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14.4 DNA Replication in Prokaryotes
After each of the first few generations, the cells were taken, isolated, and then subjected to high speeds ofcentrifugation.
The DNA was loaded into a solution of salt and spun at high speeds during the centrifugation.
Under these circumstances, the DNA will form a band according to its density.
The density of the DNA grown in 15N will be higher than the density of the DNA grown in 14N.
After one generation of growth in 14N after they had been shifted from 15N, the single band observed was intermediate in position in between the cells grown exclusively in 15N and 14N.
Either a semi-conservative or dispersive mode of replication was suggested by this.
The intermediate position between 15N and 14N is where the band of 14N DNA was formed.
If the DNA replicates in a conservative manner, the results could be explained.
The other two models were ruled out because of this.
Each of the two strands that make up the double helix serves as a template from which new strands are copied.
The new strands will complement the old strands.
When two daughter DNA copies are formed, they have the same sequence and are divided into the two daughter cells.
By the end of this section, you will be able to explain the process of DNA replication in prokaryotes.
In 42 minutes, all of the base pairs in a single circular chromosome can be replicated, starting from a single site along the chromosomes and proceeding around the circle in both directions.
1000 nucleotides are added per second.
The process is fast and painless.
Each of the structural genes plays a critical role during the process.
The addition of nucleotides requires energy from the nucleoside triphosphates.
The energy released when the bond between the phosphates is broken is used to form a bond between the incoming nucleotide and the growing chain.
There are three main types of polymerases in prokaryotes.
It's now known that both DNA pol III and DNA pol I are required for DNA synthesis and repair.
It turns out that there are specific sequence of genes that make up the origins of replication.
The origin of replication in E. coli is rich in AT sequence and is approximately 245 base pairs long.
The origin of replication is recognized by some of the proteins that bind to this site.
The hydrogen bonds between the nitrogenous base pairs are broken by the helicase.
There is a requirement for the process.
The Y-shaped structures called replication forks are formed when the DNA opens up.
Two replication forks are formed at the beginning of the replication process.
A new strand of DNA can only be extended in the direction of 5' to 3', which is why it is only able to add nucleotides in the 5' to 3' direction.
It requires a free 3'-OH group and a bond between the 3'-OH end and the 5' phosphate of the next nucleotide.
If a free 3'- OH group isn't available, it can't add nucleotides.
The template strand can now be extended by adding one-by-one nucleotides that are compatible with it.
A fork is formed when the strands of DNA are separated.
The replication fork tends to cause the DNA to become coiled.
The helix is prevented from re-forming by single-strand binding.
A primer is created by Primase.
This primer is used to make the daughter DNA strand.
The leading strand and lagging strand have different ways in which DNA is synthesised.
The RNA primer is replaced by the DNA polymerase I.
The gaps between the fragments are sealed by DNA ligase.
The topoisomerase causes temporary nicks in the helix of the double helix when it opens up.
There is a slight problem at the replication fork due to the fact that the double helix is antiparallel.
One strand is in the 5' to 3' direction while the other strand is in the 3' to 5' direction.
There is only one strand that can be synthesised continuously towards the fork.
There are new primer segments that point away from the replication fork.
The leading strand can be extended from one primer to another.
The direction of the lagging strand will be 3' to 5', and that of the leading strand will be 3' to 3'.
The sliding clamp holds the polymerase in place by binding to the DNA.
As the synthesis progresses, the primers are replaced with DNA.
The exonuclease activity of DNA pol I fills in the gaps left by the removal of the primer and the addition of DNA nucleotides.
Once the chromosomes have been replicated, the two DNA copies move into two different cells.
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