Activity 2: Transcription

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

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

When other organelles need to manufacture materials for thecell, copies of the master blueprint are made, so that theoriginals are not damaged or lost. This process is referred to astranscription. The copies made by transcription will be in the formof 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 DNAsequence in this activit represents a gene located on one of thechromosomes in the nucleus of a cell that codes for a specificprotein.

1. Since thymine isn’t found in mRNA, the base pairing rule youlearned in the previous activity must be altered. Complete Table9-2 by entering the appropriate bases.

Table 9 – 2 mRNA Base pairing

DNA base	mRNA base
A
C
T
G

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

Fill in the appropriate mRNA sequence in the second column.

1. The mRNA code is composed of a sequence of three bases alongthe mDNA molecule. The three bases are referred to as a codon.

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

1. How many codons does your mRNA strand contain?
2. What is the base sequence of the fourth codon of yourmRNA?

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 proteinsynthesis

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

Reminder: Ribosomes are the sites of protein synthesis within acell

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

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

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

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

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

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