From Gene to Protein—Transcription and Translation



Protein Synthesis:

In this activity, you will use paper models to learn more about transcription and translation. Specifically, you will model how a cell carries out transcription and translation to make the beginning of the hemoglobin molecule. Hemoglobin is a protein that helps your blood cells carry oxygen.

Complete the following table to summarize the basic characteristics of transcription and translation.

| |Original message or instructions in: |Molecule which |Location where |

| | |is synthesized |this takes place |

| | | | |

|Transcription |Nucleotide sequence in gene in DNA | |Nucleus |

| |in chromosome | | |

| | | | |

|Translation | | | |

| | | | |

Step 1: Transcription

How does the information in the DNA of the gene get copied into a message in the mRNA?

The sequence of DNA determines the sequence of mRNA. Base pairing rules are used to make sure the order is correct.

|Complementary Nucleotides for Base-Pairing: |

|Between two strands of DNA |Between DNA and RNA |

|(During Replication) |(During Transcription) |

| | |

|G (guanine) pairs with C (cytosine). |G (guanine) pairs with C (cytosine). |

| | |

|T (thymine) pairs with A (adenine). |T (thymine) in DNA pairs with A (adenine) in RNA. |

| | |

| |A in DNA pairs with U (uracil) in RNA. |

Transcription Modeling Procedure

Note: You will work with a partner to model the actual sequence of steps used by the cell to carry out transcription. You probably will be able to think of a faster way to make the mRNA, but you should follow the sequence of steps described below in order to learn how the cell actually makes mRNA. Remember, enzymes do not have a brain, eyes or hands, so transcription must proceed in a step-by-step chemical process that adds one nucleotide at a time to the growing mRNA molecule.

1. To model the process of transcription, you and your partner will need:

• Nucleus

• Single blue strand of DNA labeled "Beginning of Hemoglobin Gene"

• 9 pink RNA nucleotides

• A pink strip labeled "Second Part of mRNA"

• tape

In addition, you should prepare by completing the following chart to summarize the base-pairing rules you will need to follow as you synthesize the mRNA molecule.

| DNA nucleotide |Complementary nucleotide in RNA |

| G | |

| C | |

| T | |

| A | |

2. One of you will act as the cytoplasm which surrounds the nucleus and supplies the nucleotides and the other will act as the enzyme that attaches the nucleotides to make the RNA molecule.

Enzyme: Insert the "Beginning of Hemoglobin Gene" DNA molecule through the slot in the RNA polymerase diagram so the first two nucleotides of the DNA are on the dashed line of the RNA polymerase.

Cytoplasm: Give the first RNA nucleotide (complementary to the first DNA nucleotide) to the enzyme person.

Enzyme: Put the first RNA nucleotide in the box labeled RNA nucleotide.

Cytoplasm: Give the next RNA nucleotide (complementary to the next DNA nucleotide) to the RNA polymerase person.

Enzyme: Put this nucleotide in the box labeled "next RNA nucleotide" and join the two nucleotides together with transparent tape. (The tape represents the covalent bond that forms between the adjacent RNA nucleotides as the mRNA molecule is synthesized.)

Enzyme: Then, move the DNA molecule and the growing mRNA molecule one space to the left.

3. Repeat #2 as often as needed to complete transcription of the beginning of the hemoglobin gene, adding one nucleotide at a time to the mRNA molecule.

• Be careful to follow the base-pairing rule accurately

4. Once your mRNA is made, tape the second part of mRNA to the CUG end of the mRNA you made

• We’ll pretend you kept adding nucleotides until the mRNA molecule was complete.

Transcription Questions:

1. Notice that the process of transcription is similar to the process of DNA replication. What are some similarities between transcription and DNA replication?

2. There are also a few important differences between DNA replication and transcription. Fill in the blanks in the following table to summarize these differences.

|DNA replication |Transcription |

|The whole chromosome is copied. |___________________is transcribed. |

|DNA is made. |mRNA is made. |

|DNA is double-stranded. |mRNA is _____________ -stranded. |

|T = thymine is used in DNA, so A pairs with T in |T = thymine is replaced |

|DNA. |by ___ = uracil in RNA, |

| |so A in DNA pairs with ___ in mRNA. |

3. To summarize what you have learned, explain how a gene directs the synthesis of an mRNA molecule. Include in your explanation the words and phrases: base-pairing rules, complementary nucleotides, cytoplasm, DNA, gene, mRNA, nucleotide, nucleus, enzyme, and transcription.

• This part is worth 10 points…each correctly used word = 1 point

Step 2: Translation

In the process of translation, the sequence of nucleotides in mRNA determines the sequence of amino acids in a protein. The figure below shows an example of how transcription is followed by translation.

[pic]

(Figure 14.6 from Krogh, Biology, a Guide to the Natural World, 2005)

|mRNA codon |Amino acid |

|ACU |Threonine (Thr) |

|CAU |Histidine (His) |

|CCU |Proline (Pro) |

|CUG |Leucine (Leu) |

|GAG |Glutamic acid (Glu) |

|GUG |Valine (Val) |

The sequence of codons in the mRNA determines the sequence of amino acids in the protein. The table to the right shows the six codons that will be part of your mRNA molecule, together with the amino acid coded for by each of these codons.

Translation Modeling Procedure:

1. To model the process of translation, you and your partner will need:

• Ribosome page

• 6 green tRNA molecules

• 6 purple amino acids

• the mRNA you made during your simulation of transcription

You also will need to know which amino acid corresponds to each tRNA anti-codon. The table below shows the codons in your mRNA and the corresponding amino acids.

Use the base-pairing rules to show the tRNA anti-codon for each mRNA codon.

|mRNA codon |tRNA |Amino acid |

| |anti-codon | |

| ACU | UGA |Threonine (Thr) |

| CAU | |Histidine (His) |

| CCU | |Proline (Pro) |

| CUG | |Leucine (Leu) |

| GAG | |Glutamic acid (Glu) |

| GUG | |Valine (Val) |

Note: You will be modeling the actual sequence of steps used by the cell to carry out translation. You probably will be able to think of a faster way to make the protein, but you should follow the sequence of steps described below in order to learn how the cell actually makes proteins.

2. One of you will play the role of the ribosome where the amino acids are assembled into a protein and the other one will act as the cytoplasm, which is the source of tRNA and amino acid molecules.

Cytoplasm: Use the above table to match each tRNA molecule to the correct amino acid. Tape the amino acid to the tRNA very lightly, because they will only be joined temporarily and will separate again soon.

Ribosome: Insert the mRNA through the slot in the model ribosome, with the first three nucleotides of the mRNA in the "codon" position and the next three nucleotides in the "next codon" position.

Cytoplasm: Supply the tRNA that has the correct anti-codon to match the first codon in the mRNA.

Ribosome: Place this tRNA with amino acid in position.

Your model ribosome should look like:

Cytoplasm: Supply the tRNA that has the correct anti-codon to match the second codon in the mRNA.

Ribosome: Place the tRNA in position. (Your model should look like the picture below.) Tape these two amino acids together. The tape represents the covalent bond between the first two amino acids in the hemoglobin protein.

Ribosome: At this time, the first amino acid detaches from the first tRNA, so remove that tape.

Ribosome: Move the mRNA to the left so the second codon is in the first position in the ribosome. The matching tRNA with amino acid also moves to the first position. Also, the first tRNA is released into the cytoplasm where it would be reused in a real cell.

Cytoplasm: Put the first used tRNA in the packet.

Your model should look like:

Cytoplasm: Supply the tRNA that has the correct anti-codon to match the codon in the "next codon" position.

Ribosome: Repeat the previous steps until all of the amino acids are attached

Cytoplasm: Repeat the previous steps until all of the amino acids are attached and all of the tRNA molecules are used.

You will end up with empty tRNA molecules, an empty mRNA molecule, and a string of amino acids. The amino acid chain is your hemoglobin protein.

Questions

1. Explain the function of mRNA.

2. Explain the function of tRNA.

3. Explain why a cell needs both mRNA and tRNA in order to make a protein and how tRNA and mRNA work together to bring in the right amino acids in the right sequence as a protein is synthesized.

-----------------------

(Figure 17.7, Campbell and Reece, Biology, 2005)

slot

Additional nucleotides in mRNA…

Additional nucleotides in mRNA…

Additional nucleotides in mRNA…

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