Video Transcript
The genetic code is universal,
meaning it is conserved throughout all organisms that use DNA as genetic material on
Earth. Which theory is this strong
evidence of? (A) The theory that living
organisms appeared independently at different times. (B) The theory of homeostasis. (C) The theory of spontaneous
generation of life. (D) The theory of common ancestors
for all organisms.
In order for a gene in DNA to be
converted into a protein, it needs to go through a couple of steps. The first step is called
transcription and involves the DNA being copied to form mRNA. Then, this mRNA sequence is
translated into a sequence of amino acids to form a polypeptide. This polypeptide can then go on to
fold into a protein. Since this question is asking about
the genetic code, let’s describe how a sequence of mRNA can be translated into amino
acids.
A group of three nucleotides in
mRNA is called a codon. In this six-nucleotide sequence of
mRNA, there’s two codons. The specific sequence of a codon
corresponds to a specific amino acid. We can use a codon wheel to figure
this out. When reading a sequence of mRNA, we
start from the five prime end and work our way to the three prime end.
In the codon wheel, we’re doing the
same thing, by starting from the center, which corresponds to the five prime end,
and working our way out toward the three prime end. So the codon CCA gives us the amino
acid proline. And the codon GUA gives us the
amino acid valine. These codons and how they’re
decoded into amino acids as we’ve just demonstrated using this codon wheel
corresponds to the genetic code. This genetic code is common to all
forms of life on Earth. The codon GUA corresponds to valine
in humans, in chickens, in bacteria, and so on. It’s universal. Because all forms of life use the
same genetic code, this suggests that we all share a common ancestor that also used
the genetic code.
Therefore, the option that
correctly identifies a theory that is supported by the universal nature of the
genetic code is given by answer choice (D): the theory of common ancestors for all
organisms.
