DNA Replication

Lesson 2

• Label the ends of DNA strand as either 5’ or 3’.

5’

Leading Strand

5’

5’

3’

3’

helicase

SSBPs

gyrase

primase

5’

3’

DNA polymerase III

The leading strand is synthesized continuously during DNA replication

Lagging Strand

Okazaki fragment

5’

3’

3’

5’

5’

3’

The laggin strand is synthesized discontinuously during DNAreplication

Connecting Lagging Strands

• DNA polymerase I – removes the RNA primer

• DNA ligase – connects the sugar-phosphate backbone of Okazaki fragments

Mistakes in DNA Replication

• less than 1 error in 107 (10 million) NTs

• exonuclease – enzymes which can cut out sections of the DNA strand

• DNA polymerase I and III and exonuclease work together to fix mistakes

Nuclease

DNApolymerase

DNAligase

A distorted section of DNA.1

A nuclease enzyme cutsthe damaged DNA strandat two points and thedamaged section isremoved.

2

Repair synthesis bya DNA polymerasefills in the missingnucleotides.

3

DNA ligase seals theFree end of the new DNATo the old DNA, making thestrand complete.

4

Replicating the Ends of DNA Molecules

• The ends of eukaryotic chromosomal DNA– Get shorter with each

round of replication

Figure 16.18

End of parentalDNA strands

Leading strandLagging strand

Last fragment Previous fragment

RNA primer

Lagging strand

Removal of primers andreplacement with DNAwhere a 3 end is available

Primer removed butcannot be replacedwith DNA because

no 3 end availablefor DNA polymerase

Second roundof replication

New leading strand

New lagging strand 5

Further roundsof replication

Shorter and shorterdaughter molecules

5

3

5

3

5

3

5

3

3

• Eukaryotic chromosomals– Have repeated short nucleotide sequences, called

telomeres at their ends that do not code for amino acids.

– postpone the erosion of genes near the ends of DNA molecules (e.g. in humans 100-1,000 TTAGGG)

Figure 16.19 1 µm

• Some cells (like cervical cancer cells) have an enzyme called telomerase– Lengthens the telomeres, thus restoring original

length and compensating for shortening that occurs during DNA replication.

– Contributes to the “indestructible” nature of cancer cells

Explain the function of each of the following enzymes in DNA replication:

DNA gryase Relieves the tension produced by the unwinding DNA during replication

DNA helicase Unwinds the DNA double helix by disrupting hydrogen bonds between base pairs

DNA polymerase I

Removes RNA primers and replaces them with the appropriate deoxyribonucleotides (DNA bases) during DNA replication

DNA polymerase III

Makes complementary strands of DNA (adds deoxyribonucleoside triphosphates to the 3’ end of the elongating strand

DNA ligase Joins DNA fragments together by catalyzing the formation of a bond between the 3’ hydroxyl group and a 5’ phosphate group on the sugar-phosphate backbones

RNA primase Makes RNA primers

• Review from previous lessons– Pg. 198 Section Review - #2, 3, 7 (info from

previous lessons)

• Material from today’s lesson– Pg. 204 Section Review - #2– Pg. 205 Understanding Concepts - #8, 9, 10,

15, 18, 25, 33, 42