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Gene Expression. Supplementary reading: Chapter 19 in Campbell 7 th edition. Background. Genes serves as a recipe for building a protein molecule (these are called “structural genes”).

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gene expression

Gene Expression

Supplementary reading: Chapter 19 in Campbell 7th edition


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 protein-making machinery of the cell (ribosomes) where it is translated into a protein product.

  • 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
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
‘inducible” vs “repressible” genes
  • Try these tutorials, for further explanation:- The Tryptophan Repressor - Combination of Switches - the Lac Operon
  • operon animated explanation
  • lacoperon movie (avi) 3:24
eukaryotic gene expression
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 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” movie of mRNA editing (introns/exons)

control of transcription in eukaryotes
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.
  • Cleavage (mitotic division)
  • Morula
  • Blastula
  • Blastopore
  • Gastrulation
  • Differentiation

Deuterostomes or Protostomes

gene expression during 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)
when mutations occur in hox sequences
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)