Chapter 17 (5)

Changes in the genetic information of a cell (or virus) are called __; they are responsible for the huge diversity of genes found among organisms because __ are the ultimate source of new genes.
Chemical changes in a single base pair of a gene are called __.

If a __occurs in a gamete or in a cell giving rise to gametes, it may be transmitted to offspring and to a succession of future generations. If the __ has an adverse effect on the phenotype of an organism, the mutant condition is referred to as a __ or __.

__ within a gene can be divided into two general categories: __ and __
A __ is the replacement of one nucleotide and its partner with another pair of nucleotides.

Some substitutions are called __ because, owing to the __ of the genetic code, they have no effect on the encoded protein. (A change in a base pair may transform one codon into another that is translated into the same amino acid.)

Substitutions that change one amino acid to another one are called __. Such a mutation may have little effect on the protein: The new amino acid may have properties similar to those of the amino acid it replaces, or it may be in a region of the protein where the exact sequence of amino acids is not essential to the protein’s function.

However, the __ of greatest interest are those that cause a major change in a protein. The alteration of a single amino acid in a crucial area of a protein- like in the __ of an enzyme- will significantly alter protein activity. Occasionally, such a mutation leads to an improved protein or one with novel capabilities, but much more often such mutations are detrimental, leading to a useless or less active protein that impairs cellular function.

Substitution mutations are usually __: the altered codon still codes for an amino acid and thus makes sense, although not necessarily the right sense.

A __can also change a codon for an amino acid into a stop codon. This is called a __, and it causes translation to be terminated prematurely; the resulting polypeptide will be shorter than the polypeptide encoded by the normal gene. Nearly all nonsense mutations lead to nonfunctional proteins.

__ and __are additions or losses of nucleotide pairs in a gene. These mutations have a disastrous effect on the resulting protein more often than substitutions do.

They may alter the __of the genetic message, the triplet grouping of bases on the mRNA that is read during translation.
· Such a mutation, called a __, will occur whenever the number of nucleotides inserted or deleted is not a multiple of three. All the nucleotides that are downstream of the deletion or insertion will be improperly grouped into codons, and the result will be extensive missense, usually ending sooner or later in nonsense and premature termination. Unless the __is very near the end of the gene, the protein is almost certain to be nonfunctional.

Mutations can arise many ways. Errors in DNA replication or recombination can lead to __, __, or __ as well as to mutations affecting longer stretches of DNA.

If an incorrect base is added to a growing chain during replication that base will then be mismatched with the base on the other strand. In many cases, the error will be corrected by systems. Otherwise, the incorrect base will be used as a template in the next round of replication, resulting in a __.
Such mutations are called __. It is difficult to calculate the mutation rate. About one nucleotide in every 10¹⁰ is altered, and the change is passed on to the next generation of cells.

A number of physical and chemical agents, called __, interact with DNA in ways that cause mutations.

In the 1920s, __ discovered that X-rays caused genetic changes in fruit flies, and he used X-rays to make fruit fly mutants for his genetic studies.
· But he also recognized an alarming implication of his discovery: X-rays and other forms of high-energy radiation pose hazards to the genetic material of people as well as laboratory organisms.

__, a physical mutagen, includes UV light, which can cause disruptive __in DNA.
Chemical mutagens fall into several categories. __ are chemicals that are similar to normal DNA bases but that pair incorrectly during DNA replication. Some other chemical mutagens interfere with correct DNA replication by inserting themselves into the DNA and distorting the double helix.

Still other mutagens cause chemical changes in bases that change their pairing properties.
Researchers have developed various methods to test the mutagenic activity of chemicals. A major application of these tests is the preliminary __of chemicals to identify those that may cause cancer. This approach makes sense because most __are mutagenic, and conversely, most mutagens are __.