How DNA copies itself
Grade 9 Science Worksheet - Building Confidence
Circle the best answer.
Ari watches DNA polymerase build a new strand on an unzipped ladder. Which letters belong in the two blanks, left to right?
- T, then G
- T, then C
- U, then C
- A, then G
Bo sees DNA polymerase pause after building CATG. Which nucleotide does it add next?
- G, which pairs with the old C
- A, which pairs with the old C
- T, which pairs with the old C
- C, which copies the old C
Unzipped ladder (model, not to scale). Letters line up pair by pair from the left. Esme fills in the first and last letters of a new strand. Which letters belong in the two blanks, left to right?
- A, then T
- G, then T
- C, then A
- G, then A
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 Jun reads the job card. Which enzyme unzips the double helix at the fork?
- DNA polymerase, which breaks the hydrogen bonds between bases
- Ligase, which seals gaps in the new strand
- Helicase, which breaks the hydrogen bonds between bases
- Primase, which makes a short RNA primer
Kira reads the card. What does DNA polymerase do at the fork?
- It copies the old letters onto the new strand
- It adds matching nucleotides to the 3′ end of the new strand
- It builds without mistakes, so nothing checks the new strand
- It opens the helix by breaking the hydrogen bonds
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 Luz reads the proofreading card. DNA polymerase finds a wrong base in the new strand. What does it do?
- 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
- It releases the wrong nucleotide and adds the correct one
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 Milo compares three models of copying DNA. Which model is semiconservative replication, and why?
- Model Y, because each copy keeps one old strand
- Model Y, because each copy is made of two new strands
- Model W, because old and new pieces are mixed
- Model X, because the old strands stay together
Strand count after one round (model data)
molecule old strands new strands both copies together 2 2 copy 1 ? ? Pia reads the strand count after one round of copying. What fills the two ? cells for copy 1?
- 2 old strands and 2 new strands
- 0 old strands and 2 new strands
- 2 old strands and 0 new strands
- 1 old strand and 1 new strand
Use this
Unzipped ladder (model, not to scale). Old strand, 3′ to 5′: GCTAAC. New strand: CGATTG. The ends of the new strand are not labeled. Letters line up pair by pair from the left.
Tala sees the old strand written 3′ to 5′, left to right. How is the new strand written, left to right?
- Either way, because the ends do not change the pairing
- 3′ to 5′, because a copy keeps the direction of its old strand
- 5′ to 3′, because the two strands run in opposite directions
- 3′ to 5′, because the polymerase adds to the 5′ end
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′ ATGC 3′
- 5′ ATGG 3′
- 5′ ATGU 3′