NAME_________________________________
Date__________________________________
DNA Structure Review
Figure 1
1. The untwisted form of the DNA molecule looks like a ladder. The long strands that make up the sides of the DNA
molecule are made up of two kinds of chemical groups. Look at Figure 1. The shape with an S in it stands for the
__________________________ group.
2. The _____________________ group is represented by a circle with a P in it.
3. The steps in the center of the molecule are made from a pair of ___________________.
4. Each step is joined to the sides of the ladder by forming a bond with a ______________________ group. The
bases DO NOT connect to the ________________________groups.
5. On the sides of the ladder, each___________________________ group is followed by an alternating
______________________ group.
6. In figure 1, label the complementary bases.
Figure 2 is a short segment of one strand of DNA molecule. Label the parts indicated by the arrows.
7. ______________________
8. ______________________
9. ______________________
10. Let’s take a closer look at the steps of the DNA ladder. Each step is made of two chemical substances called bases.
Look at Figure 3. The bases are labeled ______________ and ___________________.
Figure 3
11. While all of the steps are made of two bases, not all are made from Adenine and Thymine. Some of the steps are
made of two other bases. Look at figure 4. It shows the base _______________ joined end to end with the base
___________ to form a rung.
Figure 4
Figure 5
12. Figure 5 shows a segment of a DNA molecule with five steps. The steps are labeled 1 to 5. Look at step #1 and #3.
Describe any differences you see between these steps. ____________________________________________________
__________________________________________________________________________________________________
13. In figure 5, do any of the steps contain the combination A-G? __________________________
14. Compare the connections where the two bases in each step join together. The connections were drawn to show that
“A” ALWAYS joins with base “_________” and the base “G” ALWAYS joins with base “_____________”.
15. There are four possible combinations of bases. Two of them are A-T….T-A….
Write the other two combinations ____________________________& __________________________________
16. Let us review. Which chemical groups form the long strands of the DNA molecule? Try not to look back…
______________________________________& _______________________________________
17. The _______________________________ ( steps / sides) of the DNA ladder are made of four bases A, T, G, C.
18. Name the four combinations of bases that appear in the DNA molecule.
______________________ ______________________
____________________
_______________________
19. DNA molecules control inheritance. We also know that genes control inheritance. Therefore, you can conclude that
there is a relationship between DNA and _________________________.
20. Only the bases, which form the steps of the DNA ladder, control inheritance. There are thousands of genes in any
one organism, such as a human being. Since there are only four bases, then one base ______________________
(could / could not) be one gene.
21. One base cannot be one gene, yet scientists believe that he four bases form thousands of genes. How is this
possible? One explanation is that each of the four combinations of bases is like a letter in out alphabet. Thousands of
words can be made out of the same set of 26 letters. If this explanation is correct, thousands of genes can be made from
the same set of four ____________________.
22. Look at the pairs of bases liste4d below. Circle the pairs that can occur as letters in the genetic code.
A-C
H-T
A-G
C-G
A-T
G-C
T-G
G-T
H-T
C-H
A-C
T-A
T-H
T-G
C-A
23. Although there are only 26 letters in the alphabet, hundreds of thousands of words can be formed by arranging the
letters in different orders. In a similar way, thousands of genes are formed when the four combinations of bases are
arranged in different ____________________________.
24. Figure 6 diagrams part of a DNA molecule. Fill in the circles and squares with the letters “S” and “P” to show the
correct location of sugar and phosphate groups.
Figure 6
25. Fill in each step of the ladder in Figure 6 with a pair of bases that can go together.
26. Which part of the DNA molecule contains the genetic code? ______________________________________
27 REVIEW
The DNA molecules in the chromosomes of animals and plants control heredity. The DNA molecules is a twisted
structure which, when untwisted, resembles a __________________________. The sides of the DNA molecule are long
threads or strands that are made up of alternating __________________________ and ________________________
groups. The steps are connected to the long strands of the DNA molecules and the ___________________groups. The
steps do not connect the __________________________ groups of the two strands. The steps of the ladder are made of
fours bases. They are designated by the letters ___________, _____________, ______________, and ____________.
The four letters only combine in two ways. “A” and “___________” always go together and “G” and “___________”
always pair together.