DNA Discovery & Protein Synthesis

Structure of DNA

The molecular biologists realized that in order to learn how DNA might reproduce itself and transmit inherited information, they needed to discover the structure of the DNA molecule. They would have to work “blindfolded,” in the sense that earlier studies had provided very few clues to guide them. The researchers knew that each DNA molecule contained many copies of the four types of bases, small molecules called adenine, cytosine, guanine, and thymine. The molecule also included at least one “backbone,” a long string of identical, alternating sugar and phosphate molecules. X-ray crystallography, a technique that helped chemists analyze the shape of molecules, suggested that the backbone was shaped like a coil, or helix. Austrian-born biochemist Erwin Chargaff had shown in the late 1940s that the amount of cytosine in a DNA molecule was always the same as the amount of guanine, and the same was true of adenine and thymine. However, no one knew how many backbone strands each molecule of DNA contained or how the backbones and bases were arranged within the molecule.

James Watson and Francis Crick deduced in 1953 that each molecule of deoxyribonucleic acid (DNA) is made up of two “backbones” composed of alternating smaller molecules of phosphate (P) and deoxyribose (D), a sugar. The backbones both have the shape of a helix, or coil, and they twine around each other. Inside the backbones, like rungs on a ladder, are four kinds of smaller molecules called bases. The bases always exist in pairs, connected by hydrogen bonds. Adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G).

How DNA Replicates

DNA’s structure explains its power to duplicate itself. When a cell prepares to divide, the hydrogen bonds between the bases dissolve and the DNA molecule splits along its length like a zipper unzipping. Each half then attracts bases and backbone pieces from among the molecules in the cell, forming the same pairs of bases that had existed before. The result is two identical DNA molecules.

If DNA carried hereditary information, Crick and Watson said, DNA molecules had to be able to reproduce themselves when chromosomes duplicated during cell division. The two men believed that the key to DNA’s reproduction lay in the molecule’s mirror-image structure. Just before a cell divides, they proposed, the weak hydrogen bonds between the pairs of bases in its DNA molecules break. Each molecule then splits lengthwise, like a zipper unzipping. Each base attracts its pair mate, complete with an attached backbone segment, from among free-floating materials in the cell nucleus. An adenine molecule always attracts a thymine and vice versa, and the same for cytosine and guanine. When the process is complete, the nucleus contains two identical double-stranded DNA molecules for every one that had existed before. The cell now splits, and each of the two daughter cells receives a complete copy of the original cell’s DNA. Experiments later confirmed this theory.

Use of DNA to make Protein

As a first step in making a protein, part of a DNA molecule (a gene) uses itself as a pattern to form a matching stretch of messenger RNA (mRNA). When the messenger RNA moves into the cytoplasm of the cell, it attracts matching short stretches of transfer RNA (tRNA), each of which tows a single amino acid molecule. With the help of an organelle called a ribosome, the transfer RNA molecules lock onto the matching parts of the messenger RNA, and the amino acids they carry are joined, forming a protein.

Crick and Brenner suggested that DNA makes a copy of itself in the form of RNA (ribonucleic acid), which is like DNA except that it has a different kind of sugar in its backbones, and in place of thymine it has a different base, uracil. DNA normally cannot leave a cell’s nucleus, but its RNA copy, which came to be called messenger RNA, can travel into the cytoplasm, the jellylike material that makes up the outer part of the cell. In the cytoplasm, Crick and Brenner said, the messenger RNA encounters small bodies called ribosomes. A ribosome rolls along the messenger RNA molecule and attracts from the cytoplasm the amino acid represented by each three-base “letter” of the translated DNA code. Crick believed that what he called adapter molecules (later called transfer RNA) tow the amino acids to the correct spots on the messenger RNA. The amino acids then join together, forming the protein. The messenger RNA and the ribosome release the protein molecule into the cell. Brenner and other researchers in the early 1960s proved that this theory was essentially correct.

Ref : Modern Genetics Engineering Life