Chapter 18

BiologySixth Edition

Raven/Johnson

(c) The McGraw-Hill Companies, Inc.

Not all genes are expressed all of the time.

Cells are specialized and it would be a waste of resources to make excess products when not needed.

Genes can be turned on and off.

Transcriptional Control Overview

Regulating Promoter Access:

If RNA polymerase is blocked from binding with the DNA helix, then transcription can never begin.

Promoter – specific sequence of nucleotides at one end of the gene that ‘tells’ the polymerase where to begin transcribing.

Either the promoter can be blocked by another protein, or another protein facilitates binding between the polymerase and the promoter.

Posttranscriptional Control:

Influences the mRNA or the activity of the proteins encoded by the mRNA.

Base pairs create unique bonding opportunities although the double helix is wound.

One of the helical (recognition helix) segments fits into the major groove, while the other butts up against the DNA molecule to ensure proper positioning.

This section of DNA is called the regulatory sequence.

Notice the distance between the two copies of the motif – it’s the same as the distance between the major grooves.

More than 50 types of regulatory proteins have a nearly identical sequence of 60 amino acids.

Motif is always presented to DNA in the same way.

Zinc helps to shape protein

Subunits of several hydrophobic amino acids (usually leucine).

Controlling Transcription Initiation:

Repressors – turn genes off

Activators – turn genes on

Operon – a cluster of genes that are transcribed together

Promoter – tells polymerase where to start transcription

Operator – a region where a repressor binds to

Enhancer – a region where an activator binds to

RNA polymerase binds to promoter

Unless a repressor is in the way

Repressors bind to Operators and turn genes off.

Tryptophan changes the shape of the repressor so it fits in the major grooves.

The binding of CAP to DNA makes it easier for the DNA polymerase to bind to the promoter. CAP is considered to be a trancriptional activator*.

* CAP doesn’t bind to DNA unless cAMP binds to CAP. That only happens when intracellular levels of glucose are falling and allows the use of other molecules for food. A living things response to stimuli!!

Combination of Switches

Responsible for producing three proteins that import lactose into the cell and break it down to glucose and galactose

Remember – the purpose of the lac operon is to produce proteins that breakdown lactose to glucose and galactose.

In eukaryotes the sequence TATAAA is common to many promoters and is called the TATA box.

Other sections of DNA help to bind the transcription complex to the DNA.

Rate of transcription can be regulated by the availability of activators.

Repressors can block activators.

The positioning of regulatory sites at a distance permits a large number of different regulatory sequences scattered about the DNA to influence a particular gene.

Effect of chromosome structure on gene regulation:

Histones positioned over promoters block the assembly of transcription assembly complexes.

Activators are not affected by histones.

RNA polymerase can push histones out of the way.

There are proteins that bind to clusters of 5-methylcytosine, preventing access to promoters – thus permanently turning off a gene.

Posttranscriptional Control

snRNP – small nuclear ribonuclearproteins (snurps)

Exons can be splice together in different ways.

Determined by tissue specific factors

The End.