operate efficiently by reducing the synthesis of tryptophan-producing enzymes.

Key points about repressible genes:

• Generally on and active in basic conditions.
• Turned off in response to excess of the final product.
• Helps in resource conservation and efficiency.

Positive Gene Regulation

Positive gene regulation refers to the mechanisms that increase the transcription of a gene. This type of regulation typically involves activator proteins that bind to the gene's promoter region, enhancing its expression. Think of it as giving a green light or amplifying a signal that encourages gene activity.

An example includes the catabolite activator protein (CAP) in bacteria, which increases the transcription of certain genes necessary for metabolizing sugars other than glucose when glucose levels are low.

Main attributes of positive gene regulation:

• Involves activators that promote gene expression.
• Enhances the rate of transcription.
• Essential for processes requiring upregulation under specific conditions.

Negative Gene Regulation

Negative gene regulation involves processes that reduce or inhibit the transcription of a gene. This usually occurs via repressor proteins that bind to specific DNA sequences, preventing the gene from being expressed, similar to applying a brake.

A familiar example is the lac operon again, where a repressor protein binds to the operon in the absence of lactose, keeping the genes turned off. This regulation is crucial for preventing unnecessary production of enzymes when they are not needed.

Core aspects of negative gene regulation:

• Involves repressors that inhibit gene expression.
• Reduces or shuts down transcription.
• Important for conserving resources and preventing overproduction.