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Nucleic Acid Replication and Protein Synthesis

written by: KM•edited by: Leigh A. Zaykoski•updated: 1/31/2010

An introduction to one of the most vital process of the human body - nucleic acid replication and protein synthesis.

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    DNA Replication

    There are two types of genetic materials in a cell, one is the deoxyribonucleic acids (DNA) and the ribonucleic acids (RNA) named such based from the kind of sugar that constitute as their building blocks. Nucleotides makes up DNA whose structure is consist of three parts: the pentose sugar, a phosphate groups (making up the phosphate-sugar backbone) and nitrogenous base. These nucleotide components are bonded together tightly by covalent bonds of the phosphate group of one nucleotide against the sugar group of another nucleotide. Also, hydrogen bonds between the bases (cytosine to guanine C=G; and adenine to thymine A=T) of each nucleotide also exist rendering the overall stability of the DNA molecule more popularly called as the double helix structure.

    The process of DNA replication is fairly a complex mechanism that involves a lot of check points in order to prevent DNA material damage as well as unwanted alterations on its structures and base pairs, ultimately leading to mutations. Because of the stable structure of the DNA, brought about by its covalent and hydrogen bonds, it is not difficult to fathom that the first step in replication is the unwinding of the DNA double helix by an enzyme called helicase. Helicase “untwists” the DNA helix (on A=T rich sites for it is easier to break than the triple bonds that exist between cytosine and guanine) while another enzyme, the DNA polymerase elongates each strands.

    After breaking the strand, the RNA Primase binds in the 3‘-5‘ parent chain which in turn attracts RNA nucleotides due to the presence of exposed free base ends suitable for hydrogen bondings. These RNA nucleotides are the primers for the subsequent binding of the DNA nucleotides. The binding of the RNA primase runs in opposite direction, one for the 5‘-3‘ and another for the 3‘-5‘ template.The 5‘-3‘ template is called the leading strand while the other template is the lagging. Since the DNA polymerase cannot process the 3‘-5‘ template (lagging) more RNA primase are required along with Okazaki fragments. These RNA primase are important factors since they are required for the DNA polymerase to bind to the 3‘ end. The DNA polymeraseI removes RNA fragments after it reads the strand, any remaining gaps in the strand are then closed by the action of the DNA ploymerase by actively adding complementary nucleotides into the strand; on the other hand, the ligase fills in the missing phosphate in the sugar backbone.

    Finally, when the DNA Polymerase reaches the end of the DNA molecule, then termination occurs. Since the last potion of the lagging strand lacks RNA after the primer is being removed then it is virtually impossible for the DNA polymerase to close this gap, hence this “unclosed” gap is hence dubbed as the non-coding part of the DNA also know as the telomeres. Every cell cycle, part of these telomeres are subsequently removed. In order to prevent any aberrations in DNA replication, enzymes such as the nucleases determines and removes erroneous nucleotide sequences after which, the DNA polymerase fills the gap created.

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    Protein Synthesis

    Protein synthesis is also termed as translation, and as we will see later, its mechanisms are almost similar to the process of DNA (nucleic acid) replication. First, we have to keep in mind that the process of protein synthesis occurs in the cytoplasm (where ribosomes are present) and not in the nucleus. During translation, the messenger RNA (mRNA) is programmed by the trinucleotide genetic code to produce a specific polypeptide (simple protein). Therefore, the mRNA becomes the template for the synthesis of new proteins. Before the actual translation process, activation occurs where an amino acid combines with a transfer RNA (tRNA) for it to be “charged” also called as aminoacyl-tRNA. Next is the initiation step where the small ribosome sub-unit attached itself to the 5' end of the mRNA at the start codon (three base pairs usually in AUG, GUG or UUG). Next another “charged” tRNA attaches to the ribosome that contains an anti-codon than corresponds to the codon, officially starts the elongation process. Elongation happens when the mRNA reaches the stop codon (UAA, UAG, or UGA).