DNA, the genetic material, is a double helix containing the nitrogen bases A (adenine) paired with T (thymine) and G (guanine) paired with C (cytosine). During replication, DNA "unzips," and then a complementary strand forms opposite to each original strand.

DNA specifies the synthesis of proteins because it contains a triplet code: every three bases stand for one amino acid. During transcription, mRNA is made complementary to one of the DNA strands. mRNA, bearing codons, moves to the cytoplasm, where it becomes associated with the ribosomes. During translation, tRNA molecules, attached to their own particular amino acids, travel to a ribosome, and through complementary base pairing between anticodons and codons, the tRNAs and therefore the amino acids in a polypeptide are sequenced in a predetermined way.

The following levels of control of gene expression are possible in eukaryotes: transcriptional control, post-transcriptional control, translational control, and post-translational control.

The prokaryote operon model explains how one regulator gene controls the transcription of several structural genes, genes that code for proteins. In eukaryotes, the chromosome has to decompact before transcription can begin. Transcription factors attach to DNA and turn on particular genes.

In molecular terms, a gene is a segment of DNA, and a mutation is a change in the normal sequence of nucleotides of this segment. Frameshift mutations result when a base is added or deleted and the result is a nonfunctioning protein. Point mutations can range in effect, depending on the particular codon change. Gene mutation rates are rather low, because DNA polymerase proofreads the new strand during replication and because there are repair enzymes that constantly monitor the DNA.

Cancer is characterized by a lack of control: the cells grow uncontrollably and metastasize. Cancer development is a multistep process involving the mutation of genes. Proto-oncogenes and tumor-suppressor genes are normal genes that bring on cancer when they mutate because they code for factors involved in cell growth.

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