際際滷

際際滷Share a Scribd company logo

Molecular Genetics_Lac operon .ppt

Download as PPT, PDF
A
assemu1

The document discusses gene expression in prokaryotes through the lac operon in E. coli. It explains that the lac operon consists of three genes involved in lactose metabolism regulated by a single promoter. When glucose is absent and lactose is present, an activator protein binds to DNA, lifting the repressor protein off the operator site and allowing RNA polymerase to transcribe the lac operon. When glucose is present, the repressor protein binds to the operator site, blocking transcription of the lac operon.

1 of 7
Download to read offline
Gene expression in prokaryotes Functional nit of DNA containing a cluster of genes under a single promoter Group of genes that are transcribed at the same time Multiple genes are on and off simultaneously due to the regulation of one promoter
Multiple genes are on and off simultaneously due to the regulation of one promoter
Lac operon Consists of three genes each involved in the process of sugar lactose One of them the gene for the enzyme 硫-galactosidase Hydrolysis Lactose in to Glucose and Glactose E.coli can use either glucose (mono) or Lactose (Di), How ever Lactose must be hydrolyzed first in to glucose
Gene Regulation -- The Lac Operon transacetylase Structural genes (genes that code for polypeptides) Promoter: A site where RNA polymerase attaches in order to transcribe mRNA. I gene is called a regulator gene: is transcribed to make a mRNA which is translated to a repressor protein. There is a termination signal at the end of the I gene. Operator: It is a short sequence of bases that acts like a switch that can be recognized by repressor protein
Situations When glucose is absent and lactose is present An Activator protein is needed which stabilises RNA polymerase The activator protein only works when glucose is absent In this way E.coli only makes enzymes to metabolise other sugar in the absence of glucose Activator protein: Bound to DNA Repressor protein: Lifted off operator site RNA polymerase: Sits on the promoter site Lac operon: Transcription
Situations When glucose is present and lactose is absent Activator protein: Not bound to DNA Repressor protein: Bound to operator site RNA polymerase: Blocked by the repressor Lac operon: No transcription A repressor protein is continuously synthesised , It sits on a sequence of DNA just in front of the Lac operon, the Operator site. The repressor protein block the promoter site where the RNA polymerase settles before it starts transcribing
Situations When glucose is present and lactose is present Activator protein: Not bound to DNA Repressor protein: Lifted off operator site RNA polymerase: keeps falling off promoter site Lac operon: No transcription This explains how the lac operon is transcribed only lactose is present But it dose not explain why the operon is not transcribed when both present When glucose and lactose are present RNA polymerase can sit on the promoter site but it is unstable and keeps it falling off,

More Related Content

Molecular Genetics_Lac operon .ppt

  • 1. Gene expression in prokaryotes Functional nit of DNA containing a cluster of genes under a single promoter Group of genes that are transcribed at the same time Multiple genes are on and off simultaneously due to the regulation of one promoter
  • 2. Multiple genes are on and off simultaneously due to the regulation of one promoter
  • 3. Lac operon Consists of three genes each involved in the process of sugar lactose One of them the gene for the enzyme 硫-galactosidase Hydrolysis Lactose in to Glucose and Glactose E.coli can use either glucose (mono) or Lactose (Di), How ever Lactose must be hydrolyzed first in to glucose
  • 4. Gene Regulation -- The Lac Operon transacetylase Structural genes (genes that code for polypeptides) Promoter: A site where RNA polymerase attaches in order to transcribe mRNA. I gene is called a regulator gene: is transcribed to make a mRNA which is translated to a repressor protein. There is a termination signal at the end of the I gene. Operator: It is a short sequence of bases that acts like a switch that can be recognized by repressor protein
  • 5. Situations When glucose is absent and lactose is present An Activator protein is needed which stabilises RNA polymerase The activator protein only works when glucose is absent In this way E.coli only makes enzymes to metabolise other sugar in the absence of glucose Activator protein: Bound to DNA Repressor protein: Lifted off operator site RNA polymerase: Sits on the promoter site Lac operon: Transcription
  • 6. Situations When glucose is present and lactose is absent Activator protein: Not bound to DNA Repressor protein: Bound to operator site RNA polymerase: Blocked by the repressor Lac operon: No transcription A repressor protein is continuously synthesised , It sits on a sequence of DNA just in front of the Lac operon, the Operator site. The repressor protein block the promoter site where the RNA polymerase settles before it starts transcribing
  • 7. Situations When glucose is present and lactose is present Activator protein: Not bound to DNA Repressor protein: Lifted off operator site RNA polymerase: keeps falling off promoter site Lac operon: No transcription This explains how the lac operon is transcribed only lactose is present But it dose not explain why the operon is not transcribed when both present When glucose and lactose are present RNA polymerase can sit on the promoter site but it is unstable and keeps it falling off,