Which statement about tRNAs is false?

a. They have an anticodon at their 5′ end and an amino acid attachment site at their 3′ end. b. They interact with mRNA. c. ATP is required for the charging of tRNAs with amino acids. d. They interact with ribosomes. e. Specific enzymes bind amino acids to their corresponding tRNAs.

The correct answer is a.  They have an anticodon at their 5′ end and an amino acid attachment site at their 3′ end.

Diagram showing the structure of tRNA
Diagram showing the structure of tRNA, the anticodon is indicated by CAU at the end of the one loop (Yikrazuul [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)])

RNA is a very important nucleic acid which is used to make proteins using the code from the DNA. The RNA has a slightly different structure to DNA in that it is always a single-strand, the sugar that is present in the backbone is ribose and the base uracil occurs instead of thymine.

There are three types of RNA found in eukaryotic cells, transfer RNA (tRNA), messenger RNA (mRNA) and ribosomal RNA (rRNA). These three RNA molecules are all involved in protein synthesis.

The tRNA molecule has a complex folded structure that resembles a cloverleaf. The molecule has several folded loops and hydrogen-bonded stems. The molecule also has an anticodon on one end, that is called an anticodon loop and the amino acid attaches on a 3′ end of a different section.

The enzyme aminoacyl tRNA synthase catalyzes the reaction in which amino acids bind to a tRNA molecule. The anticodon that is present dictates which amino acid can bind to the molecule.

The mRNA and rRNA are also very important in protein synthesis. The mRNA is involved in the transcription process in which the DNA code is copied and taken to the ribosome where translation with tRNA then occurs.

The rRNA is formed in the nucleolus and along with various proteins assembles to form ribosomes in the cytoplasm. This is the site where polypeptides are actually synthesized by various complex reactions.

The RNA molecule

The RNA stands for ribonucleic acid and in many ways, the molecule is similar to that of DNA. Some viruses enter a living cell and then they use their RNA to make DNA in a process known as reverse transcriptase.

However, all living organisms have DNA as the genetic material and use various types of RNA to copy the information that is on the DNA.

RNA though only consists of a single strand of nucleotides that are linked together by bonds that are formed between the ribose sugar and the phosphate group making up the backbone. The sugar in an RNA molecule is known as ribose while in DNA it is a deoxyribose sugar.

One of four nitrogen bases attaches to the sugar of RNA. The possible bases are adenine, uracil, guanine, and cytosine. It is important to understand that RNA does not have thymine, but has uracil instead.

The carbons of the ribose sugar are also numbered from 1’ to 5’ depending on where they are located relative to the oxygen atom.

The ribonucleic acid molecule is stabilized by the addition of certain functional groups, such as for example, methyl groups.

Three types of RNA have been discovered in eukaryotic cells, namely transfer RNA, messenger RNA, and ribosomal RNA. These three types of RNA all play an important role in the process of protein synthesis.

Transfer RNA

The transfer RNA or tRNA is necessary for the translation stage of protein synthesis. The molecule itself has various folds making it resemble a cloverleaf in shape.

The tRNA consists of stem sections in which there are hydrogen bonds between corresponding base pairs, and it has loops that contain some modifications of the nitrogenous bases.

Modifications that are found include methylguanosine, ribothymidine, inosine, and pseudouridine. There is both an acceptor stem and an anticodon stem. The anticodon is three nitrogen bases that are found on the loop that projects from the anticodon stem.

These three bases are known as an anticodon and they code for a particular type of amino acid. Each tRNA can bind to a specific amino acid in a reaction which is catalyzed by the enzyme aminoacyl tRNA synthase.

This enzyme helps bring about aminoacylation of the RNA at the 3′ end of the molecule. The anticodon is actually found on a separate part of the molecule which is known as the anticodon loop.

The tRNA molecules with the amino acids attached then line up at the ribosome opposite the corresponding bases, the codons, of the mRNA molecule.

Eventually, peptide bonds will form between the assembled amino acids to form a polypeptide, which will become further altered to form a functional protein.

Messenger RNA

The messenger RNA is made in the nucleus using the DNA code as a template. The idea is for the mRNA to carry the genetic code to the ribosome where the translation stage of protein synthesis can occur.

The transcription in which mRNA is made involves RNA polymerase enzymes and other enzymes which splice the RNA molecule to produce a final mRNA transcript that only contains coding genes known as exons.

The mRNA in eukaryotes has a cap made of 7-methylguanosine which occurs on the 5’end of the molecule, and a poly-A tail that is found attached to the other part, the 3’ end.

Even though the mRNA moves out of the nucleus and to the cytoplasm, it is not always immediately translated. In fact, scientists have found that there exist translator repressor proteins that bind close to the 5’ stem of the molecule to stop the process of translation from happening.

Ribosomal RNA

This RNA is made in the nucleolus of the nucleus in the case of eukaryotic cells. The rRNA binds to several different types of proteins to make the subunits of a ribosome.

All of the components that make up the ribosome are assembled in the cytoplasm to form what we know as the ribosome. This is the site of protein synthesis and thus plays a very important role in the process.

References

  1. D Pak, R Root-Bernstein, ZF Burton (2017). tRNA structure and evolution and standardization to the three nucleotide genetic code. Transcription.
  2. T Marcey (2010). tRNA structure. Retrieved from callutheran.edu.
  3. NV Bhagavan, C-E Ha (2011). RNA and protein synthesis. Retrieved from sciencedirect.com
  4. Y Wan, K Chatterjee (2019). RNA. Retrieved from Encyclopedia Britannica.
  5. H Lodish, A Berk, Sl Zipurksy, et al. (2000). Molecular Cell Biology. USA: New York, W.H. Freeman Publishers.

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