Question

Activity 2: Transcription

Imagine that you’re having a house built. The architect draws up a set of blueprints for the builder. The builder, in turn, has to hire several subcontractors for the electrical, plumbing, and heating systems. The builder doesn’t wish to give up the original set of blueprints, so copies are made for each of the subcontractors.

This same principle applies to a cell. The nucleus contains the master set of blueprints, the DNA. DNA molecules are needed to direct the activities that occur in the cytoplasm of each cell.

When other organelles need to manufacture materials for the cell, copies of the master blueprint are made, so that the originals are not damaged or lost. This process is referred to as transcription. The copies made by transcription will be in the form of another molecule called messenger RNA (mRNA)

There are several differences between DNA and mRNA:

DNA is double stranded	mRNA is single stranded
DNA contains the base thymine (T)	mRNA contains the base uracil (U)
DNA contains the sugar deoxyribose.	mRNA contains the sugar ribose

In this activity you’ll transcribe a DNA code into mRNA. The DNA sequence in this activit represents a gene located on one of the chromosomes in the nucleus of a cell that codes for a specific protein.

1. Since thymine isn’t found in mRNA, the base pairing rule you learned in the previous activity must be altered. Complete Table 9-2 by entering the appropriate bases.

Table 9 – 2 mRNA Base pairing

DNA base	mRNA base
A
C
T
G

2. Looking at Figure 9 – 2, you can see that column one contains the DNA sequence found along one side of a double helix.

Fill in the appropriate mRNA sequence in the second column.

3. The mRNA code is composed of a sequence of three bases along the mDNA molecule. The three bases are referred to as a codon.

ATG is the first triplet on the DNA sequence. What is the corresponding codon?

4. How many codons does your mRNA strand contain?
5. What is the base sequence of the fourth codon of your mRNA?

FIGURE 9-2

1 DNA	2 DNA	3 Amino Acids
A
T
G
T
A
T
G
T
T
T
T
G
A
C
G
G
G
A
G
A
C
C
C
C

Activity 3: Translation and protein synthesis

Having copied the DNA into mRNA leaves the nucleus of the cell and travels to a ribosome.

Reminder: Ribosomes are the sites of protein synthesis within a cell

Proteins are composed of chains of amino acids. A chain of amino acids is also referred to as a polypeptide chain because the amino acids are held together with a type of covalent bond called a peptide bond.

Amino acids are brought to the ribosome by another type of RNA called transfer RNA (tRNA). The codons in the mRNA determine the sequence and number of amino acids being bonded into the corresponding protein.

1. Refer to the mRNA codons in Table 9-3 to determine the amino acid sequence for your protein.

To do this, just match each codon in your mRNA sequence to the corresponding amino acid as listed in the table.

Fill in column three of Figure 9-2 with the correct amino acids.

2. How many amino acids does your protein contain?
3. What is the sixth amino acid in the polypeptide chain?
4. If the sixth triple read GAA instead of GGA, would the sequence of amino acids change? Explain your answer.
5. If the sixth triple read GGC instead of GGA, would the sequence of amino acids change? Explain your answer.
6. What is the name of the bonds that attach the amino acids together within the polypeptide chain?

Answer 1

ACTIVITY 2

Answer 1: Table 9 – 2 mRNA Base pairing (as adenine will pair to uracil in RNA and cytosine will pair with guanine only)

DNA base	mRNA base
A	U
C	G
T	A
G	C

Answer 2: ATG is the first triplet on the DNA sequence. What is the corresponding codon?

The corresponding codon will be UAC (as adenine will pair to uracil in RNA and cytosine will pair with guanine only and the mRNA code is composed of a sequence of three bases . The three bases are referred to as a codon).

Answer 3 How many codons does your mRNA strand contain?- 8 (when paired in triplets see in the given image)

We can see the figure now:

ACTIVITY 3

1. Question What is the base sequence of the fourth codon of your mRNA?

Answer: From the given figure above, it can be deduced to be AAC

2. Question Fill in column three of Figure 9-2 with the correct amino acids.

Answer Refer in the figure 9-2.

3. Question: How many amino acids do your protein contain?

Answer: From the figure 9-2, 8 amino acids are present in the protein.

4. Question What is the sixth amino acid in the polypeptide chain?

Answer: The sixth amino acid in the polypeptide chain is Proline coded by CCU codon.

5. Question: If the sixth triple read GAA instead of GGA, would the sequence of amino acids change? Explain your answer

Answer: Yes, the sequence of amino acids would change. If the DNA sequence for the sixth triple read would have been GAA, then the mRNA sequence would have been CUU that would start coding for Leucine instead of Proline.

6. Question: If the sixth triple read GGC instead of GGA, would the sequence of amino acids change? Explain your answer.

Answer: No, the sequence of amino acids would not change. If the DNA sequence for the sixth triple read would have been GGC, then the mRNA sequence would have been CCG that would code for Proline only.

7. Question What is the name of the bonds that attach the amino acids together within the polypeptide chain?

Answer: Peptide bond links the amino acid froming polypeptide chain. The bond is formed between the carboxyl group of one amino acid with that of amino group of other amino acid.