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By the end of this section, you will be able to:
  • Discuss the similarities and differences between DNA replication in eukaryotes and prokaryotes
  • State the role of telomerase in DNA replication

Eukaryotic genomes are much more complex and larger in size than prokaryotic genomes. The human genome has three billion base pairs per haploid set of chromosomes, and 6 billion base pairs are replicated during the S phase of the cell cycle. There are multiple origins of replication on the eukaryotic chromosome; humans can have up to 100,000 origins of replication. The rate of replication is approximately 100 nucleotides per second, much slower than prokaryotic replication. In yeast, which is a eukaryote, special sequences known as Autonomously Replicating Sequences (ARS) are found on the chromosomes. These are equivalent to the origin of replication in E. coli .

The number of DNA polymerases in eukaryotes is much more than prokaryotes: 14 are known, of which five are known to have major roles during replication and have been well studied. They are known as pol α , pol β , pol γ , pol δ , and pol ε .

The essential steps of replication are the same as in prokaryotes. Before replication can start, the DNA has to be made available as template. Eukaryotic DNA is bound to basic proteins known as histones to form structures called nucleosomes. The chromatin (the complex between DNA and proteins) may undergo some chemical modifications, so that the DNA may be able to slide off the proteins or be accessible to the enzymes of the DNA replication machinery. At the origin of replication, a pre-replication complex is made with other initiator proteins. Other proteins are then recruited to start the replication process ( [link] ).

A helicase using the energy from ATP hydrolysis opens up the DNA helix. Replication forks are formed at each replication origin as the DNA unwinds. The opening of the double helix causes over-winding, or supercoiling, in the DNA ahead of the replication fork. These are resolved with the action of topoisomerases. Primers are formed by the enzyme primase, and using the primer, DNA pol can start synthesis. While the leading strand is continuously synthesized by the enzyme pol δ , the lagging strand is synthesized by pol ε . A sliding clamp protein known as PCNA (Proliferating Cell Nuclear Antigen) holds the DNA pol in place so that it does not slide off the DNA. RNase H removes the RNA primer, which is then replaced with DNA nucleotides. The Okazaki fragments in the lagging strand are joined together after the replacement of the RNA primers with DNA. The gaps that remain are sealed by DNA ligase, which forms the phosphodiester bond.

Telomere replication

Unlike prokaryotic chromosomes, eukaryotic chromosomes are linear. As you’ve learned, the enzyme DNA pol can add nucleotides only in the 5' to 3' direction. In the leading strand, synthesis continues until the end of the chromosome is reached. On the lagging strand, DNA is synthesized in short stretches, each of which is initiated by a separate primer. When the replication fork reaches the end of the linear chromosome, there is no place for a primer to be made for the DNA fragment to be copied at the end of the chromosome. These ends thus remain unpaired, and over time these ends may get progressively shorter as cells continue to divide.

Questions & Answers

who is the father of biology
Sean Reply
Aristotle
Lubera
Aristotle
estella
what are prokaryotic cells
Kusubira Reply
having cell that lack membrane - bound nuclei.
Lubera
Please each person should suggest ur answers
Franklina Reply
differentiate between autelogical and synecology
Christiana Reply
differentiate between autelogical and synecology
Christiana Reply
answer please
Edmond
Autecology is the study of individual organism or individual species. It is also known as population ecology. Synecology is the study of group of organisms of different species which are associated together as a unit in form of a community.
Daniel
Autecology is the study of the environment in relation to only one species in contrast while synecology is the study of the environment affecting groups of species coexisting in an area.
Franklina
Describe the factors affecting homotasis
Grace Reply
Fluids and electrolytes Energy and nutrition Immune response mediators
Franklina
prove starch in a mango leaf
Ruth Reply
prove starch in a leaf
Ruth
draw and name the plant cell
Nalongo
wat is classification
Nalongo
levels of classification
Nalongo
classification of what
Ezenwa
Nidhipal
Nidhi
Iodine solution is used to test leaves for the presence of starch.
Franklina
Identify the caste of termites or Honey bees
Mensah Reply
Soldier Winged reproductive Worker King and queen
Franklina
what is biology
Jackson Reply
biology is a study of living organism
Brilliant
what is the myelin sheath?
Myelin sheath is an insulting layer that forms around nerves, including those in the brain and spinal cord.
Franklina
description on how an enzyme such as pepsin breaks down a substrate
linister Reply
The specific reaction catalyzed by pepsin is the acid hydrolysis of the peptide bond. This reaction will break down protein into smaller units to enable the digestive process
Franklina
what is biology
Kanzy Reply
study of life
Tufail
the study of life..meaning studying all aspects of life in humans,animals and everything inside the global
Nalukui
study of life
Memory
is the branch of science which deal with the study of living and non living things
David
Is the study of living organisms or living things
Franklina
draw the taxonomic hierarchy of classification
KC Reply
Kingdom Class Order Family Genus Species
Franklina
explain the following bush burning, deforestation, over grazing
Thomas Reply
what is biodiversity
Tracy
No idea
nana
Please what are alleles
nana
Deforestation: this is the cutting down of trees without planting new ones
Brilliant
over grazing is the continuously planting of crops in one particular land.
Friday
what is afforestation
Friday
Bush burning is the removal of the natural vegetation cover, that protects the soil surface through the use of fire. Deforestation is the actor process of establishing a forest especially on land not previously forested.
Franklina
Over grazing is the practice of grazing toi many livestock for too long a period on a land unable to recover its vegetation.
Franklina
what is physiology
Joseph Reply
the study of physical things
Nalukui
dealing with those things which you can see by your neck eyes
Odwa
Further explanation please
nana
the study of things tt interact with time,energy
David
Is the scientific study of chemistry and physics of the structure of the body
Franklina

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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