APB Unit 6.3 - Gene Expression and Regulation #GoBioRams

APB Unit 6.3 - Gene Expression and Regulation #GoBioRams

Chapter 18

We need to keep track of

Prokaryotic mechanisms

v

Eukaryotic mechanisms

First up

Prokaryotic Regulation of Gene Expression

using Operons

Prokaryotes (Archaea & Bacteria)

• These organisms have been living successfully on Earth for quite a long time because they are efficient.

• They only make what they need since having a surplus isn't very efficient.

• Prokaryotes are not horders.

First up the trp operon

This operon is involved with the production of the essential amino acid tryptophan.

Note what happens if the prokaryote

does not have enough tryptophan.

Note what happens when the prokaryote

has enough tryptophan.

trp operon

• No tryptophan - repressor is inactive.

• Already have enough tryptophan - repressor is activated with a corepressor.

• Since the repressor is always considered to be "on" the trp operon is called a repressible operon.

Adding another layer of complexity.

Note the attenuator system.

Next up

lac operon in prokaryotes.

Note that this operon is usually "off" and needs an inducer to inactivate the repressor.

This makes lac operon an inducible operon.

First up

Lactose is absent.

Next up

Lactose is present.

Prokaryotic Gene Regulation "trends"

• Inducible enzymes are usually involved in catabolic pathways and their synthesis is induced by a chemical signal; trp operon.

• Repressible enzymes are usually involved in anabolic pathways and their synthesis is repressed by high levels of the end product; lac operon.

• Although lac operon can be both inducible and repressible.

Prokaryotic Gene Regulation "trends"

• Regulation of both trp and lac operons involves the negative control of genes since the operons are switched off by the active form of the repressor.

Prokaryotic Gene Regulation "trends"

• Some operons are subject to possible control by using a stimulatory protein; catabolite activator protein (CAP) which activates transcription.

• When preferred glucose is scare, CAP is activated by binding with cyclic AMP (cAMP).

Prokaryotic Gene Regulation "trends"

• Activated CAP attaches to the lac operon promoter and increases the binding affinity of RNA polymerase which will cause faster transcription.

• When glucose levels increase, CAP detaches from the lac operon promote and transcription rates go back to normal levels.

• CAP regulates other catabolic pathways.

Multiple Choice

For the trp operon, if there is tryptophan absent, the repressor is

• Active

• Inactive

Multiple Choice

The trp operon, it does NOT use a corepressor if there is plenty of tryptophan available.

• False

• True

Multiple Choice

For the lac operon, if lactose is absent, the repressor is

• Active

• Inactive

Multiple Choice

For the lac operon, what does the allolactose inducer do if lactose is present?

• The allolactose binds with the repressor and inactivates the repressor.

• The allolactose binds to the promoter and stops gene expression.

Multiple Choice

For the lac operon, if lactose is present, glucose is scarce, and the cAMP levels are high

• the lac mRNA is transcribed.

• the lac mRNA is not transcribed.

Multiple Choice

For the lac operon, if lactose is present, glucose is present, and the cAMP levels are low

• the lac mRNA is transcribed.

• the lac mRNA is not transcribed.

Now for Eukaryotes

the more "complicated" cells.

Differential gene expression

Remember differentiation?

That's when different cells in an Eukaryote are "assigned" different roles; ex. skin v. liver cells.

Know mechanisms

• Regulating chromatin structure or heterochromatin (methylation) v. euchromatin (histone acetylation or phosphorylation of the methylated histone).

• We discussed this earlier in Unit 5 during our introduction to epigenetics.

Adding another layer of complexity

• Control elements are segments of non-coding DNA that can serve as binding sites for transcription factors, which regulate transcription.

• Proximal control elements are near the promoter while distal control elements are near the enhancer.

How do these control elements and

transcription factors work?

Transcription factories

Locations where multiple euchromatin areas are exposed.

https://jcs.biologists.org/content/124/21/3676

Alternative RNA splicing

Editing which exons to keep and in what order.

DNA Degradation, Protein Processing, &

Protein Degradation can also add

to the variety of proteins that are produced.

Ah, the variety.

RNAs

We are living in an RNA World Hypothesis.

A "short list" of different RNA

https://en.wikipedia.org/wiki/List_of_RNAs

https://www.futurelearn.com/info/courses/translational-research/0/steps/14201

The list keeps growing.

Multiple mechanisms

And we keep adding to the list:

RNA interference or RNAi

microRNAs or miRNAs

small interfering RNAs or siRNAs

long noncoding RNAs or lncRNAs

Review

• We discussed prokaryotic gene regulation of expression by comparing the trp and lac operons.

• We then discussed multiple, multiple, multiple mechanisms that eukaryotes use to differentiate gene expression.

Review

• Epigenetics, heterochromatin, euchromatin, methylation, acetylation, histones,...

• Control elements, spliceosomes, transcription factories, alternative RNA splicing,...

• Lots of different RNA mechanisms and the RNA World Hypothesis.