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MCAT Content / Control Of Gene Expression In Prokaryotes / Operon Concept Jacob Monod Model

Operon Concept, Jacob–Monod Model

Topic: Control Of Gene Expression In Prokaryotes

The operon concept states that the set of genes that are transcribed in the prokaryotes are under the control of operons. Jacob and Monod showed the organization of bacterial genes into operons.

In bacteria and archaea, structural proteins with related functions are usually encoded together within the genome in a block called an operon and are transcribed together under the control of a single promoter, resulting in the formation of a polycistronic transcript. In this way, regulation of the transcription of all of the structural genes encoding the enzymes that catalyze the many steps in a single biochemical pathway can be controlled simultaneously, because they will either all be needed at the same time, or none will be needed. For example, in E. coli, all of the structural genes that encode enzymes needed to use lactose as an energy source lie next to each other in the lactose (or lac) operon under the control of a single promoter, the lac promoter.

MCAT Operon Concept, Jacob–Monod Model

MCAT Operon Concept, Jacob–Monod Model

French scientists François Jacob (1920–2013) and Jacques Monod at the Pasteur Institute were the first to show the organization of bacterial genes into operons, through their studies on the lac operon of E. coli. For this work, they won the Nobel Prize in Physiology or Medicine in 1965. Although eukaryotic genes are not organized into operons, prokaryotic operons are excellent models for learning about gene regulation generally. There are some gene clusters in eukaryotes that function similarly to operons. Many of the principles can be applied to eukaryotic systems and contribute to our understanding of changes in gene expression in eukaryotes that can result in pathological changes such as cancer.

mcat Operon Concept, Jacob–Monod Model

Practice Questions


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MCAT Official Prep (AAMC)

Practice Exam 3 B/B Section Passage 4 Question 18

Sample Test B/B Section Question 29


Key Points

• Gene expression in prokaryotes is largely regulated at the point of transcription. Gene expression in eukaryotes is additionally regulated post-transcriptionally.

• Prokaryotic structural genes of related function are often organized into operons, all controlled by transcription from a single promoter. The regulatory region of an operon includes the promoter itself and the region surrounding the promoter to which transcription factors can bind to influence transcription.

• Jacob and Monod gave the model to explain the organization of genes into operons that control the transcription in prokaryotes. They were awarded Nobel prize for this.

Key Terms

operon: a unit of genetic material that functions in a coordinated manner using an operator, a promoter, and structural genes that are transcribed together

promoter: the section of DNA that controls the initiation of RNA transcription

polycistronic: mRNA that can encode for multiple polypeptides

archaea: are single-celled microorganisms with a structure similar to bacteria

lac operon: The lac operon is an operon that encodes proteins that allow the bacteria to use lactose as an energy source

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