BIOLOGY Chapter 13: 10th Edition Regulation of Gene … · 2 Outline Prokaryotic Regulation trp Operon lac Operon Eukaryotic Regulation Chromatin Structure Transcriptional Control

Sylv

ia S

. Ma

der

Copyright © The McGraw Hill Companies Inc. Permission required for reproduction or display

PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor

BIOLOGY 10th Edition

Regulation of Gene Activity

Chapter 13: pp. 233 - 248

1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

regulator gene promoter operator structural genes

DNA

RNA polymerase

RNA polymerase cannot bind to promoter.

mRNA

enzymes inactive repressor

a. Tryptophan absent. Enzymes needed to synthesize tryptophan are produced.

DNA

inactive repressor

b. Tryptophan present. Presence of tryptophan prevents production of enzymes used to synthesize tryptophan.

tryptophan

active repressor

5 3

2

Outline

Prokaryotic Regulation

trp Operon

lac Operon

Eukaryotic Regulation

Chromatin Structure

Transcriptional Control

Posttranscriptional Control

Translational Control

Posttranslational Control

Genetic Mutations

Cancer

3


Bacteria do not require the same enzymes all the time

Enzymes are produced as needed

Francois Jacob and Jacques Monod (1961) proposed the operon model to explain regulation of gene expression in prokaryotes

Operon is a group of structural and regulatory genes that function as a single unit

4

Prokaryotic Regulation: The Operon Model

Operon consist of three components

Promoter

DNA sequence where RNA polymerase first attaches

Short segment of DNA

Operator

DNA sequence where active repressor binds

Short segment of DNA

Structural Genes

One to several genes coding for enzymes of a metabolic

pathway

Translated simultaneously as a block

Long segment of DNA

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6

Repressible Operons: The trp Operon

The regulator codes for a repressor

If tryptophan (an amino acid) 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 present:

Combines with repressor as corepressor

Repressor becomes functional

Blocks synthesis of enzymes and tryptophan

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8

The trp Operon


When the repressor

binds to the operator,

transcription is prevented.

active

repressor

structural genes

regulator gene

promoter operator

9

The trp Operon



DNA

RNA polymerase


mRNA



DNA

inactive repressor


tryptophan

active repressor

5 3

10

Inducible Operons: The lac Operon

The regulator codes for a repressor

If lactose (a sugar that can be used for food) is absent:

Repressor attaches to the operator

Expression is normally “off”

If lactose is present:

It combines with repressor and renders it unable to bind to operator

RNA polymerase binds to the promoter

The three enzymes necessary for lactose catabolism are produced

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12

The lac Operon



DNA


RNA polymerase can bind to promoter.

active repressor

active repressor

mRNA

enzymes

active repressor

inactive repressor

b. Lactose present. Enzymes needed to take up and use lactose are produced only when lactose is present.

a. Lactose absent. Enzymes needed to take up and use lactose are not produced.

lactose

DNA

5 3

Animation













14

Action of CAP


DNA

inactive CAP

inactive CAP

active CAP

a. Lactose present, glucose absent (cAMP level high)

b. Lactose present, glucose present (cAMP level low)

DNA

cAMP

promoter CAP binding site

RNA polymerase binds

fully with promoter.

RNA polymerase does

not bind fully with promoter.

promoter operator

operator

CAP binding site

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16


A variety of mechanisms

Five primary levels of control:

Nuclear levels

Chromatin Packing



Cytoplasmic levels



Animation

17













18

Regulation of Gene Expression:

Levels of Control in Eukaryotes

functional protein

plasma

membrane

polypeptide chain

Posttranslational

control

Posttranscriptional control

Transcriptional control

Translational

control

nuclear pore

mRNA

pre-

mRNA intron exon

histones

nuclear envelope

Chromatin

structure


3

5

3

5

19

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

The levels of chromatin packing determined by

degree of nucleosome coiling

20

Chromatin Structure Regulates

Gene Expression


DNA

histone protein

a. Darkly stained heterochromatin and lightly stained euchromatin

b. A nucleosome c. DNA unpacking

H2A

H2B

H3

H1

H4 histone

tail

nucleosome

accessible

promoter

DNA to be transcribed

inaccessible

promoter

a: Courtesy Stephen Wolfe

euchromatin heterochromatin nucleolus

1 mm

chromatin remodeling complex

21

Chromatin Packing

Euchromatin

Loosely coiled DNA

Transcriptionally active

Heterochromatin

Tightly packed DNA

Transcriptionally inactive

Barr Bodies

Females have two X chromosomes, but only one is active

Other is tightly packed along its entire length

Inactive X chromosome is Barr body

22

X-Inactivation in Mammalian Females

Coats of tortoiseshell

cats have patches

of orange and black.

One X chromosome is inactivated in

each cell. Which one is by chance. Females have two

X chromosomes.

active X chromosome

inactive X

inactive X

active X chromosome

allele for

orange color

allele for

black color

cell division Barr bodies

© Chanan Photo 2004


23


Transcription controlled by proteins called

transcription factors

Bind to enhancer DNA

Regions of DNA 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

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27

Eukaryotic Transcription Factors


promoter

DNA

enhancer

transcription

activator

mediator proteins

mRNA transcription

RNA polymerase

transcription

factor complex

gene

28


Posttranscriptional control operates on primary mRNA transcript

Given a specific primary transcript: Excision of introns can vary

Splicing of exons can vary

Determines the type of mature transcript that leaves the nucleus

May also control speed of mRNA transport from nucleus to cytoplasm Will affect the number of transcripts arriving at rough

ER

And therefore the amount of gene product realized per unit time

29

Processing of mRNA Transcripts


intron intron

intron

cap

protein product 1

mRNA

RNA splicing

poly-A

tail

exon intron

protein product 2

RNA splicing

exon

a. b.

cap

A B C D E

A B C D E

A B C

C

D E

A D E B

pre-mRNA

mRNA

pre-mRNA poly-A

tail

5 3 5 3

30

Function of microRNAs


pre-mRNA

MicroRNA is cut from

a pre-mRNA and binds with

proteins to form RISC.

Complementary base pairing

between RNAs allows RISC

to bind to mRNA.

Translation

is inhibited.

The mRNA

is degraded.

mRNA

RISC

(RNA-induced

silencing complex)

microRNA

(miRNA)

proteins

or

RISC

5

3

3 5

5

3

31


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

Posttranslational Control - Affects the activity of a

protein product

Activation

Degradation rate

32

Regulation Through Gene Mutation

Mutation is a permanent change in the

sequence of bases in DNA.

No effect on protein activity

Protein is completely inactivated

Germ-line mutations occur in sex cells

Somatic mutations occur in body cells

33

Causes of Mutations

Spontaneous mutation DNA can undergo a chemical change

Movement of transposons from one chromosomal location to another

Replication Errors 1 in 1,000,000,000 replications DNA polymerase

Proofreads new strands Generally corrects errors

Induced mutation: Mutagens such as radiation, organic chemicals

Many mutagens are also carcinogens (cancer causing) Environmental Mutagens

Ultraviolet Radiation Tobacco Smoke

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35

The Ames Test For Mutagenicity

bacterial

strain

(requires

histidine)

Control

Mutation did not occur Mutation occurred

Suspected

chemical

mutagen

bacterial

strain

(requires

histidine) Plate onto petri plates

that lack histidine.

Incubate overnight bacterial

growth


36

Causes of Mutations

Ultraviolet (UV) radiation is easily absorbed

by the pyrimidines in DNA.

Cause neighboring thymine molecules next

to one another to bond together

Thymine dimers.

C G

C

A

A

G

kink

thymine

dimer


T

T

37

Causes of Mutations

Usually, these dimers are removed by DNA

repair enzymes

Deficient DNA repair enzymes leave the skin

cells vulnerable to the mutagenic effects of

ultraviolet light

Accumulation of mutation

High incidence of cancer

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39

XerodermaPigmentosome


© Ken Greer/Visuals Unlimited

40

Effect of Mutations on Protein Activity

Point Mutations Involve change in a single DNA nucleotide Changes one codon to a different codon Affects on protein vary:

Nonfunctional Reduced functionality Unaffected

Frameshift Mutations One or two nucleotides are either inserted or deleted

from DNA Protein always rendered nonfunctional

Normal : THE CAT ATE THE RAT After deletion: THE ATA TET HER AT After insertion: THE CCA TAT ETH ERA T

41

Point Mutations in Hemoglobin


b. Normal red blood cell

a.

c. Sickled red blood cell

No mutation

Val His Leu Thr Pro Glu Glu

(normal protein)

His His

(abnormal protein)

Glu Val

(incomplete protein)

Glu Stop

C T C C T C T G G A G T C A C G T G G A G

C T C C T C T G G A G T C A C G T G A G

Val His Leu Thr Pro Glu Glu

C T C C A C T G G A G T C A C G T G G A G

Val His Leu Thr Pro Glu

C T C C A T G G A G T C A C G T G G A G T

Val His Leu Thr Pro Stop

A

b, c: © Stan Flegler/Visuals Unlimited.

Val

3 5

42

Carcinogenesis

Development of cancer involves a series of

mutations

Proto-oncogenes – Stimulate cell cycle

Tumor suppressor genes – inhibit cell cycle

Mutation in oncogene and tumor suppressor

gene:

Stimulates cell cycle uncontrollably

Leads to tumor formation

43

Cell Signaling Pathway

Cell signaling pathway that stimulates a mutated tumor suppressor gene


receptor

inhibiting growth factor

cytoplasm

plasma

membrane

signal

transducers

transcription factor

nucleus

protein that is

unable to inhibit

the cell cycle

or promote

apoptosis

mutated tumor suppressor gene

44

Cell Signaling Pathway

Cell signaling pathway that stimulates a proto-oncogene


receptor

stimulating growth factor

cytoplasm

plasma

membrane

signal

transducers

transcription factor

nucleus

protein that

overstimulates

the cell cycle

oncogene

45

Review


trp Operon

lac Operon






Genetic Mutations

Cancer

Sylv

ia S

. Ma

der

Copyright © The McGraw Hill Companies Inc. Permission required for reproduction or display

PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor

BIOLOGY 10th Edition

Regulation of Gene Activity

Chapter 13: pp. 233 - 248

46



DNA

RNA polymerase


mRNA



DNA

inactive repressor


tryptophan

active repressor

5 3

BIOLOGY Chapter 13: 10th Edition Regulation of Gene … · 2 Outline Prokaryotic Regulation trp Operon lac Operon Eukaryotic Regulation Chromatin Structure Transcriptional Control

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