The mature mRNA molecules are exported through nuclear pore to the cytoplasm. The mature mRNA is translated into an amino acid sequence of a particular protein in a process called translation. Translation is facilitated by ribosomes in the cytoplasm. The coding region of each mRNA molecule is composed of codons , which are three nucleotides , representing a particular amino acid of the polypeptide chain.
Each codon in the mRNA molecule is read by the anticodon of the tRNA to bring the specific amino acid to the ribosome. The secondary structure of tRNA is a clover-leaf shape. It is composed of four loop structures known as D-loop, anticodon loop, variable loop, and T-loop.
Anticodon loop is composed of a specific anticodon that scans the complement codon in the mRNA molecule. The amino acid is loaded into the CCA tail at the end of the acceptor stem. Some anticodons form base pairs with several codons by the wobble base pairing. The secondary structure of a tRNA molecule is shown in figure 2.
The ribosome is the protein-synthesizing organelle in the cell, translating the coding sequence on an mRNA molecule into a polypeptide chain. The synthesis of rRNA occurs at the nucleolus. Both rRNA molecules combine with ribosomal proteins to form a small subunit and a large subunit. The large subunit of rRNA serves as the ribozyme that catalyzes the peptide bond formation. They are single-stranded biopolymer.
The RNA has ribose nucleotides where the nitrogenous bases are attached to the ribose sugar which are attached by the phosphodiester bonds forming the chain or strands of different lengths. In the year , R. Holley described the RNA structure. The essential and significant process of molecular biology is the flow of genetic information in a cell, which is three steps; DNA makes RNA that leads to proteins. Therefore, proteins are regarded as the workhorses of the cell, that play essential roles in the cell.
The method of transcription is appropriately controlled by three chief factors, promoter, regulator and inhibitor. In this context, we will discuss the structural as well as the functional differences between the three types of RNA in eukaryotic cells.
Role mRNA carries genetic information from the nucleus to ribosomes for the synthesis of proteins. Synthesised in Nucleus. Size In mammals, the size of the molecules is around to 12, nucleotides nt. The size of the molecule of tRNA is 76 to 90 nucleotides nt. Shape mRNA is linear in shape. Electron micrograph of a pancreatic exocrine cell section.
The cytosol is filled with closely packed sheets of endoplasmic reticulum membrane studded with ribosomes. At the bottom left is a portion of the nucleus and its nuclear envelope. Image courtesy of Prof. Orci University of Geneva, Switzerland. Merging cultures in the study of membrane traffic. Nature Cell Biology 6 , doi Each mRNA dictates the order in which amino acids should be added to a growing protein as it is synthesized.
In fact, every amino acid is represented by a three-nucleotide sequence or codon along the mRNA molecule. Figure 7: The ribosome and translation A ribosome is composed of two subunits: large and small. During translation, ribosomal subunits assemble together like a sandwich on the strand of mRNA, where they proceed to attract tRNA molecules tethered to amino acids circles.
A long chain of amino acids emerges as the ribosome decodes the mRNA sequence into a polypeptide, or a new protein.
Each tRNA molecule has two distinct ends, one of which binds to a specific amino acid, and the other which binds to the corresponding mRNA codon. During translation , these tRNAs carry amino acids to the ribosome and join with their complementary codons.
Then, the assembled amino acids are joined together as the ribosome, with its resident rRNAs, moves along the mRNA molecule in a ratchet-like motion. The resulting protein chains can be hundreds of amino acids in length, and synthesizing these molecules requires a huge amount of chemical energy Figure 8.
Figure 8: The major steps of translation 1 Translation begins when a ribosome gray docks on a start codon red of an mRNA molecule in the cytoplasm.
A second tRNA molecule, bound to two, connected amino acids, is attached to the 4 th , 5 th , and 6 th nucleotide from the left. It no longer has amino acids bound to its terminus. In step 4, the tRNA molecule that formerly had two connected amino acids attached to its terminus, has now accumulated four amino acids total. Different colored spheres represent different amino acid types, and the four spheres are connected end-to-end in a chain.
A tRNA to the right has one amino acid attached to its terminus. A tRNA molecule carrying a single amino acid is shown approaching the ribosome from the cytoplasm. In step 5, the ribosome is shown to have moved along the length of the mRNA molecule from left to right. A long chain of approximately 19 amino acids is connected to the end of the tRNA molecule.
Five tRNA molecules carrying a single amino acid each are seen floating freely in the cytoplasm surrounding the mRNA molecule. In step 6, the ribosome is disassociated from the mRNA molecule. The amino acid chain has disassociated from the tRNA and is floating freely in the cytoplasm as a complete protein molecule. The illustrated ribosome is translucent and looks like an upside-down glass jug.
The mRNA is composed of many nucleotides that resemble pegs aligned side-by-side along the molecule, in parallel. Each type of nucleotide is represented by a different color yellow, blue, orange, or green. The first three nucleotides, bound to the ribosome, are highlighted in red to represent the stop codon. In step 2, a tRNA molecule is bound to the stop codon.
At the end of the tRNA molecule opposite this point of attachment is an amino acid, represented as a sphere. In step 3, a tRNA bound to a single amino acid is attached to the 7 th , 8 th , and 9 th nucleotide from the left. In eukaryotic cells, however, the two processes are separated in both space and time: mRNAs are synthesized in the nucleus, and proteins are later made in the cytoplasm.
This page appears in the following eBook. Aa Aa Aa. Ribosomes, Transcription, and Translation. Figure 1: DNA replication of the leading and lagging strand. The helicase unzips the double-stranded DNA for replication, making a forked structure.
Figure 3: RNA polymerase at work. What Is the Function of Ribosomes?
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