Page 64 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

7.1 Mutations: Primary Tools of Genetic Analysis 223

lower than the rate of forward mutation holds true for Spontaneous Mutations Arise
most types of mutations. In one extreme example, dele- from Random Events
tions of more than a few nucleotide pairs can never revert
because DNA information that has disappeared from the Because spontaneous mutations affecting a gene occur so
genome cannot reappear spontaneously. infrequently, it is difficult to study the events that produce
them. To overcome this problem, researchers turned to
bacteria as the experimental organisms of choice. It is easy
Higher mutation rate in human sperm than to grow many millions of individuals and then search rap-
in human eggs idly through enormous populations to find the few that
New technologies that will be explained in detail in later carry a novel mutation. In one study, investigators spread
chapters have enabled researchers to determine the DNA wild-type bacteria on the surface of agar containing suffi-
sequence of the entire genome of thousands of people. By cient nutrients for growth as well as a large amount of a
comparing the genome sequences of parents and their chil- bacteria-killing substance, such as an antibiotic or a bacte-
dren, scientists have measured the human mutation rate riophage. Although most of the bacterial cells died, a few
with great precision. They found that the average value is showed resistance to the bactericidal substance and con-
about one mutation per hundred million base pairs (bp) per tinued to grow and divide. The descendants of a single
−8
gamete (or 1 × 10 ). Because the haploid human genome resistant bacterium, produced by many rounds of binary
9
is about 3 × 10 bp, each gamete contains on average about fission, formed a mound of genetically identical cells
30 mutations, and each child contains about 60 mutations— called a colony.
that is, 60 base pairs that are different than those in either The few bactericide-resistant colonies that appeared
of their parents’ genomes. You should note that this num- presented a puzzle. Had the cells in the colonies somehow
ber includes all DNA changes, only very few of which in- altered their internal biochemistry to produce a life-saving
fluence phenotype. response to the antibiotic or bacteriophage? Or did they
Interestingly, most of these 60-odd new mutations in carry heritable mutations conferring resistance to the bac-
each human are obtained from the sperm rather than the tericide? And if they did carry mutations, did those muta-
egg. Advances in genome sequencing technology have re- tions arise by chance from random spontaneous events that
cently made it possible to sequence the haploid genome take place continuously, even in the absence of a bacteri-
contained in a single sperm. (See the Fast Forward Box cidal substance, or did they only arise in response to
Crossovers Mapped in Chromosomes of Human Sperm in environmental signals (in this case, the addition of the
Chapter 5.) By comparing the genome sequences of more bactericide)?
than 100 individual sperm from the same person, the per- In 1943, Salvador Luria and Max Delbrück devised an
−8
bp mutation rate was found to be 2–4 × 10 , which indi- experiment to examine the origin of bacterial resistance
cates that most of the new mutations seen in children come (Fig. 7.5). According to their reasoning, if bacteriophage-
from the sperm rather than the egg. resistant colonies arise in direct response to infection by
The idea that sperm carry more mutations than bacteriophages, separate suspensions of bacteria contain-
oocytes makes sense. The reason is that more rounds of ing equal numbers of cells will generate similar, small
cell divisions are needed to produce human sperm than numbers of resistant colonies when spread in separate pe-
human eggs, presenting more opportunities for mutations tri plates on nutrient agar suffused with phages. By con-
to occur. Recall from Chapter 4 that human females are trast, if resistance arises from mutations that occur
born with essentially all of the primary oocytes they will spontaneously even when the phages are not present, then
ever produce. It has been estimated that the germ-line different liquid cultures, when spread on separate petri
cells of a female zygote need to undergo only 24 rounds plates, will generate very different numbers of resistant
of mitotic cell divisions to produce all of these oocytes. colonies. The reason is that the mutation conferring resis-
Male germ-line cells, on the other hand, undergo mitosis tance can, in theory, arise at any time during the growth of
continually throughout life. Starting from a male zygote, the culture. If the mutation occurs early, the cell in which
the number of cell divisions to generate a sperm at age 13 it happens will produce many mutant progeny prior to pe-
is estimated to be 36. After that, about 23 rounds of tri plating; if it happens later, far fewer mutant progeny
mitotic divisions occur per year in the male germ line, will be present when the time for plating arrives. After
meaning that at age 20, the cell lineage producing a given plating, these numerical differences will show up as fluc-
sperm has undergone 200 divisions; at age 30, 430; and tuations in the numbers of resistant colonies growing in
at age 45, 770. Therefore, in humans, most new muta- the different petri plates.
tions found in the progeny come from the sperm rather The results of this fluctuation test were clear: Most
than from the egg. Moreover, the older the father, the plates supported zero to a few resistant colonies, but a
more mutations are likely to be found in his sperm. few harbored hundreds of resistant colonies. From this