Download BIOLOGY Wednesday Sub Work

BIOLOGY Wednesday Sub Work
DIRECTIONS:
1) Read through these notes.
2) Use these notes to answer the questions on the back.
3) THIS IS DUE BY THE END OF THE HOUR TODAY!!
UNIT 4 NOTES: DNA REPLICATION
(1) Replication Basics:
a. Overall Goal = Make Identical copy of cell’s DNA molecule.
b. Basic Steps in DNA Replication:
i. Double helix is untwisted.
ii. Strands are separated.
iii. Cell needs to make a complimentary (or matching) copy of each strand.
c. End Result= End up with 2 DNA molecules, each with 1 old/original strand and 1
new/daughter strand
(2) Why Do Cells Replicate DNA?
a. It’s necessary to make more cells! If you make more cells, you have to have more
DNA inside! 
b. It happens during the S Phase of the Cell Cycle
c. What About When There’s Errors?
i. Could have zero impact on the cell (this is usually what happens).
ii. Could provide a positive advantage to the cell.
iii. Could kill/hurt the cell.
(3) DNA Replication is Semi-Conservative:
a. This was discovered by the scientists Messelson & Stahl.
i. Semi = ½ or Partial
ii. Conserve = To Keep or To Save
b. You start with 1 whole, original/old molecule of DNA….
i. You end up with 2 molecules of DNA.
ii. Each new molecule has:
1. 1-old/original strand (this is the part that is conserved)
2. 1-new strand
(4) Untwisting & Separating the DNA Strands:
a. DNA is untwisted and separated by an enzyme called Helicase.
b. DNA is opened in small chunks called Replication Forks.
c. Several helicase enzymes open up lots of replication forks at once, down the
length of the DNA molecule.
d. This increases the rate/efficiency with which a cell can replicate the DNA
molecule.
(5) Making the Complimentary Strands:
a. Once DNA is untwisted and the strands are separated….
i. An enzyme called DNA Polymerase finds matching DNA nucleotides and
pairs them up along the strands.
ii. DNA Polymerase is picky though! It can only lay down DNA nucleotides by
grabbing them at the 5’ End (the end with the phosphate).
b. The “Leading Strand”:
i. This is the strand on the original molecule which goes 3’  5’.
ii. This is called the leading strand because DNA Polymerase can move down
this strand with no problem, super fast! 
c. The “Lagging Strand”:
i. This is the strand on the original molecule which goes 5’  3’.
ii. This is called the lagging strand because remember DNA polymerase can
only lay down nucleotides starting at the 5’ end. That means the Lagging
Strand is going in the wrong direction! 
iii. To fix this problem the cell lays down RNA nucleotides first, and then DNA
Polymerase replaces them with DNA!  Yayyyyy!!
QUESTIONS
(1) What does it mean to replicate DNA?
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(2) What are the 3 basic steps in DNA Replication?
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(3) How many molecules of DNA do you end up with after replication? ____________________
(4) Why do cells need to replicate their DNA?
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(5) When do cells replicate their DNA? ___________________________________________________
(6) What is meant by saying that DNA Replication is “Semi Conservative”?
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(7) Describe the strands in each molecule after DNA Replication:
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(8) Which enzyme untwists and separates the strands in the DNA double helix?
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(9) Describe how helicase unwinds and separates the strands in DNA during replication:
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(10)
Why do cells form multiple replication forks at once along the DNA molecule, during
replication?
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(11)
What does the word complimentary mean? Or to compliment? What does it mean in DNA
replication?
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(12)
What does DNA Polymerase do for the cell during replication?
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(13)
(14)
What end of the DNA nucleotide can DNA Polymerase grab? _____________________
What is the difference between the leading and lagging strand in the DNA molecule?
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(15)
Why can’t DNA Polymerase just move down the lagging strand regularly?
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(16)
How do cells get around the problem between DNA Polymerase + the lagging strand?
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