Gene Expression

Big Idea 3: Living systems store, retrieve, transmit, and respond to

info essential to life processes.

Essential Knowledge

• 3B1: Gene regulation results in differential gene expression, leading to cell specialization.

• 3B2: A variety of intercellular and intracellular signal transmissions mediate gene expression

Much Diversity due to Gene Expression

• Each tissue in our body is very diff. despite having same DNA

• Even identical twins…

Some Basics..

Example: Lac Operon

Some Basics:

Operons

Simple gene expression mechanisms in prokaryotes

Components of Operon• Operator: sequence next to promoter (On/Off

switch) If repressor protein is on it, then off…can’t make genes

• Promoter (where RNA polymerase binds to make mRNA)

Types of Operons

• Some genes are normally “off” and can be turned “on” Presence of a molecule induces gene expression

• Some genes are normally “on” and can be turned “off” Presence of a molecule represses gene expression

Lac Operon, + Control• If lactose present, bacteria need to make lactase

to break it down.

• Lactose binds to repressor on operator repressor is released from operator

• RNA polymerase now fits onto promoter region to make mRNA protein lactase.

Negative ControlCase Study: Trp Operon

• Tryptophan fits in repressor blocks RNA polymerase.

• Once out of tryp, repressor changes shape to allow promoter available to make more tryptophan turns transcription on.

Ribosomal Genes…

• Always on!!! Always Expressed!

In Eukaryotes…

• No operons…• Gene expression involves regulatory genes,

regulatory elements, and transcription factors All work together to make product

Transcription

Factors

• Bind to specific DNA sequences

• Some activate (increase expression) and some are repressors (decrease expression)

Regulators of Protein Synthesis

RNA Splicing

• Exons spliced together.

• Introns removed.

• snRNPs/spliceosomes help remove introns.

Repressor Proteins• Inhibit transcription/translation by binding

onto operon.

• Silence genes Inactivate gene expression.

Methylation• DNA or histone methylation prevents transcription.

• Protects against restriction enzymes.

siRNA (small interfering RNA)

• Facilitates degradation of mRNA

• Inhibits translation by breaking down mRNA.

Signal

Transmission

• Within and b/t cells mediates gene expression.

• Ex: cytokines regulate gene expression to allow cell replication and division.

• Within and b/t cells mediates cell function.

• Ex: HOX genes (homeotic genes)

Signal

Transmission

Mr Anderson

• http://www.youtube.com/watch?v=3S3ZOmleAj0

Lac Operon or Trp Operon Flow Map