1. Differential Gene Expression
Almost all the cells in an organism are genetically identical
Differences between cell types result from differential gene expression, the expression of different genes by cells with the same genome
Errors in gene expression can lead to diseases including cancer
Gene expression is regulated at many stages

2. Fig. 18-6
Signal
NUCLEUS
Chromatin
Chromatin modification
DNA
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Tail
Cap
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells
Polypeptide
Protein processing
Active protein
Degradation
of protein
Transport to cellular
destination
Cellular function

3. Chromatin modification
Fig. 18-6a
Signal
NUCLEUS
Chromatin
Chromatin modification
DNA
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells
RNA processing
Tail
mRNA in nucleus
Cap
Transport to cytoplasm
CYTOPLASM

4. Overview: Conducting the Genetic Orchestra
Prokaryotes and eukaryotes alter gene expression in response to their changing environment
In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types
RNA molecules play many roles in regulating gene expression in eukaryotes

5. Transport to cellular destination
Fig. 18-6b
CYTOPLASM
mRNA in cytoplasm
Translation
Degradation
of mRNA
Polypeptide
Protein processing
Active protein
Degradation
of protein
Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells
Transport to cellular
destination
Cellular function

6. Regulation of Chromatin Structure
Genes within highly packed heterochromatin are usually not expressed
Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression

7. Histone Modifications
In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails
This process loosens chromatin structure, thereby promoting the initiation of transcription
The addition of methyl groups (methylation) can condense chromatin; the addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin
Animation: DNA Packing

8. (a) Histone tails protrude outward from a nucleosome
Fig. 18-7
Histone
tails
Amino
acids
available
for chemical
modification
DNA
double helix
(a) Histone tails protrude outward from a
nucleosome
Figure 18.7 A simple model of histone tails and the effect of histone acetylation
Unacetylated histones
Acetylated histones
(b) Acetylation of histone tails promotes loose
chromatin structure that permits transcription

9. X Inactivation in Female Mammals
In mammalian females, one of the two X chromosomes in each cell is randomly inactivated by DNA methylation during embryonic development
The inactive X condenses into a Barr body
If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10. X chromosomes Allele for orange fur Early embryo: Allele for black fur
Fig. 15-8
X chromosomes
Allele for
orange fur
Early embryo:
Allele for
black fur
Cell division and
X chromosome
inactivation
Two cell
populations
in adult cat:
Active X
Inactive X
Active X
Black fur
Orange fur
Figure 15.8 X inactivation and the tortoiseshell cat

11. The Roles of Transcription Factors
To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors
General transcription factors are essential for the transcription of all protein-coding genes
In eukaryotes, high levels of transcription of particular genes depend on control elements interacting with specific transcription factors

12. Transcription Factors
Proximal control elements on the DNA are located close to the promoter
Distal control elements, groups of which are called enhancers, may be far away from a gene or even located in an intron

13. Animation: Initiation of Transcription
An activator is a protein that binds to an enhancer and stimulates transcription of a gene
Bound activators cause mediator proteins to interact with proteins at the promoter
Animation: Initiation of Transcription

14. Promoter Activators Gene DNA Enhancer
Fig
Promoter
Activators
Gene
DNA
Distal control
element
Enhancer
TATA
box
General
transcription
factors
DNA-bending
protein
Group of
mediator proteins
RNA
polymerase II
Figure 18.9 A model for the action of enhancers and transcription activators
RNA
polymerase II
Transcription
initiation complex
RNA synthesis

15. Some transcription factors function as repressors, inhibiting expression of a particular gene
Some activators and repressors act indirectly by influencing chromatin structure to promote or silence transcription

16. Combinatorial Control of Gene Activation
A particular combination of control elements can activate transcription only when the appropriate activator proteins are present

17. Enhancer Promoter Albumin gene Control elements Crystallin gene
Fig
Enhancer
Promoter
Albumin gene
Control
elements
Crystallin gene
LIVER CELL
NUCLEUS
LENS CELL
NUCLEUS
Available
activators
Available
activators
Albumin gene
not expressed
Figure Cell type–specific transcription
Albumin gene
expressed
Crystallin gene
not expressed
Crystallin gene
expressed
(a) Liver cell
(b) Lens cell

18. Mechanisms of Post-Transcriptional Regulation
Transcription alone does not account for gene expression
Regulatory mechanisms can operate at various stages after transcription
Such mechanisms allow a cell to fine-tune gene expression rapidly in response to environmental changes

19. Animation: RNA Processing
In alternative RNA splicing, different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns
Animation: RNA Processing

20. Exons DNA Troponin T gene Primary RNA transcript RNA splicing mRNA or
Fig
Exons
DNA
Troponin T gene
Primary
RNA
transcript
Figure Alternative RNA splicing of the troponin T gene
RNA splicing
mRNA or

21. Animation: 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

22. 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

23. Protein Processing and Degradation
After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control
Proteins destined for degradation are Ubiquinated and sent for degradation either to a lysosome or Proteasomes which are giant protein complexes that bind protein molecules and degrade them
Animation: Protein Processing
Animation: Protein Degradation

24. Proteasome and ubiquitin to be recycled Ubiquitin Proteasome
Fig
Proteasome
and ubiquitin
to be recycled
Ubiquitin
Proteasome
Protein to
be degraded
Ubiquitinated
protein
Protein
fragments
(peptides)
Protein entering a
proteasome
Figure Degradation of a protein by a proteasome

25. Concept 18.4: A program of differential gene expression leads to the different cell types in a multicellular organism
During embryonic development, a fertilized egg gives rise to many different cell types
Cell types are organized successively into tissues, organs, organ systems, and the whole organism
Gene expression orchestrates the developmental programs of animals

26. A Genetic Program for Embryonic Development
The transformation from zygote to adult results from cell division, cell differentiation, and morphogenesis

27. (a) Fertilized eggs of a frog (b) Newly hatched tadpole
Fig
Figure From fertilized egg to animal: What a difference four days makes
(a) Fertilized eggs of a frog
(b) Newly hatched tadpole

28. Cell differentiation is the process by which cells become specialized in structure and function
The physical processes that give an organism its shape constitute morphogenesis
Differential gene expression results from genes being regulated differently in each cell type