10.15 - 10.16 Flashcards
Note 1 —-»
Translation (RNA→protein) can be divided into four steps, all of which occur in the cytoplasm. When the polypeptide is complete at the end of step 5, the two ribosomal subunits come apart, and the tRNA and mRNA are released. Translation is rapid; a single ribosome can make an average-sized polypeptide in less than a minute. Typically, an mRNA molecule is translated simultaneously by a number of ribosomes. Once the start codon emerges from the first ribosome, a second ribosome can attach to it; thus, several ribosomes may trail along on the same mRNA molecule. As it is made, a polypeptide coils and folds, assuming a three-dimensional shape, its tertiary structure. Several polypeptides may come together, forming a protein with quaternary structure
Which of the types of nucleic acids you’ve learned about does not participate directly in translation?
DNA
Note 2 —-»
Many inherited traits can be understood in molecular terms. For instance, sickle-cell disease results from a change in a single amino acid in one of the polypeptides in the hemoglobin protein. This difference is caused by a single nucleotide difference in the DNA coding for that polypeptide. In the double helix, one nucleotide pair is changed. Any change to the genetic information of a cell or virus is called a mutation.
Mutation
A change in the genetic material of the cell.
Note 3 —-»
A nucleotide substitution is the replacement of one nucleotide and its base-pairing partner with another pair of nucleotides.
Silent Mutation
A mutation in a gene that changes a codon to one that codes for the same amino acid as the original codon.
Missense Mutation
A change in the nucleotide sequence of a gene that alters the amino acid sequence of the resulting polypeptide.
Note 4 —-»
Some missense mutations have little or no effect on the resulting protein, but others, as in the case of sickle-cell disease, prevent the protein from performing its normal function. Nonsense mutations change an amino acid codon into a stop codon. For example, if an AGA (Arg) codon is changed to a UGA (stop) codon, the result will be a prematurely terminated protein, which probably will not function properly.
Note 5 —-»
Because mRNA is read as a series of nucleotide codons (triplets) during translation, adding or subtracting nucleotides may alter the reading frame (triplet grouping) of the genetic message. Such a mutation, which is called a frameshift mutation, occurs whenever the number of nucleotides inserted or deleted is not a multiple of three. All the nucleotides after the insertion or deletion will be regrouped into different codons.
Frameshift Mutation
A change in the genetic material that involves the insertion or deletion of one or more nucleotides in a gene, resulting in the triplet grouping of nucleotides.
Note 6 —-»
Spontaneous mutations result from errors during DNA replication or recombination. Other mutations are caused by physical or chemical agents called mutagens. High-energy radiation, such as X-rays or ultraviolet light, is a physical mutagen. Some chemical mutagens are molecules that are similar to normal DNA bases but disrupt DNA replication.
Mutagen
A chemical or physical agent that interacts with DNA and causes a mutation.
Note 7 —-»
Occasionally, a mutation leads to a protein that enhances the success of the mutant organism and its descendants. Much more often, mutations are harmful to an organism. Mutations are, however, an important source of the rich diversity of genes in the living world, a diversity that makes evolution by natural selection possible.
How could a single nucleotide substitution result in a shortened protein product?
A substitution that changed an amino acid codon into a stop codon would produce a prematurely terminated polypeptide.
