Copyright © 2009 Pearson Education, Inc. PowerPoint Lectures for Biology: Concepts & Connections, Sixth Edition Campbell, Reece, Taylor, Simon, and Dickey Chapter 11 How Genes Are Controlled Lecture by Mary C. Colavito

CONTROL OF GENE EXPRESSION Copyright © 2009 Pearson Education, Inc.

 Gene expression is the overall process of information flow from genes to proteins –Mainly controlled at the level of transcription –A gene that is “turned on” is being transcribed to produce mRNA that is translated to make its corresponding protein –Organisms respond to environmental changes by controlling gene expression Copyright © 2009 Pearson Education, Inc Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes

 An operon is a group of genes under coordinated control in bacteria  The lactose (lac) operon includes –Three adjacent genes for lactose-utilization enzymes –Promoter sequence where RNA polymerase binds –Operator sequence is where a repressor can bind and block RNA polymerase action Copyright © 2009 Pearson Education, Inc.

11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes  Regulation of the lac operon –Regulatory gene codes for a repressor protein –In the absence of lactose, the repressor binds to the operator and prevents RNA polymerase action –Lactose inactivates the repressor, so the operator is unblocked Copyright © 2009 Pearson Education, Inc.

11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes  Types of operon control –Inducible operon (lac operon) –Active repressor binds to the operator –Inducer (lactose) binds to and inactivates the repressor –Repressible operon (trp operon) –Repressor is initially inactive –Corepressor (tryptophan) binds to the repressor and makes it active –For many operons, activators enhance RNA polymerase binding to the promoter Copyright © 2009 Pearson Education, Inc.

An example of gene regulation – the Lac Operon  Lac Operon Animation

DNA RNA polymerase cannot attach to promoter Lactose-utilization genes Promoter Operator Regulatory gene OPERON Protein mRNA Inactive repressor Lactose Enzymes for lactose utilization RNA polymerase bound to promoter Operon turned on (lactose inactivates repressor) mRNA Active repressor Operon turned off (lactose absent) Protein

DNA RNA polymerase cannot attach to promoter Lactose-utilization genes Promoter Operator Regulatory gene OPERON mRNA Active repressor Operon turned off (lactose absent) Protein

DNA Protein Inactive repressor Lactose Enzymes for lactose utilization RNA polymerase bound to promoter Operon turned on (lactose inactivates repressor) mRNA

DNA Inactive repressor Active repressor Inactive repressor Active repressor Lactose Promoter Tryptophan OperatorGene lac operon trp operon

11.2 Differentiation results from the expression of different combinations of genes  Differentiation involves cell specialization, in both structure and function  Differentiation is controlled by turning specific sets of genes on or off  This is how we get cells that have the same genetic information, but different structures and functions Copyright © 2009 Pearson Education, Inc.

Muscle cell Pancreas cells Blood cells

DNA double helix (2-nm diameter) “Beads on a string” Linker Histones Metaphase chromosome Tight helical fiber (30-nm diameter) Nucleosome (10-nm diameter) Supercoil (300-nm diameter) 700 nm

11.3 DNA packing in eukaryotic chromosomes helps regulate gene expression  Eukaryotic chromosomes undergo multiple levels of folding and coiling, called DNA packing –Nucleosomes are formed when DNA is wrapped around histone proteins –“Beads on a string” appearance –Each bead includes DNA plus 8 histone molecules –String is the linker DNA that connects nucleosomes –Tight helical fiber is a coiling of the nucleosome string –Supercoil is a coiling of the tight helical fiber –Metaphase chromosome represents the highest level of packing  DNA packing can prevent transcription Copyright © 2009 Pearson Education, Inc.

11.3 DNA packing in eukaryotic chromosomes helps regulate gene expression Copyright © 2009 Pearson Education, Inc. Animation: DNA Packing

11.4 In female mammals, one X chromosome is inactive in each somatic cell  X-chromosome inactivation –In female mammals, one of the two X chromosomes is highly compacted and transcriptionally inactive –Random inactivation of either the maternal or paternal chromosome –Occurs early in embryonic development and all cellular descendants have the same inactivated chromosome –Inactivated X chromosome is called a Barr body –Tortoiseshell fur coloration is due to inactivation of X chromosomes in heterozygous female cats Copyright © 2009 Pearson Education, Inc.

Two cell populations in adult X chromosomes Early embryo Allele for black fur Inactive X Black fur Allele for orange fur Orange fur Cell division and random X chromosome inactivation Active X Inactive X Active X

11.5 Complex assemblies of proteins control eukaryotic transcription  Eukaryotic genes –Each gene has its own promoter and terminator –Are usually switched off and require activators to be turned on –Are controlled by interactions between numerous regulatory proteins and control sequences Copyright © 2009 Pearson Education, Inc.

11.5 Complex assemblies of proteins control eukaryotic transcription  Regulatory proteins that bind to control sequences –Transcription factors promote RNA polymerase binding to the promoter –Activator proteins bind to DNA enhancers and interact with other transcription factors –Silencers are repressors that inhibit transcription  Control sequences –Promoter –Enhancer –Related genes located on different chromosomes can be controlled by similar enhancer sequences Copyright © 2009 Pearson Education, Inc.

11.5 Complex assemblies of proteins control eukaryotic transcription Copyright © 2009 Pearson Education, Inc. Animation: Initiation of Transcription

Enhancers Other proteins DNA Transcription factors Activator proteins RNA polymerase Promoter Gene Bending of DNA Transcription

11.6 Eukaryotic RNA may be spliced in more than one way  Alternative RNA splicing –Production of different mRNAs from the same transcript –Results in production of more than one polypeptide from the same gene –Can involve removal of an exon with the introns on either side Copyright © 2009 Pearson Education, Inc. Animation: RNA Processing

1 or Exons DNA RNA splicing RNA transcript mRNA

11.8 Translation and later stages of gene expression are also subject to regulation  Control of gene expression also occurs with –Breakdown of mRNA –Initiation of translation –Protein activation –Protein breakdown Copyright © 2009 Pearson Education, Inc.

Folding of polypeptide and formation of S—S linkages Initial polypeptide (inactive) Folded polypeptide (inactive) Active form of insulin Cleavage

11.9 Review: Multiple mechanisms regulate gene expression in eukaryotes  Many possible control points exist; a given gene may be subject to only a few of these –Chromosome changes (1) –DNA unpacking –Control of transcription (2) –Regulatory proteins and control sequences –Control of RNA processing –Addition of 5’ cap and 3’ poly-A tail (3) –Splicing (4) –Flow through nuclear envelope (5) Copyright © 2009 Pearson Education, Inc.

11.9 Review: Multiple mechanisms regulate gene expression in eukaryotes  Many possible control points exist; a given gene may be subject to only a few of these –Breakdown of mRNA (6) –Control of translation (7) –Control after translation –Cleavage/modification/activation of proteins (8) –Breakdown of protein (9) Copyright © 2009 Pearson Education, Inc.

 Applying Your Knowledge For each of the following, determine whether an increase or decrease in the amount of gene product is expected –The mRNA fails to receive a poly-A tail during processing in the nucleus –The mRNA becomes more stable and lasts twice as long in the cell cytoplasm –The region of the chromatin containing the gene becomes tightly compacted –An enzyme required to cleave and activate the protein product is missing Copyright © 2009 Pearson Education, Inc Review: Multiple mechanisms regulate gene expression in eukaryotes

Copyright © 2009 Pearson Education, Inc Review: Multiple mechanisms regulate gene expression in eukaryotes Animation: Protein Processing Animation: Protein Degradation

NUCLEUS DNA unpacking Other changes to DNA Addition of cap and tail Chromosome Gene RNA transcript Gene Transcription Intron Exon Splicing Cap mRNA in nucleus Tail Flow through nuclear envelope Broken- down mRNA CYTOPLASM Breakdown of mRNA Translation mRNA in cytoplasm Broken- down protein Cleavage / modification / activation Breakdown of protein Polypeptide Active protein

NUCLEUS DNA unpacking Other changes to DNA Addition of cap and tail Chromosome Gene RNA transcript Gene Transcription Intron Exon Splicing Cap mRNA in nucleus Tail Flow through nuclear envelope

Broken- down mRNA CYTOPLASM Breakdown of mRNA Translation mRNA in cytoplasm Broken- down protein Cleavage / modification / activation Breakdown of protein Polypeptide Active protein

11.10 Cascades of gene expression direct the development of an animal  Role of gene expression in fruit fly development –Orientation from head to tail –Maternal mRNAs present in the egg are translated and influence formation of head to tail axis –Segmentation of the body –Protein products from one set of genes activate other sets of genes to divide the body into segments –Production of adult features –Homeotic genes are master control genes that determine the anatomy of the body, specifying structures that will develop in each segment Copyright © 2009 Pearson Education, Inc.

Animation: Development of Head-Tail Axis in Fruit Flies Cascades of gene expression direct the development of an animal Copyright © 2009 Pearson Education, Inc. Animation: Cell Signaling

Head of a normal fruit fly Antenna Eye Head of a developmental mutant Leg

Head of a normal fruit fly Antenna Eye

Head of a developmental mutant Leg

Egg cell within ovarian follicle Follicle cells “Head” mRNA Protein signal Egg cell Gene expression 1 Cascades of gene expression 2 Embryo Body segments Adult fly Gene expression 3 4

11.11 CONNECTION: DNA microarrays test for the transcription of many genes at once  DNA microarray –Contains DNA sequences arranged on a grid –Used to test for transcription –mRNA from a specific cell type is isolated –Fluorescent cDNA is produced from the mRNA –cDNA is applied to the microarray –Unbound cDNA is washed off –Complementary cDNA is detected by fluorescence Copyright © 2009 Pearson Education, Inc.

cDNA Nonfluorescent spot Fluorescent spot Actual size (6,400 genes) Each well contains DNA from a particular gene DNA microarray mRNA isolated DNA of an expressed gene DNA of an unexpressed gene Reverse transcriptase and fluorescent DNA nucleotides 1 cDNA made from mRNA 2 cDNA applied to wells 3 Unbound cDNA rinsed away 4

THE GENETIC BASIS OF CANCER Copyright © 2009 Pearson Education, Inc.

11.18 Cancer results from mutations in genes that control cell division  Mutations in two types of genes can cause cancer –Oncogenes –Proto-oncogenes normally promote cell division –Mutations to oncogenes enhance activity –A faulty p53 gene causes cancer –Tumor-suppressor genes –Normally inhibit cell division –Mutations inactivate the genes and allow uncontrolled division to occur Copyright © 2009 Pearson Education, Inc.

1.Explain how prokaryotic gene control occurs in the operon 2.Describe the control points in expression of a eukaryotic gene 3.Describe DNA packing and explain how it is related to gene expression 4.Explain how alternative RNA splicing and microRNAs affect gene expression 5.Compare and contrast the control mechanisms for prokaryotic and eukaryotic genes You should now be able to Copyright © 2009 Pearson Education, Inc.

6.Distinguish between terms in the following groups: promoter—operator; oncogene—tumor suppressor gene; reproductive cloning— therapeutic cloning 7.Define the following terms: Barr body, carcinogen, DNA microarray, homeotic gene; stem cell; X-chromosome inactivation 8.Describe the process of signal transduction, explain how it relates to yeast mating, and explain how it is disrupted in cancer development You should now be able to Copyright © 2009 Pearson Education, Inc.

9.Explain how cascades of gene expression affect development 10.Compare and contrast techniques of plant and animal cloning 11.Describe the types of mutations that can lead to cancer 12.Identify lifestyle choices that can reduce cancer risk You should now be able to Copyright © 2009 Pearson Education, Inc.