e introns and joins the exons together, and adds unique features to the ends of the transcript to make a mature mRNA. It is still not known what the exact role of the intron is, but it is believed that it may be a site for recombination, the process by which progeny derive a combination of genes different from that of either parent, resulting in novel genes with new combinations of exons, which is the key to evolution.
The RNA copy of a DNA sequence is made by an enzyme called RNA polymerase, which recognizes DNA sequences where transcription is initiated, called promoter sites - there are usually two promoter sites upstream from the beginning of every gene) (NCBI, 12004). Eukaryotic cells have three different RNA polymerases which each recognize a different class of genes. RNA polymerase II is responsible for synthesizing mRNAs from protein-coding genes, and requires a TATA sequence, found 25 to 30 nucleotides upstream from the beginning of the gene: this is the initiator sequence. Transcription terminates when the polymerase reaches a stop signal, which is not fully understood in eukaryotes. Translation by the ribosomes is the process of protein synthesis from amino acids directed by the genetic code carried on mRNA, and is initiated at the amino acid methionine and translation is terminated when the ribosome reaches one of three stop codons. Each amino acid is coded for on mRNA by a sequence of three RNA bases, known as a codon.
DNA sequences which code for proteins are called structural genes (NCBI, 2004). An insignificant fraction of the genome is made up of regulatory sequences, but these sequences provide critical functions such as indicating the beginning and end of genes, providing sites for initiating replication and recombination, and constituting receptor sites for proteins that turn genes on and off. Regulatory sequences are inherited, but are not referred to as genes. Ninety-eight percent of the genome is ...