Question

2. Explain why the replication of one of the DNA strands is continuous while the other strand is discontinuous. What causes this difference? 3. A) What are telomeres? B) Are telomeres present in both Prokaryotes and Eukaryotes? Explain why some cells have telomeres and others don't. 4. A) Draw a cell with 2 pairs chromosomes as it goes through the cell cycle and indicate the main things that happen to its chromosomes at each stage (label: sister chromatids, homologous chromosomes, G1, S, G2, Prophase, metaphase, anaphase, telophase, citokinesis, indicate the 3 checkpoints). Draw it in paper and upload a photo or scanned. B) Assuming the life of a cell starts as soon as cell division is completed and ends when the cell starts a new process of cellular division, how many times in a cell’s life does it replicate its DNA? Explain. 5. A) Draw the phases of mitosis and indicate the main events that occur in each. B) When does a multicellular organism undergo mitosis (when it wants to reproduce, all the time, only when injured, when it’s growing, or when)? 6. Discuss A) why cell division requires multiple checkpoints and B) what is the role of proto-oncogenes and tumor suppressor genes in normal cells and C) why are proto-oncogenes and tumor suppressor genes related to cancer?


Answer 1

2.

• The two strands of the DNA run antiparallel. The polymerase enzymes that add nucleotides to the primer, during replication, can add only in one direction, that is in the 3’to 5’direction. To the 3’- OH of the leading strand, new nucleotides are added.

• The energy required for phosphodiester bond formation is derived from hydrolysis of the phosphates in the NTP that is added to the existing chain. This ensures that if there is any mismatch, the nucleotide phosphate can be excised and correct nucleotide is added.

• If nucleotides can be added to the 5’end of a growing chain, the energy required for bond formation must be derived by hydrolysis of the phosphates at the 5’end. If there is any mismatch, there is no chance of repair, since the energy is derived from the existing strand and not from the in coming nucleotide. This poses a problem.

• Due to this limitation, the replication of DNA is unidirectional. But since the two strands of the DNA run antiparallel, a different strategy is adapted in replication of the lagging strand. It is discontinuous replication; okazaki fragments are added, and from the 3’end of this fragments, polymerization occurs. Technically, a large number of primers are required for lagging strand replication, each okazaki fragment acts as a primer.

3.

Telomeres are self replicating entities that are present at the ends of eukaryotic chromosomes. They are present only in eukaryotes, but not in prokaryotes. Prokaryotes have circular chromosomes and do not face the problem of end replication, of their chromosomes

Eukaryotes have linear chromosomes. DNA replication requires a primer, which is RNA in nature. To this RNA primer, nucleotides are added during replication, in 3’to 5’direction. Once the replication is over, the RNA primer is degraded. However, the polymerases are unable to add nucleotides to the 5’end. Thus, with each round of replication, eukaryotic chromosomes fall short of a small segment of DNA. To solve this problem, they have self replicating entities called Telomeres.