REVIEW

DNA Structure

Deoxyribonucleic Acid

DNADeoxyribose sugarDouble helixA -2-T, C-3-GStrands are complementaryPurines: A and GPyrimidines: T and C

Ribonucleic Acid

RNARibose SugarSingle-strandedA-2-U, C-3-GPurines: A and GPyrimidines: U and C

DNA Replication

Many proteins work together in DNA rep and repair

The two strands are complementary, each will act as a template for the new strand in rep

(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

(b) The first step in replication is separation of the two DNA strands.

(c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

(d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

A

C

T

A

G

A

C

T

A

G

A

C

T

A

G

A

C

T

A

G

T

G

A

T

C

T

G

A

T

C

A

C

T

A

G

A

C

T

A

G

T

G

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T

C

T

G

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G

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G

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T

C

Figure 16.9 a–d

DNA Replication is semiconservative

Each of the two new daughter molecules will have one old strand (parent strand) and one newly made strand

Figure 16.10 a–c

Conservativemodel. The twoparental strandsreassociate after acting astemplates fornew strands,thus restoringthe parentaldouble helix.

Semiconservativemodel. The two strands of the parental moleculeseparate, and each functionsas a templatefor synthesis ofa new, comple-mentary strand.

Dispersivemodel. Eachstrand of bothdaughter mol-ecules containsa mixture ofold and newlysynthesizedDNA.

Parent cellFirstreplication

Secondreplication

Origins of Replication

Sites where the two strands are separated Unwound by Helicase

Euks: may have hundreds or even thousandsStabilized by SSBPs until replication is

complete

Elongation

Begins at the replication fork

Is catalyzed by DNA polymerases, which add nucleotides to the 3’ of a growing strand

Figure 16.13

New strandTemplate strand5 end 3 end

Sugar A TBase

C

G

G

C

A

C

T

PP

P

OH

P P

5 end 3 end

5 end 5 end

A T

C

G

G

C

A

C

T

3 endPyrophosphate

2 P

OH

Phosphate

Leading vs. Lagging

DNA poly only add nucleotides to the 3’ endsLeading strand: the strand on which DNA

polymerase can synthesize a comp strand continuously, moving toward the replication fork

Lagging strand: DNA polymerase must work in the opposite direction, away from the fork, synthesizing Okazaki fragments Joined together by DNA ligase

Initiation

DNA poly cannot initiate rep, since they can only add nucleotides to the 3’ end

And RNA primer is required, synthesized by Primase One for the leading strand Multiple for lagging strand, each Okazaki fragment

must be primed separately

DNA Replication “Machine”

Various proteins participate in rep DNA poly, helicase, ligase, etc

They form a single, large complex or machine

Proofreading and Repairing DNA

DNA poly proofread newly synthesized DNA an replace any incorrect nucleotides

Excision repair: enzymes cut out and replace damaged stretches of DNA

Replicating the Ends of DNA Molecules

The ends of euk chromosomal DNA get shorter with each round of replication

So, the ends are made of repeating nucleotide sequences called telomeres Postpone the erosion of genes near the ends of DNA

moleculesIn germ, cancer, stem cells:

Telomerase catalyzed the lengthening of telomeres to prevent the loss of essential genes

Teach it to a 7th Grader

Draw a flow chart that uses analogies to explain the roles of each protein/enzyme involved

Must have drawings and explanationsAnalogy example:

Helicase : Pair of scissors