Download lesson viii - MisterSyracuse.com

LESSON VIII:
TRANSCRIPTION
To Address NYS Standards:
2 (Organisms inherit genetic information in a variety of ways that result in continuity of
structure and function between parents and offspring) and 4 (The continuity of life is
sustained through reproduction and development).
Behavioural Objective:
The students will be able to completely understand the process of transcription and how it
is important to the cell. Students will understand the difference between transcription and
translation, and where in the central dogma of replication this process lies.
Explanation of Lesson Plan:
This lesson will allow students to see how DNA is copied to mRNA to send out to the
ribosomes. The analogy of photocopying a cookbook will be used to illustrate how DNA
should never be removed from the nucleus, but must be replicated to be sent out.
Hook: (5 minutes)
“if you wanted to give great-great-grandmother’s delicious brownie recipe to someone,
you wouldn’t just rip it out of your cookbook and hand it over! You’d make a copy of it.
This is what we’re going to learn about today. There are two words that sound alike
mean very different things. You have to keep these straight in your head. Transcription
is going from one language to the same language. Monks, or people like Igor transcribe
books. It’s just copying them. That’s what we’re doing today. Translation is from one
language to another. That’s when some snooty Ph.D translates Dante’s Inferno from
Italian to English. Or, when we take the language of nucleotides and make it the
language of amino acids or proteins. Let’s get ready to rumble!!!
Test of Prior Learning: (2 minutes)
1. Who knows anything about making RNA?
2. What makes RNA different from DNA?
3. How are the processes similar or different?
New Learning: (30 minutes)
1. So we want to make a copy of DNA. DNA is double-stranded, but why?
2. So we don’t need a molecule that is so stable. This molecule isn’t going to be around
that long. In fact, we don’t want it to be around that long!
3. So we need to make a copy of DNA, which we’re going to do using RNA. RNA has
only one strand, so it is easer to read, and easier to break down when we need to.
4. RNA is going to carry the information on DNA out of the nucleus to the ribosomes,
where it will be turned into proteins.
5. There are a few different kinds of RNA: mRNA is messenger RNA, tRNA is transfer
RNA, and rRNA is ribosomal RNA, which is what makes up ribosomes!
6. So we make a copy of the DNA with an enzyme called RNA polymerase. It does the
same thing as DNA polymerase, except is uses RNA nucleotides.
7. These are the same as DNA, except Adenine pairs with Uracil, not Thymine. So the
pairs are A with U, and C with G. Just to confuse you.
8. So there is a helicase that opens up the DNA, and then other proteins like gyrases and
single-strand binding proteins to keep it open.
9. This time, though, we are making only a single-stranded copy. So we only need to use
one side, and there will be only one leading strand. No Okazaki fragments or anything.
Boring.
10. Simple, eh? Not so much. Let’s take a look at bacterial transcription
11. Transcription starts at a promoter site. This is a specific region of DNA with a
specific sequence of bases. It signals the start of a gene.
12. RNA polymerase attaches here, and starts adding bases, using the DNA as a template
strand. It is much slower than DNA polymerase, at only 40 bases per second.
13. It moves along until it hits the terminator. “You have been targeted for termination.”
14. This signals RNA Polymerase to fall off of the DNA, and release the new mRNA.
15. In bacteria, this is it. It’s done. Let’s take a look at eukaryotes, though.
16. The promoter usually contains the sequence TATAAA, so is called the Tata box.
This is where RNA polymerase attaches.
17. Eukaryotes also have enhancers and other sequences that can tell RNA polymerase to
go faster, slower, repeat segments, there are also silencers that can turn off RNA
polymerase. There is much more control.
18. There is a terminator in eukaryotes as well, but there are other chemicals that are
thought to stop transcription, too.
19. We’re not done yet! Eukaryotes have Introns and Exons, which prokaryotes like
bacteria don’t have.
20. Introns are slices of DNA that don’t code for anything. They are not “junk” DNA,
we just don’t know what it does. It must serve some purpose, like absorbing mutations,
etc.
21. Anyway, before mRNA leaves the nucleus, it must be processed. This involves
cutting out the introns, and going with the exons. Think, “GO WITH THE GAS.” The
exons are then ligated together, and the mRNA is exported.
Test of New Learning: (3 minutes)
1. What is the main difference between prokaryote and eukaryote transcription?
2. What is a promoter?
3. What is a terminator?