Organic Chemistry

Section 27.9 Biosynthesis of DNA: Replication 049

thymine–adenine cytosine–guanine

sugar

sugar

major groove

minor groove

sugar

sugar

major groove

minor groove

H N

CH 3 N

N

H

H

N

N

O

O

O

N

N

O

H

N

H

N H N

N

N

H N

H

N

N

PROBLEM 13

Calculate the length of a turn in:

a. A-DNA b. B-DNA c. Z-DNA

27.9 Biosynthesis of DNA: Replication

Watson and Crick’s proposal for the structure of DNA was an exciting development
because the structure immediately suggested how DNA is able to pass on genetic in-
formation to succeeding generations. Because the two strands are complementary,
both carry the same genetic information. Both strands serve as templates for the
synthesis of complementary new strands (Figure 27.11). The new (daughter) DNA

3-D Molecules:

B-helix; A-helix; Z-helix

AT

TA TA

TA TA

TA TA

TA

AT

AT AT

AT

AT

AT AT

AT

CG

CG CG

CG

GC

GC

GC

GC GC

GC

GC

GC

GC

CG

parent strands

uncoiling

3 ′-end

3 ′-end 5 ′-end

5 ′-end^3 ′-end 3 ′-end

5 ′-end

5 ′-end

daughter

daughter strand

strand

>Figure 27.11

Replication of DNA. The daughter

strand on the left is synthesized

continuously in the

direction; the daughter strand

on the right is synthesized

discontinuously in the

direction.

5 ¿¡ 3 ¿

5 ¿¡ 3 ¿

>Figure 27.10

One side of each base pair faces

into the major groove, and the

other side faces into the minor

groove.