Gene Regulation and Development

gene expression

• Cells can turn genes on or off when needed

• This depends on environmental or internal signals

  “On” = gene is transcribed

  “Off” = gene is not transcribed

• This helps with cell specialization (like how skin and nerve cells are different)

bacterial gene expression

• operons are a group of genes that can be turned on or off, they have 3 parts

  • promoter: where RNA polymerase attaches

  • operator: on/off switch

  • genes: code for related enzymes in pathway

• operons are repressible or inducible

  • repressible (on to off): transciption is usually on but can be stopped

  • inducible(off to on): transcription is usually off, but can be induced

bacterial gene expression

• regulatory gene → makes a repressor protein that binds to the operator to block RNA polymerase from transcribing the gene

  • always expressed, but at low levels

  • binding a repressor to an operator is reversible

review on allosteric enzymes

• allosteric activator → a substrate binds to the allosteric site and changes the enzyme’s shape to keep the active site open and working properly
  

• allosteric inhibitor → a substrate binds to the allosteric site and changes the enzyme’s shape so the active site closes, keeping the enzyme inactive

repressible operons - trp operon

• trp operon helps bacteria make tryptophan

• a repressible operon, so it’s usually on

• A repressor protein can turn it off

• repressor is inactive by itself

• when tryptophan levels are high, tryptophan binds to the repressor

  • this activates the repressor, which then binds to the operator

• This blocks transcription, so no more tryptophan is made until it’s needed again

inducible operons - lac operon

• controls making lactase, which breaks down lactose

• It’s an inducible operon, so it’s usually off

• normally bound to the operator, blocking transcription

• lactose (allolactose) is present, it acts as an inducer

• allolactose binds to the repressor, turning it inactive

• The repressor falls off, and the genes can now be transcribed to make lactase

eukaryotic gene expression pt.1

• eukaryotic gene expression is regulated at different stages

• chromatin structure → if DNA is tightly wrapped its less accessible for transcription

  • histone acetylation → adds acetyl groups to histones → looses DNA

  • DNA methylation → adds methyl groups to DNA → chromatin condense

• epigenetic inheritance

  • chromatin changes don’t change the DNA sequence, but they can be passed down

  • these changes can be reversed, unlike mutations

  • explain why identical twins can have different traits or health conditions

eukaryotic gene expression pt.2

• transcription initiation

  • chromatin is loosened, transcription factors can bind to special control elements (non-coding DNA parts).

  • these can increase or decrease gene expression depending on whether activators or repressors bind to them.

eukaryotic gene expression pt.3

• RNA processing

  • alternative splicing of pre-mRNA

• translation initiation

  • translation can be activated or repressed by initiation factors

  • microRNAs and small interfering RNAs can bind to

    mRNA and degrade it or block translation

eukaryotic development

• during embryonic development, cell division and

  cell differentiation occurs

  • cells become specialized in their structure and function

  • morphogenesis → physical process that gives an organism its shape

how do cells differentiate during early development?

• cytoplasmic determinants → substances in the mom egg that influence cells

• induction → cell-to-cell signals that can cause a change in green expression

• both influence pattern formation

  • “body plan” for the organism

    • homeotic genes map out body structures

    • hox genes are homeotic genes; they are known for their role in specifying the identity of body segments during embryonic development.

• apoptosis → programmed cell death

  • allows structures to take their form

  • ex: if apoptosis did not occur during the development of human hands and feet, we would be born with webbed fingers and toes