Often, this involves the excision of the first amino acid in the chain usually methionine, as this is the particular amino acid indicated by the start codon. Once a protein is complete, it has a job to perform. Some proteins are enzymes that catalyze biochemical reactions. Other proteins play roles in DNA replication and transcription. Yet other proteins provide structural support for the cell, create channels through the cell membrane, or carry out one of many other important cellular support functions.
This page appears in the following eBook. Aa Aa Aa. The ribosome assembles the polypeptide chain. What is the genetic code? More on translation. How did scientists discover how ribosomes work? What are ribosomes made of? Is prokaryotic translation different from eukaryotic translation? Figure 1: In mRNA, three-nucleotide units called codons dictate a particular amino acid.
For example, AUG codes for the amino acid methionine beige. The codon AUG codes for the amino acid methionine beige sphere.
The codon GUC codes for the amino acid valine dark blue sphere. The codon AGU codes for the amino acid serine orange sphere. The codon CCA codes for the amino acid proline light blue sphere.
The codon UAA is a stop signal that terminates the translation process. The idea of codons was first proposed by Francis Crick and his colleagues in During that same year, Marshall Nirenberg and Heinrich Matthaei began deciphering the genetic code, and they determined that the codon UUU specifically represented the amino acid phenylalanine.
Following this discovery, Nirenberg, Philip Leder, and Har Gobind Khorana eventually identified the rest of the genetic code and fully described which codons corresponded to which amino acids. Reading the genetic code. Redundancy in the genetic code means that most amino acids are specified by more than one mRNA codon.
Methionine is specified by the codon AUG, which is also known as the start codon. Consequently, methionine is the first amino acid to dock in the ribosome during the synthesis of proteins. Tryptophan is unique because it is the only amino acid specified by a single codon. The remaining 19 amino acids are specified by between two and six codons each. Figure 2 shows the 64 codon combinations and the amino acids or stop signals they specify.
Figure 2: The amino acids specified by each mRNA codon. Multiple codons can code for the same amino acid. Figure Detail. What role do ribosomes play in translation? As previously mentioned, ribosomes are the specialized cellular structures in which translation takes place. This means that ribosomes are the sites at which the genetic code is actually read by a cell. The journey from gene to protein is complex and tightly controlled within each cell.
It consists of two major steps: transcription and translation. Together, transcription and translation are known as gene expression. During the process of transcription, the information stored in a gene's DNA is passed to a similar molecule called RNA ribonucleic acid in the cell nucleus.
Both RNA and DNA are made up of a chain of building blocks called nucleotides, but they have slightly different chemical properties. Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. A technique called mass spectrometry permits scientists to sequence the amino acids in a protein. After a sequence is known, comparing its amino acid sequence with databases allows scientists to discover if there are related proteins whose function is already known.
Often similar amino acid sequences will have similar functions within a cell. The amino acid sequence also allows scientists to predict the charge of the molecule, its size, and its probable three-dimensional structure. How do scientists study the impact of proteins on genes or other proteins? A good way to study the function of the protein is to see what happens in the cell when the protein is not present.
For this scientists use model systems, such as cell culture or whole organisms, wherein they can test the function of specific proteins or genes by modifying or mutating them.
The expression level of a gene can be calculated by measuring the transcribed mRNA northern blot , the expressed protein Western Blot , or by directly staining the protein or mRNA when it is still in the cell. New techniques have changed the way we study gene expression — DNA microarrays, serial analysis of gene expression SAGE , and high-throughput sequencing allow larger screens of multiple molecules simultaneously and have opened up the possibility of new and broader kinds of questions.
To analyze large datasets and see how networks of molecules interact, a new discipline called systems biology provides the framework for these larger and more integrated understandings of regulatory networks.
Interestingly, proteins are not the only gene regulators. Termination occurs when one of the three stop codons reaches the A site. Typically a single mRNA is used to make many copies of a polypeptide simultaneously. Multiple ribosomes, polyribosomes , may trail along the same mRNA.
Translation, the movie! During and after synthesis, a polypeptide coils and folds to its three-dimensional shape spontaneously. In addition, proteins may require posttranslational modifications. Ribosomes, free and bound. Free ribosomes are suspended in the cytosol and synthesize proteins that reside in the cytosol. Bound ribosomes are attached to the cytosolic side of the endoplasmic reticulum.
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