10.6 Flashcards
Note 1 —-»
An organism’s genotype, its genetic makeup, is the heritable information contained in the sequence of nucleotide bases in DNA. The phenotype is the organism’s physical traits.
Note 2 —-»
The DNA inherited by an organism specifies traits by dictating the synthesis of proteins (or, in some cases, just RNAs). In other words, proteins are the links between genotype and phenotype. However, a gene does not build a protein directly. Rather, a gene dispatches instructions in the form of RNA, which in turn programs protein synthesis. The answer is that the DNA inherited by an organism specifies traits by dictating the synthesis of proteins (or, in some cases, just RNAs). In other words, proteins are the links between genotype and phenotype. However, a gene does not build a protein directly. Rather, a gene dispatches instructions in the form of RNA, which in turn programs protein synthesis. The molecular “chain of command” is from DNA in the nucleus of the cell to RNA to protein synthesis in the cytoplasm. The two main stages are transcription, the synthesis of RNA under the direction of DNA, and translation, the synthesis of protein under the direction of RNA.
Transcription
The synthesis of RNA on a DNA template.
Translation
The synthesis of a polypeptide using the genetic information encoded in an mRNA molecule.
Note 3 —-»
The relationship between genes and proteins was first proposed in 1902 when English physician Archibald Garrod suggested that genes dictate phenotypes through enzymes, proteins that catalyze specific chemical reactions. Garrod hypothesized that an inherited disease reflects a person’s inability to make a particular enzyme. He gave as one example the hereditary condition called alkaptonuria, in which the urine is dark because it contains a chemical called alkapton. Garrod reasoned that people with alkaptonuria inherited an inability to make an enzyme that breaks down alkapton. Years later, biochemists accumulated evidence in favor of Garrod’s proposal that cells make and break down biologically important molecules via metabolic pathways, as in the synthesis of an amino acid or the breakdown of a sugar. Each step in a metabolic pathway is catalyzed by a specific enzyme. Therefore, individuals lacking one of the enzymes for a pathway are unable to complete that pathway.
Note 4 —-»
Beadle and Tatum studied strains of the mold that were unable to grow on a simple growth medium. Each of these so-called nutritional mutants turned out to lack an enzyme in a metabolic pathway that synthesized some molecule the mold needed, such as an amino acid. Beadle and Tatum also showed that each mutant was defective in a single gene. Accordingly, they hypothesized that the function of an individual gene is to dictate the production of a specific enzyme. This work won Beadle and Tatum the 1958 Nobel Prize in Chemistry. The “one gene-one enzyme hypothesis” has since been modified. First, it was extended beyond enzymes to include all types of proteins. For example, keratin (the structural protein of hair) and the hormone insulin are two examples of proteins that are not enzymes. In addition, many proteins are made from two or more polypeptide chains, with each polypeptide specified by its own gene. For example, hemoglobin, the oxygen-transporting protein in your red blood cells, is built from two kinds of polypeptides, encoded by two different genes. In addition, many eukaryotic genes code for a set of polypeptides (rather than just one) by a process called alternative splicing.
Gene
A region of DNA that can be expressed to produce a functional product that is either a polypeptide or an RNA molecule.
What are the functions of transcription and translation?
Transcription is the transfer of information from DNA to RNA. Translation is the use of the information in RNA to make a polypeptide.
