The two cores have different abilities to dissociate from DNA. This corresponds to the need to synthesize 

a continuous leading strand (where polymerase remains associated  with the template) and 

a discontinuous lagging strand (where polymerase repetitively dissociates and reassociates)

TLS Online TPP Program

#Id: 6359

#Unit 2. Cellular Organization

Operon consist

Physiology/Mechanism

Molecular biology/Biochemistry of component

Effect of mutations

TLS Online TPP Program

#Id: 6360

#Unit 2. Cellular Organization

Repressors impede access of RNA polymerase to the promoter and Activators enhance the RNA polymerase–promoter interaction.

TLS Online TPP Program

#Id: 6361

#Unit 2. Cellular Organization

Repressors bind to specific sites on the DNA. In bacterial cells, such binding sites, called operators

Regulation by means of a repressor protein that blocks transcription is referred to as negative regulation. Activators provide a molecular counterpoint to repressors; they bind to DNA and enhance the activity of RNA polymerase at a promoter; this is positive regulation.Repressor binding to DNA is regulated by a molecular signal called effector

TLS Online TPP Program

#Id: 6362

#Unit 2. Cellular Organization

Activators that stimulate this kind of promoter work by triggering a conformational change in either RNA polymerase or DNA; that is, they interact with the stable closed complex and induce a conformational change that causes transition to the open complex. This mechanism is an example of allostery.

TLS Online TPP Program

#Id: 6363

#Unit 2. Cellular Organization

There are 4 genes (I, Z, Y, A), 2 Promoters and 3 lac operators in Lac operon

The major O1 lac operator lies adjacent to promoter

The O1 operator extends from position -5 just upstream of the mRNA start point to position +21 within the transcription unit.

TLS Online TPP Program

#Id: 6364

#Unit 2. Cellular Organization

Effects of mutations in the three lac operators All three operators are essential for effective repression.