Protein Synthesis

BACKGROUND

DNA carries the information for the synthesis of all the proteins of an organism. Proteins are large and complex molecules, composed of hundreds of amino acids. There are twenty different amino acids. The sequence of the nucleotides in the DNA molecule determines the sequence of amino acids in a protein molecule. The information encoded in the nucleotide sequence of DNA is not used directly in protein synthesis. Instead, the information carried in the DNA molecule is encoded, or transcribed, into a special type of RNA, messenger RNA (mRNA). This process is called transcription. In transcription, the nucleotide thymine is replaced by uracil, so the pairing of nucleotides becomes A-U, while C-G remains the same. Once transcription has finished, the mRNA passes out of the nucleus and travels to a ribosome. Here the mRNA is read, and the correct amino acids are brought into specified places by another type of RNA, transfer RNA (tRNA). The amino acids are then linked together by peptide bonds to form the protein originally specified by DNA. OBJECTIVES In this activity you will:

Learn how the transcription of DNA occurs during protein synthesis.

Become familiar with the code by which the information in mRNA is translated.

Use paper models to see how translation of mRNA occurs during protein synthesis.

MATERIALS 1/2-in transparent tape Glue Sticks PROCEDURES AND OBSERVATIONS During transcription, the DNA double helix unzips. As the hydrogen bonds between the two DNA strands break, nucleotides present in the cell line up next to the nucleotides of one DNA strand to form mRNA. RNA contains uracil in place of the thymine in DNA. Thus in the pairing of RNA nucleotides with DNA, uracil pairs with adenine and cytosine with guanine. The nucleotides in the newly formed mRNA are complementary to the nucleotides of the DNA segment on which it formed. For example, where the DNA contained guanine, the RNA, contains cytosine. After the mRNA is formed, it passes out of the nucleus and into the cytoplasm. a. One strand of DNA has the base sequence:

C G A T T G G C A G T C A T. Determine the sequence of bases in the complementary strand of mRNA that would form next to this DNA strand.

Write the sequence of bases in the complementary mRNA strand below.

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The information carried on the mRNA is in a code the genetic code. A group of three nucleotides on a molecule of mRNA is called a codon; each codon specifies one of the twenty amino acids, except for three codons that are stop, or termination, signals. There are sixty-four codons in the genetic code. Most amino acids are coded for by more than one codon. The genetic code and the abbreviations for the twenty amino acids are given in Figure 1. GENETIC CODE

	U	C	A	G	Abbreviations for the Amino Acids
UU	phe	phe	phe	phe
UC	ser	ser	ser	ser	gly ala val ile leu ser thr pro asp glu lys arg asn gin cys met trp phe tyr his stop	glycine alanine valine isoleucine leucine serine threonine proline aspartate glutamate lysine arginine asparagine glutamine cysteine methionine tryptophan phenylalanine tyrosine histidine termination
UA	tyr	tyr	stop	stop
UG	up	up	stop	trp
CU	leu	leu	leu	leu
CC	pro	pro	pro	pro
CA	his	his	gin	gin
CG	arg	arg	arg	arg
AU	stop	stop	stop	met
AC	thr	thr	thr	thr
AA	asn	asn	lys	lys
AG	ser	ser	arg	arg
GU	val	val	val	val
GC	ala	ala	ala	ala
GA	asp	asp	glu	glu
GG	gly	gly	gly	gly

Use the genetic code to read the codons below. Find the first two letters of the codon in the left column of the chart. When you have found them, follow the row across to the column beneath the last letter of the codon. This is the abbreviation of the amino acid specified. Find the name of amino acid and write it in the space provided. (If there is more than one amino acid, separate the names by dashes.)

U U A:

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G A G:

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UAU CUA:

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AUC UUG:

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AAG AGU UCG:

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AAA UUU GGG:

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CCA GCU AGA GGG UGG CUG UCA:

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Molecules of tRNA are formed in the nucleus and migrate into the cytoplasm. There are twenty different types of tRNA, one for each type of amino acid. Each type of tRNA can grab one kind of amino acid molecule. Each tRNA has a three-base segment called an anticodon, which is complementary to a codon on the mRNA. In protein synthesis, a tRNA molecule carrying an amino acid molecule becomes attached briefly to the mRNA at a codon complementary to its anticodon. Then a tRNA complementary to the adjacent codon attached on the mRNA. A peptide bond forms between the two amino acid molecules carried by the tRNAs. Amino acids are added one at a time to the growing chain as the mRNA strand is "read." This continues until a termination (stop) codon is encountered. After each peptide bond is formed, the tRNA is released to go and pick up a new amino acid in the cytoplasm. Determine the anticodon for each codon below. Write it in the space provided.

G G U:

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C G C:

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A U G:

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U C G:

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A A A:

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C U G:

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Print and cut out the mRNA and tRNA models with amino acids. Spread them out on the table in front of you. Then cut out the mRNA strands and tape them together.
Starting from the left of the mRNA strand, find a tRNA molecule with an anticodon complemen-tary to the first codon. With a small piece of tape, attach the tRNA to the mRNA strand, anticodon to codon.
Then go on to the next codon and find a tRNA with the complementary anticodon. Tape the tRNA in place to the mRNA. Also, use a small piece of tape between the two amino acids to represent a peptide bond.
Once the peptide bond has been formed, the tRNA molecule attached first is released. Carefully cut the tape attaching the first tRNA to the mRNA, and cut the line that separates the tRNA and the amino acid. You may set the tRNA model aside and discard it later.

termination codons

When the termination codon is read, the strand of amino acids is released.

Repeat steps c and f along the mRNA strand. When you have used up all the tRNA-amino acid models provided, you will notice that there is one codon left on the mRNA—a termination codon. Cut the tape between the mRNA and the tRNA, and the line between the last tRNA and amino acid, thus releasing the chain of amino acids.

Write the sequence of the amino acids formed by translation of the mRNA strand.
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CONCLUSIONS AND APPLICATIONS

Write the mRNA that would be transcribed from the following strand of DNA. Then read the mRNA, and write the names of the amino acids coded for.

DNA G T A T A C C A G T C A T T T G T C mRNA amino acids

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Suppose the bases of the DNA in question 1 were not transcribed correctly and the mRNA read: CAAAUGGUCAGUAAACAG How many mistakes were made in transcription? Write the amino acids that would be formed by translation of this mRNA.
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Suppose the bases of the DNA were not transcribed correctly and the mRNA read: CACAUGGUUAGUAAGCAG How many mistakes were made in transcription? Write the amino acids that would be formed by translation of the mRNA.
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Sometimes a mistake occurs in the translation of an mRNA strand. Suppose that the reading of the mRNA strand in question 1 began, by mistake, at the second nucleotide instead of the first. The first codon would be AUA. Write the sequence of amino acids that would be formed.
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Protein Synthesis

BACKGROUND

U

C

A

G

UU

UC

UA

UG

CU

CC

CA

CG

AU

AC

AA

AG

GU

GC

GA

GG

U U A:

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G A G:

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UAU CUA:

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AUC UUG:

AAG AGU UCG:

AAA UUU GGG:

CCA GCU AGA GGG UGG CUG UCA:

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G G U:

C G C:

A U G:

U C G:

A A A:

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C U G: