The replication forks are extended with the help of helicase.
The replication fork is covered with single-strand binding proteins to prevent it from reverting to its original state.
Topoisomerase prevents supercoiling by binding at the region ahead of the fork.
Primers are made from the DNA strand.
The 3'-OH end of the primer is being added to by DNA polymerase III.
The leading strand and lagging strand continue to be degraded.
exonuclease activity is the reason for the removal of RNA primers.
Adding dNTPs fills the gaps.
The gap between the two DNA fragments is sealed by a substance called DNA ligase.
The functions of each of the enzymes are summarized in Table 14.1.
Binds single-stranded DNA to prevent it from reverting back.
The full process of DNA replication is reviewed here.
Eukaryotic genomes are larger and more complex than prokaryotic ones.
There are a number of different linear chromosomes.
The human genome has 3 billion base pairs per haploid set of chromosomes, and 6 billion base pairs are replicated during the S phase of the cell cycle.
Humans can have up to 100,000 origins of replication across the genome.
The autonomously replicating sequence is found on the chromosomes in yeast.
The origin of replication in E. coli is similar to these.
Fourteen DNA polymerases are known in eukaryotes, of which five are known to have major roles during replication and have been studied.
They are called pol a, pol b, pol g, pol d, and pol e.
The same steps are used for replication in prokaryotes.
The DNA needs to be made available as a template.
Eukaryotic DNA is bound to histones and forms structures called nucleosomes.
Histones must be removed and replaced in order to account for the lower rate of replication in eukaryotes.
Some chemical modifications may be made to the chromatin so that it can slide off the proteins or be accessible to the DNA replication machinery.
A pre-replication complex is made at the beginning of replication.
The replication process begins with the recruitment of hilcase and other proteins.
The energy from the hydrolysis opens up the helix.
The forks are formed at the origin of the replication.
Over-winding, or supercoiling, occurs in the DNA ahead of the fork when the double helix opens.
The action of topoisomerases resolves these.
DNA pol can start synthesis with the help of the primer.
There are three major DNA polymerases involved: a, d and e. The lagging strand is synthesized by pol d, but the leading strand is continuously synthesised by pol e. As pol d runs into the primerRNA on the lagging strand, it takes over from the DNA template.
The displaced primerRNA is removed and replaced with a new one.
The fragments in the lagging strand are joined after the replacement of the primer with DNA.
The gaps are sealed by DNA ligase.
The chromosomes are linear.
The DNA pol can only add nucleotides in the 5' to 3' direction.
The end of the chromosomes is reached in the leading strand.
Each of the short stretches of the lagging strand is initiated by a separate primer.
There is no way to replace the primer on the 5' end of the lagging strand when the fork reaches the end of the linear chromosome.
repetitive sequences that code for no particular genes are tomeres.
Humans have a six-base-pair sequence that is repeated 100 to 1000 times.
The genes are protected from being deleted as cells continue to divide.
telomerase adds the telomeres to the ends of the chromosomes.
It is attached to the end of the chromosomes, and the DNA strand is 3' long.
Once the 3' end of the lagging strand template is long enough, DNA polymerase can add the nucleotides to the ends of the chromosomes.
The ends of the chromosomes are duplicated.
The ends of linear chromosomes are maintained by telomerase.
Stem cells and germ cells can be active with telomerase.
It isn't active in adult cells.
The discovery of telomerase by Elizabeth, Carol W. Greider, and Jack W. Szostak earned them the prize in 2009.
One of the scientists who discovered how telomerase works is Elizabeth Blackburn.