Translation: Protein Synthesis: Videos & Practice Problems

Translation is a crucial process in protein synthesis that occurs in three main steps: initiation, elongation, and termination. The initiation step begins with the assembly of ribosomes, which are composed of a small and a large ribosomal subunit, each made up of proteins and ribosomal RNA (rRNA).

During initiation, messenger RNA (mRNA) binds to the small ribosomal subunit. This mRNA contains codons, which are sequences of three nucleotides that code for specific amino acids. The first tRNA, carrying methionine and possessing the anticodon UAC, pairs with the start codon AUG on the mRNA. This pairing is facilitated by hydrogen bonds forming between the complementary nucleotides, where uracil replaces thymine in RNA.

The initiation step concludes when the large ribosomal subunit joins the small subunit, forming a complete ribosomal complex. This complex is essential for the subsequent steps of translation, as it provides the necessary environment for elongation and the synthesis of polypeptides.

In the process of translation, the initiation step is crucial for setting the stage for protein synthesis. This step involves several key components and actions that must occur for translation to begin effectively. One of the primary actions is the binding of messenger RNA (mRNA) to the small ribosomal subunit. This binding is essential as it positions the mRNA correctly for translation.

Another important aspect of initiation is the binding of the initiator tRNA, which carries Methionine (abbreviated as MET). This tRNA recognizes the start codon, AUG, on the mRNA through complementary base pairing, ensuring that the correct amino acid is incorporated at the beginning of the polypeptide chain.

Additionally, the combination of the large ribosomal subunit with the small subunit complex marks the conclusion of the initiation phase, allowing the ribosome to become fully functional for the elongation phase of translation.

However, it is important to note that the activation of tRNA through aminoacyl tRNA synthetase, while a critical step in the overall process of translation, does not occur during the initiation phase. This activation step happens prior to initiation, where tRNA molecules are charged with their respective amino acids, preparing them for use in translation.

In summary, the initiation step of translation includes the binding of mRNA to the small ribosomal subunit, the binding of Methionine tRNA to the start codon AUG, and the combination of ribosomal subunits, while the activation of tRNA is not part of this specific phase.

In the process of translation, elongation is a crucial step where the ribosome synthesizes a polypeptide chain by adding amino acids one by one. Initially, a second transfer RNA (tRNA) approaches the ribosome and binds to the next codon on the messenger RNA (mRNA) strand. This follows the initiation phase, where the ribosomal subunits come together, and the first tRNA, carrying methionine, pairs with the start codon.

During elongation, the newly arrived tRNA, which carries a different amino acid (for example, serine), docks into the ribosome's second binding site. A peptide bond forms between the methionine and the second amino acid, facilitated by the ribosome. This bond formation occurs as the methionine detaches from its tRNA, allowing it to link with the amino acid on the second tRNA. As a result, the first tRNA is left empty and is released from the ribosome.

Following this, a process known as translocation occurs, where the entire ribosomal complex shifts to the right, moving to the next codon on the mRNA. This movement exposes a new codon for the next tRNA to bind, while the empty tRNA is discarded. This cycle of adding amino acids and shifting continues, allowing the polypeptide chain to grow longer with each iteration.

It is important to note that multiple ribosomes can translate a single mRNA strand simultaneously, creating several polypeptide chains at once. This efficiency in protein synthesis is vital for cellular function. Overall, elongation is characterized by the sequential addition of amino acids through peptide bonds, facilitated by the coordinated action of tRNAs and the ribosomal machinery.

In the study of protein synthesis, understanding the relationship between amino acids and their corresponding mRNA codons is crucial. Each amino acid is represented by a specific sequence of three nucleotides, known as a codon. For instance, the amino acids Phenylalanine, Valine, and Arginine can be translated into mRNA codons using a codon chart.

Phenylalanine (Phe) can be encoded by several codons, including UUU and UUC. When examining the options for codons, UUU is a valid choice. Next, Valine (Val) is represented by codons such as GUU, GUC, GUA, and GUG. Lastly, Arginine (Arg) has multiple codons, including CGU, CGC, AGA, and AGG. For this example, AGA is one of the codons that corresponds to Arginine.

By utilizing the codon chart effectively, we can determine that the sequence of codons for the amino acids Phenylalanine, Valine, and Arginine could be represented as UUU for Phenylalanine, followed by a suitable codon for Valine, and AGA for Arginine. Therefore, the only option that matches this sequence is option D, which includes UUU as the codon for Phenylalanine.

Translation is a crucial process in protein synthesis, concluding with the termination step when the ribosome encounters a stop codon. The three stop codons are UAA, UAG, and UGA. Upon reaching a stop codon, a release factor protein binds to it, initiating the termination phase.

During this phase, the release factor attaches to the stop codon, leading to the hydrolysis of the growing peptide chain from the last tRNA. This hydrolysis results in the release of the completed peptide chain, which is formed by amino acids linked together by peptide bonds.

Once the peptide chain is released, the ribosomal complex disassembles. The tRNA, now devoid of any attached amino acids, detaches, and both the large and small ribosomal subunits separate. The mRNA strand is left free in the cytoplasm, ready for potential re-use in future translation processes.

It is also important to note that methionine, which is associated with the start codon AUG, is typically removed from the protein backbone after translation is complete. This marks the end of the translation process, having successfully synthesized the desired peptide chain.

To determine the sequence of a peptide translated from a given DNA template, the first step is to transcribe the DNA into messenger RNA (mRNA). The DNA template is read in the 3' to 5' direction, while the mRNA is synthesized in the 5' to 3' direction. In this process, it is essential to remember that RNA uses uracil (U) instead of thymine (T). Therefore, the base pairing rules change slightly: adenine (A) pairs with uracil (U), and guanine (G) pairs with cytosine (C).

For example, if the DNA template sequence starts with A, it will transcribe to U in mRNA. Continuing this process, each nucleotide in the DNA is converted to its complementary RNA base. After transcription, the resulting mRNA sequence is divided into codons, which are groups of three nucleotides. Each codon corresponds to a specific amino acid based on the genetic code.

Once the mRNA is formed, the next step is to decode the codons to identify the corresponding amino acids. For instance, if the mRNA codons are UAC and CCA, they translate to tyrosine (Tyr) and proline (Pro), respectively. Continuing this process for all codons will yield the complete amino acid sequence of the peptide.

In summary, the transcription of the DNA template into mRNA involves replacing thymine with uracil and pairing the bases correctly. After obtaining the mRNA, decoding the codons allows for the identification of the amino acids, leading to the final peptide sequence. In this case, the correct answer is option B, which corresponds to the identified amino acids.