Operons & Prokaryotic Gene Expression Part 2- MCAT Biology | MedSchoolCoach

Operons & Prokaryotic Gene Expression Part 2

MCAT Biology - Chapter 2 - Section 2.2 - Gene Expression - Operons & Prokaryotic Gene Expression Part 2
  1. Home
  2. »
  3. MCAT Masterclass
  4. »
  5. Biological and Biochemical Foundations of Living Systems
  6. »
  7. Biology
  8. »
  9. Operons & Prokaryotic Gene Expression Part 2- MCAT Biology
Play Video about Operons & Prokaryotic Gene Expression Part 2 - Gene Expression - MCAT Biology

Sample MCAT Question - Operons & Prokaryotic Gene Expression Part 2

Which of the following is true regarding positive inducible operons?

a) Inducer molecules bind to the activator, preventing transcription

b) Operators bind to DNA, stimulating transcription

c) Inducer molecules bind to the operator, activating transcription

d) Inducer molecules bind to activator, stimulating transcription

Click to Reveal the Answer

D is correct.

A positive inducible operon is an operon that is usually inactive under normal conditions. In this way, the activator protein usually cannot bind to the DNA to stimulate transcription. When an inducer molecule is present, it will bind to the activator protein, causing a conformational change that allows the activator protein to bind to the DNA to stimulate transcription.

Get 1-on-1 MCAT Tutoring From a Specialist

With MCAT tutoring from MedSchoolCoach, we are committed to help you prepare, excel, and optimize your ideal score on the MCAT exam.

 

For each student we work with, we learn about their learning style, content knowledge, and goals. We match them with the most suitable tutor and conduct online sessions that make them feel as if they are in the classroom. Each session is recorded, plus with access to whiteboard notes. We focus on high-yield topics if you’re pressed for time. If you have more time or high-score goals, we meticulously cover the entire MCAT syllabus.

Operons & Prokaryotic Gene Expression

In prokaryotes, as in other organisms, gene expression is tightly regulated. Structural proteins that have related functions are encoded together within the genome in various blocks called operons. An operon is a DNA sequence that includes a promoter, an operator, and the genes that are regulated together. The promoter is a DNA sequence that is recognized by RNA polymerase for transcription. The operator is a DNA sequence that a repressor can bind to, and repressors are proteins that can either allow or inhibit transcription. The set of genes in an operon are transcribed together by way of the promoter, creating a polycistronic transcript. In other words, multiple proteins are encoded by a single mRNA transcript. This form of gene expression is actually quite efficient. Instead of producing separate mRNAs for each protein required for a biochemical process, prokaryotes instead produce a single mRNA encoding all the proteins necessary for that process.

Positive Control of Prokaryotic Gene Expression

Prokaryotic gene expression can be controlled in two ways: positive control and negative control. Negative control was discussed in a previous post. This post will cover positive control of prokaryotic gene expression.

 

In positive control, an activator protein binds to DNA to stimulate transcription. The activator protein usually binds to a site on the DNA that is separate from the operator protein. The activator protein then is essentially the opposite of a repressor protein used in negative control of prokaryotic gene expression. Remember that when a repressor protein binds to DNA, it prevents transcription. When an activator protein binds, it activates or enhances transcription. Also, similar to repressor proteins, activator proteins can bind to inducer molecules. When this happens, it causes a conformational change in the activator protein, either allowing or preventing the activator protein from binding to DNA. If binding between the activator and the inducer enables transcription, the operon is said to be positive inducible. If the same binding inhibits transcription, the operon is said to be positive repressible.

Positive Inducible Operons

Positive inducible operons are operons whose expression is usually inactive. However, the presence of an inducer molecule can turn on the transcription of this type of operon. Figure 1 shows how a positive inducible operon works. Under normal conditions, the activator protein cannot bind to DNA to stimulate transcription. At first then, transcription does not occur. However, when an inducer molecule is present, the inducer molecule binds to the activator protein. This process causes a conformational change in the activator protein that allows it to bind to the DNA to stimulate transcription.

Positive Inducible Operon - MCAT Biology
Figure 1. Positive Inducible Operon

Positive Repressible Operons

Unlike positive inducible operons, positive repressible operons usually have transcription activated in their normal state. However, the presence of an inducer molecule can prevent or inhibit transcription from occurring. Figure 2 shows a positive repressible operon. Under normal conditions, the activator protein will bind to DNA, which will stimulate the production of mRNA molecules. However, when an inducer molecule is present, the inducer molecule will bind to the activator protein. This process prevents the activator protein from binding the DNA and stops transcription.

Positive Repressible Operon - MCAT Biology
Fig 2. Positive Repressible Operon

Explore More MCAT Masterclass Chapters

Take a closer look at our entire MCAT Masterclass or explore our Biochemistry lessons below.

Interview - Job interview

One-on-One Tutoring

Are you ready to take your MCAT performance to a whole new level? Work with our 99th-percentile MCAT tutors to boost your score by 12 points or more!

See if MCAT Tutoring can help me

Talk to our enrollment team about MCAT Tutoring

Medical College Admission Test - MCAT Physics

MCAT Go Audio Course

Engaging audio learning to take your MCAT learning on the go, any time, any where. You'll be on the way to a higher MCAT score no matter where you are. Listen to over 200+ lessons.

Stock photography - Image

MCAT Practice Exams

Practice makes perfect! Our mock exams coupled with thorough explanations and in-depth analytics help students understand exactly where they stand.

MCAT Prep App Mobile

MCAT Prep App

Access hundreds of MCAT videos to help you study and raise your exam score. Augment your learning with expert-created flashcards and a question banks.