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Chapter 1, 15, 16, 18, 21-23

BIOL 243 Chapter Notes - Chapter 1, 15, 16, 18, 21-23: Codling Moth, Genetic Variation, Dna Replication


Department
Biology
Course Code
BIOL 243
Professor
Gordon Chua
Chapter
1, 15, 16, 18, 21-23

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Theme 3 Note s
Se ction 1.4: Evolution
- Ce lls from the ba s ic unit of a ll life: DNA, RNA, a nd prote ins carry out the molecular functions
of a ll ce lls ; and me ta bolic rea ctions build a nd brea k down ma cromolecules
- Both the unity and the diversity of life are e xplaine d by the proce s s of evolution, or change
ove r time
- When the re is va riation within a population of orga nisms , and whe n tha t va ria tion ca n be
inhe rite d (tha t is , when it ca n be pa ss e d from one genera tion to the next), the variants be st
suited for growth and reproduction in a given e nvironme nt will contribute dis proportionate ly to
the ne xt ge ne ra tion. Darwin re a lize d tha t farme rs have us e d this principle for thousa nds of
years to se lect for crops with high yie ld or improved re s istance to drought a nd dise a s e
- The ca pa city for Darwinian evolution ma y be life’s mos t fundame nta l prope rty
- Variation is s o commonplace , it ha rdly ga ins any a tte ntion
- Variation that ca n be inherited provide s the raw ma terial on which evolution acts
- The ca us es of va riation among individua ls within a s pe cies a re us ua lly groupe d into two
broa d ca tegories:
- Variation among individua ls is s ome times due to diffe re nces in the environme nt; this is
ca lle d environme nta l varia tion. Exa mple : among apples on the same tree some ma y ha ve
good expos ure to sunlight; some ma y be higher in the s ha de ; some were lucky e nough to
es ca pe the fema le codling moth, whos e egg de ve lops into a cate rpillar tha t ea ts its way
into the fruit
- The other ma in caus e of varia tion a mong individua ls is differe nce s in the ge ne tic ma te ria l
tha t is tra nsmitted from pa re nts to offspring; this is known a s gene tic variation. Differe nces
among individuals’ DNA ca n le a d to differe nces among the individuals’ RNA a nd prote ins ,
which affe ct the molecula r functions of the ce ll and ultimate ly ca n le a d to physica l
diffe re nces tha t ca n be obs erve d
- In all se xual orga nisms , fe rtilization produce s unique combinations of ge nes , which e xpla ins
in pa rt why sis te rs a nd brothe rs with the same pa re nts ca n be s o differe nt from one anothe r
- Ge ne tic varia tion a rises from muta tions. Muta tions a ris e e ithe r from ra ndom errors during
DNA replica tion or from environme ntal fa ctors such as UV radiation, which ca n dama ge DNA.
If the se mutations a re not corre cte d, the y a re pa s s e d on to the ne xt ge ne ration
- In nature, most mutations that ha rm growth and reproduction die out afte r a ha ndful of
genera tions
- Mutations tha t a re ne ithe r harmful nor be neficial ca n pe rsis t for hundre ds or thous a nds of
genera tions . Muta tions that are be neficial to growth and re production can gra dua lly be come
incorpora te d into the ge ne tic make up of e very individua l in the species . Tha t is how evolution
works : The ge ne tic ma keup of a population cha nge s over time
- Evolutionary the ory pre dicts tha t ne w s pe cie s arise by the diverge nce of populations through
time from a common ance s tor. As a re s ult, clos ely re lated s pe cies are like ly to res e mble ea ch
othe r more clos e ly than they do more dista ntly re lated species
- On a phyloge ne tic tree , the tips or bra nches re pres e nt the different groups of orga nisms,
nodes (whe re line s s plit) repre s e nt the most rece nt common ance stor (last common ance stor
— LCA), a nd time runs from left to right
- Evolutionary the ory pre dicts tha t primate s should show a nes te d pa ttern of simila rity, a nd this
is wha t morphologica l a nd molecular obs e rvations revea l
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- Biologists call the full s e t of e volutiona ry re la tionships among a ll orga nisms the tree of life
- The tree of life has 3 major bra nches repre s e nting the 3 doma ins (ba cte ria , archa e a ,
eukaryote s )
- The LCA of a ll living orga nisms , which form a root to the tree, is thought to lie be tween the
bra nch le a ding to Bacte ria a nd the bra nch lea ding to Archa e a and Euka rya
- The tree of life makes predictions for the orde r of appe a ra nce of differe nt life-forms in the
fos s il re cord. The gre ate r tre e also pre dicts that all re cords of a nima l life should be pre ce ded
by a long interval of microbial evolution
- Share d features sometime s imply inhe ritance from a common a ncestor
- Molecular studie s such a s comparisons of DNA se quence and foss ils s how tha t the close
simila rity be twe e n humans a nd chimpa nze es refle cts de s ce nt from a common a ncestor. The ir
diffe re nces reflect wha t Darwin ca lle d “descent with modification” — e volutiona ry cha nge s
tha t ha ve a ccumulated over time s ince the two linea ges s plit
- At a broa de r sca le, the funda me ntal feature s s ha re d by all orga nisms reflect inhe rita nce from
a common ance s tor that lived billions of yea rs ago, And the differe nce s that cha racte rize the
ma ny bra nche s on the tree of life ha ve forme d through the continuing action of e volution
since the time of our e arlie s t a nce s tors
- Nes te d patte rns of s imila rity among living orga nisms a nd the succes s ion of foss ils in the
geologic record fit the predictions of e volutiona ry the ory
- Bacte ria are ide a l to s e e evolution in a ction, be caus e of the ir short life s pan, and therefore ,
ra pidly incre a sing number of ge nera tion. The fact tha t the y live for a shorter period of time
me a ns that evolution works quicke r on them, making it e a s ier to se e evolution occurring
- Bacte ria are also ide a l to s tudy evolution in a ction beca us e the y re produce ra pidly a nd ca n
form populations with millions of individua ls. Large popula tion size me a ns tha t muta tions a re
like ly to form in ne arly e very genera tion, even though the proba bility that any individua l ce ll
will acquire a muta tion is s ma ll. (In contras t to bacte ria , think about trying evolutionary
expe rime nts on e le pha nts )
Se ction 15.1: Ge notype a nd Phe notype
- Without muta tions , e volution would not be pos s ible. Muta tions ge nera te the occa sional
favourable variants tha t allow orga nisms to evolve and become adapte d to their e nvironme nt
ove r time
- The ge netic makeup of a ce ll or orga nism constitutes its ge notype . a population with a ge ne
pool tha t has ma ny varia nts in ma ny differe nt ge ne s will cons ists of orga nisms with nume rous
diffe re nt ge notypes
- We differ from one pe rs on to the ne xt in a very la rge number but very sma ll fra ction of
nucle otide s
- Ge ne ticists us e the te rm polymorphism to re fer to a ny gene tic diffe re nce among individuals
tha t is s ufficiently common that it could almost ce rtainly be pre s e nt in a group of 50 ra ndomly
chos e n individua ls. For example , is ma ny people have a n A-T pa ir a t a particular site in the
genome, but ma ny othe rs ha ve a G-C pair a t the sa me site, this differe nce is a polymorphism.
Of course , the polymorphis m is the res ult of a muta tion. All individua ls once ha d the s a me
genotype at this s ite, but a muta tion occurre d a t this s ite s ome time in the pa st, and is now
commonly found in the popula tion
- Phenotype is an individual’s obs e rva ble chara cteris tics or traits, s uch a s height, we igh, e ye
color, and so forth. The phenotype ma y be visible, as in thes e chara cteristics, or ma y be s e e n
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in the de velopme nt, physiology, or beha viour of a ce ll or orga nisms . For example, colour
blindne ss and lactos e intolera nce a re phe notype s
- The phe notype res ults in pa rt from the ge notype: A ge notype with a muta te d ge ne for an
enzyme that would norma lly me ta bolize la ctos e ca n lead to the phe notype of lactos e
intolera nce. However, the e nvironme nt also commonly plays an important role, s o it is mos t
accura te to sa y tha t a phe notype re s ults from a n inte raction betwe e n the genotype a nd the
environme nt
- Beta globin is a subunit of he moglobin that ca rrie s oxyge n in re d blood ce lls
- The differe nt forms of a ny ge ne are ca lle d alle les, and the y corre s pond to diffe re nt DNA
se que nces (polymorphis ms) in the ge nes
- A s ma ll cha nge, such a s the following: one amino acid of the be taglobin prote in is affecte d,
this cha nge ca n have a dra ma tic effect on the function of the prote in, s ince the amino acid
se que nce de termines how a prote in folds , a nd prote in folding in turn de te rmine s the prote ins
function
- An individua l who inhe rits a n a lle le of the same type from e ach pa re nt is said to be
homozygous
- Individua ls who inhe rit differe nt type s of a lle les from the ir pare nts are he te rozygous
- When a muta tion occurs in the coding sequence of a ge ne it ma y ha ve no effect on the amino
acid seque nce , or it ma y re s ult in a cha nge in the amino acid sequence , introduce a stop
codon, or s hift the re a ding frame
- Ma ny of the muta tions that a lter a mino acid seque nce are ha rmful. Harmful muta tions are
ofte n eliminate d in one or a few ge nera tions be caus e the y decre a s e the survival a nd
the refore the ca pa city for reproduction of the individua ls tha t ca rry them
- Some time s , ha rmful mutations pers ist in a population
- For e xa mple, ma ny polymorphisms incre as e s usce ptibility to pa rticula r dise a s e s . One such
polymorphism incre a ses the risk of emphys e ma , a condition ma rked by loss of lung tis s ue .
Emphyse ma pa tients s uffer from s hortne ss of bre a th even whe n a t re st, a whe e zy cough,
and incre a s e d blood pres s ure in the a rte ries of the lungs. Without proper trea tme nt, the
dise as e progre s s e s to res piratory fa ilure or conge s tive hea rt failure , a nd eventua l de ath
- In ge notype -by-environme nt inte ra ctions, a phe notype is the re s ult of a n interplay be tween
gene s a nd the e nvironment
- Neutral muta tions are thos e which ha ve e ither no effe ct on the orga nis m, or ha ve effects tha t
are not a ss ociate d with diffe re nces in s urvival or reproduction
- Ma ny muta tions ha ve natura l effe cts on orga nisms be ca us e the y occur in noncoding DNA.
Neutral muta tions are there fore es pe cially like ly to occur in orga nisms with la rge ge nome s
and abundant noncoding DNA
- Some time s common, see mingly ha rmless ge netic variations occur in coding sequence
- One example in huma n populations is the tas te r phe notype as sociate d with pe rception
of a bitter taste from certa in che mica ls, including phe nylthiocarba mide (PTC). The
minima l conce ntration for P TC tasting varies a mong individua ls, but be ing able to taste a
conce ntra tion of 0.5 millimolar or le s s is often take n a s the cutoff be twe en the ta ste r a nd
conteste r phe notype s . The a bility or ina bility to tas te PTC is due la rge ly, but not
exclus ively, to a lle les of a single ge ne ca lle d TAS2R38 tha t encode s a test re ce ptor in
the tongue. Nonhuma n prima te s a re homozygous for an a lle le of this ge ne known a s the
PAV a lle le , so ca lle d be cause the prote in it e ncode s ha s the amino acids proline (P),
alanine (A), and valine (V) a t s pe cific pos itions . The most common a llele ass ociate d with
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