Regulating Gene Expression WITH OVER GENES IN EVERY CELL, HOW DOES THE CELL KNOW WHAT GENES TO EXPRESS AND WHEN TO EXPRESS THEM?

Purpose of gene regulation  Every cell in your body contains the same genetic material (with the exception of your gametes)  But, your cells don’t need to produce every possible protein all the time! (You only need one cookie recipe at a time!)  The purpose of gene regulation is to tell specific cells what proteins to produce and when to produce them.  Why do certain cells only express certain genes?  How do they do this?  How do cells know what genes they need to express?

Bacterial Gene Regulation  Prokaryotes in Domain Bacteria only have exons, all of their genetic material codes for proteins  Prokaryotes express genes that are necessary for their immediate survival  This helps conserve energy  To do this, genes on bacterial DNA are organized into operons, clusters of genes that are regulated and expressed together

 Operons clusters of genes that are regulated and expressed together  Many genes under one ON/OFF switch  An operon contains a promoter sequence, an operato r and genes encoding for similar proteins  The promoter sequence is a sequence of DNA that defines where transcription begins, this is where the RNA polymerase binds to the DNA strand.  The operator sequence is a segment of DNA in front of the promoter that repressors can bind to  Repressors are proteins that bind to the DNA and block (repress) transcription of a gene

Lac Operon  Example of an operon Example of an operon  Start at 1:50  Questions to think about…  What is the function of the Lac Operon in prokaryotic cells?  What biomolecule causes the Lac Operon to begin transcribing the genes/producing mRNA?  When the above biomolecule binds to the repressor protein, it the repressor active or inactive?  What will happen in the cell when the repressor is inactivated?  Is this process and example of positive or negative feedback?

Gene regulation in eukaryotes  Gene regulation in eukaryotes is quite different than gene regulation in prokaryotes because:  Many eukaryotes are multicellular and have different tissue types  Eukaryotes contain both introns and exons in their genetic sequences  Different genes need to be on (expressed) or off (not expressed) in different types of cells. Therefore, gene expression must be regulated. Check in: T/F: Every cell in a eukaryotic organism has the same DNA T/F: Every cell in a eukaryotic organism produces the same array of proteins

Levels of gene regulation  Genes can be regulated at the  DNA level  RNA level  Protein level  They can be regulated through  Time  SpaceAbundance

DNA Level #1  Chromatin can be loosely coiled (Euchromatin), or tightly coiled (heterochromatin)  When chromatin is loosely coiled, the DNA can unwind enough to let RNA polymerase attach and transcription can occur  When chromatin is tightly coiled it cannot unwind to allow RNA polymerase to attach and transcription will NOT occur Heterochromatin

DNA Level #2  Eukaryotic cells have sites- the TATA box and an enhancer region - that transcription factors bind to and control when the gene is expressed.  The TATA box is where the RNA polymerase binds to begin transcription  The Enhancer region is a section of DNA where transcription factors bind  Transcription factors are proteins that bind to the DNA strand and either enhance or repress the production of mRNA transcripts.

Transcription Factors  Regulate the rate at which mRNA strands are produced  Example: Steroid receptor proteins  In mammals hormones can be proteins or steroids  Steroids (for example testosterone) can enter the cell and interact with steroid receptor proteins to control gene expression  When testosterone enters a cell, it binds to a steroid receptor protein, which enters the nucleus and is a transcription factor that activates the genes responsible for the male traits.

Gene Regulation at the RNA Level  Splicing (in eukaryotes only)  Before the mRNA can leave the nucleus, it needs to be processed because it contains both introns and exons. The introns are spliced out and the exons are glued back together and “expressed” into a protein  Introns are int ervening  Exons are ex pressed  Alternative splicing  Specific exons are glued together to form different proteins  Allows one gene to give rise to multiple distinct proteins

Protein Level  Modifying a protein after translation  Takes place in the Golgi apparatus  Occurs through  Folding  Adding specific chemical groups  Phosphorylation (adding phosphate group)  Cutting parts off

Review  Review regulation of eukaryotic gene expression Review  Click on control of gene expression in eukaryotes  Check In: What are 3 ways a eukaryotic cell can regulate gene expression?

Levels of gene regulation  Genes can be regulated at the  DNA level  RNA level  Protein level  They can be regulated through  Time  SpaceAbundance

Gene Regulation Through Time  Not all genes are expressed throughout an organism’s entire life.  What types of genes are only expressed early in life? What types of genes are always expressed?  Transcription factors that regulate growth and development are only expressed when an organism is growing and developing.  Genes that are always expressed control daily cellular functions like the replication of DNA, metabolism and cell repair. NucleosomeDNA helicaseATP Synthase

Gene Regulation Through Time Cont.  Some genes can be turned on or off quickly in response to the environment  Genes that regulate development and cell differentiation are only expressed at particular time periods during development

Gene Regulation Through Space  Developmental regulatory genes are genes that tell other genes what to do during development  Hox genes – a specific family of developmental regulatory genes genes that regulate an organism’s body plan (where the legs, wings, tail, head, arms etc are)  Expressed in only certain parts of the body  Video  Start 0:44 end 6:17 Video

Gene Regulation Through Abundance of mRNA Transcripts/Proteins  Certain cells in a developing embryo will express more of particular genes (they contain a higher concentration of mRNA transcripts and proteins)  The gradient in which these genes are expressed will dictate the directionality of the embryo  The head region and tail region  Parts of the arm that are close to the body vs further from the body

Recap: What are the 6 ways that the type of gene and the timing of its expression can be regulated?  Genes can be regulated at the  DNA level  RNA level  Protein level  They can be regulated through  Time  SpaceAbundance

What else is effecting gene expression? Why can there be so many differences between identical twins? …. ……A Preview of Epigenetics  Epigenetics – How the environment influences gene  Modifications to DNA due to environmental changes that causes genes to be expressed or not expressed  DNA Methylation is a common epigenetic factor. In this process methyl groups (CH 4 ) are added to DNA which blocks transcription.  DNA Methylation can be caused by stress, diet, exercise, etc. etc.