The Genetic Code

The linear sequence of (the four types of) bases in DNA forms the genetic code. A series of three adjacent bases (triplets) in a strand of DNA or mRNA coding for a specific amino acid is known as a codon. There are 64 (4x4x4) possible codons. All but three code for a specific amino acid. The three codons that do not code for any amino acids are known as the stop codons, and indicate where protein chain is to end. Each codon specifies only one amino acid, but there may be several codons specifying for the same amino acid. At a given time, only one strand of DNA is active in protein synthesis. This is known as the "sense strand." Assume that the top strand (3' --> 5') from Figure 1 is the sense strand of the DNA. Read the codons from the 3' end of this strand toward the 5' end. Look at the DNA genetic code table provided.

Question 1:  Which amino acids are coded by the five codons?

1. ___ - ___ - ___

2. ___ - ___ - ___

3. ___ - ___ - ___

4. ___ - ___ - ___

5. ___ - ___ - ___

Question 2:  Which one is the start codon? ___ - ___ - ___.

Question 3:  Is there a (chain terminating) stop codon?

Question 4:  From the genetic code table, determine how many stop codons are there?

Question 5:  Do the stop codons code for amino acids?

Check your answers



The transcription of DNA is catalyzed by RNA polymerase enzyme, which causes the separation of the two strands of DNA. This triggers the copying of the sense strand of DNA from it's 3' end toward it's 5' end to form a single strand of mRNA. The mRNA grows (RNA ribonucleotides added one at a time) from 5'--->3' direction. Thus mRNA is not only anti- parallel, but also the mirror image of the sense strand of DNA.

Question 6: Transcribe the DNA sense strand (3'--->5' direction) given below to make a single strand of mRNA from this strand. Remember the rule of base pairing, and the rules followed in copying DNA.

3'end T A C A T A G G C C A T A T T 5' DNA
'end    .   .   .  .   .   .    .   .   .   .   .  .    .   .  .  . ' mRNA

                                Figure 2

Translation of the Genetic Code

You have seen that transcription results in the synthesis of mRNA. It also produces two other  types of RNA: ribosomal RNA (rRNA), and transfer RNA (tRNA). The rRNA combines with ribosomal proteins to form ribosomes. Ribosomes are transported to the cytoplasm where they form sites for protein synthesis.

Transfer RNAs, once synthesized, also are transported to the cytoplasm where they pick up the amino acids. Each tRNA receives only one amino acid. There is a different transfer RNA for each amino acid. Thus, 61 different tRNA species are found. The attachment of an amino acid to the tRNA is mediated by the enzyme aminoacyl synthetase. The enzyme is specific for an amino acid and for a particular tRNA. Each tRNA carries an unpaired three-base sequence (called anticodon) that is complementary to one of the codons of mRNA. The anticodon of tRNA recognizes the codon of mRNA, thereby forming the basis for translating the mRNA codons and placing the amino acids in the correct sequence in a protein.





Answers to Questions

Answers to Question #1

1. Methionine     2. Proline    3. Lysine    4. Valine    5. None   (Because the stop codon does not code for any amino acid.)

Answer to Question #2

The start codon in DNA is l;ocated at the 3' end of the gene. If you lokk at Fig. 1, that codon is TAC.


Answer to Question #3

Look at the 5' end of DNA. It is ATT.   This is the stop codon.


Answer to Question #4

There are 3 stop codons in DNA. Thes eare: ATT, ATC and ACT.

Answer to Question #5

The stop codons are chain terminating and they do not code for any amino acids.

Answer to Question #6

Follow the rules for copying the DNA:

1. Copy DNA from 3' to 5' end

2. Follow the rule of base pairing: A-T, G-C.

3. Make the new starnd antiparallel.

5' ... AUG UAU CCG GUA  UAA ...3'

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