control of gene expression at transcription- The Lac

lecture 11

What is one of the most important ways cells regulate what RNAs and proteins are expressed?

Controlling gene expression at the level of transcription

How does controlling transcription regulate gene expression?

It controls when and how often a gene is transcribed (specifically, the initiation of transcription)

What kind of proteins are specialised for switching genes on and off?

Gene regulatory proteins

How do different cell types in a multicellular organism carry out specialized functions despite containing the same genome?

• Nearly all cells of a multicellular organism contain the same genome.

• Differences between cells depend upon precise control of gene expression.

• Each cell type uses a different collection of genes from its total repertoire

Give examples of specialised cells and their functions mentioned in the sources.

1. B cells of the pancreas express insulin.

2. B lymphocytes express Antibodies (Ab).

3. Nerve cells express neurotransmitters such as acetylcholine

What are the two essential structural regions of all genes regarding transcription?

Promoter region and terminator region

What enzyme performs the process of transcription?

RNA Polymerase

What are the stages of transcription carried out by RNA Polymerase?

1. Initiation, 2. Elongation. 3. termination

What is the role of the promoter region in transcription?

1. attracts RNA polymerase

2. correctly orients the enzyme to begin making an RNA copy of the gene

Besides the promoter, what other DNA sequences are involved in gene regulation?

Most genes have regulatory DNA sequences

What acts as a switch to control transcription?

A combination of regulatory DNA sequence and gene regulatory proteins

How does the complexity of gene expression control in eukaryotes compare to bacteria?

more complex in eukaryotes than in bacteria

Where is eukaryotic gene expression primarily controlled, and by what?

• initiation of transcription.

• It is controlled by proteins that bind to specific regulatory sequences and modulate RNA polymerase activity, similar to bacteria

How is gene expression regulated in the many differentiated cell types of multicellular organisms?

: By the combined actions of multiple different transcriptional regulatory proteins

What additional levels of complexity exist in eukaryotic transcription control?

The packaging of DNA into chromatin and its modification by methylation

Where are the simplest and most understood examples of gene regulation found?

bacteria

Describe the genome of E. coli according to the source.

single circular DNA molecule of about 4.6 x 10^6 nucleotide pairs and encodes ~4300 proteins

Do E. coli make all encoded proteins simultaneously? How do they regulate protein synthesis?

• Only a fraction of proteins are made at any one time.

• Bacteria regulate gene expression according to the food sources available in the environment

What bacterial gene regulation example is highlighted?

The Lac Operon

Define an operon.

• set of adjacent genes in a bacterial genome.

• They are transcribed from a single promoter into a single long mRNA, which is then translated into separate proteins.

• common in bacteria

What does the Lac operon encode proteins for?

To import and digest lactose in the microbial environment

When is Lac operon expression induced?

• in the presence of lactose.

What are the genes in the Lac Operon structure?

lacZ, lacY, and lacA.

All 3 are expressed together under the control of 1 promoter

What are the functions of the genes in the Lac Operon?

• lacY codes for lactose permease (transports lactose into the cell)

• . lacZ and lacA code for enzymes (b-galactosidase and transacetylase) that split lactose

What is the role of the lacI gene?

• located outside the operon

• codes for an allosteric repressor protein

What is the operator sequence in the Lac Operon?

regulatory DNA sequence that lies within the promoter for the lactose operon, immediately upstream of the genes

Describe the state of the Lac Operon when Lactose is present.

• The Repressor is Inactive, and the Operon is ON.

• Lactose is split into allolactose, an isomer of lactose.

• Allolactose binds to the repressor protein, making it inactive so it cannot attach to the operator

• . RNA polymerase can then bind to the promoter and transcribe t

What molecule is formed when lactose is split and binds to the repressor?

Allolactose,

. It binds to the repressor, causing a conformational change that inactivates it

Describe the state of the Lac Operon when Lactose is absent.

• The Repressor is Active, and the Operon is OFF.

• Allolactose is not present.

• The active repressor protein binds to the operator sequence, blocking RNA polymerase access to the promoter, so transcription is switched off

What happens when the repressor protein binds to the operator sequence?

RNA polymerase cannot gain access to the promoter, and transcription of the operon genes is switched off

What happens when the repressor protein cannot bind to the operator sequence?

RNA polymerase can bind to the promoter and transcribe the three genes of the lac operon