For "JOYMERCY" Only!!! Assign #4

Regulation of Gene Activity

and Gene Mutations

Chapter 15

Mader: Biology, 9th Ed.

Prokaryotic Regulation

 Operon – group of structural and

regulating genes that function together as

a unit

 Regulator genes – located outside of the

operon, codes for a repressor that

controls whether the operon is active or

not

Mader: Biology, 9th Ed.

Prokaryotic Regulation

 Operon Model consists of the following components:

 Promoter – short DNA sequence where RNA polymerase first attaches

 Operator – short DNA sequence where active repressor binds

 Structural Genes – 1 to several genes coding for enzymes of metabolic pathway that are translated as a unit

Mader: Biology, 9th Ed.

Operon

Mader: Biology, 9th Ed.

trp Operon

 Regulator codes for a repressor that

ordinarily is unable to attach to the

operator

 RNA polymerase binds to the promoter,

the structural genes are expressed, and

the product enzymes are produced

Mader: Biology, 9th Ed.

trp Operon

 If tryptophan is absent, repressor is unable to attach to the operator (expression is normally “on”)  RNA polymerase binds to the promoter

 Enzymes for synthesis of tryptophan are produced

 If tryptophan is already present, the enzymes are not needed and the operon is turned off  Repressible operon

 Tryptophan is the corepressor, binds to repressor

 Blocks synthesis of enzymes and tryptophan

Mader: Biology, 9th Ed.

trp Operon

Mader: Biology, 9th Ed.

lac Operon

 3 structural genes are adjacent to one

another on the chromosome and are

under control of a single promoter and

single operator

 Regulator gene codes for a lac operon

repressor that ordinarily binds to the

operator and prevents transcription

Mader: Biology, 9th Ed.

lac Operon

 When lactose is present, it binds to the repressor which undergoes a change, preventing it from binding to the operator

 Lactose is the inducer of the lac operon

 The enzymes are inducible enzymes, and the entire unit is an inducible operon

 If lactose is absent, repressor attaches to the operator

 Expression is normally “off”

Mader: Biology, 9th Ed.

lac Operon

Mader: Biology, 9th Ed.

Eukaryotic Regulation

 A variety of mechanisms regulates gene expression in eukaryotic cells

 Five primary levels of control:  Chromatin packing

 Transcriptional Control

 Posttranscriptional Control

 Translational Control

 Posttranslational Control

Mader: Biology, 9th Ed.

Chromatin Structure  Eukaryotic DNA associated with

histone proteins  Together make up chromatin

 As seen in the interphase nucleus

 Nucleosomes  DNA wound around balls of eight

molecules of histone proteins

 Looks like beads on a string, each bead a nucleosome

 Presence limits access to DNA, and euchromatin becomes genetically active when histones no longer bar access to DNA

Mader: Biology, 9th Ed.

Organization of

Chromatin

Mader: Biology, 9th Ed.

Chromatin Structure Organization of Chromatin

 Euchromatin  Loosely coiled DNA

 Transcriptionally active

 Heterochromatin  Tighly packed DNA

 Transcriptionally inactive, genes are not being expressed in that particular cell

 Example of heterochromatin in a cell  Barr Bodies - Inactive X chromosome in

females

 This chromosome is not producing gene products

Mader: Biology, 9th Ed.

X-Inactivation

Mader: Biology, 9th Ed.

Transcriptional Control

 Transcription Factors – DNA-binding

proteins that control transcription

 Different combination is believed to regulate

activity of any particular gene

 Enhancers - Regions where factors that

regulate transcription can also bind

 Always present in cell, but most likely have to be

activated before they will bind to DNA

Mader: Biology, 9th Ed.

Transcription Factors

Mader: Biology, 9th Ed.

Posttranscriptional Control  Posttranscriptional control begins once there is

a primary mRNA transcript

 Processing of primary transcript  Differential excision of introns

 Differential splicing of exons

 Varies the mature mRNA that leaves the nucleus

 Speed of mRNA transport from nucleus to cytoplasm  Affect number of transcripts arriving at ER

 Can determine the amount of gene product realized per unit time following transcription

Mader: Biology, 9th Ed.

Processing of mRNA Transcripts

Mader: Biology, 9th Ed.

Translational Control

 Translational Control - Determines degree

to which mRNA is translated into a protein

product

 Presence of 5’ cap

 Length of poly-A tail on 3’ end

 Masking of mRNA

 Life of mRNA

 Influence of hormones

Mader: Biology, 9th Ed.

Posttranslational Control

 Posttranslational Control - Affects the

activity of a protein product

 Activation of protein product

 Degradation rate of a protein

Mader: Biology, 9th Ed.

Genetic Mutations

Effect of Mutations on Protein Activity

 Point Mutations

 Involve change in a single DNA nucleotide

 Therefore a change in specific codon

 Nonsense mutations – introduction of a stop codon

as a result of mutation

 Affects on protein vary

 Nonfunctional

 Reduced functionality

 Unaffected

Mader: Biology, 9th Ed.

Point Mutation

Nonsense

Mutation

Mader: Biology, 9th Ed.

Genetic Mutations

Effect of Mutations on Protein Activity

 Frameshift Mutations - Occur because

one or more nucleotides are either

inserted or deleted from DNA; renders

proteins nonfunctional

 Normal: THE CAT ATE THE RAT

 After deletion: THE ATA TET HER AT

 After insertion: THE CCA TAT ETH ERA T

Mader: Biology, 9th Ed.

Nonfunctional

Proteins

 Single nonfunctional

protein can have a

dramatic effect on the

phenotype

 Androgen insensitivity

due to faulty receptor for

androgens (male sex

hormones)

 Cells are unable to

respond to male

hormones such as

testosterone

Mader: Biology, 9th Ed.

Carcinogenesis

 Development of cancer involves a series of

various types of mutations

 Proto-oncogenes – stimulate cell cycle

 Tumor-suppressor genes – inhibit cell cycle

 Mutation in oncogene and tumor-suppressor

gene

 Stimulates cell cycle

 Leads to tumor formation

Mader: Biology, 9th Ed.

Carcinogenesis

Mader: Biology, 9th Ed.

Causes of Mutations

 Replication Errors

 1 in 1,000,000,000

replications

 DNA polymerase

 Proofreads new strands

 Generally corrects error

Xeroderma Pigmentosum

Mader: Biology, 9th Ed.

Causes of Mutations

 Environmental Mutagens – environmental

agent that increases the chance of

mutation

 Carcinogens - Mutagens that increase the

chances of cancer  UV Radiation

 Tobacco Smoke

Mader: Biology, 9th Ed.