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18.24 DNA and Heredity

Millennia ago humans observed, in many species, that offspring tend to share the characteristics of their parents. You might have recognized that you have the same eye or hair color as your parents, that members of your family grow tall, or short, or that some medical conditions are common in your family. The concept that parental traits are passed on and manifested in offspring was termed heredity and, though it was observed and used for millennia, it is only in the last hundred and fifty years that the governing mechanisms of heredity have been determined. 

In 1865, an Augustinian monk named Gregor Mendel published his experiments in plant hybridization. Mendel described the transfer of characteristics in pea plants from one generation to the next. The significance of his work was not fully recognized until 1900 when three scientists, Carl Correns, Hugo de Vries and Erich von Tschermak-Seysenegg, realized that Mendel's experiments described an individual unit of heredity. This individual unit has become known as the gene. The exact components of the "gene" were unknown. Indeed they are still being defined today. In 1944 Oswald Avery, Colin McLeod and Maclyn McCarty and in 1952 Alfred Hershey and Martha Chase published the results of their experiments that showed that a molecule called deoxyribonucleic acid (DNA) was the chemical component of which genes are made. 

DNA is a long chain polymer, made up of monomers called nucleotides. The nucleotides in DNA bond end to end, forming long strings of DNA. These long strings can be tens or hundreds of millions of nucleotides in length. There are many nucleotides involved in various processes in an organism, but the nucleotides that make up DNA come in four flavors. These flavors are distinct because the sugar
 molecule in each bares a different component, called a base, adenine (A), cytosine (C), guanine (G) or thymine (T). In 1953, James Watson and Francis Crick famously discovered that, in a cell, two strands of DNA line up with each other, forming a ladder like structure. Parts of the nucleotide form the stiles of the ladder, while the bases of each nucleotide meet in the middle, forming the rungs. Importantly, Watson and Crick realized that an adenine base in one strand will only connect with a thymine base in the opposite strand and that a cytosine base will only connect with a guanine base. This property of DNA, called "base pairing", means that as long as the sequence of nucleotides of one strand of DNA is known, it is possible to deduce the sequence of nucleotides in the opposite strand. This property is important for the function of DNA in heredity for two reasons. 

This property of DNA suggested a mechanism to biologists that would allow a DNA sequence to be replicated, so that it could be passed from parent to offspring. Due to the relatively weak bonds between the A - T, and C - G base pairs, the DNA ladder can be spilt, lengthways down the middle, yielding two, single strands. By using the rules of base pairing a new, complimentary strand can be made for each of the old, parent strands. This process, called DNA replication, yields two, identical, double-strands of DNA, one of which is be transferred to each daughter cell during cell division. In 1958 Matthew Meselson and Frank Stahl discovered this method of DNA replication, called semi-conservative replication. 

Base pairing of DNA is also important because it allows information, coded by alternating sequences of nucleotides, to be translated into a physical organism. The sequence of bases present in the DNA of every organism is different. A cell can read the sequence of bases in a strand of DNA and, by using complimentary base pairing, translate the information into a specific protein, the building blocks of any organism. These proteins are vital for cells as structural elements and as mediators of the chemical reactions of life. 

The genetic information contained in an organism's genes is passed on from one generation to the next. Mutations naturally arise in any sequence of nucleotides contained within genes. Often times these mutations have little or no effect on the organism. Occasionally, however, these mutations can have a negative impact on the organism. We see this frequently with many genetic human diseases. A mutation in the sequence of nucleotides results in the lack of production of certain key proteins necessary for the normal functioning of some biological mechanisms. On the other hand, a naturally occurring mutation can have a positive effect. If an organism possesses a mutation that, when the organism is placed in a unique situation, provides an advantage for that organism, then the mutation can be considered positive. In this way, species of organisms can change over time in response to environmental selective pressures as a result of offspring inheriting advantageous characteristics.