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DNA nucleotides are composed of long chains of bases. A triplet code is a sequence of three bases along a single strand of DNA. Each triplet code is read and calls for a specific amino acid. Recall that there are 4 bases in DNA (Guanine, adenine, cytosine and thymine) and 20 amino acids that are linked together in different arrangements to make various proteins. The 4 bases can also be arranged into 64 different triplet codes which sequence of three bases. Sixtyone of the codes are matched up to one of the 20 amino acids and a given amino acid can be specified by more than one triplet code. While the remaining three triplet codes act as stop signals and end the protein chain rather than adding an amino acid. As the triplet codes are read and the appropriate amino acid is added to the growing chain and the final result being a protein as determined by the DNA information. So the genetic code is universal in all cells.
Transcription; mRNA Synthesis.
First item of business in protein synthesis is the unraveling of the DNA double helix and separation of the two strands of nucleotides. One of the strands will act as a template and will determine the sequence of RNA nucleotides and the template strand is determined by the presence of a specific sequence of DNA nucleotides called the promoter. The sequence is located near the beginning of the gene and RNA polymerase is the enzyme that joins together the aligned ribonucleotides into a strand. When the triplet codes reach a stop sequence or stop signal then RNA polymerase ends the chain and releases then RNA transcript. Finally a series of adenine nucleotides called the poly A tail is added to the end of the transcribed RNA strand. The tail is vital and it gives the signal necessary to allow the RNA to move out of the nucleus and then bind to ribosomes in the cytoplasm where proteins will be synthesized from the encoded information.
In DNA the three base sequences are called triplet codesin RNA the three bases sequences that specify one amino acid are called codons and codons are analogous in function. So the entire sequence of nucleotides in the entire template strand is transcribed into a primary RNA transcript but Only certain segments of this gene actually code for amino acids and the segments are called exons while the non-coding segments in between exons are called introns. The introns are spliced off of the gene by a spliceosome to form a continuous sequence of exons, the sequence is now called mRNA.
Translation; Polypeptide Synthesis.
After removing the introns the mRNA moves out into the cytoplasm through the nuclear pores and binds to a ribosome and each ribosome is composed of proteins and a class of RNA called ribosomal rRNA and which is a strand transcribed from the DNA in the nucleolus.
Transfer RNA (tRNA) is the link between an amino acid and its mRNA codon since the clover leaved shaped molecule of tRNA can combine with both. Transfer of RNA is synthesized in the nucleus before it moves out into the cytoplasm. An enzyme called aminoacyl-tRNA synthetase, there are 20 of these, specific to each amino acid, links specific amino acids to tRNA molecules. The tRNA molecule and amino acid are then base paired to mRNA with a three base sequence called the anti-codon and the anti-codon specifies the amino acid.
Protein assembly is a three-stage process.
Initiation of the polypeptide chain begins by binding an anti-codon in an amino acid tRNA complex to the corresponding codon in the mRNAââ‚¬"ribosome complex. This initial binding is driven by enzymes called initiation factors, the activity of these enzymatic factors regulate the rate of protein synthesis and the initiation phase is the slowest of the three phases in the assembly process. Elongation of the polypeptide chain is the second phase where each amino acid brought to the chain on a tRNA molecule is linked by a peptide bond to the end of the growing protein chain, the free tRNA is then released from the ribosome and will go attach to another amino acid.
The ribosome acts as a Reader and when it reaches a termination sequence in the mRNA, the link between the polypeptide chain and tRNA is broken and the completed protein is then released from the ribosome and the ribosome is available for the next mRNA strand coming from the nucleus.
As small protein emerge from the ribosome they undergo folding and the larger proteins will fold within the recess of a small hollow protein chamber called chaperones. If anything is to be added here in the protein chain such as a carbohydrate or lipid derivatives then these occur at the chaperone site. Eventually, the mRNA molecules are broken down into nucleotides by cytoplasmic enzymes. Mitochondrial DNA does not have introns becouse eachMi tochondria have the complete set of machinery to produce its own proteins and the nuclear DNA supplies the rest.
Regulation of Protein Synthesis:
Signals from within or outside the cell can turn on/off the transcription of genes and this regulation is performed through allosteric or covalent modulation of a class of enzymes called transcription factors. Pre-initiation complex at the promoter region forms these factors and activates or represses the initiation process, such as the separation of DNA strands, activation of RNA polymerase.There are only 4 types of nitrogen-containing bases that can compose RNA nucleotides: 1, adenine (a) 2, uracil (U) 3, guanine (G) 4, cytosine (C). Amino acids are 20 different ones. Considering only one nucleotide (a 1:1 coding) it would be impossible to codify all amino acids. Considering two nucleotides there would be an arrangement of 4 elements (2 x 2) resulting in a total of only 16 possible codifier units (4 x 4). Nature may know combinatory analysis since it makes a genetic code by arrangement of the 4 RNA bases, 3 x 3, which are provided 64 different triplets (4 x 4 x 4). Therefor each triplet of nitrogen containing bases of RNA codifies one amino acid of a protein so as these triplets appear in sequence in the RNA molecule, sequential amino acids codified by them are bound together to make polypeptide chains(example, a UUU sequence codifies the amino acid phenylalanine, as well the UUC sequence; the ACU, ACC, ACA and ACG sequences codify the amino acid threonine; and so on for all possible triplet sequences and all other amino acids)
Each sequence of three nitrogen-containing bases of RNA that codifies one amino acid is called a codon and this codon is the codifier unit of the genetic code. Since there are 20 amino acids and 64 possibilities of mRNA codons and it is expected some amino acids to be codified by more than one codon and these are really happens.