Concept 16.2: Many proteins work together in DNA replication and repair

It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.

–J.D. Watson and F.H.C. Crick, Molecular structure of nucleic acids: a structure for deoxyribose nucleic acids, Nature 171:737-738

(1953).

The Basic Principle: Base Pairing to a Template Strand

0Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication.

0In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules.

Figure 16.9-3

(a) Parent molecule (b) Separation ofstrands

(c)“Daughter” DNA molecules,each consisting of oneparental strand and onenew strand

A

A

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C

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G

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(a) Conservativemodel

(b) Semiconservativemodel

(c) Dispersive model

Parentcell

Firstreplication

Secondreplication

0Experiments by Matthew Meselson and Franklin Stahl supported the semiconservative model .

0Experiment: They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope.

0Results:0The first replication produced a band of hybrid DNA,

eliminating the conservative model.0A second replication produced both light and hybrid

DNA, eliminating the dispersive model and supporting the semiconservative model.

© 2011 Pearson Education, Inc.

Figure 16.11

Bacteriacultured inmedium with15N (heavyisotope)

Bacteriatransferred tomedium with14N (lighterisotope)

DNA samplecentrifugedafter firstreplication

DNA samplecentrifugedafter secondreplication

Less dense

More dense

Predictions: First replication Second replication

Conservativemodel

Semiconservativemodel

Dispersivemodel

21

3 4

EXPERIMENT

RESULTS

CONCLUSION

Replication Stations

10 minutes at each station.Make sure to complete your notes packet.

Replicating the Ends of DNA Molecules

0Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes.

0The usual replication machinery provides no way to complete the 5 ends.0Once the RNA primer is removed from the 5’ end, it cannot

be replaced with DNA because there is no 3’ end for nucleotide addition.

0Repeated rounds of replication produce shorter and shorter DNA molecules with uneven ends.

Figure 16.20

Ends of parentalDNA strands

Leading strandLagging strand

Last fragment Next-to-last fragment

Lagging strand RNA primer

Parental strand Removal of primers andreplacement with DNAwhere a 3 end is available

Second roundof replication

Further roundsof replication

New leading strand

New lagging strand

Shorter and shorter daughter molecules

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0Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called telomeres.

0Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules.

0It has been proposed that the shortening of telomeres is connected to aging.

Telomeres are stained orange in this mouse chromosome.

An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells (to prevent essential genes from going missing in the gametes).

0The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions.

0There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist.