Beginning within the nucleus, the first step leading to the synthesis of a polypeptide is

A. Translation of an RNA nucleotide sequence into a sequence of amino acids

B. Transferring of information from DNA to messenger RNA

C. Removal of introns from RNA and the stitching together of exons

D. Linking of nucleotides to form a polypeptide

E. Translation of a DNA nucleotide sequence into a sequence of amino acids

Diagram showing processing of mRNA
Diagram showing processing of mRNA

The correct answer is B. Transferring of information from DNA to messenger RNA

Protein synthesis is a complex process in which the genetic code is essentially translated into amino acids which are then linked together to form polypeptides.

The first step is transcription which involves the messenger RNA being made using the DNA coding strand as a template. First, the DNA double helix has to unwind and the bonds between the two polynucleotide strands have to be broken.

RNA polymerase then brings in free RNA nucleotides to the DNA where they link together in the same sequence of the DNA nitrogen bases. In essence, the RNA is copying the genetic code that is present on the DNA molecule.

The mRNA transcript undergoes splicing so that only the coding bases are present in the final strand. This then exits the nucleus via the nuclear pores and goes to the ribosome in the cytoplasm.

Specific enzymes in the cytoplasm find and link the correct amino acid to each transfer RNA molecule based on the anti-codon (triplet of bases) on the RNA.  

The tRNA then carries the amino acid to the ribosome where the mRNA is. Here the tRNA lines up with the corresponding three bases on the mRNA.

That is the anti-codon of tRNA aligns with the codon of the mRNA. Eventually, all the amino acids are lined up in accordance with the genetic sequence of bases on the mRNA. These link together and further bonding occurs to form a polypeptide chain.

DNA and RNA

DNA and RNA are the two nucleic acids that are involved in the process of polypeptide synthesis. The two molecules both consist of nucleotides comprising a sugar and phosphate backbone and a nitrogen base. However, there are some important differences between the two types of nucleic acids.

The DNA is deoxyribonucleic acid which is named for the presence of a deoxyribose sugar. The RNA is ribonucleic acid and is named for the ribose sugar that the molecule has.

Both nucleic acids consist of polynucleotide strands, but in RNA it is only one single strand, while in DNA it is a double-stranded structure that twists into a double helix.

There are some subtle differences with the nitrogen bases as well, in that DNA has thymine, adenine, cytosine, and guanine, while RNA has uracil instead of thymine, and then adenine, cytosine and, guanine.

Base pairing rules are important for understanding how RNA lines up with DNA in transcription and translation. Adenine can only bond with uracil and cytosine with guanine. Thus, when bases line up the uracil of RNA can only bond with or line up with the adenine of the DNA or RNA.

Transcription

The first part of transcription involves copying the genetic code that is present on the DNA molecule, in other words, transferring the information from DNA to RNA.

Only one of the two strands making up the double helix of DNA contains the code, and this is known as the coding strand. This will be the strand that is used as a template molecule for the formation of the messenger RNA (mRNA) molecule.

The mRNA molecule is constructed from free RNA nucleotides that base pair with the corresponding nitrogen bases on the DNA coding strand.

Before this can occur the double helix has to unwind and the hydrogen bonds between the bases on the two DNA strands have to be broken.

Once the coding strand is available the RNA polymerase enzyme can start to catalyze the reaction. The process begins at a promoter region on the DNA molecule. This is a set of bases that is present just before the code begins.

The polymerase enzyme brings RNA nucleotides in, and they are formed based on the template, the sequence of bases on the DNA strand. It is extremely important that the sequence is copied correctly since this is the basis for the formation of a functional protein.

In eukaryotic cells, the process of transcription occurs in the nucleus of the cell. Once the process is complete, other enzymes work to splice the coding regions together.

These regions are known as exons and are surrounded by non-coding regions known as introns. The introns are removed and RNA molecule modified prior to exiting the nucleus.

Translation

The second stage of protein synthesis is the translation, which occurs at the ribosomes in the cytoplasm. The structure of these ribosomes is quite complex, they consist of two subunits and have active sites where the process of translation takes place.

Two other types of RNA now become involved in the process of polypeptide synthesis. These are ribosomal RNA (rRNA) and transfer RNA (tRNA). The rRNA is responsible for the formation of the ribosomal subunits.

The tRNA is important in bringing amino acids to the mRNA strand once it is at the ribosome in the active site. The tRNA molecule contains three nucleotide bases on one end of the molecule. This sequence of nitrogen bases is known as an anti-codon.

This triplet code matches a specific amino acid. The other end of the tRNA molecule is a loop structure to which an amino acid can attach. Activating enzymes are involved in the process by with the tRNA finds and attaches to the correct amino acid.

How the code is read

The genetic code then is read in groups of three bases at a time, and what is also important to understand is that the process involves a stop and start codon. The codons are the triplet bases on the mRNA molecule.

Once the mRNA arrives at the ribosome the code is read and translated from start to stop. The start codon is always the amino acid methionine, in prokaryotes and eukaryote cells.

The enzyme aminoacyl-tRNA synthase ensures that each tRNA molecule links to the correct amino acid. The tRNA then lines up with the corresponding bases of the mRNA at the ribosome.

In other words, the codon and anti-codon have to contain corresponding nitrogen bases in the correct sequence. The process continues until a stop codon is reached. This will be the triplet code of UAG, UGA, or UAA. These bases signals that the process should halt.

After the translation has ended the amino acids link together by peptide bonds and then later link and fold in complex ways. Functional polypeptides are large and complex biological macromolecules that form either a tertiary or quarternary structure.

References

  1. H Lodish, A Berk, Sl Zipurksy, et al. (2000). Molecular Cell Biology. USA: New York, W.H. Freeman Publishers.
  2. PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants. Worth Publishers.
  3. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing.
  4. Y Wan, K Chatterjee (2018). RNA. Retrieved from Encyclopedia Britannica.
  5. Editors of Encyclopedia Britannica (2018). DNA. Retrieved from Encyclopedia Britannica.

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