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Gene Expression
Supplementary reading: Chapter 19
in Campbell 7th edition
Background
Genes serves as a recipe for building a protein
molecule (these are called “structural genes”)
When a particular protein is
needed by the cell, the
corresponding gene, made of
DNA, is turned "on," or
transcribed into messenger
RNA, which then carries the
"protein recipe" to the proteinmaking machinery of the cell
(ribosomes) where it is
translated into a protein product.
Overview
• Concept of "genes" -vs- "junk" sequences. See Chromosome
11 Flyover animation. (introns, exons, pseudogenes,
transposons, etc)
• Not all genes are active in all cells
• Not all genes in a given cell are activated all the time
• There MUST be some way to control when a gene is turned
"on" or "off"
• The activation of a gene results in transcription (mRNA made)
which in turn results in the formation of a protein
• Chromosomes are really made up of structural genes and their
ON and OFF switches ("regulatory genes")
– "ON" = transcription and protein formation, "OFF" = not transcribed, no
protein made
Prokaryotes (ex: bacteria)
• Much less DNA (than eukaryotes, like humans) so much easier to study
• The lac operon model (Jacob, Monod, Lwoff) 1965 Nobel Prize
• The OPERON consists of:
–
–
–
–
Regulator gene- codes for the repressor protein
Promotor gene- attachment site for RNA polymerase enzyme, to begin transcription
Operator gene- attachment for repressor
Z, Y, a,= structural genes- code for polypeptides (in this case, for enzyme)
•
•
When the RNA polymerase enzyme is not blocked by the repressor (genes are
"ON") it will move along the structural genes causing them to be transcribed
into mRNA; this results in the enzyme being made ("expressed").
When the repressor molecule is on the operator, transcription is "blocked", so
no enzyme is made. In this case, presence of lactose "induces" transcription
by essentially removing the repressor from the operator.
‘inducible” vs “repressible” genes
• Try these tutorials, for further explanation:
- The Tryptophan Repressor
- Combination of Switches - the Lac Operon
• operon animated explanation
• lac operon movie (avi) 3:24
Eukaryotic Gene Expression
Chromosome structure: chromatin (DNA helices) are
wrapped around a central histone protein core.
"nucleosomes": DNA wrapped around histones
(proteins); forms beadlike loops in the chromatin.
"euchromatin":
uncoiled DNA that is
being transcribed
"heterochromatin":
tightly coiled DNA;
contains inactive
genes
picture comparison
Introns and Exons
• Introns: “inert” noncoding sections of eukaryotic genes that are
transcribed but not translated.
• Exons: codons for protein synthesis
Pre-RNA (initial transcript) contains useful information (from
exons) - coding for protein- interspersed with some
“extra“ noncoding (intron) sequences. It must be modified
before the ribosome can make the protein it calls for.
mRNA Cleavage : “spliceosomes” cut and splice only the
necessary info together, before it reaches the ribosome for
translation.
Watch this
“Modification of mRNA : prokaryotes -vs- eukaryotes”
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter15/animations.html#
short movie of mRNA editing (introns/exons)
Control of Transcription in Eukaryotes
• Regulatory proteins, called
“transcription factors”
help place the RNA
polymerase at the correct
spot on the promotor.
• “Enhancer” sequences
(often located elsewhere
on chromosome) are
“looped in” to initiate
gene expression by
bringing activators on
contact with the
polymerase.
Chapter 21 in Campbell 7th ed.
“The Genetic Basis of
Development”
Morphogenesis (from the Greek morphê shape and genesis
creation, literally, "beginning of the shape") is the biological process
that causes an organism to develop its shape. It is one of three
fundamental aspects of developmental biology along with the control
Embryology
• Cleavage
(mitotic
division)
• Morula
• Blastula
• Blastopore
• Gastrulation
• Differentiation
Deuterostomes or Protostomes
Differentiation into Germ Layers
Phylogeny: based on embryological
development
Gene Expression during Development
Unsurprisingly, special “master genes” have been found to
control cell specialization (roles) as well as the
placement/location of certain bodily structures, like
appendages
• Homeotic Genes are regulatory genes
that determine where certain
anatomical structures, such as
appendages, will develop in an
organism during morphogenesis.
• Specific DNA sequences found within a
homeotic gene are referred to as
“homeoboxes” (Hox)
• These "master genes" are conserved
from flies to mice to humans
(determine location of body parts in
human embryos as well).
• Can even be found in fungi and plants
(control flower development)
Morphogenesis
When mutations occur in Hox sequences…
Very well-studied in fruit flies (located on the 3rd chromosome)
"Antennapedia" is a hox gene first discovered in Drosophila which controls
the placement of legs. Scientists can induce mutation in this, causing legs to
grow in the place where antennae are normally found (left photo)
Also can manipulate the pbx (post-bithorax) and bx (bithorax) genes to this
effect (right photo)
Specific examples of genes active in
embryological development
• The Hedgehog (Hh) family of proteins transmits information
to embryonic cells required for proper development.
• Different parts of the embryo have different concentrations of
hedgehog signaling proteins.
– The use of a single protein for such a wide variety of functions is
possible because cellular responses to Hh depend on the type of
responding cell, the dose of Hh received, and the time cells are
exposed to Hh.
• The pathway also has roles in the adult. Diseases associated
with the malfunction of this pathway include basal cell
carcinoma.
There are several homologues of these genes
throughout the animal kingdoms
Ex: in vertebrates Desert Hedgehog (Dhh), Indian Hedgehog
(Ihh), and Sonic Hedgehog (Shh)
•Weird example of SHH gone wrong: Janus cats. See “Frank
and Louie”, who recently died @ age
15http://www.telegram.com/article/20141204/NEWS/312049633
•Named for the Roman god Janus, who was usually
portrayed as having two faces, domestic cats with two faces
are extremely rare
•Janus cats are thought to have too much of the sonic
hedgehog (SHH) protein, which plays a role in forming an
animal's face during development. In some experiments,
chick embryos exposed to an excess of SHH were born with
two beaks and eyes spaced far apart.