Download Across the tree of life, from bacteria to humans, clocks use oscillating

Circadian Rhythms: Lecture 5
The Plot Thickens...
What makes the clock tick? Hands of the SCN
“Main Oscillator” (SCN cells)
But what about at the molecular level?
Cells contain a newly discovered protein (clock protein)
that regulates gene function and which shows 24-hr
variations in cellular levels that appears to account for
24-hr variations in neuronal activity
GENES
(contain the instructions that tells a cell
what its job will be)
Genes that underlie our Biological Clock
How do reseachers study this?
How do you begin studying genes
1. Simple Model
2. Short-life span (inherited mutations)
Drosophila (Fruit Fly)
Ideal for Genetic Studies
1. Short Life Span: breed & interbreed 1,000s
of generations
2. Size of fly is Small: Lab set up minimal!
Genotype = gene
Phenotype = outward behavior attributed
to function of the gene
So…..We want to know if there is a particular
gene (genotype) responsible for circadian
rhythms (phenotype)
Start backwards  we don’t know the gene but we do
know the behavior  disruption in circadian rhythms
We need to measure circadian rhythms in
Drosophila = Active during the day = 12L/12D cycle
Measurement of Activity in the Fruit FLy
Get the phenotype = disruption in 12L/12D Cycle
Mutant Genotype
Konopka & Benzer (1970)
1. Fed the flies mutagens
29 hr cycle
2. 2,000 of their progeny (200th try)
19 hr cycle
No cycle
3. Found 3 flies showing disrupted 12L/12D cycles
..first to tie behavior to gene… (Period)
16 years later Michael Young (1986)
Found that 3 flies had alterations in a single gene
X Chromosome
Period: PER
PER is somehow involved in
-producing circadian rhythm
- setting the length of the rhythm
Is this the only gene involved?
Start from the drawing board
Fed flies mutagen
7,000 Progeny
ONE Fly!..had no circadian rhythm
Mutation on Chromosome 2
Named it TIMELESS = TIM….
So do these two genes work
together?
Genes are made of DNA- located on Chromosome
DNA: instructions for making proteins
(A=T G=C)
DNA never leaves the cell
DNA
Instructions are copied onto RNA – mRNA
(transcription)
mRNA
nucleus
ER-ribosomes mRNA: leaves nucleus enters the cytoplasm
with the molecular recipe for amino acids
Cell body
Rough ER (ribosomes) – translation
amino acids
PER/TIM Proteins
proteins
Do PER & TIM work together?
So, isolated both PER & TIM (genes)
PER Protein & TIM Protein
Put them together
PER TIM
stuck together
HEY DUDE ..they must interact with each other inside the cell!
After many experiments
Discovered a clock-like loop
PER & TIM genes are active until their
proteins begin to bind to each other (heterodimer)
They form complexes that enter the nucleus
& shut down their own genes (with the help
of Clock & Cycle)
After a few hours - enzymes
degrade the complexes &
Viola..the cycle begins again!
FANTASTIC WEBSITE!!!!!
http://learn.genetics.utah.edu/features/clockgenes/
So…the SCN now known as “one of the main biological
Clocks…however now we know that genes are
involved also
….These genes have recently been identified in humans
and rodents!
Across the tree of life, from bacteria to
humans, clocks use oscillating levels of proteins
in feedback loops to keep time. Perhaps more
amazing, fruit flies and mice — separated by
nearly 700 million years of evolution — share
the very same timekeeping proteins.
— "First Runner Up: A Remarkable Year for
Clocks," Science, 1998.
Biological Events Linked to Certain times in the 24 hour cycle
1:00 am – Pregnant women are most likely to go into labor
2:00 am – Levels of growth hormone are highest
4:00 am – Asthma attacks are most likely to occur
6:00 am – onset of menstruation is most likely to occur
- Insulin levels lowest
- BP & HR begin to rise
7:00 am Hayfever symptoms hi
8:00 am Risk for heart attack &
stroke highest
6:00 pm urinary flow is highest
9:00 pm pain threshold lowest
11:00 allergic responses most
likely