Unit 4:  The genetic revolution                                                       1   2   3   4  

2.4. Expression of the genetic information

2.4.1. The genetic code

The DNA instructions are codified a sequence of nucleotides. However the cell needs to transform this information into proteins to be able to perform its vital functions.

Proteins are macromolecules formed by little molecules named amino acids. There are only 20 different amino acids, but they combine forming long chains that give as a result thousands of different proteins. 

The cell uses the genetic code to translate the DNA language to the proteins language. This code is the relationship of correspondence between mRNA nucleotides (and consequently, DNA) and amino acids.

The genetic code has the following properties:

• It is organized in triplets or codons: each group of three nucleotides (triplet) codifies one amino acid, but there are also codons which do not codify any amino acid. These non-sense codons are used as signals to indicate the start (start codon) and the end (stop codons) of the gen.

• It is degenerated: there are more codons tan amino acids, so that each amino acid can be codified by more than one codon. In each codon the two first nucleotides are usually, fixed and the third, variable.

• It is non-overlapping: one nucleotide only belongs to a unique codon.

• Its reading is without stops: The sequence is a continual line with no breaks between the codons.

• It is universal: The same triplet in different species codifies the same amino acids.

2.4.2. The synthesis of proteins

The transformation of a gene (sequence of nucleotides) into a protein (sequence of amino acids) is not a direct process. It is possible to distinguish several phases in it. The set of those phases is known as the central dogma of molecular biology.  

The instructions to build proteins are stored in the nucleus of the cell. However, the ribosomes that have to make them are in the cytoplasm. So that it is necessary to transfer the information from the nucleus to the cytoplasm. This process is carried out in two phases: transcription and translation.

a) Transcription

This is the process of copying DNA in the format of messenger RNA. It is performed in the nucleus.

             that is going to be transcribed.

             are attaching in a sequential way. Just one strand of DNA is transcribed

             (template strand).

             is closed again.

Animation: mRNA synthesis

Animation: Transcription (McGraw Hill)

b) Translation

This is the process of transforming mRNA into protein. It is performed in the cytoplasm.

 1^st)  mRNA transfers the information from the nucleus to the cytoplasm.

 2^nd) Ribosomes “read” the instructions stored in mRNA and “translate them,

       placing each amino acids in the correspondent place of the massage,

        following the genetic code.

 3^rd) Amino acids are transported by transfer RNA (tRNA). Each one of the twenty 

       amino acids is transported by one specific tRNA. This is possible because

       each tRNA molecule contain a complementary triplet of nucleotides of one

       of the codons of the mRNA. This triplet is the anticodon.

Animation: Protein synthesis

Animation: Protein synthesis (McGraw Hill)

2.4. What is a codon?  

2.5. Give an example demonstrating that the genetic code is degenerated.

       Use the chart.

2.6. How can be explain from the evolutionary point of view, that all living

       beings have the same genetic code?

2.7. This is the sequence of a piece of a strand of DNA:

…T T C G T A T C G G C A T T G C A A …

a. Write down its correspondent mRNA.

b. Which amino acid corresponds to each codon?