Regulation of Gene Expression

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mRNA Degradation

The life span of mRNA molecules in the cytoplasm is a key to determining protein synthesis

Eukaryotic mRNA is more long lived than prokaryotic mRNA

The mRNA life span is determined in part by sequences in the leader and trailer regions

Animation: mRNA Degradation

Slide 52

Initiation of Translation

The initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to sequences or structures of the mRNA

Alternatively, translation of all mRNAs in a cell may be regulated simultaneously

For example, translation initiation factors are simultaneously activated in an egg following fertilization

Animation: Blocking Translation

Slide 53

Protein Processing and Degradation

After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control

Proteasomes are giant protein complexes that bind protein molecules and degrade them

Animation: Protein Degradation

Animation: Protein Processing

Slide 54

Fig. 18-12

Proteasome

and ubiquitin

to be recycled

Proteasome

Protein

fragments

(peptides)

Protein entering a

proteasome

Ubiquitinated

protein

Protein to

be degraded

Ubiquitin

Slide 55

Concept 18.3: Noncoding RNAs play multiple roles in controlling gene expression

Only a small fraction of DNA codes for proteins, rRNA, and tRNA

A significant amount of the genome may be transcribed into noncoding RNAs

Noncoding RNAs regulate gene expression at two points: mRNA translation and chromatin configuration

Slide 56

Effects on mRNAs by MicroRNAs and Small Interfering RNAs

MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNA

These can degrade mRNA or block its translation

Slide 57

Fig. 18-13

miRNA-

protein

complex

(a) Primary miRNA transcript

Translation blocked

Hydrogen

bond

(b) Generation and function of miRNAs

Hairpin

miRNA

miRNA

Dicer

3

mRNA degraded

5

Slide 58

The phenomenon of inhibition of gene expression by RNA molecules is called RNA interference (RNAi)

RNAi is caused by small interfering RNAs (siRNAs)