Gene Expression

Objectives:

1. Explain how genes expression is controlled in prokaryotes.

2. Explain how gene expression is controlled in eukaryotes.

Lac Operon

Promoter

Operator

Repressor

Transcription factors

Gene expression

Cellular differentiation

Stem cell

Pluripotent; Multipotent

Epigenetics

Methylation

Histone Modification

Vocabulary

Consider the following…• Do all of the cells in your body

carry out the same processes? • Do all of the cells in your body

make the same proteins?• Do all of the cells in your body

contain the same genes?• What is the connection between

genes and protein production?• How is it possible for different

types of cells to exist in your body?

Gene Expression All somatic cells

contain a complete set of chromosomes

Not all genes get transcribed and translated.

Example: Skin cells will not

translate genes to make hemoglobin, or insulin.

Cell Differentiation•Differentiation is when cells become specialized in structure and function•It results from selective gene expression, the turning on and off of specific genes.

Stem cells • Pluripotent (can become any type of cells) or multipotent (many types of cells)

Stem cell researchPotential to help cure/treat

conditions involving damaged cells (replace cells of damaged spinal cords, or cardiac muscles, etc.)

Tutorial on stem cells: http://www.stemcellresearch.umich.edu/overview/tutorial.html

Stem cells & Therapeutic Cloning

Use embryonic stem cells to create specialized human cells.

Goal: Cure disease

Embryo does not develop into a human.

Allowed in U.S. (state by state) w/ restrictions

Why do people bank cord blood?http://www.thecelebworth.com/top-10-cord-blood-banks/

Sources of Stem Cells1.Embryonic cells (after fertilization to a few weeks)2.Placenta & Umbilical cord blood3.Bone marrow in adults

What Controls Which Genes Get Translated (Expressed)?

Prokaryotes (bacteria) Eukaryotes

Operon System

Promoter and Operator sequence before genes

Switch on/off genes

Transcription factors

Prokaryotes: Gene ExpressionOperon = Promoter + Operator + GenesPromoter = where RNA polymerase binds.Operator = Where repressor protein binds.

Operon OffRepressor protein binds to operator when lactase is not needed. Stops transcription of

genes.Operon OnLactose present binds to repressor protein. Repressor released, genes

transcribed.

Gene Expression in Prokaryotes & The Lac Operon

Animation

Prokaryotes: lac operon systemOperon: sequence of instructions for turning on/off transcription. Located before gene sequences.

Includes “promoter” and “operator” sequences.

Promoter: RNA polymerase binds to; “starting line” for transcription.

Operator: site whereRepressor protein binds and STOPS TRANSCRIPTION (when proteins are not needed)

lac operon systemRepressor protein: binds to operator site prevents transcription Repressor proteins alternate shape! Lactose present: binds to repressor protein & changes repressor shape; repressor DOES NOT fit on operator site; genes transcribed; proteins for lactose digestion made.

Lactose not present; repressor shape allows it to bind to operator and STOP transcription (proteins not needed).

Animation of lac operon systemhttp://www.sumanasinc.com/webcontent/animations/content/lacoperon.htmlMcGraw Hill animation of lac operonhttp://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation27.html

Eukaryotic Gene Expression: Transcription Factors

• More complex than prokaryotes

More elaborate (than prokaryotes)Genes are not controlled in clusters

Eukaryotes: Gene ExpressionProteins called transcription factors regulate transcription by binding to promoters or RNA polymerase

Turned ON or OFF by chemical signals in the cell

Eukaryotes: Gene Expression

Epigentics: Gene Expression

Epigenetics

Environmental factors (chemicals or temperatures) can activate or deactivate genes and influence the expression of those genes in future generations.

http://theweek.com/article/index/238907/epigenetics-how-our-experiences-affect-our-offspring

The Agouti Micehttp://www.pbs.org/wgbh/nova/body/epigenetic-mice.html

Epigenetics

• Heritable traits that do not involve changes in the underlying DNA sequence (“in addition to changes to the genetic sequence”)

• Used to describe any aspect other than DNA sequence that influences the development of an organism.

• Involves chemical modifications that “mark” certain genes with a distinct signature; “biological punctuation”

ex) Doctors v. Doctor’sNOVA clip (14min)

DNA Packing Helps Regulate Eukaryotic Gene Expression

•A single chromosome contains app. 4cm of DNA•Coiling and folding enables all this DNA to fit in the nucleus•This packing prevents gene expression by blocking transcription (protein contact with DNA)•Some regions of interphase chromosomes (chromatin) are highly packed like mitotic chromosomes •The genes in these packed regions are generally not expressed

How are genes turned on or off?

1. Methylation: Methyl groups added to DNA turn off transcription.

2. Histone modification:Proteins bind to histones (proteins that DNA is wrapped around) and causes histones to tighten DNA coiling turn off transcription

What causes methylation or histone modification????

Gene Expression: X chromosome Inactivation

• Female mammals inherit 2 X chromosomes, but do not make twice as much X-coded proteins

• One X in each somatic cell condenses into a compacted, inactive Barr body.

• The same X is not turned off in every cell

X Inactivation & Cat Fur - Tortoiseshell

X Inactivation & Cat Fur - Calico

Turning Eukaryotic Genes On & Off

•Eukaryotic RNA polymerase needs assistant transcription factor proteins•Activator proteins bind to enhancers (not adjacent to the gene)•DNA bends & interacts with other transcription factors, facilitating correct RNA polymerase attachment Gene Switches

Alternative RNA Splicing• More than one type of polypeptide can result from a

single gene• Different exons are spliced together as a result of

alternative splicing

Homeotic Genes• Master control genes that

regulate the genes that actually control the anatomy of body parts

• Discovered by studying bizarre fruit fly mutations

• Mutation in a single gene led to legs growing out of head in place of antennae

Epigenetics NOVA introduction 13 minutes http://www.pbs.org/wgbh/nova/body/epigenetics.html

Agouti mice videohttp://www.pbs.org/wgbh/nova/body/epigenetic-mice.html

Gene switches (PBS) slide showhttp://www.pbs.org/wgbh/nova/body/gene-switches.html

Ghost in Your Genes (PBS)https://www.youtube.com/watch?v=8oUJQkUk6P8

Epigenome at a Glancehttp://learn.genetics.utah.edu/content/epigenetics/

Articles for Epigenticshttp://discovermagazine.com/2013/may/13-grandmas-experiences-leave-epigenetic-mark-on-your-genes

http://theweek.com/article/index/238907/epigenetics-how-our-experiences-affect-our-offspring#