How DNA copies itself

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Use this

   Fork picture (model, not to scale). Each new strand pairs with the old strand beside it. Letters line up pair by pair from the left.

2. 1.

   Ari reads a fork that is opening toward the right. Which new strand is built in short pieces, and why?

   • Strand Y: its 3′ end points away from the fork, so it is built in pieces
   • Neither: polymerase builds both new strands in one piece
   • Strand X: its 3′ end points away from the fork, so it is built in pieces
   • Strand X: the polymerase adds to the 5′ end, so it must restart

3. Fork card (model data)

   strand	length in nucleotides	how it is built
   leading strand	120	one piece from one primer
   lagging strand	120	pieces of 40 nucleotides each with its own primer

4. 2.

   Bo counts the primers one fork needs to copy both strands. How many?

   • 6: one for each 20 nucleotides
   • 3: one for each lagging piece
   • 4: one for the leading strand and three for the lagging strand
   • 2: one for each strand

5. Use this

   Bands in the tube (textbook facts), 3 rows. Columns: generations in 14N, bands seen. 0, one band low in the tube (all 15N). 1, one band halfway between the 15N and 14N positions. 2, two bands: one halfway and one at the 14N position.

6. 3.

   Jun reads the Meselson and Stahl bands. Which model do the data from generation 1 alone rule out?

   • Conservative copying, which predicts a heavy band and a light band
   • Dispersive copying, which predicts a heavy band and a light band in the first round
   • Nothing, because conservative copying also makes a middle band
   • Semiconservative copying, which predicts a heavy band alone

7. Sample size (model data)

   generation in 14N	molecules in the sample
   0	100
   1	200
   2	400

8. 4.

   Kira applies the semiconservative model to the 400 molecules. How are they spread across the bands?

   • 100 in the middle band and 300 in the light band
   • 100 in the heavy band and 300 in the light band
   • 400 in a single band between the middle and light positions
   • 200 in the middle band and 200 in the light band

9. DNA in one cell (model data)

   stage	DNA units
   start of G1	6
   end of G1	6
   end of S phase	12
   end of mitosis	12
   each cell after cytokinesis	6

10. 5.

    Luz reads the DNA card for one cell. When did the amount of DNA double, and why?

    • In cytokinesis, because the cell splits and doubles its DNA
    • In S phase, because the cell copied its DNA
    • In G1, because the cell grows and makes more DNA
    • In mitosis, because the chromosomes are copied as they line up

11. Use this

    Notebook (textbook facts), 4 rows. Columns: line, note. W, Helicase breaks the hydrogen bonds between base pairs at the fork. X, DNA polymerase adds nucleotides to the 3′ end of the new strand. Y, After one round, one copy holds both old strands and the other holds both new strands. Z, Each new strand is built by pairing bases with an old strand.

12. 6.

    Milo checks a notebook about replication. One line is wrong. Which line, and why?

    • Line X: nucleotides are added to the 5′ end of the new strand
    • Line W: DNA polymerase is the enzyme that breaks those hydrogen bonds
    • Line Z: a new strand repeats the letters of its old strand
    • Line Y: each copy keeps one old strand and gets one new strand

13. Use this

    Notebook (textbook facts), 4 rows. Columns: line, note. W, Most copying errors are fixed by DNA polymerase as it works. X, Mismatch repair fixes some errors after copying is finished. Y, An error that no repair step fixes stays in the DNA as a mutation. Z, Pairing rules are strict, so a copy has no errors.

14. 7.

    Pia checks a notebook about copying errors. One line is wrong. Which line, and why?

    • Line W: DNA polymerase opens the helix, so it cannot fix errors
    • Line Y: a leftover error is repaired at the next round
    • Line X: repair happens during copying, not after it
    • Line Z: errors do happen, and repair fixes most of them

15. Mismatch repair card (textbook facts)

    strand	methyl groups on adenine
    old strand	present
    new strand	absent

16. 8.

    Ren reads the card about mismatch repair in E. coli. A mismatch is found after copying. Which claim and reason fit?

    • No repair is needed, because copying makes no mistakes
    • Repair removes the base from either strand at random, because both look alike
    • Repair removes the base from the old strand, because it has the methyl groups
    • Repair removes the base from the new strand, because only the old strand has methyl groups

17. Use this

    Each copy holds one old strand and one new strand. The figure does not say which is which. Letters line up pair by pair from the left.

18. 9.

    Tala wants to find the new strand in copy 1 from the letters alone. Which claim and reason are correct?

    • The letters can tell, because the new strand uses U in place of T
    • The letters cannot tell, because a new strand and the other old strand pair with the same old strand
    • The letters can tell, because the new strand is longer than the old strand
    • The letters can tell, because the new strand repeats its old strand's letters

19. Use this

    Finished copy before the next round (model, not to scale). No repair step fixed the fourth pair. Letters line up pair by pair from the left.

20. 10.

    Zee sees one pair that no repair step fixed. Which claim and reason fit the next round of copying?

    • Both new molecules carry the change, because each copies the leftover T
    • One new molecule keeps a changed pair, a mutation, because the leftover T is copied
    • Neither new molecule changes, because polymerase proofreads the template first
    • Both new molecules stay unchanged, because the old strand overrides the new strand