DNA Replication

DNA Replication DNA is a self-replicating molecule This is vital as DNA is the genetic material of all cells DNA will duplicate itself before cellular reproduction

Why does DNA Replicate? When cells go through Mitosis, they will split into two cells Mitosis makes any identical copy of an original cell In order for the cell to be identical it must copy the DNA exactly. Each cell must have the exact same DNA and same number of chromosomes for the cell to be viable.

When does this occur DNA is replicated during the S (Synthesis) stage of the cell cycle. This process if complicated and involves numerous proteins to aid in the replication of DNA

Where does this occur? DNA replication takes place in the nucleus or eukaryotes Prokaryotes do this in the cytoplasm As the DNA carries the instructions for the cell, it is extremely important To protect the DNA this takes place in the nucleus

Overview of Replication The DNA will “split” at the nitrogenous bases Complementary nucleotides will be added to the ‘unzipped’ strands This will make 2 new strands of DNA

Semi-Conservative Replication This process is called semi-conservative replication There is always half of the DNA that is preserved the same from the original cell This way each new cell gets a half of the DNA from the old cell

DNA Replication Details Each strand contains a complete and complimentary copy of the information found on the other strand. Eg - A is on one side T must be on the other side If the Hydrogen-bonds are broken between base pairs, the one side has all the information necessary to make a new strand. All that has to be done is to make sure that the bases match up. The strand that is read to make a complimentary copy is called the template.

Strands One strand is called the leading strand The leading strand is 3` to 5` Leading strands are replicated continuously One strand is called the lagging strand This is 5` to 3` Lagging strands are replicated in sections

Replication Enzymes Replication is a chemical process Since this will involve biological chemical reactions, it will involve enzymes to speed up the rate of these reactions. Each reaction has its own enzyme i.e. – each enzyme has its own specific job

Replication Enzymes Helicase enzyme- breaks the Hydrogen bonds between the nitrogen bases. Topoisomerase – helps with the uncoiling of the double helix DNA primase – makes RNA primers that attach to ‘unzipped’ DNA Helps DNA Polymerase by adding primers

Replication enzymes DNA polymerase – adds nucleotides to an existing DNA strand Works 5` to 3` Cannot make DNA from scratch. They require a primer DNA Ligase – Joins the original parental strand with the new strand of DNA.

DNA Replication DNA replication begins at an origin of replication site DNA Helicase begins to separate the hydrogen bonds while topoisomerase begins to help unwind the strands Each separated strand will serve as a template during replication

DNA Replication The leading Strand DNA Primase adds and RNA primer to the start of the DNA strand DNA Polymerase (III) attaches at the site of the RNA primer and continuously attaches the appropriate nucleotides to match the leading strand’s bases DNA polymerase only works from 3` to 5`

DNA Replication The Leading Strand DNA Polymerase (I) removes the RNA primers and fills in the gaps with DNA DNA ligase bonds the unjoined sections of the newly made strand

DNA Replication The Leading Strand The leading strand continues to work forward as it replicates This process is easy for DNA Polymerase as it is moving the same direction as Helicase

DNA Replication The Lagging Strand The lagging strand is much more complicated for DNA to replicate This is because DNA Polymerase can only work in one direction This happens using a discontinuous process Small sections are made and then joined together

DNA Replication The Lagging Strand Helicase splits the strands creating a replication fork DNA Primase adds an RNA primer DNA Polymerase (III) attaches to the primer and adds complementary nucleotides This happens in a small section called an Okazaki Fragment DNA Polymerase is actually moving AWAY from the direction of the replication fork

DNA Replication The Lagging Strand Closer to the replication fork, DNA primase has added another primer DNA Polymerase (III) attaches and adds another Okazaki Fragment This process continues and adds more DNA along the lagging strand

DNA Replication The Lagging Strand DNA Polymerase (I) removes the RNA primers and fills in the gaps with DNA DNA ligase bonds the unjoined sections of the newly made strand

Accuracy DNA replication is a highly accurate process DNA replication in bacterial makes about one mistake every 100 million nucleotides DNA polymerase is not only highly accurate but also works to proof-read the new DNA strand