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In a DNA double helix an adenine of one strand always pairs with a(n) _____ of the complementary strand, and a guanine of one strand always pairs with a(n) _____ of the complementary strand.

thymine ... cytosine

After DNA replication is completed, _____.

each new DNA double helix consists of one old DNA strand and one new DNA strand

The first step in the replication of DNA is catalyzed by _____.


The action of helicase creates _____.

replication forks and replication bubbles

Why is the new DNA strand complementary to the 3' to 5' strands assembled in short segments?

DNA polymerase can assemble DNA only in the 5' to 3' direction

The synthesis of a new strand begins with the synthesis of a(n) _____.

RNA primer complementary to a preexisting DNA strand

An old DNA strand is used as a _____ for the assembly of a new DNA strand.


Short segments of newly synthesized DNA are joined into a continuous strand by _____.


After replication is complete, the new DNAs, called _________, are identical to each other.

daughter DNA

The enzyme that can replicate DNA is called

DNA Polymerase

During DNA replication, an open section of DNA, in which a DNA Polymerase can replicate DNA, is called a

replication fork

The new DNA strand that grows continuously in the 5 to 3 direction is called the

leading strand

____________ are the short sections of DNA that are synthesized on the lagging strand of the replicating DNA.

Okazaki Fragments

What catalyzes DNA synthesis?

DNA Polymerase

What are primers?

short sequences that allow the initiation of DNA synthesis

Which part of a deoxynucleoside triphosphate (dNTP) molecule provides the energy for DNA synthesis?

phosphate groups

Which of the following enzymes creates a primer for DNA polymerase


Which of the following enzymes is important for relieving the tension in a helix as it unwinds during DNA synthesis?


One cycle of replication will produce ___daughter DNA molecules, each with one parental strand and one newly synthesized strand.


A second cycle of replication produces ____ daughter DNA molecules.



cuts, swivels, and rejoins DNA strands ahead of the replication fork


breaks the hydrogen bonds between parental DNA strands and unwinds the double helix

Single-strand binding proteins

bind to the single strands of DNA, preventing them from re-forming hydrogen bonds with each other and allowing synthesis to occur in both strands.

DNA Polymerase III

synthesizes the new strands, but requires an existing 3 hydroxyl to add nucleotides


creates short RNA primers, initiating DNA synthesis on both template strands

DNA Polymerase I

removes the RNA primers and replaces them with DNA nucleotides

On the lagging strand, DNA ligase

joins Okazaki fragments by forming phosphodiester bonds between them, thus completing DNA replication.

leading strand

daughter strand elongates toward replication fork, only one primer needed, and made continuously

lagging strand

made in segments, daughter strand elongates away from replication fork, multiple primers needed

both strands

synthesized 5 to 3

Step one of lagging strand synthesis

A new fragment begins with DNA polymerase III binding to the 3' end of the most recently produced RNA primer, primer B in this case, which is closest to the replication fork. DNA pol III then adds DNA nucleotides in the 5' to 3' direction until it encounters the previous RNA primer, primer A.

Step two of lagging strand synthesis

DNA pol III falls off and is replaced by DNA pol I. Starting at the 5' end of primer A, DNA pol I removes each RNA nucleotide and replaces it with the corresponding DNA nucleotide. (DNA pol I adds the nucleotides to the 3' end of fragment B.) When it encounters the 5' end of fragment A, DNA pol I falls off, leaving a gap in the sugar-phosphate backbone between fragments A and B.

Step three of lagging strand synthesis

DNA ligase closes the gap between fragments A and B.

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