Chapter 11 – Gene Expression

11.1 – 11.3 Skip 11.4

AP Biology Radjewski 2014

Some rats are genetically programmed to prefer alcohol to water

Review

• DNA RNA Protein

Examples

1. When an extracellular signal binds to its receptor, it sets in motion a signal transduction pathway that may end in some genes being activated and some being repressed.

2. During cell cycle, cyclins are synthesized only at specific points.

In some cases:

• Gene expression is modified to counteract changes in environment to help maintain homeostasis

• In other cases, gene expression changes so that the cell can perform a specific function.– For example, all of our cells carry the genes to encode

keratin (protein in hair) and hemoglobin.• But keratin is only made in specialized epithelial cells and

hemoglobin is only made by developing RBC’s– In contrast, all cells express the genes that encode for

enzymes for metabolism

Two types of genes

• Constitutive genes– Actively expressed all

the time

• Inducible genes– Expressed only when

their proteins are needed by the cell

Genes are subject to positive and negative regulation

• Positive regulation – there is a binding of an activator that stimulates transcription

• Negative regulation – there is a binding of a repressor, that prevents transcription

What are viruses?• Non living particle that can only

reproduce within a host cell• Not cellular• Can have ds DNA, ssDNA, dsRNA or

ssRNA• Takes over the host cells protein

synthesis machinery within minutes of entering the host

• 2 types of reproductive cycles– Lytic– Lysogenic

Lytic Cycle• Lytic – “break”, meaning host cell is destroyed

afterwards• 6 steps

1. Bacteriophage (virus infecting a bacteria cell) infects a host cell – viral DNA enters

2. It uses the bacterium’s RNA polymerase to transcribe early genes

3. One early protein shuts down host (bacterial) gene transcription

4. Another protein stimulates viral genome replication5. Another protein stimulates late gene transcription6. New viral capsid proteins and a protein lyses the

host cell

HIV Review

• Human immunodeficiency virus• Typically infects only cells of the immune

system that express a surface receptor called CD4.

• Proteins on the membrane are involved in the infection of new cells, which HIV enters by direct fusion of the viral envelope with the host plasma membrane

• It is a retrovirus

Retrovirus• Genome is single stranded RNA• Carries an enzyme, reverse transcriptase that makes a

DNA strand that is complementary to the RNA, while at the same time degrading the RNA and making a second DNA strand that is complementary to the first

• The resulting dsDNA becomes integrated into the host’s chromosome, where it resides and the virus can become dormant.

• Eventually cellular triggers result and stimulates transcription of the viral DNA, resulting in mRNAs that are translated into viral proteins, and in new copies of the viral genome

Negative regulation of HIV

• Normally a host cell has a negative regulatory system that can repress the expression of invading viral genes.

• However HIV can counteract this with a virus-encoded protein called Tat (Transactivator of transcription)

• Tat binds to the viral mRNA along with proteins that allow RNA polymerase to transcribe the viral genome.

Without Tat (normal Human)

With Tat (HIV infected humans)

Operon

• Cluster of genes with a single promotor that code for proteins in the DNA of bacteria – Codes for 3 lactose-metabolizing enzymes in E.

Coli

• Called the lac operon– Example of negative regulation

• 4 major components

Component #1

• Promotor region– Region of DNA to which the RNA polymerase

attaches to begin transcription

Component #2

• Structural genes– Contain DNA sequences that code for several

enzymes

Component #3• Operator– Located between the promotor and the structural

genes– Region of DNA that is able to control RNA

polymerase’s access to structural genes– It’s like a switch that can turn the operon on or off

Component #4• Repressor protein– A substance that can prevent gene expression by

binding to the operator and prevents RNA polymerase from transcribing the structural genes

– Transcription would resume when the repressor is removed by a molecule called an inducer

Background Information

• When you consume milk, the disaccharide lactose is soon present in your intestinal tract and available to the E. Coli living there.

• Before E. Coli can absorb lactose, it must first make beta-galactosidase, the enzyme that breaks down lactose into glucose and galactose.

• 3 enzymes are needed to metabolize lactose:1.B-galactoside permease – moves sugar into

cell2.B-galactosidase – hydrolyzes lactose to

glucose and galactose3.B-galactoside transacetylase – transfers acetyl

groups from acetyl CoA to certain B-galactosides. Role is unclear.

• It is in E. Coli’s best interest to focus its energy on using available nutrients

• Therefore E. Coli should only make the enzyme when lactose is present.

• How does E. Coli do this?– By an operon!

So if lactose is absent:

Repressor binds to operator and it prevents RNA polymerase from binding to promotor, so transcription is blocked. No mRNA is produced, so no enzyme is produced.

And if lactose is present:Lactose binds to repressor. The repressor becomes inactive!

Binds to promotor

Operator is free so transcription takes place and the enzymes are made!!

Inducible System

• Lac operon is called an inducible system• Allolactose (alternative form of lactose) is the

inducer and it leads to the synthesis of enzymes in the lactose-metabolizing pathway by binding to the repressor protein and preventing its binding to the operator.

Trp Operon

• Trp – tryptophan, an amino acid• A co-repressor is involved. It is a molecule that binds

to the repressor, causing it to change shape and bind to the operator, thereby inhibiting transcription.

• When tryptophan is adequately present in the cell, it is energy efficient to stop making the enzymes for tryptophan synthesis

• Therefore tryptophan functions as a co-repressor and binds to the repressor of the trp operon!– This causes the repressor to bind to the trp operator to

prevent transcription.

Tryptophan is inadequate/absent

Tryptophan is present/too much!

Repressible System

• Trp operon• The product of a metabolic pathway (the co-

repressor) binds to the repressor protein, which is then able to bind to the operator and block transcription.

Eukaryotic Cells

• Can also regulate the transcription of large stretches of DNA (containing many genes) by reversible, non-sequence-specific alterations to either the DNA or the chromosomal proteins

• These alterations can be passed on to daughter cells after mitosis or meiosis

• Are called Epigenetic changes (not mutations)

DNA Methylation• 1-5% of cytosines in the

DNA are chemically modified by the addition of a methyl group to form 5-methyl-cytosine.

• Catalyzed by the enzyme DNA methyltransferase

• Usually occurs when C’s that are adjacent to G’s

• Areas rich in the methylation are called CpG islands, and are abundant in promotors

DNA Methylation continued…

• This change in DNA is heritable– When DNA is replicated, an enzyme called

maintenance methylase catalyzes the formation of 5-methylcytosine in the new DNA strand

• But it is reversible by demethylase, which catalyzes the removal of the methyl group from cytosine.

• Methylated DNA binds specific proteins that are involved in the repression of transcription; thus heavily methylated genes tend to be inactive