Storage of Genetic Information

Most DNA in plants resides in the nucleus. Because DNA directs all cellular activities, it is important to protect DNA and allow for its permanent storage. The nuclear envelope probably helps to protect fragile DNA molecules from mechanical shearing by components of the cytoskeleton (see Chapter 3).

The genetic information in a DNA molecule resides in the sequence of nucleotides along the rungs of the ladder. The sequence GAATCC, for example, codes for a different set of amino acids than CTTAGA. Early scientists were fooled by the simplicity of a DNA molecule and did not believe it could provide information for the survival of all organisms on earth. However, the DNA in a typical plant cell contains millions or billions of base pairs. Abarley plant with 5 billion base pairs of DNA in its nucleus could produce 45,ooo,ooo,ooo different DNA sequence combinations! This provides more than enough genetic variability for the plant.

At this point, we need to introduce some important concepts, beginning with that of the gene. We are all familiar with the idea that genes control physical traits, such as eye color in people and height in plants. How do they do that? From a molecular perspective, a gene is a segment of DNA (several thousand base pairs long) that directs the synthesis of a protein. That protein is then used by the cell as structural or storage material, or it influences the activities of the cell by acting as an enzyme. As you recall from Chapter 2, enzymes are organic catalysts. Any activity in the cell, from photosynthesis to cellwall construction, absolutely depends on enzymes to facilitate chemical reactions. Therefore, all activities of the cell are controlled by enzymes, which require genes for their synthesis. Each kind of organism has many different genes in its genome, which is a term for the sum total of DNA in an organism's chromosomes. A relatively simple organism such as a bacterium typically has several thousand genes in its genome. On the other hand, more complex eukaryotic organisms usually have between 10,000 and 100,000 genes per genome. The DNA message in a gene is read in three-nucleotide groups, called codons. If we compare DNA nucleotides to letters of the alphabet, then codons are comparable to words that are organized into gene sentences; all gene sentences in a genome make up the book that constitutes the whole organism. The differences between organisms lie in the differences in the composition of their genomes. Organisms that are very dissimilar—for example, bacteria and higher plants—will have many differences in their DNA sequences. The genomes of similar organisms, such as domestic plants and their wild relatives, are very similar, yet different enough to account for dramatic differences in appearance.

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