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Transcript of Polypeptide Synthesis
What is it ?
Before synthesis of a protein occurs, the corresponding RNA molecule has to be made and this is done through RNA transcription. One strand of the DNA double helix is used as a template by the RNA polymerase to synthesise an mRNA. ( messenger RNA). This then travels from the nucleus to the cytoplasm. The coding mRNA sequence can be described as a unit of three nucleotides called a codon.
After transcription, the new RNA strand is released and the two unzipped DNA strands bind together again to form the double helix. A single gene on a DNA strand can produce enough RNA to make thousands of copies of the same protein in a very short time.
The first step in transcription is the partial unwinding of the DNA double helix strand so that the portion of DNA that codes for the needed protein can be copied. Once the DNA molecule is unwound at the correct location, an enzyme called RNA polymerase helps line up nucleotides to create a complementary strand of mRNA. Since mRNA is a single-stranded molecule, only one of the two strands of DNA is used as a template for the new RNA strand. The other is expelled.
The ribosome binds to the mRNA at the start codon which is akways (AUG), coding for the amino acid Methionine. this is recognized only by the initiator tRNA. The ribosome proceeds to the elongation phase of protein synthesis by moving along each time translation takes place. During this stage, complexes, composed of an amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon. The ribosome moves from codon to codon along the mRNA.
So basically, different Amino acids are added one by one per codon, translated into polypeptidic sequences dictated by DNA and represented by mRNA. At the end, a release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome. Each codon is a step in the process.
In order for a cell to make proteins, only the relevant instructions for those proteins are accessed in the DNA nucleotide sequence. Since the DNA instructions must remain in the nucleus, an intermediate molecule – messenger RNA (mRNA) – is created; this carries a transcribed copy of the relevant instructions from the nucleus to the ribosomes in the cytoplasm. The ribosomes can be considered as the ‘machinery’ that translated the message carried by the mRNA into a cell product such as protein.
DNA consists of long chain of nucleotides wound into a double helix. The sequence of nucleotide bases determines the meaning of the message – because it codes for the sequence of RNA nucleotides and ultimately the sequence of amino acids that form the polypeptide chain.
Like DNA, RNA is a nucleic acid made from a chain of nucleotides, but it differs from DNA in the following ways:
- Most RNA is single-stranded
- The sugar in RNA is ribose sugar
- RNA has a nitrogenous base uracil (U) instead of thymine.
There are three types of RNA:
- messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA):
- mRNA is single-stranded and is not twisted into a helix. They are found in both the nucleus and cytoplasm. They function as an intermediate molecule, carrying information from DNA in the nucleus to the ribosomes in the cytoplasm.
- tRNA molecules occur in the cytoplasm. Each one is 75 nucleotides long and twisted into the shape of a clover leaf. On one end of the tRNA there are three unpaired bases called the anticodon, which attach the tRNA to its complementary bases on the mRNA strand. The other end of the tRNA is able to bind with an amino-acid temporarily. Each tRNA molecule will only attach to one particular amino acid. The specific sequence of these three bases at the anticodon end determines which amino acid will be carried by the tRNA.
- rRNA forms a structural part of the ribosomes
Genes vary in the functions that they carry out in the cells of an organism. Some genes produce proteins that become part of the structure and functioning of the organism. These genes are termed structural genes.
Some genes, however, produce proteins that control the action of other genes. These genes are termed regulator genes and their actions determine whether other genes are active or not and, if active, the rate at which their products are made. These genes are important in the embryonic development.
Ribosomes have three binding sites: one for mRNA and two for tRNA. The two tRNA sites are the A site and P site. tRNA is made up of many nucleotides that bend into the shape of a cloverleaf. At its tail end, tRNA has an acceptor stem that attaches to a specific amino acid. At its head, tRNA has three nucleotides that make up an anticodon.
An anticodon pairs complementary nitrogenous bases with mRNA. For example if mRNA has a codon AUC, it will pair with tRNA’s anticodon sequence UAG. tRNA molecules with the same anticodon sequence will always carry the same amino acids, ensuring the consistency of the proteins coded for in DNA.
DNA, RNA and GENES
DNA and RNA
3 types of RNA