BIOLOGY 1510 CHAPTER 16.

What is gene expression?

Conversion of a genotype into a phenotype (aka transcription and translation)

Why do cells need to control gene expression?

Adaptation to different environmental conditions, directing development and homeostasis, Differentiation of cells (different cells need to express different proteins)

Regulatory proteins

Specialized molecules that control when and how strongly genes are expressed, or how other cellular processes proceed, acting as molecular switches, dimmers, and signal integrators (repressors and activators)

Post transcription alterations

mRNA splicing, translation repressor proteins, mRNA degradation, protein degradation

Where does control of gene expression occur?

All levels of transcription and translation, but most common at initiation of transcription through regulatory proteins (regulatory proteins can repress or activate transcription)

Control of gene expression - Repressor proteins

Decreases the frequency of transcription initiation, binding to DNA at operator regions blocks RNA polymerase, negative control of transcription (i.e turn off protein production)

Control of Gene expression - activator proteins

• increases the frequency of transcription initiation

• Binding to DNA at enhancer region enhances the binding of RNA polymerase

  • Positive control of transcription (i.e increase how often transcription occurs)

Operons

multiple genes that are part of a single transcription unit having a single promoter

Operon systems are common in

Bacteria

Lac operon

A series of genes that produce proteins required for metabolism (i.e. breakdown) of lactose

When lactose is present in Lac operon and needs to be broken down

Transcription is allowed & Lac repressor protein is absent

When lactose is absent in Lac operon

Lac repressor protein binds to operator region & blocks RNA polymerase from initiating transcription

Trp operon

a series of genes, that produce proteins required to produce tryptophan, an essential amino acid (protein production is only necessary when trytophan is absent in the environment)

When trytophan is present in Trp operon

Trp repressor protein binds to operator region & Blocks RNA polymerase from initiating transcription

When tryptophan is absent in Trp operon

Transcription is allowed & Trp repressor protein is absent

Activator proteins

Increase the frequency of transcription ( rare in prokaryotes, common in eukaryotes)

Specific transcription factors

Increase the frequency of transcription, bind at enhancer region, binding causes looping of DNA , interaction of RNA polymerase II and activators enhances transcription

DNA looping

Has multiple activators

Post transcription alterations

mRNA splicing, Translation repressor proteins, mRNA degradation, protein degradation (old, unused, incorrectly folded or simply no longer needed. Done via the addition of ubiquitin to “tag” the protein. Then degraded by proteasome)

What is negative control of transcription?

A repressor decreases or prevents transcription.

What is positive control of transcription?

An activator increases transcription.

What happens to the lac operon when lactose is present?

The repressor is inactivated and cannot block the operator, allowing transcription.

What happens to the lac operon when lactose is absent?

The repressor binds the operator and blocks transcription.

What happens to the trp operon when tryptophan is present?

Tryptophan activates the repressor, which binds the operator and blocks transcription.

What happens to the trp operon when tryptophan is absent?

The repressor remains inactive, so transcription can occur.

What is mRNA splicing as a form of gene regulation?

Introns are removed and exons are joined together before the mRNA is translated.

What do translation repressor proteins do?

Bind to mRNA and prevent or decrease translation.

How can mRNA degradation regulate gene expression?

Destroying mRNA prevents it from being translated, reducing protein production.

How can protein degradation regulate gene expression?

Proteins that are damaged, incorrectly folded, old, or no longer needed can be marked for destruction.

What is ubiquitin?

A small protein used to tag proteins for degradation.

What is a proteasome?

A protein complex that breaks down proteins that have been tagged for degradation by ubiquitin.

DNA → mRNA pairing rule

DNA A → RNA U

DNA T → RNA A

DNA C → RNA G

DNA G → RNA C

reverse if going backwards