The Molecular Basis of Inheritance

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Synthesis of leading and lagging strands during DNA replication

Slide 35

Priming DNA Synthesis

DNA polymerases cannot initiate the synthesis of a polynucleotide

They can only add nucleotides to the 3 end

The initial nucleotide strand

Is an RNA or DNA primer

Slide 36

Only one primer is needed for synthesis of the leading strand

But for synthesis of the lagging strand, each Okazaki fragment must be primed separately

Slide 37

Slide 38

Other Proteins That Assist DNA Replication

Helicase, topoisomerase, single-strand binding protein

Are all proteins that assist DNA replication

Slide 39

A summary of DNA replication

Slide 40

The DNA Replication Machine as a Stationary Complex

The various proteins that participate in DNA replication

Form a single large complex, a DNA replication “machine”

The DNA replication machine

Is probably stationary during the replication process

Slide 41

Proofreading and Repairing DNA

DNA polymerases proofread newly made DNA

Replacing any incorrect nucleotides

In mismatch repair of DNA

Repair enzymes correct errors in base pairing

Slide 42

Figure 16.17

Nuclease

DNA

polymerase

DNA

ligase

A nuclease enzyme cuts

the damaged DNA strand

at two points and the

damaged section is

removed.

Repair synthesis by

a DNA polymerase

fills in the missing

nucleotides.

3

DNA ligase seals the

Free end of the new DNA

To the old DNA, making the

strand complete.

In nucleotide excision repair

Enzymes cut out and replace damaged stretches of DNA

Slide 43

Replicating the Ends of DNA Molecules

The ends of eukaryotic chromosomal DNA

Get shorter with each round of replication

Slide 44

Eukaryotic chromosomal DNA molecules

Have at their ends nucleotide sequences, called telomeres, that postpone the erosion of genes near the ends of DNA molecules