Control Over Gene Expression

1
Control Over Gene Expression

• Control Mechanisms
• Examples from Bacteria
• Control in Eukaryotic Cells
• Cancer Cells

2
Types of control mechanisms

• Regulatory proteins
• Intervene before, during, or after transcription
  or translation
• Negative control
• Slowing down or stopping gene activity

3
Types of control mechanisms

• Positive control
• Promote or enhance gene activity
• May involve non-coding base sequences

4
Types of control mechanisms

• Chemical modification
• Methylation may inactive genes
• Acetylation

5
Example of gene control in bacteria

• Promoter
• Base sequences that signal the start of a gene
• Operator
• Binding site for repressor protein that prevents
  transcription

6
Example of gene control in bacteria

• Operon
• A unit of genetic material that functions in a
  coordinated manner by means of an operon, a
  promotor, an done or more structural genes that
  are transcribed together

7
Example of gene control in bacteria

• Lac operon
• When lactose is absent, the repressor binds to a
  set of operons
• Promoter is inaccessible and transcription will
  not occur

8
Example of gene control in bacteria

• When lactose is present, a converted form binds
  to and changes the shape of the repressor
• It cant bind, so transcription proceeds

9
Example of gene control in bacteria

• If no glucose is present, the lac operon is used
  more
• CAP (an activator protein) makes a promoter more
  inviting to RNA polymerase.
• It must be bound to cAMP

10
Genetic control in eukaryotic cells

• Transcriptional control
• Gene amplification, chemical modification, DNA
  rearrangement
• Transcript processing
• Alternative splicing

11
Genetic control in eukaryotic cells

• Translational controls
• Length of poly A tail
• Inactivation of transcripts
• Post-translational control
• Modification of proteins

12
Examples of control mechanisms

• Which genes are active at any given time?
• Depends on the type of organism, developmental
  stage, and controls
• Homeotic genes
• master genes that interact with each other
• Lead to formation of body tissues and organs

13
Examples of control mechanisms

• X chromosome inactivation
• Occurs in all female placental mammals
• All except 3 dozen genes on 1 X chromosome are
  inactivated
• Condensed chromosome is also called a Barr body
• Alleles inactivated in cells form a specific
  tissue region may lead to differential gene
  expression
• Calico coat in cats

14
Examples of control mechanisms

• X chromosome inactivation

15
Examples of control mechanisms

• Hormones
• Signaling molecules that can stimulate or inhibit
  gene activity
• e.g. ecdysome insect hormone

16
Examples of control mechanisms

• Sunlight as a signal
• Phytochrome blue-green pigment that is involved
  in a variety of cellular responses active from
  results from exposure to red wavelengths of light

17
Cancer cells

• Oncogenes mutations that may induce cancer
• Insertion of viral DNA
• Radiation and chemical exposure
• Naturally occurring mutations

18
Cancer cells

• Cancer cell characteristics
• Plasma membrane becomes more permeable
• Abnormal proteins form
• Cancer cells grow and divide abnormally
• Metastasis
• Movement of cancer cells away from origin