Three bases constitute a codon, that stand for one amino acid. Out of the 64 possible 3-base codons, 61 specify amino acids while the other 3 are stop signals. The ribosomes scan a messenger RNA 3-bases at a time and bring in the corresponding amino acids to link to the growing protein chain. When they reach a stop signal, they release the completed protein.
Gene expression is the process by which a gene product (RNA or polypeptide) is made. Two steps (transcription and translation) are required to make a polypeptide from the instructions in a DNA gene.
In transcription, an enzyme called RNA polymerase makes a copy of one of the DNA strands (an RNA copy).
In translation, this RNA (messenger RNA or mRNA) carries the genetic instructions to the cell’s protein factories called ribosomes. The ribosomes “read” the genetic code in the mRNA and put together a protein according to its instructions.
Ribosomes already contain molecules of RNA (ribosomal RNA or rRNA). Experience shows that ribosomes are non-specific translation machines that can make an unlimited number of different proteins, according to the instructions in the mRNAs that visit the ribosomes.
The process of translation
Transcription and Translation
DNA is a double helix – two DNA strands wound around each other. The bases of each strand are on the inside of the helix, and a base on one strand pairs with one on the other in a very specific way. DNA has only four different bases:
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Wherever we find an A in one strand, we always find a T in the other; wherever we find a G in one strand, we always find a C in the other. The strands are complementary. If we know the base sequence of one, we automatically know the sequence of the other.
The process of replication takes one strand (after they come apart) and enzymes build new partners for them using the old strands as templates following the Watson-Crick base-pairing rules. This process is called semi-conservative replication since one strand of the parental double helix is conserved in each of the daughter double helices.
A Schematic Diagram of DNA Replication
All living things carry out countless chemical reactions and that these reactions are catalysed (accelerated) by proteins called enzymes. Many of these reactions take place in sequence, so that one chemical product becomes the substrate (starting material) for the next reaction. These sequences of reactions are called pathways, and the substrates within a pathway are called intermediates.
Mutagens are used to introduce mutations into genes and then observe the effects of these mutations on biochemical pathways.
Most genes contain the information for making one polypeptide. Genes do more than one thing:
- first, they are replicated faithfully;
- second, they direct the production of RNAs and proteins;
- third; they accumulate mutations and so allow evaluations.
Experiments that establish relationships between genes, proteins, cells and functions.
Pathway for Gene Expression
From DNA to Protein
Eric Fosler-Lussier published this tutorial on Markov Models and Hidden Markov Models. Great introductory resource!
The PDF can be downloaded from the original site at this address.