How does genetic code carry genetic information?

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ashley227 eNotes educator| Certified Educator

Living organisms' genetic code is stored in their DNA. There are viruses that utilize RNA instead of DNA as their genetic code, but viruses are not considered to be living because they do not contain at least one cell, which is the smallest unit of life. DNA is a double-stranded chain of monomer nucleotides. A nucleotide of DNA consists of a deoxyribose, a phosphate group, and a nitrogenous base. It is the order of the nitrogenous bases (adenine, thymine, cytosine, and guanine) that actually specifies the "genetic code." We abbreviate these bases as A, T, C, and G.  These bases are paired within the 2 strands so an A in one strand is hydrogen bonded to a T in the other strand and a C in one strand is hydrogen bonded to a G in the other strand. The base pairing gives DNA the appearance of a ladder; the rungs are the paired bases and the sides are the deoxyribose sugars and phosphates. To give DNA its double helix appearance, the ladder would be twisted. 

The sequence of nucleotides is broken up into genes which provide the instructions for making the proteins needed by the cell. All cells containing DNA contain the same instruction set, but only a subset of the genes in the DNA are actually expressed in each cell (depending on the needs of the cell).

In eukaryotes, the DNA is housed in the nucleus of the cell and cannot leave. It can therefore not take the instructions to the ribosome (where proteins are made). The DNA code is then transcribed into a RNA message (mRNA). mRNA is also a nucleic acid, but it is able to leave the nucleus to take the message to the ribosome.  

The process of making the mRNA message is called transcription. Only the template strand of the DNA (the one typically shown as the bottom strand, going from 3' to 5') is used as the template to make the mRNA, which is single-stranded, not double-stranded like DNA. The base pairing rules are still followed, except that there are no Ts in RNA; they are replaced by Us (uracil).  

Here is an example:

Sense DNA:       ATG  CCC  GGG  TAC  GGA

Template DNA:   TAC GGG  CCC   ATG  CCT

mRNA:              AUG  CCC  GGG  UAC  GGA

In eukaryotes, the RNA is then processed (the molecule consists of a lot more bases than what is shown in the example above) with a 5' cap and a poly-A tail after any introns have been removed. This allows the mRNA to leave the nucleus and make its way to a ribosome where the message can be translated into a chain of amino acids (on its way to becoming a protein).

The ribosome reads the mRNA three base pairs at a time. Each three base pair segment is called a codon. This is why the sequence is broken into three base pair segments above. Each codon specifies a specific amino acid or stop that is brought over by a transfer RNA (tRNA) with an anticodon that is complimentary to the mRNA codon. For example, if the mRNA codon is AUG, the anticodon is UAC. Remember there are no Ts in RNA. You can use a codon table to figure out what the amino acid sequence will be. The ribosome forms peptide bonds between the amino acids to create a chain.

Using the link to the table I have included, the amino acid chain for our example would be

Methionine-Proline-Glycine-Tyrosine-Glycine

The functions of our cells are directly tied to the proteins that are produced.

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