8.7
Replication in Eukaryotes
In eukaryotic cells, DNA replication is highly conserved and tightly regulated. Multiple linear chromosomes must be duplicated with high fidelity befo…
DNA replication in eukaryotic chromosomes initiates at multiple origins of replication, which are identified and bound by the origin recognition complex, or ORC.
The ORC then recruits helicases to unwind the DNA, producing a replication bubble with two replication forks.
The two forks move in opposite directions and disrupt the nucleosomes ahead of them. These nucleosomes are then reassembled on the daughter strands, maintaining the chromatin structure.
At each fork, RNA primers provide the site for DNA polymerase to elongate the leading strand and the Okazaki fragments of the lagging strand.
An RNase enzyme then removes these primers, and the DNA polymerase fills the gaps. Finally, DNA ligase seals the fragments together.
However, as the last primer is removed from the lagging strand at the end of the linear chromosome, it produces an overhanging stretch of the template DNA.
An enzyme called telomerase extends this overhanging stretch with non-coding DNA to prevent the loss of coding DNA during subsequent replication cycles.
The replication continues until the adjacent replication bubbles merge and the whole chromosome is duplicated.
Q1: Why does eukaryotic DNA replication start at multiple origins instead of one?
Eukaryotic chromosomes are much larger than prokaryotic genomes, making replication from a single origin too slow. Multiple origins of replication allow simultaneous replication across the chromosome, dramatically reducing the time needed to duplicate the entire genome before cell division. The origin recognition complex identifies and binds these origins to initiate replication.
Q2: What is the role of RNA primers in eukaryotic DNA replication?
RNA primers are short nucleotide sequences synthesized by primase that provide the starting point for DNA polymerase to bind and begin copying DNA. Because DNA polymerase cannot initiate synthesis de novo, primers are essential for both leading and lagging strand synthesis. After replication, RNase enzymes remove these primers and DNA polymerase fills the resulting gaps.
Q3: How do leading and lagging strands differ during eukaryotic DNA replication?
The leading strand is synthesized continuously in the 5' to 3' direction as the replication fork advances. The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments, which are 100–200 base pairs long. This difference occurs because DNA polymerase can only synthesize in one direction, requiring the lagging strand to be built in segments that are later joined together.
Q4: What happens to nucleosomes during eukaryotic DNA replication?
As replication forks move along the chromosome, they disrupt nucleosomes ahead of them. These nucleosomes are then reassembled on the newly synthesized daughter strands, maintaining the chromatin structure and preserving gene regulation patterns. This ensures that epigenetic information is properly inherited during cell division.
Q5: Why do telomeres shorten with each cell division in eukaryotes?
Eukaryotic chromosomes are linear, and because of how the lagging strand is synthesized, the terminal RNA primer cannot be fully replaced with DNA. This leaves a gap at the chromosome end, causing a small amount of telomeric DNA to be lost with each replication cycle. Over many cell divisions, telomeres gradually shorten, serving as a marker of cellular aging.
Q6: How does telomerase prevent chromosome degradation?
Telomerase is an enzyme expressed in certain cell populations, such as germ cells and stem cells, that extends telomeres by adding non-coding repetitive DNA sequences to chromosome ends. This lengthening compensates for the DNA lost during replication, allowing these cells to undergo additional cell cycles before telomeres become critically short and trigger cellular senescence.
Q7: What is the role of DNA ligase in completing eukaryotic DNA replication?
DNA ligase seals the gaps between adjacent DNA fragments, particularly the Okazaki fragments on the lagging strand after RNA primers are removed and gaps are filled by DNA polymerase. This enzyme catalyzes the formation of phosphodiester bonds, creating a continuous DNA backbone and completing the replication process.