Translation - AP Biology

Ribosomes are non-membranous organelles that direct protein synthesis by reading mRNA and joining amino acids into strands of polypeptides. Ribosomes exist in both free and membrane-bound states. They are synthesized in both the nucleolus and cytoplasm. The components that make up these non-membranous organelles are rRNA molecules and a variety of proteins. Ribosomes have a large and a small subunit, together called the translational apparatus. The small ribosomal subunit reads the mRNA strand and the large ribosomal subunit joins amino acids into polypeptides.

The enzyme aminoacyl-tRNA synthetase joins tRNA molecules with a corresponding amino acid. First, an amino acid is bound to aminoacyl-tRNA synthetase using ATP. Then, the tRNA molecule containing the corresponding anticodon binds to the enzyme. The correct tRNA molecule is identified by aminoacyl-tRNA synthetase by its anticodon sequence and other areas of its structure. Last, the tRNA molecule covalently bonds to the amino acid and is released from aminoacyl-tRNA synthetase.

The large ribosomal subunit has three sites that interact with tRNA molecules—the peptidyl “P” site, the aminoacyl “A” site, and the exit “E” site. The P-site holds the tRNA corresponding to the most recently added amino acid, which is attached to the growing polypeptide chain by a peptide bond. The A-site holds the tRNA with the next amino acid to be added to the chain. Finally, the E-site holds the free tRNA without an amino acid that was previously in the P-site.

The anticodon of any codon will be the RNA nucleotides that complement the codon sequence. In RNA, adenine (A) complements uracil (U) while cytosine (C) complements guanine (G). Hence, for the codon 5' AUG 3', the complements will be 3' UAC 5'. Note that many of the incorrect answers contain thymine (T), a nucleotide not found in RNA.

The start codon for any strand of RNA begins with the codon that codes for the amino acid methionine. This is the first amino acid in a polypeptide chain. The abbreviation for methionine is: Met.

When translated from a ribosome on the rough endoplasmic reticulum (RER), the protein is moved through the RER until it is released in a lipid vesicle that can transport the protein to the plasma membrane, where the lipid vesicle fuses with the lipid membrane and the protein is secreted outside the cell. Smooth endoplasmic reticulum does not have ribosomes on it and is used for the production and transport of lipids and in detoxification. No ribosomes are found in the nucleus or directly on the plasma membrane. Ribosomes in the cytosol translate proteins that stay inside the cell.

In messenger RNA, each codon is three nucleotides that codes for a particular amino acid during translation. Purines and pyrimidines are types of nucleotides on DNA and RNA. The genetic code is redundant, but each codon only codes for one amino acid.

Translation is the process of converting an mRNA codon sequence into protein via the ribosome, so that is the correct answer. Apoptosis is programmed cell death. Transcription is close, but it is the process of making RNA from DNA. Glycolysis is the process of creating two pyruvate molecules from glucose, and produces two ATP.

mRNA is the template used during translation. The mRNA strand is read and "translated" into a polypeptide by tRNA.

DNA would be the template for transcription, not for translation.

Transcription leads to the production of hnRNA (heterogeneous nuclear RNA), which primarily consists of pre-mRNA and must go through processing and modification to form mRNA and leave the nucleus.

The other three choices, mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA), all play active roles in the process of translation. mRNA serves as the codon template. tRNA matches anticodons to mRNA and carries amino acids. rRNA forms a large part of the ribosome structure and plays a functional role as the site of translation.

Every codon is composed of three RNA nucleobases, and codes for a specific amino acid; however, there can be multiple codons that code for one amino acid. The start codon, AUG, signals the beginning of translation and codes for methionine.

While there is only one start codon (AUG), but there are three different stop codons (UGA, UAG, and UAA) that can each signal for the end of translation, or termination. During the translation process, tRNA is used to bring amino acids (corresponding to the codons in the mRNA sequence) to the ribosome, which become attached via peptide bonds to form a polypeptide.

The genetic code is degenerative, meaning that multiple codons can code for the same amino acid. It is also unambiguous: a particular codon will always code for one amino acid. That being said, it would be wrong to assume that an amino acid will only have one codon, as an amino acid can have multiple different codons that code for it.

In translation, the mRNA is positioned in the ribosome and read in the 5'-to-3' direction. Initiation of translation is triggered by a tRNA attached to a methionine entering the P site of the ribosome. The mRNA will then be read, and additional amino acids will be added to the chain, which grows in the P site. New tRNA enters the A site and old tRNA exits the E site, but the amino acid chain is always anchored to the tRNA in the P site.

Stop codons are a signal for the ribosome to recruit a release factor. Release factors are proteins that dissociate the translation complex and release the polypeptide chain.

There are no tRNAs that match stop codons and there is no "special" terminal amino acid. Chaperones are involved in folding proteins, but they are not involved in the termination of translation.

Elongation continues until a stop codon occupies the A-site of the ribosome. The stop codon is a three-base signal present within the mRNA. There are three stop codons: UAG, UAA, and UGA.

There are three principle steps to translation. Initiation occurs when the ribosomes encounters the start codon, AUG, and recruits a methionine tRNA. Elongation of the polypeptide occurs as the ribosomes continues to recruit tRNA molecules and build the peptide chain. Termination occurs when the ribosome encounters a stop codon and releases the completed polypeptide.

Translation, the process of synthesizing a polypeptide from an mRNA template, primarily occurs in the cytoplasm. Another possible answer would be the rough endoplasmic reticulum. Both the rough endoplasmic reticulum and cytoplasm contain ribosomes, which are essential for translation.

The mitochondria are essential for cellular respiration, and are the site of the citric acid cycle and electron transport chain. The nucleus houses DNA and synthesizes ribosomes (in the nucleolus). The Golgi apparatus modifies and packages proteins in vesicles after translation is complete.

After DNA is transcribed into RNA, the RNA goes through post-transcriptional modifications and is then sent out of the nucleus to the cytoplasm. From there, the mRNA is brought to the ribosomes, some located on the rough endoplasmic reticulum and some free-floating, in order to be translated into proteins. Proteins are then packaged and transported to their respective locations for usage.

The nucleolus is responsible for synthesizing and assembling ribosomal subunits. The nucleus houses DNA and is the site of transcription, but not translation. Mitochondria are essential for cellular respiration and ATP synthesis. Lysosomes digest cellular wastes and defective proteins.

Missense mutations are point mutations that cause a single amino acid in a protein to be changed. This may or may not affect the functionality of the protein. When one amino acid is replaced by another amino acid from the same class, such as replacing one polar amino acid with another, functionality is usually retained. When an amino acid from a different class is used, such as replacing an acidic amino acid with a basic amino acid, the protein folding may be affected and functionality may fail.

The other answers describe other types of mutations. Silent result in no change to the protein primary structure. Nonsense mutations cause early termination. Frameshift mutations shift the reading frame of the codon sequence, severely altering the protein composition.