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Chapter 14

BIOL 243 Chapter Notes - Chapter 14: Peptide, Covalent Bond, Dna Mismatch Repair


Department
Biology
Course Code
BIOL 243
Professor
Gordon Chua
Chapter
14

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Chap te r 14 Note s
- Section 14.1: The Ra te a nd Nature of Muta tions
- Mos t mutations a re s ponta ne ous , occurring by cha nce in the a bse nce of a ny a s s ignable
ca us e . The y occur randomly, unconnecte d to a n organism’s ne e ds — it make s no
diffe re nce whe ther the muta tion would be nefit the orga nis m
- The most common muta tion is the s ubs titution of one nucle otide pair for a diffe re nt
nucleotide pa ir. Nucleotide s ubs titution muta tions a re ne verthe le ss re lative ly rare , and
the ir rate of occurre nce differs among orga nis ms
- Mos t mutations a re due to e rrors in re plica tion
- RNA virus e s a nd retrovirus es have a rela tively high rate of muta tion be cause RNA is a
le s s s ta ble mole cule than DNA, but more importantly be caus e the re plica tion of the s e
ge nome s lacks a proofreading function. For other genome s , the rate of mutation pe r
nucleotide pre DNA re plica tion re fle cts differe nce s in fidelity of replica tion a nd the
efficie ncy of proofre a ding a nd other mecha nis ms of DNA re pa ir
- Ave ra ging muta tion rate s conceals many de ta ils . First, ce rta in nucle otide s a re e s pe cia lly
prone to mutation and ca n e xhibit ra te s of muta tion that are gre a te r tha n the a verage by
a fa ctor of 10 or more . S ite s in the genome that are e s pe cia lly muta ble a re ca lle d
hots pots. Second, in s ome multicellula r orga nisms , the ra te s of muta tion differ be tween
the s e xes . Finally, the rate s for the multice llular a nima ls de pends on the type of ce ll: A
dis tinction must be ma de betwee n muta tions tha t occur in ge rm cells (haploid ga me te s
and the diploid ce lls tha t give ris e to the m) a nd muta tions that occur in s oma tic cells (the
othe r ce lls of the body). In mamma ls, the ra te of muta tion pe r nucleotide per re plica tion
is gre a te r in s omatic ce lls tha n in germ ce lls
- The rate of muta tion per ge nome per ge ne ra tion depends on the s ize of the ge nome a nd
the number of ce ll divisions pe r ge ne ra tion
- The rate of muta tion per nucle otide pe r re plica tion in mos t multice llular organisms is low
- Huma ns ha ve the s ma llest ra te of muta tion per nucle otide pe r re plica tion, huma ns a ls o
ha ve the la rgest rate of mutation pe r ge nome per ge nera tion. This para dox a ris e s
be ca us e huma ns ha ve a large genome a nd undergo ma ny cell divis ions per ge ne ra tion
- About 80% of the ne wly a ris ing muta tions in a zygote come from the fa the r. This is
be ca us e, in a huma n male at a ge 30, the diploid germ ce lls ha ve gone through a bout
400 cycle s of DNA re plica tion a nd ce ll divis ion be fore meios is take s pla ce , a s compare d
with about 30 cycle s in fe ma le s. More ove r, while the numbe r of ne wly a ris ing muta tions
from the mother rema ins a pproxima te ly cons ta nt with the mother’s age , the number of
ne wly a ris ing muta tions from the fa the r increas es with age
- Muta tions can take pla ce in a ny type of cell. Thos e that take pla ce in e ggs a nd s perm
and the ce lls that give ris e to them are ca lle d germ line muta tions a nd those in non-
re productive ce lls are calle d s oma tic muta tions . This dis tinction is important be caus e
soma tic muta tions a ffe ct only the individua l in which they occur — the y a re not
tra ns mitted to future ge ne ra tions beca us e they only occur in re productive ce lls
- For germ line muta tions, it is the ra te of muta tion pe r ge nome per ge nera tion that
ma tte rs more . Germ line mutations are importa nt to the e volutionary process a s the y a re
pa s s e d from one gene ra tion of orga nis m to the next, the y may eventua lly come to be
pre s e nt in ma ny individua ls de s cende d from the original carrie r
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- For somatic muta tions, the muta tion ra te that matters is the rate of muta tion pe r
nucleotide pe r replica tion. Although soma tic muta tions a re not trans mitted to future
ge ne ra tions , they a re tra ns mitte d to daughte r ce lls during mitos is. He nce , a s oma tic
muta tion affects not only the ce ll in which it occurs , but a lso all the ce lls tha t desce nd
from it
- Mos t cance r cells re s ults from muta tions in s oma tic cells. In s ome ca s e s, the muta tion
increas e s the a ctivity of a ge ne that promote s ce ll growth a nd divis ion, which in other
ca s e s , it de cre a s e s the activity of a gene that re s tra ins cell growth a nd divis ion. In e ither
event, the mutant ce ll and its desce ndants e s ca pe from one of the norma l control
proce s se s. Fortunate ly, a s ingle s oma tic muta tion is us ua lly not s ufficient to ca us e
ca nce r — us ua lly two or thre e more mutations in different ge ne s are required to de ra il
control of norma l ce ll divis ion s o e xtensive ly that ca ncer res ults
- To ca us e ca ncer, mutations mus t occur seque ntially in a single cell line . Thre e ke y
muta tions tha t have be e n implicate d in the origin of invasive colon cance r: p53, Ra s , a nd
AP C. Ea ch muta tion occurs randomly, but if, by chance , a mutation in the Ra s gene
occurs in a ce ll that is derive d from one in which the AP C ge ne ha s be e n mutate d, the
cell’s proge ny forms a polyp. Anothe r cha nce muta tion in the s ame ce ll line , which now
ca rries muta tions in both the APC a nd Ra s ge ne s , could lead to maligna nt ca ncer
- Each type of cancer is ca us e d by its own pa rticula r s equence of mutations , a lthough
muta tions in s ome gene s a re implica ted in s e ve ra l diffe re nt type s of cance r. An e xample
is the p53 ge ne, the non muta nt product of which de te cts DNA damage a nd s lows the
ce ll cycle to a llow time for DNA re pa ir. Muta tions in p53 a re one s te p in the muta tiona l
progre s sion of ma ny diffe re nt type s of ca nce r, including colon ca nce r and bre ast ca nce r
- The effe ct of a ge rm line mutation is therefore to re duce the number of a dditional
muta tions tha t would othe rwise be ne ce ss a ry to produce ca nce r ce lls
- Any mutation that incre a s e s the risk of dis e a s e in an individua l is known a s a genetic risk
factor for that dise a s e . For colon ca nce r, the major ge netic ris k factors are muta tions in
AP C, Ra s, a nd p53
- A risk fa ctor does not ca us e the dise a s e , but it make s the dis e ase more like ly to occur.
For the ge ne s implica te d in colon cancer, e a ch is a risk factor be caus e when it is
muta te d, fe we r a dditiona l muta tions are ne e de d to bring a bout tutor growth
- Not a ll ge ne tic ris k fa ctors are known for a ll dis e a s e s
- Your pe rs ona l ge nome ca n be of gre a t value in ide ntifying disea s e s for which you ca rry
ris k fa ctors
- Jos hua a nd Es ther Lederbe rg ca rrie d out a now-fa mous e xperiment:
- Bacte rial ce lls we re grown and forme d colonie s on a gar pla te in the a bs e nce of
antibiotic. Then, us ing replica pla ting, a te chnique they inve nte d, the Le de rbe rgs
tra ns fe rred the s e colonie s to new pla te s conta ining a ntibiotic. Only ba cte ria that were
re s ista nt to the a ntibiotic grew on the ne w plate s . Be cause re plica plating pre se rved
the a rra ngement of the colonies, the Le de rbergs were a ble to go ba ck to the original
pla te a nd ide ntify the colony tha t produce d the a ntibiotic-re s istant ba cteria
- Replica pla ting a llowed the Le derbergs to is olate a pure culture of a ntibiotic-res is ta nt
ba cte ria , e ve n though the origina l ba cteria never were e xpos e d to a ntibiotic. This
re s ult supporte d the first hypothe s is : The role of the environment is not to cre a te
spe cific mutations , but inste a d to s e le ct for the m. The principle the Le de rbe rgs
de monstrated is true of all orga nis ms s o fa r examine d
- Section 14.2: Small-S ca le Muta tions
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