(figure available in print form)
1. To Learn the current information about the various kinds of RNA.
2. To follow the sequence of DNA—RNA—Protein.
3. To produce 20 amino acids from four nucleotides.
Watson and Crick include in their published material this very simple model. It demonstrates the basic relationships between DNA, RNA and protein.
1. DNA is the
for its own duplication.
2. DNA is the template (code tape) for the synthesis of RNA, (this is called
3. RNA is the template in protein synthesis, (this is called
Please note that after the replication of DNA the arrows only go in one direction.
Make an imaginary chain of DNA and predict the chain of RNA it would produce. (Remember that uracil (U) replaces thymine in RNA)
III. The RNA produced in the nucleus carries the genetic message of the DNA into the cytoplasm of the cell where raw materials for the production of amino acids and, ultimately, proteins await organization. This RNA is caLled
and is designated as mRNA. It is almost always a single strand, and has no regular hydrogen bonded structure. The DNA also seems to organize a chain of mRNA in the nucleus much longer than the chain of mRNA found in the cytoplasm. This has led scientists to believe that it is edited and some material is removed before mRNA leaves the nucleus. This longer form in the nucleus is called
heterogeneous nuclear RNA
and it contains gibberish sequences called
. These introns form dangling material or loops and snip off, and the
(the sensible segments) leave the cell via the endo-plasmic reticulum, and go forth to pass on the code. The introns intrigue scientists at this time since their function is not yet understood. Some hypotheses are:
1. They allow for less damage to the sensible segments. (If there’s damage, it can happen in this extra material).
2. They serve as spacers and help regulate the system.
3. They control this RNA splicing mechanism.
4. They are freed and initiate the code for a companion section and the protein it will transcribe.
When the mRNA has exited into the cytoplasm it serves as a template for a new form of RNA called transfer RNA or tRNA. It is activated by ATP. This form of RNA seems to have two affinities, one to bond to some particular amino acid and one for its codon. Therefore, it can bring an amino acid to site A on the ribosome and from there the amino acid attaches to a protein building from a chain of amino acids at site P
. Voila! Proteins for cell growth and for export.
There seems to be only two sites per ribosome.
However, when cell metabolism is taught, students learn that there are 20 amino acids in human cells. How then, can only four kinds of nucleotides in DNA or RNA accomplish separate coding for the synthesis of all twenty? We will use a colored grid; each color will represent one of the nucleotides of RNA. We’ll label them U, C, A and G. These could only code for four at the most. Now we’ll overlay a similar set of color strips and find sixteen possible combinations UU, UC, UA, UG, CU, CA, etc. One more set of strips combining each of these doubles with the original four brings about 64 combinations, more than enough
to cue the synthesis of all 20 amino acids. In fact, these
codons do just that and, in many cases, more than one triplet causes the synthesis of the same amino acid.
(figure available in print form)
Do the write out sheet for codons. Place the overlay, (clear plastic) over the write out and it is clear which triplets code for which. In some cases only doubles are necessary to bring an amino acid together and some triplets serve as
of the chains of amino acids. Formylmethianine is the activator. Chains grow from the 3’ end (third carbonatom) to the 5’ or vice versa.
1. Take chain of these codons and “synthesize” a chain of amino acids. CUC, GUG, GCU, GAC, CAU, UAA.
2. Make up a chain of codons and predict the amino acids.
3. In your text, find the polypeptide
for normal hemoglobin and list their codons.