Chapter 13 Section 2: DNA Replication
Opening Activity DNA is considered to be a relatively stable molecule. What gives it this stability, even though the hydrogen bonds between the nitrogen bases are easily broken? Write out your answer, then share and compare your ideas with other students.
Content Objectives I will be able to identify: How DNA replicates, or makes a copy of itself. What the roles of proteins are in DNA replication. How DNA replication is different in prokaryotes and eukaryotes.
Chapter 13 Section 2: DNA Replication Key Vocabulary Terms Adapted from Holt Biology 2008
DNA Replication The process of making a copy of DNA
DNA Helicase An enzyme that unwinds the DNA double helix during DNA replication
DNA Polymerase An enzyme that catalyzes the formation of the DNA molecule.
Replication Forks A replication fork is the mechanism by which a strand of DNA is synthesized. Unzip the DNA and it looks like a fork, ie fork in road, not eating fork.
Chapter 13 Section 2: DNA Replication Supplementary Words Adapted from Holt Biology 2008
Histones Remember: Long molecules of DNA are tightly wound around proteins called histones
Replication bubbles By starting replication at many sites along the chromosome. Two distinct replication forks form at each start site, and replication occurs in opposite directions. This process forms replication bubbles along the DNA molecule.
Chapter 13 Section 2: DNA Replication Notes Adapted from Holt Biology 2008
Your Turn Activity
DNA Replication Because DNA is made of two strands of complementary base pairs, if the strands are separated then each strand can serve as a pattern to make a new complementary strand.
The process of making a copy of DNA is called DNA replication. DNA Replication
In DNA replication, the DNA molecule unwinds, and the two sides split. Then, new bases are added to each side until two identical sequences result. DNA Replication
DNA Replication, continued As the double helix unwinds, the two complementary strands of DNA separate from each other and form Y shapes. These Y-shaped areas are called replication forks.
DNA Replication, continued At the replication fork, new nucleotides are added to each side and new base pairs are formed according to the base-pairing rules.
DNA Replication, continued Each double-stranded DNA helix is made of one new strand of DNA and one original strand of DNA.
Three steps in replication: 1. Unwinding and separating the DNA strands 2. Adding complimentary bases 3 Formation of two identical molecules
The replication of DNA involves many proteins that form a machinelike complex of moving parts. Each protein has a specific function. Replication Proteins
Proteins called DNA helicases unwind the DNA double helix during DNA replication. These proteins wedge themselves between the two strands of the double helix and break the hydrogen bonds between the base pairs. Replication Proteins
Proteins called DNA polymerases catalyze the formation of the DNA molecule by moving along each strand and adding nucleotides that pair with each base. Replication Proteins
Replication Proteins, continued DNA polymerases also have a proofreading function. During DNA replication, errors sometime occur and the wrong nucleotide is added to the new strand.
Replication Proteins, continued If a mismatch occurs, the DNA polymerase can backtrack, remove the incorrect nucleotide, and replace it with the correct one.
Function of enzymes (proteins) of replication DNA Helicase unwinds the helix
Function of enzymes (proteins) of replication DNA polymerase: add new nucleotides to the open DNA strand proofread to prevent errors
Review Questions 1. What is the first step in DNA replication? Which enzyme performs this step? Unwinding and separating DNA strands DNA Helicase
Review Questions 2. When in the cell cycle does DNA replication occur? Interphase (S)
Review Questions 3. What would happen if DNA polymerase didn t correct errors? It would change the DNA code causing mutations or a change in the function of the cell.
Prokaryotic and Eukaryotic Replication Prokaryotic cells usually have a single chromosome which is a closed loop attached to the inner cell membrane. Replication in prokaryotes begins at one place along the loop. This site is called the origin of replication.
Eukaryotes and prokaryotes replicate their chromosomes differently. Eukaryotic cells often have several chromosomes which are linear and contain both DNA and protein. In eukaryotic cells, replication starts at many sites along the chromosome. This process allows eukaryotic cells to replicate their DNA faster than prokaryotes.
Prokaryotic and Eukaryotic Replication Click to animate
Prokaryotic and Eukaryotic Replication Two distinct replication forks form at each start site, and replication occurs in opposite directions. This process forms replication bubbles along the DNA molecule. Replication bubbles continue to get larger as more of the DNA is copied.
The smallest eukaryotic chromosomes are often 10 times the size of a prokaryotic chromosome. Eukaryotic chromosomes are so long that it would take 33 days to replicate a typical human chromosome if there were only one origin of replication. Some prokaryotes replicate their DNA and form two new cells in 20 minutes.
Prokaryotic and Eukaryotic Replication Human chromosomes are replicated in about 100 sections that are 100,000 nucleotides long, each section with its own starting point. Because eukaryotic cells have multiple replication forks working at the same time, an entire human chromosome can be replicated in about 8 hours.
Summary In DNA replication, the DNA molecule unwinds, and the two sides split. Then, new bases are added to each side until two identical sequences result. The replication of DNA involves many proteins that form a machinelike complex of moving parts. In prokaryotic cells, replication starts at a single site. In eukaryotic cells, replication starts at many sites along the chromosome.