Section 13.3DNA Replication

I Can…

• LS 1.3 I can explain how the structure of DNA relates to its ability to self-replicate.

Key Questions

1. What is the role of DNA polymerase in copying DNA?

2. How does DNA replication differ in prokaryotic cells and eukaryotic cells?

Vocabulary

• Replication

• DNA polymerase

• Telomere

Copying the Code

• Base pairing in the double helix explains how DNA can be copied.

• Each base on one strand pairs with one—and only one—base on the opposite strand.

• The strands are said to be complementary.

1. The Replication Process

2. The Role of Enzymes

3. Telomeres

The Replication Process

• Replication occurs during the S phase of the cell cycle.

• The two strands of the DNA molecule separate (forming replication forks).

• Two new complementary strands are synthesized following the rules of base pairing.

• Each strand of the original molecule serves as a template.

• DNA replication is semiconservative (each new DNA molecule has one original strand and one new strand).

The Role of Enzymes

• Helicase “unzips” and unwinds the DNA strands by breaking hydrogen bonds between the base pairs.

• DNA polymerase joins nucleotides to the DNA polymer.

• DNA polymerase also “proofreads” each new DNA strand.

Copying DNA

Direction

of replicationDirection

of replication

replication fork

DNA polymerase

new nucleotides

being added

Telomeres

• The tips of eukaryotic chromosomes are known as telomeres.

• The ends of DNA molecules are difficult to replicate.

• Telomerase adds short, repeated segments to the ends of chromosomes as they are replicated.

• These telomeres do not code for anything.

• They help prevent genes on the ends of chromosomes from being damaged or lost during replication.

• Telomerase is often switched off in adult cells.

• In cancer cells, the enzyme can be activated, enabling these cells to grow and reproduce rapidly.

Replication in Living Cells

• Prokaryotic Cell DNA• Single, circular DNA molecule

• Located in the cytoplasm

• Eukaryotic Cell DNA• Up to 1000x more DNA

• Found in the nucleus

• Packaged into chromosomes

Prokaryotic DNA Replication

• Regulatory proteins bind to a single starting point on the chromosome.

• This triggers the beginning of the S phase.

• Replication starts at a single point and proceeds in two directions until the entire chromosome is copied.

Prokaryotic DNA Replication

replication fork

replication fork

new DNA

unreplicated DNA

Eukaryotic DNA Replication

• Eukaryotic chromosomes are generally much larger and more complex.

• Replication may begin at dozens or hundreds of places on the DNA molecule.

• Proceeds in both directions until each chromosome is completely copied.

• Several proteins check DNA for chemical damage or base pair mismatches prior to replication.

• However, the system is not always perfect.

• Damaged regions are sometimes replicated (which may alter genes).

Eukaryotic DNA Replication

Replication

forks

Unreplicated DNA

New DNA

Section 13.3 Exit Ticket

1. Why does DNA need to replicate itself before cell division?

2. What is the role of enzymes during DNA replication?

3. What are the leading and lagging strands? How are the mechanisms of DNA replication for these two strands different?

4. Why is DNA replication described as a semi-conservative process?

The End