Ribosome Structure & Initiation, Extension, and Termination
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Need help preparing for the Biology section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Ribosome Structure, Initiation, Extension, and Termination of Translation. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Ribosome Structure
To understand the process of translation, it is first essential to understand the structure of ribosomes. Ribosomes are molecular complexes made up of both ribosomal RNA and ribosomal proteins. These RNA-protein complexes are responsible for translating mRNA into proteins. Within the cell, ribosomes can be found either in the cytosol as free ribosomes or bound to the rough endoplasmic reticulum, as membrane-bound ribosomes. The type of ribosome used to translate a protein depends on the type of protein that needs to be translated.
For example, cytosolic proteins are translated by free ribosomes, whereas proteins found in the membrane of the cell are translated by membrane-bound ribosomes. A ribosome is made up of two subunits. There is a small subunit, as well as a large subunit. As the mRNA is translated, it will be sandwiched between both subunits. For the MCAT exam, it is important to know the difference between prokaryotic and eukaryotic ribosomal subunits.
Prokaryotic vs. Eukaryotic Ribosomes
Both prokaryotic and eukaryotic ribosomes have one large and one small subunit. The unit of each subunit is denoted by S, which measures the sedimentation rate of each subunit derived from centrifugation. For the MCAT exam, it is not important to understand the derivation process of the S-values. However, it is important to know which S-values correspond to which subunits on prokaryotic and eukaryotic ribosomes.
For prokaryotes, the small subunit is 30S, and the large subunit is 50S. The combined ribosome is 70S. Contrastingly, in eukaryotes, the small subunit is 40S, and the large subunit is 60S. The combined ribosome is 80S. It is interesting to note that several antibiotics work by targeting the various ribosomal subunits in prokaryotes and disrupting translation. The antibiotics will not harm eukaryotic ribosomal subunits because the structures are so different.
Initiation
The first step of translation is initiation. The process of initiation is defined by the ribosome assembling around the target mRNA, and the tRNA attaching to the start codon. This process is different for both prokaryotes and eukaryotes. In prokaryotes, ribosomes bind to what is called the Shine-Dalgarno sequence. The Shine-Dalgarno sequence is a specific sequence of nucleotides before the start codon. In essence, this sequence recruits the ribosomes to the mRNA molecule. Also, in prokaryotes, unlike in eukaryotes, both transcription and translation coincide, meaning that before the RNA polymerase is finished producing the mRNA, translation has already begun.
In eukaryotes, ribosomes are recruited by the 5’ cap on mRNA. Recall that the 5’ cap is one of the post-transcriptional modifications that convert pre-mRNA into the mature form of mRNA. Furthermore, in eukaryotes, translation starts at what is called the Kozak sequence. The Kozak sequence is a nucleic acid sequence that functions as the translation initiation site in eukaryotic mRNA and contains the start codon. For the MCAT exam, it is important to be able to distinguish the Shine-Dalgarno sequence in prokaryotes from the Kozak sequence in eukaryotes. Also, in eukaryotes, transcription occurs in the nucleus, while translation takes place in the cytosol or on the membrane of the rough endoplasmic reticulum.
Extension (or Elongation)
In eukaryotes, the start codon always codes for methionine. The most common start codon is AUG. When AUG is in the Kozak sequence, it will recruit a tRNA molecule with the amino acid methionine. After this occurs, the second step in translation, the extension or elongation of the peptide chain, is ready to take place.
Termination
At some point during translation, the ribosome will encounter one of three stop codons. It is important to note that the stop codons do not code for a tRNA. In other words, there is no tRNA with an anticodon that matches any of the three stop codons. Instead, a release factor protein binds at the stop codon. When this happens, the ribosome/mRNA complex will become disrupted and break apart. This action will release the polypeptide chain and signal the end of translation.
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Ribosome Structure
To understand the process of translation, it is first essential to understand the structure of ribosomes. Ribosomes are molecular complexes made up of both ribosomal RNA and ribosomal proteins. These RNA-protein complexes are responsible for translating mRNA into proteins. Within the cell, ribosomes can be found either in the cytosol as free ribosomes or bound to the rough endoplasmic reticulum, as membrane-bound ribosomes. The type of ribosome used to translate a protein depends on the type of protein that needs to be translated.
For example, cytosolic proteins are translated by free ribosomes, whereas proteins found in the membrane of the cell are translated by membrane-bound ribosomes. A ribosome is made up of two subunits. There is a small subunit, as well as a large subunit. As the mRNA is translated, it will be sandwiched between both subunits. For the MCAT exam, it is important to know the difference between prokaryotic and eukaryotic ribosomal subunits.
Prokaryotic vs. Eukaryotic Ribosomes
Both prokaryotic and eukaryotic ribosomes have one large and one small subunit. The unit of each subunit is denoted by S, which measures the sedimentation rate of each subunit derived from centrifugation. For the MCAT exam, it is not important to understand the derivation process of the S-values. However, it is important to know which S-values correspond to which subunits on prokaryotic and eukaryotic ribosomes.
For prokaryotes, the small subunit is 30S, and the large subunit is 50S. The combined ribosome is 70S. Contrastingly, in eukaryotes, the small subunit is 40S, and the large subunit is 60S. The combined ribosome is 80S. It is interesting to note that several antibiotics work by targeting the various ribosomal subunits in prokaryotes and disrupting translation. The antibiotics will not harm eukaryotic ribosomal subunits because the structures are so different.
Initiation
The first step of translation is initiation. The process of initiation is defined by the ribosome assembling around the target mRNA, and the tRNA attaching to the start codon. This process is different for both prokaryotes and eukaryotes. In prokaryotes, ribosomes bind to what is called the Shine-Dalgarno sequence. The Shine-Dalgarno sequence is a specific sequence of nucleotides before the start codon. In essence, this sequence recruits the ribosomes to the mRNA molecule. Also, in prokaryotes, unlike in eukaryotes, both transcription and translation coincide, meaning that before the RNA polymerase is finished producing the mRNA, translation has already begun.
In eukaryotes, ribosomes are recruited by the 5’ cap on mRNA. Recall that the 5’ cap is one of the post-transcriptional modifications that convert pre-mRNA into the mature form of mRNA. Furthermore, in eukaryotes, translation starts at what is called the Kozak sequence. The Kozak sequence is a nucleic acid sequence that functions as the translation initiation site in eukaryotic mRNA and contains the start codon. For the MCAT exam, it is important to be able to distinguish the Shine-Dalgarno sequence in prokaryotes from the Kozak sequence in eukaryotes. Also, in eukaryotes, transcription occurs in the nucleus, while translation takes place in the cytosol or on the membrane of the rough endoplasmic reticulum.
Extension (or Elongation)
In eukaryotes, the start codon always codes for methionine. The most common start codon is AUG. When AUG is in the Kozak sequence, it will recruit a tRNA molecule with the amino acid methionine. After this occurs, the second step in translation, the extension or elongation of the peptide chain, is ready to take place.
Termination
At some point during translation, the ribosome will encounter one of three stop codons. It is important to note that the stop codons do not code for a tRNA. In other words, there is no tRNA with an anticodon that matches any of the three stop codons. Instead, a release factor protein binds at the stop codon. When this happens, the ribosome/mRNA complex will become disrupted and break apart. This action will release the polypeptide chain and signal the end of translation.
MEDSCHOOLCOACH
To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach
IOS Link: https://play.google.com/store/apps/de...
Apple Link: https://apps.apple.com/us/app/mcat-pr...
#medschoolcoach #MCATprep #MCATstudytools
4 سال پیش
در تاریخ 1399/06/24 منتشر شده
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