Overview: The Role of DNA
DNA directs the synthesis of proteins in cells, whether they ultimately form cellular structure or control chemical reactions as enzymes. The process takes place in a series of steps, including transcription and translation. It is a complex procedure that is part of the genetic code of every cell.
DNA and RNA
DNA is located in the nucleus of every cell, but protein synthesis takes place in organelles called ribosomes at are located throughout the cytoplasmic reticulum, outside the nucleus. Communication between the cell nucleus and the ribosomes is mediated by another type of nucleic acid, called RNA, short for ribonucleic acid. It is also a sequenced molecule, but with only one strand. One type (mRNA) acts as a messenger from the DNA in the nucleus, another type (tRNA) transfers amino acids to the ribosomes, and the third type (rRNA), in the ribosomes themselves, joins amino acids together to form proteins.
Transcription
The genetic code contained in DNA is transferred to mRNA by a process called transcription. It differs from replication in meiosis in that only one strand of DNA takes part in the process, and the nitrogen base uracil is in mRNA rather than thymine. It is similar because the mRNA strand combines with the active DNA strand by using the combinations of the nitrogen bases like rungs on a ladder.
Codons and Amino Acids
The code contained in mRNA directs the selection of particular amino acids to be combined in the correct sequence to synthesize the correct protein. Each series of three-letter directions, called a codon, is connected in the long strand of mRNA, and represents one of the 20 types of amino acid. For example, one of the ways the amino acid alanine is represented is by the three letter code GCU, for guanine, cytosine, and uracil. There is one codon sequence that starts all protein synthesis, and one that stops it.
Translation
After the mRNA finishes transcribing the code from the active strand of DNA, the DNA strands rejoin. Then the complete mRNA strand moves out of the nucleus to the ribosomes. It moves from ribosome to ribosome with the help of the rRNA contained in the ribosomes. Proteins are synthesized from amino acids in the correct sequence using the tRNA. After the protein is completely synthesized, the mRNA breaks down into its components, so that the cycle can begin again.
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