Gene Regulation

Chapter 13

I. Prokaryotic and Eukaryotic GenomesA. all of the genetic material possessed

by an organism or group of organisms

B. both contain many 1,000's of protein coding genes1. prokaryotes = most genes code

for protein as usual• only a small amount of

noncoding DNA2. eukaryotes = much of genome

does not code for protein• many noncoding regions

and repetitive DNAC. many coding genes are active

(expressed) only part of the time1. they are controlled by some

mechanism2. inducible vs. constitutive genes

Nonprotein-coding DNA sequences in various groups

II. Control of Gene ExpressionA. mRNA initially contains many noncoding

regions (introns)1. modified before leaving nucleus

a. introns removedb. coding regions (exons) remain c. primary mRNA mature mRNA

2. introns left in place no translation3. removing exons different protein

• alternative RNA splicing

B. all genes contain two basic parts1. coding region (“cistron”)

a. codes for mRNA and its protein b. one coding region may contain several

subcoding regions• produce proteins that work

together

2. regulatory regiona. regulates transcription of coding

regionb. promoter

• RNA polymerase binds to begin transcription

c. operator i. controls expression of coding reg.ii. turns a gene “on” or “off” iii. constitutive genes lack one

Fig. 13.8 Processing of mRNA transcripts

III. Prokaryotic Gene RegulationA. characterized by two recurring themes

1. most genes contain many subcoding regions in the coding region2. transcription is prevented when a repressor binds to the operator

• repressoro a special protein produced by a regulatory gene

B. operon model of prokaryotic gene regulation1. components of an operon

a. regulator gene produces the repressor proteinb. protein coding gene (usually with subcoding regions)

• has a regulatory region = promoter and operator 2. transcription of all subcoding regions is regulated by the repressor

a. repressor is bound to operator no transcription occurs• RNA polymerase is blocked gene is “off”

b. repressor is not bound to operator transcription occurs as usual• RNA polymerase not blocked gene is “on”

Page 234 Operon model

Fig. 15.2 The lac operon

IV. Eukaryotic Gene RegulationA. express only a fraction of their genes at any

given timeB. eukaryotic genes also consist of regulatory and

coding region1. regulatory region = promoter and operator2. coding region rarely contains subcoding reg.

C. changes in the chromatin itself1. can affect the availability of genes 2. increased packaging conceals genes makes

them less accessible3. decreased packaging makes genes more

accessible 4. chromatin can also be modified chemically

a. DNA methylationi. methyl groups (-CH3) attached to DNA

basesii. diminishes transcription

b. histone acetylationi. acetyl groups (-COCH3) attached to

chromatinii. increases transcription

Fig. 13.4 Levels at which control of gene expression occurs in eukaryotic cells

Fig. 12.10 Levels of chromatin structure

D. initiation of transcription1. transcription factors

a. proteins that help determine when/where genes turned “on”b. bind to a gene’s promoter in response to certain stimuli

i. activates operator gene turned “on”ii. help RNA polymerase begin transcription

2. control elementsa. noncoding segments of DNA

• lie outside of regulatory region of any particular geneb. can inhibit transcription – silencers c. can stimulate transcription – enhancers

Fig. 13.7 Initiation of transcription

E. RNA regulators1. noncoding RNA (introns) that can regulate DNA, RNA, or proteins

• microRNA2. amount of regulatory RNA increases with organism complexity

• RNA regulators allow for an increase in complexity

F. translation • ribosomes and mRNA can be blocked from assembling at initiation

G. modification of the protein 1. protein processing is also subject to regulation

• assembly into its various structures, etc. 2. mechanisms keep a protein functional or make it nonfunctional3. proteasomes

H. degradation of mRNA itself1. each mRNA has a characteristic lifespan2. special sequences in the mRNA determine this

I. other control mechanisms increase diversity in gene expression

V. TransposonsA. genes that can move from one place to another in a genome

1. “jumping genes”2. transposition

• actual mechanism is complex

B. more common in prokaryotes than eukaryotesC. can significantly affect gene expression

1. jumping into the middle of a coding sequence • prevents the normal functioning of that gene

o its expression is halted or altered2. jumping into the regulatory region of another gene

• may either increase or decrease expression

D. some transposons carry genes themselves• activated when they are inserted near another gene’s promoter

Transposons