Chapter 18 - Regulation of Gene Expression

Question 1

acetylation

Answer 1

The addition of an acetyl group, CHCO3, to an organic molecule. The acetylation of histones makes chromatin less compact so that transcription can occur.

Question 2

chromatin

Answer 2

The complex of DNA and proteins that makes up eukaryotic chromosomes.

Question 3

deacetylation

Answer 3

The removal of an acetyl group, CHCO3, from an organic molecule. The deacetylation of histones makes chromatin more compact so that transcription is prevented.

Question 4

DNA methylation

Answer 4

The presence of methyl groups on the DNA bases (usually cytosine). Methylation can affect transcription, typically lowering the amount.

Question 5

epigenetic inheritance

Answer 5

Inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence of a genome.

Question 6

gene expression

Answer 6

The process by which information encoded in DNA directs the synthesis of proteins.

Question 7

histone

Answer 7

A small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in chromatin structure.

Question 8

inducer

Answer 8

A small specific molecule that binds to a bacterial repressor protein and changes the repressor’s shape so that it cannot bind to an operator, thus switching an operon on.

Question 9

inducible operon

Answer 9

An operon which is normally turned off but can be stimulated or induced when a repressor molecule binds allosterically with an inducer molecule. The inducer makes the repressor inactive and unable to bind to the operator thus allowing transcription of the operon’s genes.

Ex. The lac operon.

Question 10

operator

Answer 10

In bacterial DNA, a sequence of nucleotide near the start of an operon to which an active repressor can attach. The binding of the repressor protein prevents RNA polymerase from attaching to the promoter and transcribing the genes of the operon.

Question 11

operon

Answer 11

A unit of genetic function found in bacteria, consisting of a promoter, an operator and a coordinately regulated cluster of genes whose products function in a common pathway.

Question 12

regulatory gene

Answer 12

A gene that codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes.

Question 13

repressible operon

Answer 13

An operon which is normally turned on but can be inhibited or repressed when an active repressor molecule binds to the operator thus blocking transcription of the operon’s genes. Ex. The trp operon.

Question 14

repressor

Answer 14

A protein that inhibits gene transcription

Question 15

Controlling Gene Expression

Answer 15

Although most cells contain the entire genome for that species, only a relatively small portions of genes may be transcribed in a given cell type at any given moment

o There are many ways to control this selective gene expression

Question 16

Prokaryote regulation of gene expression

Answer 16

Prokaryotes can regulate biochemical pathways by feedback inhibition of either by:
o Enzyme inhibition
o Operon inhibition

Question 17

Operon Model of Prokaryotes

Answer 17

• Operons are coordinated clusters of related genes controlled by a single promoter region with typically a single operator that turns the genes on
• Operons can be repressible or inducible
o Repressible Operons: Normally on
 Repressible operons can be turned off by active repressor proteins made by regulatory genes outside of the operon
o Inducible Operons: Normally off
 Inducible operons can be turned on by inducer molecules binding to repressor molecules, making them inactive and unable to bind to the operators
 The lac operon, which involves the manufacture of several enzymes needed to metabolize lactose, is an inducible operon
• When lactose is present, it will be made into allolactose which acts as an inducer to turn on the operon by making the repressor inactive

Question 18

Gene regulation in Eukaryotes

Answer 18

• Eukaryotic DNA is packaged tightly into chromatin by wrapping around histone proteins
o Histone tails are positively charged; attracted to negatively charged DNA
o Tight wrapping prevents gene transcription
o Telemere and centromere regions (heterochromatin) always tightly wrapped preventing gene transcription in those areas

• Chromatin can be modified by:
o Acetylation of histones: (adding of acetyl groups) Lessens attraction between histones and DNA: Transcription occurs
o Methylation of DNA: (adding of methyl groups) Typically prevents transcription
 Methylation patterns can be altered during an organism’s lifetime thus possibly changing gene expression (epigenetic inheritance)
 Methylation patterns can be passed on to offspring
 Epigenetics can have possibly profound implications for health and influence of environment on an organism’s gene expression

Question 19

What are some mechanisms for regulating metabolic pathways?

Answer 19

Metabolic pathways typically have several steps each involving multiple enzymes that catalyze the conversion of reactants and products in a series of chemical reactions. Such pathways become inefficient if they are allowed to keep making more and more end product, even after sufficient amounts of product have been made.

One way to regulate the pathway is for end products to act as inhibitors on the enzymes needed earlier in the pathway. If enough end product is made, the pathway will shut down in a negative feedback manner. After it has been shut down long enough and if more end product is needed, the inhibition will go away and the earlier enzymes can begin to catalyze their chemical reactions to make more product.

Another way to regulate these pathways is to control the expression of the enzymes in the first place. In this manner, end products may act as repressors, shutting off the transcription of enzymes needed in the pathway. In another negative feedback loop, if the end product are not made in enough quantity to repress transcription, more product will be made.

Question 20

What is an operon and how does it work in prokaryotic gene regulation?

Answer 20

Operons are present in prokaryotes and refer to the combination of a single promoter region that contains an operator and a series of genes that code for proteins or enzymes that are needed together for one metabolic pathway. Having the single promoter/operator for all of the genes ensures the coordinated control of expression for the genes when needed.

Question 21

What is the role of repressors in operons?

Answer 21

A repressor is a protein made by a regulatory gene separate from the operon’s genes. The repressor is capable of binding to the operator which effectively blocks RNA polymerase from being able to initiate transcription and thus turning off the operon so no gene products are made. Repressors are typically allosterically regulated so they can be in an active or inactive form.

Question 22

What’s the difference between a repressible and an inducible operon?

Answer 22

A repressible operon is one that is normally on so that the gene products are made. The binding of repressor molecule will turn off the operon.

An inducible operon is one that is normally off. An inducer molecule will bind with the repressor, making it inactive so that it can not bind with the operator, thus turning the operon on.

Repressible operons typically work in anabolic pathways because the end product of the pathway can act as a corepressor to shut off the operon when enough product has been made.

Inducible operons work well in catabolic pathways which break down molecules for metabolic uses. If the molecules are not present, there is no need for the operon to make the enzymes needed to break down the missing molecules.

Question 23

Discuss the lac operon as an example of an inducible operon.

Answer 23

The lac operon involves the genes responsible for breaking down lactose so that E. coli can utilize it as an energy source. E. coli will only use lactose if there in not enough glucose in the environment so therefore, the operon only needs to be on when there is lactose present and glucose is low.

Because the lac operon is normally off, it is considered inducible. The lac operon has a repressor molecule that is made inactive by binding allosterically with allolactose, a product of lactose. Therefore, if lactose (and consequently allolactose) is present, the allolactose will act as an inducer by binding with the repressor, making it not able to bind with the operator and thus turning on the operon.

Once the operon is on, it will produce 3 enzymes required for the import and continued metabolism of lactose.

Question 24

What are some ways eukaryotes control gene expression through chromatin structure?

Answer 24

Eukaryotes have their linear DNA strands wrapped around histone proteins forming nucleosomes. The DNA and proteins together form chromatin.

If the chromatin is tightly packed together in areas, the transcription of genes in those areas will be turned off or reduced.

If the chromatin is more loosely packed, transcription can occur.

The denseness of chromatin can be influenced in many ways, one of which is by acetylation of the histone proteins. The tails of histones are positively charged which attracts them to the negatively charged DNA.

When this attraction is strong, the chromatin is tightly packed and transcription is less. The acetylation makes the histones less positive so the attraction to the DNA is weakened, thus making the chromatin less packed and transcription can occur.

Another way to regulate transcription is to chemically modify the DNA itself by adding methyl groups. Typically, this methylation will reduce transcription.