Download Protein Synthesis Overview IP

Biology I IP
Name: ____________________________________
Period: _________
Protein Synthesis Overview IP
The Central Dogma
Your DNA contains genes that determine your physical characteristics, such as eye color. How
can this organic molecule control your characteristics? DNA contains instructions for all
the proteins your body makes. Proteins, in turn, determine the structure and function of all
your cells. What determines a protein’s structure? It begins with the sequence of amino
acids that make up the protein. Instructions for making proteins with the correct sequence of
amino acids are encoded in DNA.
1. How does DNA control your physical characteristics? ________________________________
______________________________________________________________________________
2. What do proteins do in your cells? _______________________________________________
3. What are proteins made of? _____________________________________________________
As you know, DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain
in the nucleus, but proteins are made by ribosomes in the cytoplasm. How do the instructions in
DNA get to the ribosomes outside the nucleus? Another type of nucleic acid is responsible for
copying and transporting the instructions.
This nucleic acid is RNA, or ribonucleic acid. RNA is a small molecule that can squeeze through
pores in the nuclear membrane. It carries the information from DNA in the nucleus to a ribosome
and then helps assemble the protein. In short:
DNA → RNA → Protein
Discovering this sequence of events was a major milestone in molecular biology. It is called
the central dogma of molecular biology. There are two processes involved in the central dogma:
transcription and translation.
4. Which organelle is responsible for making proteins? _________________________________
5. Where in the cell are proteins made? ______________________________________________
6. Which nucleic acid is responsible for transporting DNA instructions out of the nucleus? _____
7. How does RNA get from the nucleus to the cytoplasm? _______________________________
8. What are the two processes involved in the central dogma?
Transcription
9. What does the verb “transcribe” mean? Look it up!
Transcription is the first step of protein synthesis. Transcription takes place in the nucleus, and
it uses DNA as a template to make an mRNA molecule (the “m” in mRNA stands for
messenger). mRNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where
translation occurs. Translation reads the genetic code in mRNA and makes a protein.
10. Where does transcription take place? ____________________________________________
11. What is made during transcription? ______________________________________________
12. Why do you think that the RNA made in transcription is called messenger RNA?
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13. Where does the mRNA go after it has been made? __________________________________
Transcription takes place in three steps: initiation, elongation, and termination.
Initiation occurs when the enzyme RNA polymerase binds to a region of a gene called
the promoter. This signals the DNA to unwind so the enzyme can ‘‘read’’ the bases in one of the
DNA strands. The enzyme is now ready to make a strand of mRNA with a complementary
sequence of bases.
Elongation is the addition of nucleotides to the mRNA strand. RNA polymerase reads the
unwound DNA strand and builds the mRNA molecule, using complementary base pairs (like
DNA Replication). One exception exists during transcription: RNA uses a base called uracil (U)
instead of thymine (T). Therefore, an adenine (A) in the DNA pairs with an uracil (U) in the
RNA.
Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop
(termination) sequence in the gene. The mRNA strand is complete, and it detaches from DNA
and leaves the nucleus
14. What are the three steps of transcription? _________________________________________
15. Which enzyme is involved in transcription? _______________________________________
16. How is RNA Polymerase similar to DNA Polymerase, from DNA replication (use your past
notes to help you!)?
______________________________________________________________________________
17. Write the complementary RNA strand for A-G-T-C-C-T-A-A Hint: U instead of T!
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18. How does RNA Polymerase “know” when to end transcription?
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Figure 1: Transcription
Observe Figure 1.
Which strand of DNA (5’  3’ or 3’  5’) is used as the template strand? _________________
In which direction (5’  3’ or 3’  5’) is the mRNA made? ___________________
Translation
19. In your own words, what does the verb “translate” mean?
Translation is the second part of protein synthesis, which takes place in the cytoplasm. It is the
process in which the genetic code in mRNA is read, one codon at a time, to make a protein. A
codon is a sequence of three mRNA bases (for example, ACG). Each codon is like a “word” that
is read by the ribosome, and each codon corresponds to a specific amino acid. The ribosome
“reads” the codons in the mRNA and translates the codons into an amino acid sequence.
20. Where does translation take place? ______________________________________________
21. What is made during translation? _______________________________________________
22. What is a codon? ____________________________________________________________
24. How many codons are in the following mRNA: AAGCCUGGAUCCUUA? _____________
25. In your own words, why do you think this process is called translation?
______________________________________________________________________________
After mRNA leaves the nucleus, it moves to a ribosome, which consists of rRNA and proteins.
The ribosome reads the sequence of codons in mRNA. Molecules of tRNA (transfer RNA) bring
amino acids to the ribosome in the correct sequence.
26. What is a ribosome made of? ___________________________________________________
27. Which molecule brings amino acids to the ribosome? _______________________________
To understand the role of tRNA, you need to know more about its structure:
Each tRNA molecule has an anticodon for the amino acid it carries. An anticodon is a sequence
of 3 bases, and is complementary to a codon in mRNA. For example, the amino acid methionine
(Met for short) matches the codon AUG, so the anticodon is UAC. Therefore, methionine would
be carried by a tRNA molecule with the anticodon UAC. Wherever the codon AUG appears in
mRNA, a UAC anticodon on a tRNA temporarily binds to the codon.
While bound to the mRNA, the tRNA gives up its amino acid. Bonds form between adjacent
amino acids as they are brought one by one to the ribosome, forming a polypeptide chain. The
chain of amino acids keeps growing until a stop codon is reached.
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28. Label the tRNA, amino acid, codon, and anticodon in the figure above.
29. What is an anticodon? ________________________________________________________
30. What happens when tRNA binds to mRNA? ______________________________________
31. What is a chain of amino acids called? ___________________________________________
32. What happens when a stop codon is reached? _____________________________________