10.7 - 10.9 Flashcards
Note 1 —-»
Genes provide the instructions for making specific proteins. But a gene does not build a protein itself. The bridge between DNA and protein synthesis is the nucleic acid RNA: DNA is transcribed into RNA, which is then translated into protein. Genes provide the instructions for making specific proteins. But a gene does not build a protein itself. The bridge between DNA and protein synthesis is the nucleic acid RNA: DNA is transcribed into RNA, which is then translated into protein. Information within the cell flows as DNA→RNA→ protein. Transcription and translation are linguistic terms, and it is useful to think of nucleic acids and proteins as having languages. To understand how genetic information passes from genotype to phenotype, we need to see how the chemical language of DNA is translated into the different chemical languages of proteins.
Note 2 —-»
The pink strand underneath the enlarged DNA segment represents the results of transcription: an RNA molecule. The process is called transcription because the nucleic acid language of DNA has been rewritten (transcribed) as a sequence of bases on RNA. Notice that the language is still that of nucleic acids, although the nucleotide bases on the RNA molecule are complementary to those on the DNA strand. The purple chain at the bottom, represents the results of translation, the conversion of the nucleic acid language to the polypeptide language. Like nucleic acids, polypeptides are polymers, but the monomers that compose them are the 20 different kinds of amino acids. Again, the language is written in a linear sequence, and the sequence of nucleotides of the RNA molecule dictates the sequence of amino acids of the polypeptide. The RNA acts as a messenger carrying genetic information from DNA.
Note 3 —-»
During translation, there is a change in language from the nucleotide sequence of the RNA to the amino acid sequence of the polypeptide. How is this translation achieved? Recall that there are only four different kinds of nucleotides in DNA (A, G, C, T) and in RNA (A, G, C, U). In translation, these four nucleotides must somehow specify all 20 amino acids. Consider if each single nucleotide base were to specify one amino acid. In this case, only four of the 20 amino acids could be accounted for, one for each type of base. What if the language consisted of two-letter code words? If we read the bases of a gene two at a time—AG, for example, could specify one amino acid, whereas AT could designate a different amino acid—then only 16 arrangements would be possible (4^2), which is still not enough to specify all 20 amino acids. However, if the base code in DNA consists of a triplet, with each arrangement of three consecutive bases specifying an amino acid—AGT specifies one amino acid, for example, while AGA specifies a different one—then there can be 64 (that is, 4^3) possible code words, more than enough to specify the 20 amino acids. Thus, triplets of bases are the smallest “words” of uniform length that can specify all the amino acids. Indeed, the 64 triplets allow for more than one to represent an amino acid.
Triplet Code
A set of three-nucleotide-long “words” that specify the amino acids for polypeptide chains.
Codons
A three molecule sequence in mRNA that specifies a particular amino acid or polypeptide termination signal; The basic unit of the genetic code.
What is the minimum number of nucleotides necessary to code for 100 amino acids?
300
Genetic Code
The set of rules that dictates the amino acid translations of each mRNA nucleotide triplet.
Note 4 —-»
The first codon was deciphered by synthesizing an artificial RNA molecule using just uracil. No matter where this message started or stopped, it could contain only one type of triplet codon: UUU. When this “poly-U” was added to a test-tube mixture containing ribosomes and the other ingredients required for polypeptide synthesis, a polypeptide was translated that contained a single amino acid: phenylalanine (Phe). Thus, the RNA codon UUU must specify the amino acid phenylalanine. By variations on this method, the amino acids specified by all the codons were soon determined.
Translate the RNA sequence CCAUUUACG into the corresponding amino acid sequence.
Pro-Phe-Thr
Note 5 —-»
Transcription is the transfer of genetic information from DNA to RNA.
Note 6 —-»
After separation of the two DNA strands, one strand serves as a template for a new RNA molecule; the other DNA strand is unused. The transcription enzyme RNA polymerase moves along the gene, forming a new RNA strand by following the base-pairing rules-but remember that in RNA, U replaces T. A specific nucleotide sequence called a promoter acts as a binding site for RNA polymerase and determines where transcription starts. RNA polymerase adds RNA nucleotides until it reaches a sequence of DNA bases called the terminator, which signals the end of the gene.
RNA Polymerase
A large molecular complex that links together the growing chain of RNA nucleotides during transcription, using a DNA strand as a template.
Terminator
A special sequence of nucleotides in DNA that marks the end of a gene.
How does RNA polymerase recognize the start and end of the gene?
Special DNA sequences mark the start (promoter) and end (terminator) of a gene.
