DNA Replication: A Closer Look The copying of DNA is remarkable in its speed and accuracy More than...

DNA Replication: A Closer Look

• The copying of DNA is remarkable in its speed and accuracy

• More than a dozen enzymes and other proteins participate in DNA replication

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Getting Started

• Replication begins at special sites called _____________________, where the two DNA strands are separated, opening up a ______________________


Animation: Origins of ReplicationAnimation: Origins of Replication

Fig. 16-12a

Origin of replication Parental (template) strand

Daughter (new) strand

Replication fork

Replication bubble

Double-stranded DNA molecule

Two daughter DNA molecules

(a) Origins of replication in E. coli

0.5 µm

Fig. 16-12b

0.25 µm

Origin of replication Double-stranded DNA molecule

Parental (template) strandDaughter (new) strand

Bubble Replication fork

Two daughter DNA molecules

(b) Origins of replication in eukaryotes

• Replication fork

• Helicases

• Single-strand binding protein

• Topoisomerase

• Primer

• Primase


Fig. 16-13

Topoisomerase

Helicase

PrimaseSingle-strand binding proteins

RNA primer

55

5 3

3

3

STOPPED HERE!

Synthesizing a New DNA Strand

• Enzymes called ___________________ catalyze the elongation of new DNA at a replication fork

• Most DNA polymerases require a _________ and a ________________


Fig. 16-14

A

C

T

G

G

G

GC

C C

C

C

A

A

AT

T

T

New strand 5 end

Template strand 3 end 5 end 3 end

3 end

5 end5 end

3 end

Base

Sugar

Phosphate

Nucleoside triphosphate

Pyrophosphate

DNA polymerase

Antiparallel Elongation

• DNA has an antiparallel structure

• DNA polymerases add nucleotides only to the free 3end of a growing strand; therefore, a new DNA strand can elongate only in the 5to3direction


Fig. 16-15

Leading strand

Overview

Origin of replicationLagging strand

Leading strandLagging strand

Primer

Overall directions of replication

Origin of replication

RNA primer

“Sliding clamp”

DNA poll IIIParental DNA

5

3

3

3

3

5

5

5

5

5

Fig. 16-16Overview


Leading strand

Leading strand

Lagging strand

Lagging strand


Template strand

RNA primer

Okazaki fragment

Overall direction of replication

12

3

2

1

1

1

1

2

2

51

3

3

3

3

3

3

3

3

3

5

5

5

5

5

5

5

5

5

5

53

3

Fig. 16-16a

Overview


Leading strand

Leading strand

Lagging strand

Lagging strand


12

Fig. 16-16b1

Template strand

5

53

3

Fig. 16-16b2

Template strand

5

53

3

RNA primer 3 5

5

3

1

Fig. 16-16b3

Template strand

5

53

3

RNA primer 3 5

5

3

1

1

3

35

5

Okazaki fragment

Fig. 16-16b4

Template strand

5

53

3

RNA primer 3 5

5

3

1

1

3

35

5

Okazaki fragment

12

3

3

5

5

Fig. 16-16b5

Template strand

5

53

3

RNA primer 3 5

5

3

1

1

3

35

5

Okazaki fragment

12

3

3

5

5

12

3

3

5

5

Fig. 16-16b6

Template strand

5

53

3

RNA primer 3 5

5

3

1

1

3

35

5

Okazaki fragment

12

3

3

5

5

12

3

3

5

5

12

5

5

3

3

Overall direction of replication

Table 16-1

Fig. 16-17

OverviewOrigin of replication

Leading strand

Leading strand

Lagging strand

Lagging strandOverall directions

of replication

Leading strand

Lagging strand

Helicase

Parental DNA

DNA pol III

Primer Primase

DNA ligase

DNA pol III

DNA pol I

Single-strand binding protein

5

3

5

5

5

5

3

3

3

313 2

4

http://www.dnalc.org/resources/3d/04-mechanism-of-replication-advanced.html

http://www.dnalc.org/resources/3d/04-mechanism-of-replication-advanced.html

Proofreading and Repairing DNA

• DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides

• Mismatch repair: ______________________________________________________________________________

• Nucleotide excision repair: ______________________________________________________________________________


Fig. 16-18

Nuclease

DNA polymerase

DNA ligase

Replicating the Ends of DNA Molecules

• Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes

• The usual replication machinery provides no way to complete the 5 ends, so repeated rounds of replication produce shorter DNA molecules


Fig. 16-19

Ends of parental DNA strands

Leading strand

Lagging strand

Lagging strand

Last fragment Previous fragment

Parental strand

RNA primer

Removal of primers and replacement with DNA where a 3 end is available

Second round of replication

New leading strand

New lagging strand

Further rounds of replication

Shorter and shorter daughter molecules

5

3

3

3

3

3

5

5

5

5

• Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called _______________________

• Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules

• It has been proposed that the shortening of telomeres is connected to aging


Fig. 16-20

1 µm

• ________________– cells that make gametes in the ovaries and testes

• What would happen if chromosomes of germ cells became shorter with every cell cycle?

• An enzyme called ________________ catalyzes the lengthening of telomeres in germ cells


• The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions

• There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist


How DNA is packaged

• http://www.dnalc.org/resources/3d/08-how-dna-is-packaged-advanced.html

You should now be able to:

1. Describe the contributions of the following people: Griffith; Avery, McCary, and MacLeod; Hershey and Chase; Chargaff; Watson and Crick; Franklin; Meselson and Stahl

2. Describe the structure of DNA

3. Describe the process of DNA replication; include the following terms: antiparallel structure, DNA polymerase, leading strand, lagging strand, Okazaki fragments, DNA ligase, primer, primase, helicase, topoisomerase, single-strand binding proteins


4. Describe the function of telomeres

5. Compare a bacterial chromosome and a eukaryotic chromosome


DNA Replication: A Closer Look The copying of DNA is remarkable in its speed and accuracy More than...

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