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 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
   • 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

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?

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

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?

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

   • 400 in a single band between the middle and light positions
   • 100 in the middle band and 300 in the light band
   • 200 in the middle band and 200 in the light band
   • 100 in the heavy band and 300 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 mitosis, because the chromosomes are copied as they line up
    • In G1, because the cell grows and makes more DNA

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 W: DNA polymerase is the enzyme that breaks those hydrogen bonds
    • Line Z: a new strand repeats the letters of its old strand
    • Line X: nucleotides are added to the 5′ end of the new 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, The cell copies its DNA in S phase, before mitosis begins. X, DNA polymerase checks each base it adds against the old strand. Y, Both new molecules have the same base sequence as the original. Z, DNA is copied during mitosis, while the chromosomes line up.

14. 7.

    Noah checks a notebook about the cell cycle and copying. One line is wrong. Which line, and why?

    • Line W: the cell copies its DNA while it divides
    • Line X: copying makes no mistakes, so nothing is checked
    • Line Z: the cell copies its DNA in S phase, before mitosis
    • Line Y: the original molecule stays whole and the other is new

15. 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.

16. 8.

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

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

17. Mismatch repair card (textbook facts)

    strand	methyl groups on adenine
    old strand	present
    new strand	absent

18. 9.

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

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

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
    • Both new molecules stay unchanged, because the old strand overrides the new strand
    • 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