- 1. Gene Control in EukaryotesIn eukaryotic cells, the ability
to express biologically active proteins comes under regulation at
several points:1. Chromatin Structure: The physical structure of
the DNA, as it exists compacted into chromatin, can affect the
ability of transcriptional regulatory proteins (termed
transcription factors) and RNA polymerases to find access to
specific genes and to activate transcription from them. The
presence modifications of the histones and of CpG methylation most
affect accessibility of the chromatin to RNA polymerases and
transcription factors.2. Epigenetic Control: Epigenesis refers to
changes in the pattern of gene expression that are not due to
changes in the nucleotide composition of the genome. Literally
"epi" means "on" thus, epigenetics means "on" the gene as opposed
to "by" the gene.
2. 3. Transcriptional Initiation: This is the mostimportant mode
for control of eukaryotic geneexpression. Specific factors that
exert control includethe strength of promoter elements within
theDNA sequences of a given gene, the presence orabsence of
enhancer sequences (which enhance theactivity of RNA polymerase at
a given promoter bybinding specific transcription factors), and
theinteraction between multiple activator proteins andinhibitor
proteins.4. Transcript Processing andModification: Eukaryotic mRNAs
must be cappedand polyadenylated, and the introns must beaccurately
removed (see RNA Synthesis Page). Severalgenes have been identified
that undergo tissue-specificpatterns of alternative splicing, which
generatebiologically different proteins from the same gene. 3. 5.
RNA Transport: A fully processed mRNA must leave the nucleus in
order to be translated into protein.6. Transcript Stability: Unlike
prokaryotic mRNAs, whose half-lives are all in the range of 1 to 5
minutes, eukaryotic mRNAs can vary greatly in their stability.
Certain unstable transcripts have sequences (predominately, but not
exclusively, in the 3-non-translated regions) that are signals for
rapid degradation.7. Translational Initiation: Since many mRNAs
have multiple methionine codons, the ability of ribosomes to
recognize and initiate synthesis from the correct AUG codon can
affect the expression of a gene product. Several examples have
emerged demonstrating that some eukaryotic proteins initiate at
non-AUG codons. This phenomenon has been known to occur in E. coli
for quite some time, but only recently has it been observed in
eukaryotic mRNAs. 4. 8. Small RNAs and Control of Transcript
Levels: Within the past several years a new model of gene
regulation has emerged that involves control exerted by small
non-coding RNAs. This small RNA-mediated control can be exerted
either at the level of the translatability of the mRNA, the
stability of the mRNA or via changes in chromatin structure.9.
Post-Translational Modification: Common modifications include
glycosylation, acetylation, fatty acylation, disulfide bond
formations, etc.10. Protein Transport: In order for proteins to be
biologically active following translation and processing, they must
be transported to their site of action.11. Control of Protein
Stability: Many proteins are rapidly degraded, whereas others are
highly stable. Specific amino acid sequences in some proteins have
been shown to bring about rapid degradation. 5. Gene Control in
Prokaryotes genes are clustered into operons: gene clusters
thatencode the proteins necessary to perform coordinatedfunction
prokaryotic genes that encode the proteins necessaryto perform
coordinated function are clustered intooperons. 6. The lac operon
consists of one regulatory gene(the i gene) and three structural
genes (z, y, and a).The i gene codes for the repressor of the lac
operon.The z gene codes for -galactosidase (-gal), for
thehydrolysis of the disaccharide, lactose into its monomericunits,
galactose and glucose. y gene codes for permease,increases
permeability of the cell to -galactosides.The a gene encodes a
transacetylase. During normal growth on a glucose-based medium,the
lac repressor is bound to the operator region ofthe lac operon,
preventing transcription. However, inthe presence of an inducer of
the lac operon, therepressor protein binds the inducer and is
renderedincapable of interacting with the operator region of
theoperon. RNA polymerase is thus able to bind at thepromoter
region, and transcription of the operon ensues. 7. The lac operon
is repressed, even in the presence of lactose, if glucose is also
present. This repression is maintained until the glucose supply is
exhausted. The repression of the lac operon under these conditions
is termed catabolite repression and is a result of the low levels
of cAMP that result from an adequate glucose supply. 8.
Transcription DNA is transcribed to make RNA (mRNA, tRNA, andrRNA)
Transcription begins when RNA polymerase binds tothe promoter
sequence Transcription proceeds in the 5 3 direction Transcription
stops when it reaches theterminator sequence 9. The Process of
Transcription Figure 8.7 10. The Process of Transcription Figure
8.7 11. RNA Processing in Eukaryotes Figure 8.11 12. Translation
mRNA is translated incodons (threenucleotides) Translation of
mRNAbegins at the startcodon: AUG Translation ends atnonsense
codons: UAA,UAG, UGAFigure 8.2 13. The Genetic Code 64 sense codons
on mRNAencode the 20 amino acids The genetic code isdegenerate tRNA
carries thecomplementary anticodonFigure 8.2 14. The Process of
Translation Components needed to begin translations come
together.Figure 8.9 15. The Process of Translation On the assembled
ribosome, at tRNA carrying the first amino acid in paired with the
start codon on the mRNA.the place where this firsts tRNA sits is
called the p site.A tRNA carrying the second amino acid approaches.
Figure 8.9 16. The Process of Translation The second codon of the
mRNA pairs with a tRNA carrying the second amino acids joins to the
seconds by a peptide bond. This attaches the polypeptide to the
tRNA in the p site.Figure 8.9 17. The Process of TranslationThe
ribosome moves along the mRNA until the second tRNA is in the p
site. The next codon to be translated is brought into the a site.
The firsts tRNA now occupies the e site.Figure 8.9 18. The Process
of Translation The second amino acid is paired with the start codon
on the mRNA. Is release from the e site.Figure 8.9 19. The Process
of Translation The ribosome continues to move along the mRNA and
new amino acids are added to the polypeptideFigure 8.9 20. The
Process of Translation When the ribosome reaches a stop codon, the
polypeptide is released. Figure 8.9 21. The Process of Translation
Finally, the last tRNA isreleased ,and theribosome comes apart.The
released polypeptideforms a new protein. Figure 8.9 22. Regulation
Constitutive genes are expressed at a fixed rate Other genes are
expressed only as needed Repressible genes Inducible genes
Catabolite repression 23. Operon Figure 8.12 24. Inducible operon
(lac operon)Figure 8.12 25. Inducible operon (lac operon)Figure
8.12 26. Repressible operon (trp operon)Figure 8.13 27. Repressible
operon (trp operon)Figure 8.13 28. The trp operon encodes the genes
for the synthesis oftryptophan. This cluster of genes regulated by
arepressor that binds to the operator sequences. Theactivity of the
trp repressor for binding the operatorregion is enhanced when it
binds tryptophan known asa corepressor. Since the activity of the
trp repressor isenhanced in the presence of tryptophan, the rate
ofexpression of the trp operon is graded in response tothe level of
tryptophan in the cell. Expression of the trp operon is also
regulatedby attenuation. 29. Attenuation The attenuator plays an
important regulatory role in prokaryotic cells because of the
absence of the nucleus in prokaryotic organisms. The attenuator
refers to a specific regulatory sequence that, when transcribed
into RNA, forms hairpin structures to stop transcription when
certain conditions are not met 30. CATABOLITE REPRESSION Many
inducible operons are not only controlled by their respective
inducers and regulatory genes, but they are also controlled by the
level of glucose in the environment. The ability of glucose to
control the expression of a number of different inducible operons
is called CATABOLITE REPRESSION. 31. Catabolite Repression(a)
Growth on glucose or lactose alone (b) Growth on glucose and
lactose combined Figure 8.14 32. Lactose present, no Lactose +
glucose glucose presentFigure 8.15