How DNA copies itself

Grade 9 Science Worksheet

Circle the best answer.

1. 1.

   Bo watches DNA polymerase extend the new strand TAG. Which nucleotide does it add next, and to which end?

   • C, added to the 3′ end
   • C, added to the 5′ end
   • A, added to the 3′ end
   • G, added to the 3′ end
2. 2.

   Esme copies an old strand written 5′ to 3′. Which is the new strand, written 3′ to 5′, left to right?

   • TAGCCG
   • UAGCCG
   • ATCGGC
   • GCCGAT

3. Read this first

   Copying one molecule (model data)

   round	molecules	molecules that keep an old strand
   0	1	1
   1	2	2
   2	?	?

4. 3.

   Jun follows one molecule through two rounds of copying. What fills the two ? cells for round 2?

   • 4 molecules; 2 keep an old strand
   • 4 molecules; 4 keep an old strand
   • 4 molecules; 1 keeps both old strands
   • 3 molecules; 2 keep an old strand

5. Strand count (model data)

   round	molecules	strands in all	old strands	new strands
   0	1	2	2	0
   1	2	4	2	2
   2	4	8	2	6
   3	8	16	2	?

6. 4.

   Luz counts strands, not molecules. What fills the ? cell for round 3?

   • 8, because each molecule has one new strand
   • 14, because 2 of the 16 strands are old
   • 12, because each round adds 6 new strands
   • 16, because the old strands are used up
7. 5.

   Milo checks a new strand against its old strand. Which pair is a mismatch that proofreading or repair would flag?

   • No pair needs fixing, because copying makes no mistakes
   • The first pair, old T across from new A
   • The third pair, old C across from new C
   • The fourth pair, old G across from new C
8. 6.

   Noah reads a new strand with one wrong base. Which pair is the mismatch, and which letter is removed to fix it?

   • The fifth pair; the new G is removed
   • The sixth pair; the new G is removed
   • The sixth pair; the old A is removed
   • The first pair; the new C is removed
9. 7.

   Pia reads a finished ladder. How many pairs are mismatches that proofreading or repair would flag?

   • Five: the pairs whose two letters differ
   • Zero: copying follows the pairing rules, so errors do not occur
   • One: the fourth pair
   • Two: the fourth and fifth pairs

10. Fork steps (textbook facts)

    step	what happens
    W	Ligase seals the gaps between pieces
    X	DNA polymerase extends the primer
    Y	Helicase opens the helix
    Z	Primase makes a short RNA primer

11. 8.

    Ren puts the steps at a replication fork in order. Which order is correct?

    • X, Y, Z, W
    • Y, Z, X, W
    • Z, Y, X, W
    • Y, X, Z, W

12. Proofreading steps (textbook facts)

    step	what the polymerase does
    W	Checks the new base against the old strand
    X	Adds a base to the new strand
    Y	Adds the correct base
    Z	Releases the wrong base

13. 9.

    Tala puts the steps of proofreading in order. Which order is correct?

    • X, Z, W, Y
    • X, W, Z, Y
    • W, X, Z, Y
    • X, W, Y, Z
14. 10.

    Zee follows one Okazaki fragment on the lagging strand. Which order is correct?

    • X, Y, W, Z
    • X, Z, Y, W
    • X, Y, Z, W
    • Y, X, Z, W