Page 87 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

246 Chapter 7 Anatomy and Function of a Gene: Dissection Through Mutation

Figure 7.25 Fine structure mapping of the bacteriophage T4 rII genes. (a) A phage cross between a point mutation and a
deletion removing the DNA at the position of the mutation cannot yield wild-type recombinants. The same is true if two different deletion
mutations overlap each other. (b) Large deletions divide the rII locus into regions; finer deletions divide each region into subsections. Point
mutations, such as 271 (in red at bottom), map to region 3 if they do not recombine with deletions PT1, PB242, or A105 but do recombine
with deletion 638 (top). Point mutations can be mapped to subsections of region 3 using other deletions (middle). Recombination tests map
point mutations in the same subregion (bottom). Point mutations 201 and 155 cannot recombine to yield wild-type recombinants because
they affect the same nucleotide pair. (c) Benzer’s fine structure map. Hotspots are locations with many independent mutations that cannot
recombine with each other.
(a) Using deletions for rapid mapping (b) Portion of the rIIA deletion map at increasing resolutions
Point mutation within Point mutation outside
deletion limits deletion limits
PT1
m m
PB242
A105
Overlapping deletions Nonoverlapping deletions Region missing
in deletion
638
Cannot produce wild-type Produce wild-type
progeny by recombination progeny by recombination

Regions 1 2 3 4

(c) Fine structure of the rII region PT8
164
Each box represents an independent
occurrence of a mutation at this site.
H88
PB82

Subsections A B C D E

201 155 Point
271 279 240 mutations
0.15 0.055 0.12 Map units
Many mutations
at a site create a B cistron A cistron
hotspot. Fine structure
of subsection

neither chromosome carries the proper information at the region deleted from the other phage chromosome, providing
location of the mutation. However, if the mutation lies out- a rapid way to find the general location of a mutation.
side the region deleted from the homologous chromosome, Using a series of overlapping deletions, Benzer di-
wild-type progeny can appear (Fig. 7.25a). This is true vided the rII region into a series of relatively small regions,
whether the mutation is a point mutation affecting one or a or intervals. He could then assign any point mutation to an
few nucleotides, or is itself a large deletion. Crosses be- interval by observing whether the point mutation recom-
+
tween any uncharacterized mutation and a known deletion bined to give rII progeny when crossed with the series of
thus immediately reveal whether the mutation resides in the deletions (Fig. 7.25b).