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10.2.5.2. Replication in eukaryotes

DNA replication in eukaryotes

The replication of DNA in eukaryotes is similar to that of prokaryotes, but have some differences:

  • The DNA of eukaryotic cells is strongly associated with histones, forming nucleosomesIn replication, the strand that serves as the template for the lead strand stays with the histones and the two twist to form the old nucleosomes. The strand that serves as a pattern for the lag, and the lag strand, wrap around new histone octamers to form new nucleosomes.
  • The DNA of the eukaryotic chromosome is much longer than prokaryotic DNA, in addition to being bound to histones, the replication process is much slower. To speed up the process, there are a hundred replication origins on the eukaryotic chromosome, about a hundred replication bubbles. These replication initiation points are called replicons. Replication has a single origin in prokaryotes, while in eukaryotes there are multiple.
  • In eukaryotes, Okazaki fragments are smaller (about one hundred to two hundred nucleotides ) than in prokaryotes (1000 to 2000 nucleotides).
  • DNA replication in eukaryotes is completed until reaching the telomere, the end of the chromosome. By eliminating the last primer RNA, the delayed strand remains incomplete, since DNA polymerase cannot fill the gap because it cannot do so in the 3'→ 5' direction. This causes the telomere to shorten each time the cell divides, which is associated with aging and cell death processes.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Madrid, September 2016, option A, question 5.

Regarding the DNA replication of eukaryotic cells:

a) In the following double-stranded DNA molecule, the arrow indicates the opening direction of the double helix. Indicate from which chain (A) or (B) the conductive strand will be synthesized and from which the retarded strand. Explain  why one strand is synthesized continuously and the other discontinuously (0.75 points).

5´- ATGACGTGA - 3´

3´- TACTGCACT - 5-

b) If the percentage of bases in one of the two strands of double-stranded DNA is: A = 30%, T = 28%, G = 22% and C = 20%, what  will be the percentage of bases in the complementary strand? (0.5 points).

c) The DNA polymerase, Primasse and Helicase proteins participate in the initiation phase. Indicate the function that each one of them performs  (0.75 points).