Eukaryotic gene expression can be
at many stages, from the
Eukaryotic gene regulation:
Chromatin modification: acetylation of histone "tails".
Transcription control: activators can stimulate transcription.
RNA processing: alternative RNA splicing.
Enzymes can add negative acetyl groups (-COCH) to histone tails.
Histone acetylation loosens chromatin structure, making the DNA
accessible to transcription.
Such chromatin modifications may be passed to future generations of
cells in a process called epigenetic inheritance.
Activator proteins bind to an
enhancer region in the DNA.
A DNA-bending protein brings the bound activators closer to the
The activators bind to transcription factors and mediator proteins,
forming a transcription initiation complex.
The complex promotes the binding of RNA polymerase to the promoter,
enhancing gene expression.
The primary transcripts of some genes can be spliced in many ways,
generating different mRNA molecules.
In this example one mRNA molecule ends up with the green exon and the
other with the purple exon.
With alternative splicing, an organism can produce more than one type
of polypeptide from a single gene.
Eukaryotic gene regulation:
mRNA degradation: degradation of an mRNA in the cytoplasm limits
Protein processing: many proteins are activated by signal
molecules in transduction pathways.
Protein degradation: proteasomes degrade proteins at the end of
their useful lifespan.
Unneeded proteins can be tagged with the protein ubiquitin.
The tagged protein is then chopped up by protein complexes called
Multicellular organisms develop from a single-celled
to cells of many different types through cell
Differences between cells is mainly due to differential gene
expression, even though different cells share genomic equivalence
(have the same genes).
Morphogenesis encompasses the processes that give shape to the
organism and its parts. It takes just one week for cell division,
differentiation, and morphogenesis to transform a fertilized frog egg
into a hatching tadpole.
- Differential distribution of
in the egg can lead to subsequent
The distribution of cytoplasmic determinants, such as RNA, proteins,
and organelles, may be uneven in the unfertilized egg
Such uneven beginnings may affect expression of genes after the first
Induction by nearby cells.
The cells at the bottom of this early embryo release signal molecules
that induce nearby cells to change their gene expression
establish the axes of the body in Drosophila.
Asymmetrically distribution of these molecules in the unfertilized egg
eventually lead to differentiation of specialized segments of the
One important cytoplasmic determinant is mRNA produced by the bicoid
Gradients of bicoid mRNA and bicoid protein in egg and early embryo
leads to normal development of body segments.
A mutation in the mother's bicoid gene leads to tail structures at
both ends in the Drosophila mutant embryo.
- Expression of genes for tissue-specific proteins result in cell
and leads to observable
MyoD is a "master regulatory gene" whose expression commits the cell
to becoming skeletal muscle.
If a myoblast cell produces the MyoD protein (a transcription factor),
it binds to enhancers of many target genes.
The cell is now determined to be a skeletal muscle cell.