How DNA copies itself

Grade 9 Science Worksheet - Building Confidence

Circle the best answer.

1. 1.

   Ari watches DNA polymerase build a new strand on an unzipped ladder. Which letters belong in the two blanks, left to right?

   • U, then C
   • A, then G
   • T, then C
   • T, then G
2. 2.

   Bo sees DNA polymerase pause after building CATG. Which nucleotide does it add next?

   • T, which pairs with the old C
   • G, which pairs with the old C
   • C, which copies the old C
   • A, which pairs with the old C

3. Read this first

   Enzyme job card (textbook facts)

   enzyme	job at the fork
   helicase	breaks hydrogen bonds between paired bases so the helix opens
   DNA polymerase	adds nucleotides to the 3′ end of the growing new strand
   ligase	seals the gaps between pieces of the new strand
   primase	makes a short RNA primer that starts a new strand

4. 3.

   Jun reads the job card. Which enzyme unzips the double helix at the fork?

   • DNA polymerase, which breaks the hydrogen bonds between bases
   • Primase, which makes a short RNA primer
   • Helicase, which breaks the hydrogen bonds between bases
   • Ligase, which seals gaps in the new strand
5. 4.

   Kira reads the card. What does DNA polymerase do at the fork?

   • It builds without mistakes, so nothing checks the new strand
   • It copies the old letters onto the new strand
   • It opens the helix by breaking the hydrogen bonds
   • It adds matching nucleotides to the 3′ end of the new strand

6. Proofreading card (textbook facts)

   step	what the polymerase does
   adds a base	pairs a new nucleotide with the old strand
   reads the base	checks that the new base paired correctly
   fixes a wrong base	releases the wrong nucleotide and adds the correct one

7. 5.

   Luz reads the proofreading card. DNA polymerase finds a wrong base in the new strand. What does it do?

   • It releases the wrong nucleotide and adds the correct one
   • It changes the old strand to match the wrong base
   • It stops, and the cell starts over with new strands
   • It leaves the wrong base, because copying makes no mistakes

8. Three ways to copy a double helix (model data)

   model	what each new double helix holds
   W	short old and new pieces mixed along both strands
   X	one helix holds both old strands and the other holds both new strands
   Y	each helix holds one old strand and one new strand

9. 6.

   Milo compares three models of copying DNA. Which model is semiconservative replication, and why?

   • Model X, because the old strands stay together
   • Model Y, because each copy keeps one old strand
   • Model W, because old and new pieces are mixed
   • Model Y, because each copy is made of two new strands
10. 7.

    Noah copies the old strand TACGGA once. Which choice describes the partner strand in copy 1?

    • ATGCCT, a new strand built by pairing
    • TACGGA, a new strand that repeats the old letters
    • AUGCCU, a new strand that uses U in place of T
    • ATGCCT, the other old strand that stayed beside it

11. Strand count after one round (model data)

    molecule	old strands	new strands
    both copies together	2	2
    copy 1	?	?

12. 8.

    Pia reads the strand count after one round of copying. What fills the two ? cells for copy 1?

    • 0 old strands and 2 new strands
    • 2 old strands and 2 new strands
    • 1 old strand and 1 new strand
    • 2 old strands and 0 new strands
13. 9.

    Ren watches the new strand ATG grow. Which end does DNA polymerase add the next nucleotide to?

    • The 3′ end of the old strand, on the left
    • The 5′ end of the new strand, on the left
    • The 5′ end of the old strand, on the right
    • The 3′ end of the new strand, on the right
14. 10.

    Zee sees the new strand 5′ ATG 3′ on the old strand 3′ TACGG 5′. The polymerase adds one nucleotide. What does the new strand read now?

    • 5′ CATG 3′
    • 5′ ATGG 3′
    • 5′ ATGC 3′
    • 5′ ATGU 3′