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Objectives:

1. Explain how it has been possible for the somatic cells in all the different parts of your body, given their genetic similarity (give or take a few mutations, of course!), to have developed such different characteristics and functions.

2. Describe the various kinds of gene regulation that occur in prokaryotic and eukaryotic cells.

3. Describe the interaction of hormones, cell membrane receptor proteins and intracellular receptor proteins, transcription factors, and the promotor, regulator, operator, and structural regions in the transcription of a gene.

Web resources:

• Graphics Gallery at Access Excellence.

• The Dictionary of Cell Biology

Review of Gene Expression

DNA---------->RNA---------->PROTEIN

• The Central Dogma of Molecular Biology (see also Fig. 16.1, text)

• Protein Synthesis ŇYour FriendÓ (see also chapter 16, text)

Overview

• As an embryo develops, cells differentiate because different genes are "turned on or off."

• Differentiated cells don't have different genetic material (after all, we all developed from a fertilized cell egg to produce 100 trillion cells by mitosis and we can grow a new plant from a single root cell or a new frog from a tadpole intestine cell).

• Typically, cells are different because they express different groups of genes.

Why regulate gene expression?

• control the types and quantities of proteins (gene products) produced
  some are needed in large quantities (ribosomal proteins)

  some are need in small quantities (many enzymes)

  muscle cells need lots of contractile proteins - brain cells donŐt

  intestinal cells produce lots of digestive proteins cells of the retina donŐt

  unregulated expression is energetically costly!

• respond to the environment by turning on (or off) specific genes or groups of genes
  the lac operon (producing digestive enzymes only when the food molecule is available)

• turn genes on or off in the correct sequence during development
  changes induced by hormones during puberty

What Genes Get Controlled?

• cell reproduction
• development and differentiation
  • homeotic genes -- during development, genes need to be expressed in a particular sequence within particular groups of cells. One way to accomplish this is to have a "master gene" whose product "turns on" a sequence of coordinated events. See Lewis fig. 11.8.

• metabolism
• reproduction
• cell specialization

Terminology

• Gene Product: the protein that is produced by the gene.

• Structural region of the gene: the region of DNA that codes for the production of a protein that is the product of the gene.

• Promotor region of the gene: a control sequence near the start of a gene where RNA polymerase attaches.

• Regulator region of the gene: a gene than codes for a repressor protein. This protein binds to the operator and prevents RNA polymerase from binding to the promotor.

• Operator region of the gene: The "on/off" switch of transcription. The sequence of DNA where the repressor protein binds.

The lac operon

(This gene is normally switched off, but activated when the enzymes which digest lactose, milk sugar, are needed)

• Three structural regions of this gene are all regulated by the same promotor region.

• The 3 enzymes produced by these structural regions are necessary to break down the milk sugar lactose as a source of food.

• When lactose is present, it acts as an inducer and binds to the repressor protein so that the repressor cannot bind to the operator region of the gene (it does this by changing the shape of the repressor) and the cell begins transcribing the mRNA for the three enzymes. - The gene is turned on.

• When lactose is absent or has been digested away, there is none present to bind to the repressor protein and the repressor protein binds to the operator. This blocks (represses) RNA polymerase from binding to the DNA and prevents transcription. - The gene is turned off.

• Remember! The repressor protein is a gene product too! So, for this to work the repressor must always be expressed.

• Access Excellence's Induction of the Lac Operon

• The lac operon

• The repressed lac operon

• The induced lac operon

The trp operon

(This gene is normally switched on,producing enzymes whichmake tryptophan, ans amino acid, but is deactivated when enought tryptophan has been made.)

• Five structural genes code for the enzymes needed for the production of tryptophan

• when tryptophan is present, it binds to the repressor protein so that the repressor CAN bind to the operator. Transcription is blocked. This prevents overproduction of tryptophan.

• when tryptophan is absent, the repressor protein cannot bind to the operator. Transcription proceeds, genes are transcribed, enzymes are synthesized, and the tryptophan is manufactured.

• the trp operon is normally ON and has to be turned off in the presence of tryptophan.

  • Diagram of trp operon

The control of gene expression is much more complex in eukaryotes than in prokaryotes. Why? It involves a large number of distinct but interacting mechanisms. Repressor and activator proteins are required, and these can involve regions quite far away from the gene.

Steps in Gene Expression

1. Transcriptional control [DNA---->primary mRNA]
   • transcription is controlled by DNA-binding proteins called transcription factors (TF's)

   • These proteins MUST be bound to the DNA in order for RNA polymerase to be able to bind and initiate transcription.

   • Different combinations of TF's are necessary to regulate transcription of different genes.

   • groups of TF's bind to the promotor region of the gene and often to other segments of DNA found far away from the gene. Thus, transcription of a gene is controlled by multiple sites on the DNA.

   • Transcription factors (see also Fig. 16.4, text)

   • Structures of general transcription factor complexes (from the Richmond Lab) With a 3-D illustration

   • Zipper Proteins (TFs) that bind to DNA.

2. Posttranscriptional control [primary mRNA---->mature mRNA]
   • mRNA molecules are processed before they leave the nucleus (that is, a leading and trailing sequence of DNA is added)

   • differential processing of these molecules can vary the type of mRNA that leaves the nucleus and ultimately the protein produced

   • affects the length of time it takes for mRNA to travel to the cytoplasm

   • Transcription of DNA to mRNA and mRNA Processing (see also Fig. 16.10, text)

   • Introns and exons (Fig. 16.8, text)
     • Much of the DNA in eukaryotes does not code for protein

     • Most eukaryotic genes contain long noncoding regions called introns that are INbetween the EXpressed parts of genes which are called exons.

     • The regions of the mRNA produced by the intron regions of the DNA are cut out of the mRNA before it leaves the nucleus.

3. Translational control [mRNA---->protein]
   • mRNA molecules in cytoplasm may be degraded and recycled to make more RNA.

   • This varies the amount of gene product that is produced (as a mRNA that's degraded quickly won't express much protein).

4. Posttranslational control
   • the proteins produced may require a nother chemical to be biologically active, or may be inactivated by some chemical signal.

Hormones: a chemical certain cells release into the bloodstream, which carries it to another part of the body where it alters other cells' activities. Also, individual cells may produce hormones that act locally.

• only certain cells respond to a particular hormone

Water soluble hormones (Text, fig. 33.5)

• cannot cross cells' fatty plasma membrane so never enters cell

• instead, it binds to receptors on the cells' membrane

• this activates intracellular messengers that ultimately activates enzymes that produce the effects associated with the hormone.

Fat soluble hormones (Text, fig 33.6)

• small, lipid-soluble hormones that can pass through cell membranes

• it enters the cell's nucleus where it stimulates a particular gene to direct the manufacture of a particular protein