How does DNA actually work?



Although DNA gets most of the glory, teams of molecules are involved in getting genetic instructions read, understood and carried out within the cell .

Copying DNA
DNA contains regions called gene sequences, which are coded instructions to produce important molecules. However, DNA is held in the nucleus of the cell, where it is protected from radiation and chemical damage.

For the DNA instructions to be read and used, a type of RNA (or ribonucleic acid) carries a copy of the DNA instructions out of the nucleus. This ‘messenger RNA’ (or mRNA) is one of a number of RNA types with different functions in the cell.

RNA is chemically very similar to DNA, except the spine uses a slightly different sugar, and contains the base uracil rather than thymine. An mRNA copy of the DNA instructions is created in the nucleus; it forms a complementary strand to the DNA.
So a DNA sequence of:
AATCTCGGA, would become
UUAGAGCCU in mRNA

The process of making this mRNA copy of the DNA instructions is called transcription.The information in the mRNA is then packaged for transport out of the nucleus.



Building proteins
Once outside the nucleus, the information encoded in the mRNA can be read by molecular machines (called ribosomes) responsible for building molecules called proteins. Proteins perform a vast number of roles in our bodies; they can be used in the cell where they’re produced or transported elsewhere in the body.

DNA is made up of four bases, A, C, G, and T. Proteins, however, are made up of 20 building blocks – called amino acids. How is one code translated into the other?

At the ribosome, the mRNA is read in groups of three bases (codons), each of which identifies/specifies a particular amino acid. Each amino acid is carried to the protein-making factory on another type of RNA, transfer RNA (tRNA).

The transfer RNAs (tRNA) contain sequences (called anticodons) that are complementary to the triplet combinations on the mRNA. Each tRNA, as defined by its anticodon, carries a specific amino acid. As a result, the order of amino acids in the protein being built – and the type of protein produced – is determined by the DNA sequence.

So if the mRNA code was:
UUAGAGCCU, the tRNAs would be
AAU CUC GGA

We now know which anticodons match to which amino acids. This code even has some punctuation – some of the anticodons refer to Start or Stop. In our example above the amino acid sequence would be:
Asparagine-Leucine-Glycine

The process of creating an amino acid chain, that will fold into a protein, from the mRNA strand is called translation.There are thousands of different proteins performing a range of different functions in the body. These differences are created by the different sequence of amino acids, instructed by the base sequence of the DNA.

Alcohol dehydrogenase: Just one of the many proteins found in the human body.



Copying the code
DNA produces a messenger using the process of transcription. Watch an amazing animation of the view from inside the cell.

Copyright 2003 Howard Hughes Medical Institute

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Reading the code
Genes within the DNA code for proteins. See how the base sequence is translated into an amino acid sequence.

Copyright 2003 Howard Hughes Medical Institute

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