Chapter 16 Regulation of Gene Activity

Objective: Understand how genes are regulated in both prokaryotes and eukaryotes to understand and appreciate how phenotypes are affected by varying genotypes Chapter 16 pgs.248-257 Prokaryotic Regulation -trp operon -lac operon Eukaryotic Regulation -Transcriptional control -Posttranscriptional control -Translation control -Posttranslational control

Food for thought? How would you design an intelligent being to react to the environment?

Consider that.... Every cell of the body contains ALL the DNA for the organism. Would you want to turn on all 25,000 genes in our body at the same time? Why or why not? Not all genes are necessary all the time Cells must then have the ability to turn genes on and off  

Francois Jacob and Jacques Monod Experiments with E. Coli showed that it is capable of regulating the expression of its genes An operon consists of the following elements 1.  Promoter   -  where RNA polymerase attaches, signaling the start of the gene 2.  Operator  -  where a repressor binds, stopping the transcription of that gene 3.  Structural Genes - genes coding for the enzyme, they are transcribed as a unit

The trp Operon: The Repressible Operon   It exists in the "on" state and controls the production of tryptophan. If tryptophan is already present, it binds to the repressor and prevents more tryptophan from being made

The trp Operon exists in off position, tryptophan present

The lac Operon: The inducible operon This region is normally in the "off" position, it turns on when lactose is present

The lac Operon: The inducible operon The first well-understood example of how gene expression is controlled Prokaryotic gene expression Fig.16.2 pg. 249

Logic of Lac operon Lactose is an energy source but it is not always present 2. Genes encoding enzymes that metabolize lactose are only needed (efficiency!!!) when lactose is present 3. If lactose and glucose are both present, glucose is the preferred energy source (efficiency!!!)

Repressors versus Inducers The tryp operon is a repressible operon, it is normally on but can be turned off when tryptophan is present The lac operon is an inducible operon because it is normally off but can be turned on when lactose is present These are both examples of FEEDBACK LOOPS

Tying it All Together: Lac Operon

Transcriptional control in Eukaryotes You may expect transcriptional control in eukaryotes to Involve the organization of chromatin Include regulatory proteins such as the repressor proteins from the lac operon.

In eukaryotes, a variety of mechanisms regulate gene expression 1.  chromatin structure 2.  transcriptional control 3. post transcriptional control 4.  translational control 5.  post translational control

Chromatin Structure: Euchromatin vs Heterochromatin Euchromatin is generally more active and in interphase is not condensed Heterochromatin DNA is not active and during interphase is highly compact

Chromatin Structure: Euchromatin vs Heterochromatin Euchromatin is generally more active and in interphase is not condensed Heterochromatin DNA is not active and during interphase is highly compact

Chromatin Structure DNA is wound around a core of eight protein molecules called histones, the result resembles beads on a string (nucleosome). A nucleosome the DNA double helix is wound around a core of eight histones w/ four primary types: H2A, H2B, H3 and H4.

Nucleosome structure

H3 and H4 histone conservation The amino acid sequence of H3 and H4 vary little to none between organisms. For instance, H4 of a pea varies by two amino acids from the H4 of cattle. What does this suggest? Very few mutations and histones have very significant functions (activating and deactivating genes)

Heterochromatin Chromatin condenses to become compact chromosomes at the time of cell division. How does it do this? H1 coils into a fiber that has six nucleosomes per turn (30 nm thickness)

30nm fiber

Further Levels of Chromosomal Structure

Heterochromatin How do we know that sections of heterochromatin are inactive? Barr Bodies

BARR BODIES In females, chromatin of one of the X chromosomes inactivates, this inactive chromosome is called a BARR BODY Evidence for barr bodies 1.  Women who are heterozygous for duchenne muscular dystrophy have patches of muscle tissue that are degenerative 2.  Women who are heterozygous for a condition that causes no sweat glands have patches of skin that are normal, and patches that lack sweat glands

Barr Body in Cats

Genotype:     XB XO The black patches are XB and the orange patches are XO, the cat is multicolored because not all X's are activated

Transcription factors Although no operons like those of prlaryotic cells have been found in eukaryotic cells, transcription is controlled by DNA-binding proteins called transcription factors (Tf’s. Every cell contains many different types of Tf’s and a different combination regulates the activation of a particular gene. Tf’s bind to a promoter (TATA box) adjacent to a gene and will often bind to an enhancer well upstream from the promoter. This Tf complex attracts RNA polymerase to transcribe a mRNA

Tf complex Physical contact w/ the enhancer is possible by DNA looping on itself

Posttranscriptional Control Posttranscriptional control begins once there is an mRNA transcript. Speed of transport of mRNA from nucleus into cytoplasm can affect the amount of gene product realized at time of transcription. mRNA splicing

Translational and Posttranslational Control Translational control begins when the process mRNA molecule reaches the cytoplasm. The longer mRNA (length of poly-A tail) remains in cytoplasm the more product there will be. Posttranslational Control begins once a protein has been synthesized. Finally, most metabolic pathways are regulated by feedback inhibition.

Feedback Inhibition When the end product of the pathway is present and binds at an allosteric site on the first enzyme pathway, the enzyme changes shape. Now the first reactant of the pathway cannot bind to the active site and the pathway is shut down

Gene Mutations Point Mutations - single base changes, causing a change in the amino acid structure (protein)

Frameshift mutation A base is added or deleted and changes the reading frame G A T    C A T    A A A  G A T    A  C A   T A A  A

What happens when you have a nonfunctional protein? Hemophilia PKU Cystic Fibrosis Androgen Insensitivity - caused by a faulty receptor for androgens, individual is a chromosomal male but appears female  (hermaphrodite)

Cancer and Genetics Growth of cancer often begins with the loss of the TUMOR SUPPRESSOR GENE  ONCOGENES activate, cell division occurs uncontrollably Breast Cancer Gene BRCA1  (brak-uh)

Transposons http://www.thenakedscientists.com/HTML/articles/article/jamilcolumn1.htm/ Barbara McClintock studying maize corn found that controlling elements could move from one location to another on a chromosome    --  Transposons or Jumping Genes

In maize corn a colorless kernel results from the inability to create purple pigment. If the transposon jumps to another location, the cells regain the ability to make the pigment, this creates a speckled pattern on the kernel

Introns or Exons Exons = parts of DNA that are transcribed and code for a protein Introns = sections of DNA that are not transcribed, function unknown   sometimes once called this "JUNK DNA" - scientists now believe this is not accurate

What is Epigenetics? See this article from Learn.Genetics Video at Nova narrated by Neil DeGrasse Tyspon