Central Dogma of cell biology

Campbell et al., Chapter 17

TRANSPARENCY (Fig. 17.2)
DNA
transcription
RNA
translation
protein
As you will eventually see, there is more to it than this, like RNA processing, but first:

TRANSPARENCY (Fig. 17.4) 3 letter code for aa's (codons)
Note, with 20 amino acids, and 4 x 4 x 4 = 64 codons, there is redundancy, called "degeneracy."
For instance, (top left) for phenylalanine, code can be UUU or UUC.
(It is interesting to contemplate that if a mutation converted UUU to UUC or vice versa, the amino acid would still be the same.)

Study question: Is this code for RNA or DNA?
Answer: RNA (You can tell when you notice that U is one of the four bases specified)

TRANSPARENCY (Fig. 17.3) combining the information from the last 2 figures, here is a diagram of what nucleotide sequences might be for DNA and RNA and the amino acid sequence would be.
(It is interesting to contemplate that only one of the two DNA strands would work, the sense strand).
There are 3 stop codons
Methionine is the start codon

TRANSPARENCY (Fig. 17.23) On the topic of mutations, consider that if CTT changes to CAT, Glu -> Val, the sickle cell anemia mutation.
When one amino acid is changed to another, this is a missense mutation, as covered in the next figure.

TRANSPARENCY (Fig. 17.24 A and B) Changes in DNA, categories:
(1) change base in degenerate 3rd position - no effect
(2) change a base that matters - "missense" - the protein will have a different amino acid
Sickle cell anemia is severe. The Siamese cat presents an interesting minor (temperature sensitive) example in that it makes melanin in the cool extremities (permnissive temperature) when the enzyme is not denatured by the body's core temperature (restrictive temperature); there would be a conservative amino change.
(3) change base so that there is a stop codon - "nonsense" the protein will not be full length
(4) insertion or deletion - many amino acid changes and/or premature stop
To understand this last point, I introduce the expression "open reading frame;" even though you could conceivably start anywhere, only if you start in right place the right one of 3 nucleotides) for a normal (non-mutant) gene will the reading proceed for a reasonable distance without hitting a stop codon.



This page was last updated 7/22/02

return to Bio 104 Syllabus

return to Stark home page