biol 3301 - genetics ch10a - dna structure st

8/18/2019 BIOL 3301 - Genetics Ch10A - DNA Structure St

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The Genetic Material Must

Exhibit Four Characteristics

• For a molecule to serve as the genetic

material, it must be able – to replicate

– to store information

– to express information

– to allow variation by mutation


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• The iscovery of the genetic role of !"#

began with research by Freeric$ Griffith in

%&'()• *e stuie Streptococcus pneumoniae, a

bacterium that causes pneumonia in mammals)

– +ne strain, the strain, was harmless – avirulent) – The other strain, the - strain, was pathogenic –

virulent)

Copyright© 2002 Pearson Education, Inc., publishing as Benjamin Cummings


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!"# #s Transforming #gent

• %&.., #very, Mac/eo, an McCarty

emonstrate that the transforming principle

was !"# an not protein


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Experiment of *ershey an

Chase• *ershey an Chase emonstrate that !"#,an not protein, enters the bacterial cell

uring bacteriophage infection an irects

viral reprouction)


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• 0n %&1', #lfre *ershey an Martha Chase

showe that !"# was the genetic material of the

phage T')• The T' phage, consisting almost entirely of !"#

an protein, attac$s Escherichia coli 2 E ) coli3, a

common intestinal bacteria of mammals)• This phage can 4uic$ly turn an E ) coli cell into

a T'5proucing factory that releases phages

when the cell ruptures)



6/30

Chemical -tructure of !"#

• "ucleoties are the builing bloc$s of !"#)

• They consist of

– a nitrogenous base – a pentose sugar, an

– a phosphate group)


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"itrogenous 6ases

• The nitrogenous bases can be purines or

pyrimiines)

•The purines contain 777777777777777777777

– aenine 2#3

– guanine 2G3)

• The pyrimiines contain 77777777777777777777

– cytosine 2C3 – thymine 2T3

– uracil 283


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-ugars

• # pentose sugar

– ibose

– !eoxyribose


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#ll Carbons 0n -ugars #re

"umbere• The C19 is the location of the phosphate

group on a nucleotie)


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"ucleoties

• # nucleosie contains the nitrogenous base

an the pentose sugar)

•# nucleotie is a nucleosie with a phosphategroup ae


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The !"# nucleotie• 6uiling bloc$ of !"#

• !eoxyribose 2pentose sugar3, with :9 –+*

• ;hosphate 2on 19 carbon3

• "itrogenous base

– purine

• aenine

• guanine

– pyrimiine

• thymine

• cytosine

P CO

N

N

base

sugar

5’

3’OH


12/30

The -tructure of !"# *ols the

>www)naftb)org>naftb>%&>conc

ept>inex)html 5 for animation


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Chagraff ules

• Erwin Chagraff – if it9s true, the number of

#,T, C, G shoul be e4ual in all organisms


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Chagraff9s ules

• Chargaff showe that

– the amount of # is proportional to T an

– the amount of C is proportional to G, but

– the percentage of C ? G oes not necessarily

e4ual the percentage of # ? T


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!"# Crystals @ere #nalyAe 6y B5

ay !iffraction• B5ray iffraction of !"# showe a :).

angstrom perioicity, characteristic of a

helical structure, '# iameter, etc


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@atson5Cric$ Moel

• %3 two long polynucleotie chains coile aroun a

central axis

• '3 the chains are anti5parallel 5 their C519 to C5:9orientations run in opposite irections

• :3 nitrogenous bases are perpenicular to the axis

of the helix= they are stac$e on top of one

another, :). angstroms apart, an are locate on

the insie of the structure


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@atson5Cric$ Moel

• .3 the nitrogenous bases of opposite chains are paire to one another as a result of the formationof hyrogen bons 5 only #5T an G5C bons

occur) • 13 Each complete turn of the helix is :. angstroms

long – % bases exist in each chain per turn

• D3 There is a maor groove an a minor grovealong the axis


18/30Copyright© 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• The phosphate

group of one

nucleotide is attached

to the sugar of the

next nucleotide in

line.

• The result is a“backbone” of

alternating

phosphates and

sugars, from whichthe bases project.


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• 0n aition, @atson an Cric$ etermine that

chemical sie groups off the nitrogen bases

woul form hyrogen bons, connecting the twostrans)

– 6ase on etails of their

structure, aenine woulform two hyrogen bons

only with thymine an

guanine woul form three

hyrogen bons only

with cytosine)

– )



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• The base5pairing rules ictate the combinationsof nitrogenous bases that form the rungs of!"#)

• *owever, this oes not restrict the se4uence ofnucleoties along each !"# stran)

• The linear se4uence of the four bases can bevarie in countless ways)

• Each gene has a uni4ue orer of nitrogen bases)



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!"# -tructure

• !"# strans are antiparallel

• !"# strans are complementary

• ;hosphoiester an hyrogen bons


22/30

"ucleoties

• "ucleoties are lin$e by a phosphoiester

bon between the phosphate group at the

777777777777777777777 an the +* group

on the 77777777777777777777)

•!"# stran is always going from

7777777777777


23/30

@atson5Cric$ Moel -uggest

!"# eplication Mechanism• 0t has not escape our notice that the

specific pairing we have postulate

immeiately suggests a possible copyingmechanism for the genetic material

@atson, Cric$, %&1:, "ature, H.:1D, pp

I:I5I:(


24/30

Three Forms of !"#

There are three forms of !"#=

%) The # or ehyrate 2ry3 form, use in

crystallography) This twists in a right hane helixan has %% base pairs per turn)

') The 6 or wet form, ' nanometers wie, twisting

in a right hane helix, with % base pairs per

turn):) The J form, similar to 6 but twisting in a left

hane helix) -ometimes create by mutation)


25/30

The -tructure of "# 0s

Chemically -imilar to !"#, but

-ingle -trane


26/30

"# -tructure

• 0n "#, the sugar ribose replaces

eoxyribose of !"# an uracil replaces

thymine)


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"# -tructure

• Most "#s are single strane although

some "#s form

777777777777777777777777 regions as they

fol into ifferent seconary structures)

• 0n aition, some viruses have a ouble5

strane "# genome)


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"# is Complementary to !"#

• #ll "#s originate as complementary

copies of one of the two strans of !"#

• !"# serves as the template for theirsynthesis)


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Types of "#

• There are three classes of cellular "#s=

– Messenger "# 2m"#3,

– ibosomal "# 2r"#3, an

– Transfer "# 2t"#3)


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Forms +f "#

• r"#s are components of ribosomes for

protein synthesis,

•m"#s are the 7777777777777777777for protein synthesis,

•t"#s carry 7777777777777777 for protein

synthesis)

biol 3301 - genetics ch10a - dna structure st

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