ACTIVITY 1 : INTRONS OUT!
- Jul 21
- 3 min read
Updated: Jul 30
"Anything found to be true of E. coli must also be true of elephants."Jacques Monod, 1965 Nobel laureate for the discovery of bacterial operons |
Monod, who co-discovered how bacteria control gene expression, thought that since all life shares common ancestry, fundamental genetic rules should apply equally to everything from single-celled bacteria to mammals. In 1977, Dr. Phil Sharp, in collaboration with others, challenged this fundamental understanding.
Student Directions :
Your team will examine electron microscope images taken by Dr. Claire Moore, a colleague of Dr. Phil Sharp. The images depict RNA collected from the cytoplasm mixed with one-half (the 3’ to 5’ template) of a DNA strand from the nucleus. When the two single strands are mixed, they form a double-stranded DNA-RNA hybrid, a process called hybridization.
Activity Resources :
Activity Steps...
1 : Working with your team, examine the six Hybridization Cards.
Discuss and record the patterns you notice in the three colored portions on the cards...
Color | Patterns Our Group Notices in the DNA-RNA Hybridization Images |
Black | |
Pink | |
Green |
2 : To understand what these cards are depicting, your team will now model what is shown using yarn.
3 : Stretch out the longer strand from 3’ to 5’ at your lab station. This represents one-half of a DNA strand from the nucleus, the template strand taken from an adenovirus (a family of DNA viruses).
4 : Beneath the template strand, stretch out the shorter strand from the 5’ GTP cap to the 3’ poly-A tail. This represents an mRNA (messenger RNA) from the cytoplasm of the same adenovirus.
5 : Model hybridization by matching the same colors of yarn together. If the colors do not match, create a loop. How many loops did you form? ___________
√ Teacher checkpoint : Was this done correctly? ______
6 : Shift the mRNA yarn model at your lab station to showcase the two green “tails” shown on card #4.
√ Teacher checkpoint: Is your model a close representation of the image depicted on card #4? __________
7 : These mRNA green “tails” were initially puzzling to Dr. Sharp and his colleagues. They already knew after transcription:
The 5’ GTP cap is added to the 5’ end of mRNA to help with ribosomal recognition.
The poly-A tail is added to the 3’ end of mRNA to make the mRNA more stable.
8 : Using a metric ruler, measure the length of the yarn in centimeters.
3’ to 5’ DNA template
Color | Length (cm) |
Black | |
Pink |
mRNA from the cytoplasm
Color | Length (cm) |
Black | |
White |
Are the lengths of the black yarn different or identical? _________________
9 : How are the lengths affected by the 5’ GTP cap and poly-A tail being added to mRNA after transcription? To answer this, add up the length of the black and white yarn for the mRNA. How long is it? _________
Does this number match the length of the DNA template (black yarn only)? _________
10 : Still puzzled by the “tails”, specifically the tail on the 5’ end of mRNA shown in the electron microscope DNA-RNA hybridization images, a colleague named Dr. David Baltimore suggested analyzing a longer 3’ to 5’ template DNA from the adenovirus and repeating hybridization.
To model this, your teacher will provide you with a longer 3’ to 5’ template taken from the same adenovirus. As before, model hybridization by matching the same colors of yarn together, and if the colors do not match, create loops.
How many loops did you form? _________
Are the “mystery tails” present or absent? _________________
Do the lengths of the black yarn match? _________
How long are they? _________
11 : This model represents what Dr. Phil Sharp discovered in 1977 — ”split genes” containing introns (non-coding DNA segments) and exons (coding DNA segments). What color are the introns in your yarn model? ___________
What color are the exons in your yarn model? ___________
12 : Watch a 5-minute segment of the film 🎥 Cracking the Code : Phil Sharp and the Biotech Revolution.
13 : Examine the prokaryote and eukaryote gene images above, and revisit Jacques Monod’s quote at the start of this worksheet. Support or refute Dr. Monod’s understanding of genes based on Dr. Sharp’s discoveries.
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