Biology Notes Genetics and DNA

Genetic diseases in humans are caused by mutations. Often these diseases are the result of a problem in metabolism, in particular, the lack of an enzyme used in one step of a metabolic pathway. For example, galactosemia is a genetic disease that results from the inability to completely breakdown the sugar lactose. Individuals with the disease lack a functional enzyme 3 in this metabolic pathway. On the basis of this type of information, and other evidence, the one gene-one protein hypothesis has been proposed. It states that the function of a gene is to provide the information to make a particular protein, a polypeptide, that will be used as an enzyme or a structural protein.

 Translating the RNA message, written as a sequence of nucleotides, into protein language, written as a sequence of amino acids, requires the use of a decoder, called the genetic code. Nucleotides in the RNA message are read 3 at a time. Each nucleotide triplet, called a codon, is translated, using the genetic code, into a particular amino acid. The genetic code is universal; it is used by nearly all living things, from bacteria to humans.

 Transcription is the process by which RNA is assembled from a DNA template. The 2 strands of the DNA double helix are held together by bonds between complementary base pairs. DNA encodes genetic information in the sequence of bases along one strand. The portion of the DNA molecule that is a single gene, or coding region, is bounded by termination and promoter sites. A molecule of RNA polymerase binds to the promoter site. It moves along the DNA separating the 2 strands of the double helix. Each now unpaired base will bind to a nucleotide in the vicinity which has the appropriate complementary base. In the synthesis of RNA, uracil is the nucleotide complementary to adenine. This process ceases when RNA polymerase reaches the termination site. The DNA strands bind to one another once again as the new RNA molecule moves away. This RNA is a copy, or transcript, of the message contained in the gene.

 
Translation is the process by which the information contained in messenger RNA is used to direct the synthesis of a polypeptide. This information is carried in the sequence of bases on the messenger RNA molecule. The polypeptide can be assembled once RNA binds to a ribosome. Ribosomes consist of large and small subunits. The large subunit has 2 binding sites for transfer RNA, the P and A sites. Transfer RNA carries an amino acid, which will be incorporated into the polypeptide. Its anticodon is a triplet complementary to the codon on messenger RNA that specifies that particular amino acid. A ribosome assembles on the start codon, AUG. Transfer RNA with the anticodon UAC and carrying the amino acid methionine, binds to the codon. The transfer RNA is in the P site of the ribosome. The A site is available for a second transfer RNA with an anticodon complementary to the second messenger RNA triplet. The amino acid carried by the second transfer RNA binds to the methionine. The first transfer RNA leaves the P site and the second transfer RNA moves there, still bound to messenger RNA. This brings the third messenger RNA codon to the now-empty A site and the appropriate transfer RNA can bind to it. The third amino acid is added to the chain and translocation occurs once again. This process of polypeptide chain elongation continues until a stop codon is reached. A release factor binds to the A site. It carries no amino acid, but facilitates release of the polypeptide and the messenger RNA from the ribosome.