Protein Synthesis

Synthesizing a protein molecule requires that the correct amino acid building blocks be present in the cytoplasm. Furthermore, these amino acids must align in the proper sequence along a strand of messenger RNA. A second kind of RNA molecule, synthesized in the nucleus and called transfer RNA (tRNA), aligns amino acids in a way that enables them to bond to each other. A transfer RNA molecule consists of only seventy to eighty nucleotides and has a complex three-dimensional shape. The two ends of the tRNA molecule are most important for the "connector" function.

At one end, each transfer RNA molecule has a specific binding site for a particular amino acid. There is at least one type of transfer RNA molecule for each of the twenty amino acids. Before the transfer RNA can pick up its amino acid, the amino acid must be activated. Special enzymes catalyze this step. ATP provides the energy to form a bond between the amino acid and its transfer RNA molecule.

The other end of each transfer RNA molecule includes a region, called the anticodon, that contains three nucleotides in a particular sequence unique to that type of transfer RNA. These nucleotides bond only to a specific complementary mRNA codon. In this way, the appropriate transfer RNA carries its amino acid to the correct place in the sequence, as prescribed by the mRNA.

DNA Makes History

In July 1918, the last tsar of Russia, Nicholas II, and his family, the Romanovs, met gruesome deaths at the hands of Bolsheviks in a town in the Ural Mountains of central Russia. Captors led the tsar, tsarina, four daughters and one son, plus the family physician and three servants, to a cellar and shot them, bayoneting those who did not die quickly. The executioners stripped the bodies and loaded them onto a truck, which would take them to a mine shaft where they would be left. But the truck broke down, and the bodies were instead placed in a shallow grave, then damaged with sulfuric acid so that they could not be identified.

In July 1991, two Russian amateur historians found the grave, and based on its location, alerted the government that the long-sought bodies of the Romanov family might have been found. An official forensic examination soon determined that the skeletons were from nine individuals. The sizes of the skeletons indicated that three were children. The porcelain, platinum, and gold in the teeth of some of the skeletons suggested that they were royalty. The facial bones were so decomposed from the acid that conventional forensic tests were not possible. But one very valuable type of evidence remained — DNA. Forensic scientists extracted DNA from bone cells and mass-produced it for study using a technique called the polymerase chain reaction (PCR) described in Clinical Application 4.3.

By identifying DNA sequences specific to the Y chromosome, which is found only in males, the DNA detectives could tell which of the skeletons were from males. Then they delved into the DNA in mitochondria. Because these organelles pass primarily from mother to offspring, identifying a mitochondrial DNA pattern in a woman and children would establish her as their mother. This was indeed so for one of the women (with impressive dental work) and the children.

But a mother, her children, and some companions does not a royal family make. The researchers had to connect the skeletons to the royal family. Again they turned to DNA. Genetic material from one of the male skeletons shared certain rare DNA sequences with DNA from living descendants of the Romanovs. This man also had aristocratic dental work and shared DNA sequences with the children! The mystery of the fate of the Romanovs was apparently solved, thanks to the help of DNA.

DNA fingerprinting is a general term for several techniques that are increasingly being used to compare the genetic material of individuals, to confirm or rule out relationships — such as blood relatedness, presence at a crime scene, or to identify accident victims. Recent applications of DNA fingerprinting have exonerated several jailed innocent people, and identified Thomas Jefferson as a possible father of a son of his slave Sally Hemings. DNA fingerprinting applications aren't confined to humans. It was used, for example, to identify the two strains of cultivated grapes that can be bred to yield sixteen popular varieties of wine grapes. ■

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