The sequences along the protein production. A protein is a compound formed by small molecules called amino acids that determine the structure and function. The amino acid sequence is in turn determined by the base sequence of DNA nucleotides. Each sequence of three bases, called a triplet, is a genetic code word or codon, which specifies a particular amino acid. Thus, the triplet GAC (guanine, adenine, cytosine) is the codon for the amino acid leucine, while the CAG (cytosine, adenine, guanine) corresponds to the amino acid valine. Therefore, a protein composed of 100 amino acids is encoded by a segment of 300 nucleotides of DNA. Of the two polynucleotide chains that form a DNA molecule, only one, called parallel, containing the necessary information for the production of an amino acid sequence determined. The other, called antiparallel, helps replication.
Protein synthesis begins with the separation of the DNA molecule into its two strands. In a process called transcription, a part of the parallel strand acts as a template to form a new chain called messenger RNA or mRNA (see ribonucleic acid). The mRNA leaves the cell nucleus and attaches to the ribosomes, specialized cellular structures that serve as focal protein synthesis. Amino acids are transported to the ribosomes by another type of RNA called transfer RNA (tRNA). It starts a process called translation consists of the linking of amino acids in a sequence determined by the mRNA to form a protein molecule.
A gene is a nucleotide sequence of DNA that specifies the order of amino acids in a protein via an intermediary mRNA molecule. The substitution of a DNA nucleotide by another containing a different base causes all descendant cells or viruses containing this same sequence of bases altered. As a result of substitution can also change the amino acid sequence of the resulting protein. This alteration of a DNA molecule is called mutation. Almost all mutations are the result of errors during the replication process. Exposure of a cell or virus to radiation or certain chemicals increases the likelihood of mutations. REPLICATION
In almost all cellular organisms, replication of DNA molecules takes place in the nucleus, just before cell division. Begins with the separation of the two polynucleotide chains, each of which then acts as a template for the assembly of a new complementary strand. As the original string is opened, each of the nucleotides of the two resulting strings attract other complementary nucleotide previously formed by the cell. Nucleotides are joined together by links hydrogen to form the rungs of a new molecule of DNA. As the complementary nucleotides are fitted into place, an enzyme called DNA polymerase binds linking the phosphate group of one sugar molecule to the next, in order to build the string side of the new DNA molecule. This process continues until it has formed a new polynucleotide chain along the old, being rebuilt and a new molecule with double-helix structure
0 comments:
Post a Comment