Science - Chemistry
DNA Carries Heredity
Every living system has a blueprint for replication, or making copies of itself. This blueprint is commonly called heredity. The key structure of the hereditary process is the long, spiral DNA molecule. DNA consists of two complementary strands coiled around each other to form a twisting ladder called a double helix (see Genetics). The strands are made up of varying sequences of chemical groups which are called nucleotides. A nucleotide consists of a sugar and a phosphate group plus either of two purine bases--adenine (A) and guanine (G)- or either of two pyrimidine bases - thymine (T) and cytosine (C).
DNA contains the genetic code for making proteins from smaller molecules called amino acids. Each base on a strand of DNA pairs only with its complement on the other strand; that is, A pairs only with T, and G pairs only with C. Moreover, each set of three bases on a strand, such as AAA, AGC, GGG, or CGT, codes for a specific amino acid (or in the case of a few triplets, for an end to the protein-making process). Thus, a base triplet corresponds to a particular amino acid in the same way that a unit of the Morse telegraph code corresponds to an alphabet letter. In this manner, DNA directs the sequencing of the amino acids that grow into proteins.
In many organisms, DNA is restricted to the cell nucleus, while protein synthesis goes on at the endoplasmic reticulum, a system of membrane-lined tubes in the cytoplasm. Ordinarily attached to the endoplasmic reticulum are the ribosomes, "workbenches" for protein construction. Since the ribosomes are away from the nucleus, the building code must somehow be communicated from DNA to the ribosomes. This is done through ribonucleic acid (RNA). RNA is closely related to DNA and can carry genetic messages. First, DNA unwinds and separates its strands so that complementary strands of RNA can be assembled on them. A strand of so-called messenger RNA (mRNA) then travels out of the nucleus to the ribosomes, where protein synthesis begins.
The mRNA strand, like its DNA "parent," contains the total genetic information needed for sequencing amino acids into a particular protein. Imagine a protein containing only the two amino acids A and B strung out in this unvarying sequence: A--B--A--B--A--B (the sequence is deliberately shortened because proteins usually contain several hundred amino acids). A strand of mRNA has the series of complementary base triplets that codes for this sequence. However, another type of RNA called transfer RNA (tRNA) must carry the amino acids to the ribosome for assembly. When the mRNA code calls for amino acid A, the appropriate tRNA carries it in a form ready for peptide bonding with the next amino acid in line. In a peptide bond, the tail-end carbon atom of one amino acid is linked to the nitrogen atom of the next. When the code calls for it, another tRNA carries amino acid B. Bit by bit, the polypeptide chain grows to the desired length, guided by the mRNA directions. At the end of the operation, the newly formed protein is kicked off the ribosome. The protein instantly folds up in the most stable way. Synthesis proceeds at a fast pace. A protein containing 400 amino acids can be synthesized in about 20 seconds. (For more information about the role of DNA in protein synthesis, see Genetics.)
Of all the molecules that DNA could direct to be built, one might wonder why the information encoded in DNA is limited solely to the manufacture of protein. The reason is that so long as DNA can direct the making of protein enzymes, no other direction is necessary because enzymes aid in the building of all other cell molecules.
Most of the details of protein synthesis have been omitted from this discussion so that key events could be stressed. However, one procedure merits mention. Before an amino acid can be assembled into a polypeptide chain, it must first be modified to a so-called acyl amino acid, which is more reactive than an unmodified one. This important acyl conversion is powered by the energy stored in a molecule called adenosine triphosphate (ATP).
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