1.DNA MOLECULES ARE LONG POLYMERS MADE UP OF REPEATING NUCLEOTIDES.

2. A NUCLEOTIDE IS MADE OF A SUGAR (DEOXYRIBOSE), A PHOPHATE GROUP AND A NITROGEN BASE (ADENINE, THYMINE, GUANINE OR CYTOSINE).

3. ADENINE AND GUANINE ARE PURINES (DOUBLE RINGS). WHILE CYTOSINE AND THYMINE ARE PYRIMIDINES (SINGLE RINGS).

4. DNA IS MADE OF TWO STRANDS THAT ARE ANTIPARALLEL (RUN IN OPPOSITE DIRECTIONS). FREE PHOSPHATE END = 5’ END. THE FREE HYDROXIDE (OH) END = 3’ END. THE 5’ PHOSPHATE END REACTS WITH THE 3’ OH OF THE PREVIOUS NUCLEOTIDE.

5. NITROGEN BASES (A,T,C,G) ARE HELD TOGETHER BY HYDROGEN BONDS (= the rungs of the ladder).

6. The base-pairing rule states that thymine bonds with adenine while cytosine bonds with guanine.

1.Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”. 2. A eukaryotic chromosome may have hundreds or even thousands of origins of replication.

3. THE DNA IS UNWOUND AT VARIOUS POINTS WITH THE AID OF THE ENZYME HELICASE.

4. Replication proceeds in both directions from each origin, until the entire molecule is copied. 5. At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating

In eukaryotes, DNA replication begins at may sites along the giant DNA molecule of each chromosome. Two daughter DNA molecules Parental (template) strand Daughter (new) strand 0.25 µm Replication fork Origin of replication Bubble In this micrograph, three replication bubbles are visible along the DNA of a cultured Chinese hamster cell (TEM).

6. Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork New strand 5 end Phosphate Base Sugar Template strand 3 end 5 end3 end 5 end 3 end 5 end 3 end Nucleoside triphosphate DNA polymerase Pyrophosphate

7. The antiparallel structure of the double helix (two strands oriented in opposite directions) affects replication. DNA polymerases add nucleotides only to the free 3  end of a growing strand; therefore, a new DNA strand can elongate only in the 5  to  3  direction. -Also, single-stranded binding proteins (SSB’s) are added along the open strand of DNA to stabilize the strands allowing replication to occur.

8. Along one template strand of DNA, called the leading strand, DNA polymerase can synthesize a complementary strand continuously in the 5’ to 3’ direction, moving toward the replication fork. 9. To elongate the other new strand, called the lagging Strand (going from 3’ to 5’), DNA polymerase must work in the direction away from the replication fork The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase.

10. THE ENZYME PRIMASE MAKES A PRIMER RNA STRAND SO THE DNA POLYMERASE HAS A FREE 3’ OH TO BEGIN ITS BUILDING OF THE NEW DNA IN THE OKAZAKI FRAGMENTS.

Parental DNA 5 3 Leading strand Okazaki fragments Lagging strand DNA pol III Template strand Leading strand Lagging strand DNA ligase Template strand Overall direction of replication

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