Which one of the following is true of tRNAs?

A. There are four types of tRNA.

B. tRNAs carry special sequences known as codons.

C. tRNAs are double-stranded.

D. Each tRNA binds a particular codon.

E. All of the above.

F. None of the above.

Diagram of tRNA molecule showing the cloverleaf shape and loops
Diagram of a tRNA molecule showing the cloverleaf shape and loops

The correct answer is F. None of the above.

Ribonucleic acid (RNA) is the nucleic acid that is responsible for transcribing the DNA code and translating this code into proteins. The basic structure of an RNA nucleotide is a phosphate group, a ribose sugar and one of the four nitrogen bases, adenine, uracil, cytosine or guanine.

There are three types of RNA that are found in cells, messenger RNA, ribosomal RNA, and transfer RNA. These are single-stranded molecules but nonetheless they do have a complex structure that is adapted for their particular function.

Messenger RNA is formed in the nucleus using DNA as a template. The enzyme RNA polymerase is involved in this process which is known as transcription.

The transcript is modified and non-coding sequences are removed. The final messenger RNA molecule then leaves the nucleus and moves to a ribosome in the cytoplasm.

Ribosomes are composed of ribosomal RNA which is attached in a complicated way to several proteins. This type of RNA is made in the nucleolus and is the most complex in the structure of the three types of ribonucleic acid.

Transfer RNA has the shape of a cloverleaf and has a code on one side (anticodon) and attaches to an amino acid on the other side.

Each code is known as an anticodon which dictates what amino acid to attach to. Once ready the tRNA moves to the ribosome where it aligns with the mRNA and protein translation commences.

RNA

RNA is ribonucleic acid, which is one of the three types of nucleic acids that occur in living cells. The other types of nucleic acids that are found are DNA and ATP.

The RNA is a single-stranded molecule that is comprised at the most basic level of a nucleotide. Each nucleotide consists of a ribose sugar, a phosphate group, and a nitrogen base. There are four types of nitrogen base that are found in this nucleic acid, namely, adenine, uracil, cytosine, and guanine.

Ribonucleic acid is necessary for the synthesis of proteins and this is how the genetic code is actually expressed in cells. Enzymes act to stabilize the RNA molecules by attaching various functional groups, for instance, methyl groups to specific parts of the molecule.

There are three different types of RNA that scientists have discovered. These are messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). All three are involved in eukaryotic protein synthesis

mRNA

This RNA molecule is an integral part of protein synthesis since it is how the DNA code is copied. The mRNA is made by using the coding strand of a cell’s DNA as a template. Free nucleotides of mRNA are assembled with the help of the enzyme RNA polymerase. The nucleotides are aligned by complementary bases with the DNA strand.

This process by which mRNA copies the code from DNA is known as transcription, and it occurs in the nucleus. The assembled mRNA is then further modified and the non-coding base sequences, the introns are spliced by other enzymes.

The final mRNA transcript that leaves the nucleus for the cytoplasm only consist of exons which are coding bases and every three bases form what is known as a codon.

It is this codon that will form the basis for protein translation. The mRNA transcript moves out of the nucleus and into the cytoplasm where it positions itself into a ribosome.

Gene regulation is achieved by ensuring that there is a correct balance between the quantity of mRNA being made versus the quantity being degraded. Degradation of the mRNA is important as it ensures the halting of protein production.

rRNA

The ribosomal RNA is a molecule that combines with various proteins (up to 80 different types), to form the ribosome. Prokaryotic cells also have rRNA forming the ribosomes but they have a somewhat different structure when compared with eukaryotic nucleic acids.

In fact, eukaryotic rRNA molecules tend to be larger than those molecules found making up ribosomes in bacterial cells.

These molecules are made in eukaryotic cells, in the nucleolus of the nucleus along with tRNA molecules, and they are among the most complex of all the different RNA types that are found in a living cell.

tRNA

The tRNA molecules are the nucleic acids that are important in the translation stage of protein synthesis which occurs in the cytoplasm of the cell.

The tRNA can be described as having the shape of a cloverleaf, with one end containing the anticodon and the opposite end of the molecule attaching to an amino acid.

The structure of this molecule is quite complex with several loops present, with the anticodon being present on one of these loops.

The enzymes known as aminoacyl tRNA synthase catalyzes the reaction in which a tRNA molecule attaches to an amino acid. Each molecule of the RNA can only attach to one specific amino acid.

The amino acid that attaches to the tRNA depends on what the triplet of bases making up the anticodon is since this is the code for a specific amino acid. The tRNA molecule lines up with the mRNA at the ribosome after the amino acid has been attached to it.

This is done with the anticodon opposite the complementary codon on the mRNA. This process is actually very complex and involves various enzymes and binding sites on the ribosome.

Once the amino acids are all lined up, peptide bonds form between them to form a polypeptide. This is only the first stage of the formation of the protein though. In fact, much more bonding and folding of the structure occurs until a functional protein is finally generated.

References

  1. RL Dorit, WF Walker, RD Barnes (1991).  Philadelphia: USA, Saunders College Publishing.
  2. NV Bhagavan, C-E Ha (2011). RNA and protein synthesis. Retrieved from sciencedirect.com
  3. Y Wan, K Chatterjee (2018). RNA. Retrieved from Encyclopedia Britannica.
  4. H Lodish, A Berk, Sl Zipurksy, et al. (2000). Molecular Cell Biology. USA: New York, W.H. Freeman Publishers.
  5. D Pak, R Root-Bernstein, ZF Burton (2017). tRNA structure and evolution and standardization to the three nucleotide genetic code. Transcription.

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