DNA REPLICATION

What does it mean to replicate? The production of exact copies of complex molecules, such as DNA molecules, that occurs during growth of living organisms When does replication occur? DNA replication takes place during the synthesis (S) phase of the cell cycle.

Early Models of DNA Replication Conservative Replication The original helix of DNA is completely copied to make a new helix. Dispersive Replication The original helix of DNA is broken down and interspersed with new pieces of DNA.

The Accepted Model of Replication Semi-Conservative Replication In this model, the parental strands of DNA separate. Each strand serves as a template in order to produce DNA molecules that have one strand of parental DNA and one strand of new DNA.

General Information Since DNA is antiparallel the strands are replicated differently. Replication begins with the original strands unwinding at a location called the REPLICATION FORK. One strand follows continuous, smooth replication and is called the LEADING STRAND. The leading strand is synthesized in the 5’ to 3’ direction. One strand is synthesized in several small pieces and is not continuous. This strand is called the LAGGING STRAND. The lagging strand is also synthesized in the 5’ to 3’ direction.

DNA Replication: The Leading Strand 1. DNA HELICASE unwinds and unzips the DNA double helix by breaking the hydrogen bonds between the bases. 2. Proteins called SINGLE-STRANDED BINDING PROTEINS bind with the DNA to keep the strands separate during replication. 3. RNA PRIMASE adds a short segment of RNA, called an RNA primer, to the DNA strand. 4. DNA POLYMERASE attaches to the RNA primer and begins adding DNA bases to the 3’ end of the strand, therefore synthesizing new DNA. Polymerase adds by matching the correct base pairs to the new strand. 5. RNAse H removes the RNA primers initially put on the strand.

Practice Building DNA If your original strand had the following base sequence, what bases would RNA primase add to create the RNA primer? ACAGTT A G If your original strand had the following base sequence, what bases would DNA polymerase add to create a complementary strand OF DNA? ACAGTT A G

DNA Replication: The Lagging Strand 1. DNA HELICASE unwinds and unzips the DNA double helix by breaking the hydrogen bonds between the bases. 2. Proteins called SINGLE-STRANDED BINDING PROTEINS bind with the DNA to keep the strands separate during replication. 3. RNA PRIMASE adds a short segment of RNA, called an RNA primer, to the DNA strand. Since the strand is synthesized discontinuously, many primers are added. 4. DNA polymerase adds bases to the RNA primers to form many small fragments, known as OKAZAKI FRAGMENTS. 5. RNAse H removes the RNA primers initially put on the strand and the Okazaki fragments are joined by DNA LIGASE.

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Comparing Replication Eukaryotes DNA is able to unwind in multiple areas along the strand during replication Occurs in the nucleus Prokaryotes DNA is unwound at one origin of replication and takes place in both directions. Occurs in the cytoplasm