Video Transcript
Which of the following is true
about the fate of an mRNA strand after being translated? (A) After translation, mRNA strands
are converted to rRNA and are used in the production of ribosomes. (B) mRNA strands are converted to
tRNA. (C) mRNA strands can bind to other
ribosomes to begin new rounds of translation. (D) Each mRNA strand is translated
only once before being degraded in the cytoplasm.
A gene is a section of DNA that
codes for a particular functional unit, for instance, a protein. You will recall that in order for a
gene to be expressed as a protein, it first must be transcribed to the mRNA. This mRNA is then translated in a
process called translation to a polypeptide chain. A polypeptide is a long chain of
amino acids, which are the building blocks of proteins. This polypeptide chain, in
association with other chains, can then fold into the corresponding protein for that
gene.
The question asks us what happens
to an mRNA strand after it has been translated. Let’s go through the answer choices
to find the correct answer.
Answer option (A) claims that mRNA
is converted into rRNA or ribosomal RNA. To decide if this is correct or
not, we need to discuss what ribosomes are. A ribosome is made up of two
subunits called the large subunit and the small subunit. Those subunits are assembled in a
region in the nucleus called the nucleolus and then transported to the cytoplasm
where they are involved in translation. The ribosomal subunits are composed
of protein and rRNA. The proteins are built up like
every other protein in the cytoplasm through the translation of the corresponding
mRNA and then move back into the nucleus and nucleolus. In contrast to that, the rRNA is
transcribed directly from the DNA, which codes for the rRNA in the nucleolus. Answer option (A) is therefore
incorrect.
Answer option (B) claims that mRNA
is converted into tRNA. tRNA, or transfer RNA, is the second major component needed
for translation, next to the ribosomes. A tRNA is a specialized adapter RNA
molecule that is able to bring amino acids, for example, asparagine, or Asn for
short, to the site of translation for the synthesis of a polypeptide. All tRNA molecules have the same
distinct characteristic cloverleaf shape. This distinct shape is due to the
single-stranded RNA being able to fold upon itself due to complementary base-pairing
rules. tRNA therefore needs a very specific sequence of nucleotides, like the one
you could see here: CUCCC followed by some other nucleotides followed by GGGAG. This specific sequence allows the
tRNA to fold correctly.
An mRNA has its nucleotides ordered
in such a way that it encodes a specific protein and will with the highest
probability not have these exact sequences needed for tRNA folding. mRNA can
therefore not be converted into tRNA. Answer choices (C) and (D) say that
either mRNA is being reused or that mRNA is used only once. Our body generally needs to produce
a large amount of proteins. To produce an mRNA for every single
protein in our body, we’d use a very large amount of energy.
It therefore makes sense that the
mRNA can be reused for further translations until the mRNA is eventually
degraded. And this is actually the case. Not only that, but in addition, a
single mRNA can be translated by numerous ribosomes in a structure called a polysome
or polyribosome to allow for a simultaneous synthesis of a single polypeptide. So the correct answer is given by
answer option (C). mRNA strands can bind to other ribosomes to begin new rounds of
translation.
