Pyrimidines – single rings each with six sides.
e.g. cytosine (C) and thymine (T)
purines – double rings comprising of a six-sided and five-sided ring.
e.g. adenine (A) and guanine (G)
Condensation reactions join the nucleotides together, the reaction between the nucleoside and phosphoric acid. A phosphodiester bridge is formed between the 3 carbon atom of one pentose sugar and 5 carbon atom of another sugar. The hydrogen bonds join bases across the strands.
Protein synthesis
Protein synthesis involves three phasese, the transfer of the coden information from the nucleus to the cytoplasm (transcription), this forms a strand of mRNA which leaves the nucleus and attaches to a ribosome on the RER or in the cytoplasm. The second phase is the conversion of that information into polypeptides on the ribosomes (translation). The translation of the code in triplets or codons occurs when tRNA molecules with a complementary anticodon attach to the ribosome leaving a specific amino acid. the mRNA strand moves along to translate the next coden to form a polypeptide strand. The last phase is called translocation which is a process of the ribosome move on the mRNA. These three stages involve another nucleic acid, ribonucleic acid (RNA). There are three types of RNA:
mRNA – messenger RNA is 3-5% of the total RNA in a cell, which is formed in the nucleus during the process of transcriotion and which carries the instructions from the DNA to the ribosomes.
tRNA – transfer RNA is a clover leaf shaped molecule and is up to 15% of the total RNA in the cell, which is present in the cytoplasm and which picks up amino and transports them to the ribosomes for assembly into polypeptides.
rRNA – ribosomal RNA is 80% of the total RNA in a cell, which is a component of the ribosomes on which the polypeptide chains are biult up.
The importance of DNA in protein synthesis
DNA is the molecule which controls the synthesis of proteins. The sequence of purine and pyrimidine base along the DNA molecule determine the sequence of amino acids in a protein molecule (the genetic code). Proteins are used for growth and repair and also as enzymes, in which form they catalyse all other cellular activities.
Three bases coded for the position of one amino acid in a protein molecule. This was called triplet code or coden. Protein synthesis relies on the effective communication of the coded information held in the DNA to the sites of protein manufacture, the ribosomes in the cytoplasm. Since DNA is part of larger structures (chromosomes), which are unable to move from the nucleus, intermediate messenger molecules are needed. These are messenger RNA molecules.
To begin with, the DNA duplex unzips to expose the base sequence on the coding strand. RNA nucleotides then move in and align themselves according to the rules of base pairing (A-U and G-C) with U replacing T in the RNA molecule. During transcription only the coding parts of the DNA are copied (the exons). Non coding parts or introns are ignored. The completed mRNA molecule detaches from the DNA template and exits the nucleus via the nuclear pores, moving into the cytoplasm. That’s the whole process of transcription. DNA is very important in the stage of transcription.
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