Control over Genes

1
Control over Genes
2
Skin Cancer

• Basal Cell Carcinoma
• Squamous Cell Carcinoma
• Malignant Melanoma

3
Changes in DNA Trigger Cancer

• Ultraviolet radiation can cause breaks

4
Gene Control

• Which genes are expressed in a cell depends
  upon
• Type of cell
• Internal chemical conditions
• External signals
• Built-in control systems

5
Mechanisms of Gene Control

• Controls related to transcription
• Transcript-processing controls
• Controls over translation
• Post-translation controls

6
Regulatory Proteins

• Can exert control over gene expression through
  interactions with-DNA
• RNA
• New polypeptide chains
• Final proteins

7
Control Mechanisms

• Negative control
• Regulatory proteins slow down or curtail gene
  activity
• Positive control
• Regulatory proteins promote or enhance gene
  activities

8
Chemical Modifications

• Methylation of DNA can inactivate genes
• Acetylation of histones allows DNA unpacking and
  transcription

9
Gene Control in Prokaryotes

• No nucleus separates DNA from ribosomes in
  cytoplasm
• When nutrient supply is high, transcription is
  fast
• Translation occurs even before mRNA transcripts
  are finished

10
The Lactose Operon
operator
regulatory gene
gene 1
gene 2
gene 3
operator
transcription, translation
promoter
lactose operon
repressor protein
11
Low Lactose

• Repressor binds to operator
• Binding blocks promoter
• Transcription is blocked

12
High Lactose
allolactose
lactose
mRNA
RNA polymerase
gene 1
operator
operator
promoter
13
Lac-Operon Analogy

• Like an elephant that likes to sit on a railroad
  track blocking the train.
• The only thing that the elephant likes better
  than to sit on his spot on the track is peanuts.
• If peanuts are available, the elephant leaves the
  track and eats the peanuts.
• While he is gone the train can proceed down the
  track until the elephant goes back to sit on the
  track after eating all the peanuts.

14
Controls in Eukaryotic Cells

• Control of transcription
• Transcript processing controls
• Controls over translation
• Controls following translation

15
Most Genes Are Turned Off

• Cells of a multicelled organism rarely use more
  than 5-10 percent of their genes at any given
  time
• The remaining genes are selectively expressed

16
Homeotic Genes

• Occur in all eukaryotes
• Master genes that control development of body
  parts
• Encode homeodomains (regulatory proteins)
• Homeobox sequence can bind to promoters and
  enhancers

17
Hormones

• Signaling molecules
• Stimulate or inhibit activity in target cells
• Mechanism of action varies
• May bind to cell surface
• May enter cell and bind to regulatory proteins
• May bind with enhancers in DNA

18
Vertebrate Hormones

• Some have widespread effects
• Somatotropin (growth hormone)
• Others signal only certain cells at certain times
• Prolactin stimulates milk production

19
Phytochrome

• Signaling molecule in plants
• Activated by red wavelengths, inactivated by
  far-red wavelengths
• Changes in phytochrome activity influence
  transcription of certain genes

20
Controlling the Cell Cycle

• Cycle has built-in checkpoints
• Proteins monitor chromosome structure, whether
  conditions favor division, etc.
• Proteins are products of checkpoint genes
• Kinases
• Growth factors

21
Oncogenes

• Have potential to induce cancer
• Mutated forms of normal genes
• Can form following insertions of viral DNA into
  DNA or after carcinogens change the DNA

22
Cancer Characteristics

• Plasma membrane and cytoplasm altered
• Cells grow and divide abnormally
• Weakened capacity for adhesion
• Lethal unless eradicated

23
Apoptosis

• Programmed cell death
• Signals unleash molecular weapons of
  self-destruction
• Cancer cells do not commit suicide on cue