The Mitotic Cell Cycle: Question 2

Worked solution

Part (a): Sister chromatids and the centromere

Sister chromatids are the two identical copies of a single chromosome that are produced when the chromosome is replicated during S phase. Because they come from copying the same original DNA molecule, the two sister chromatids are genetically identical to one another. They remain physically attached to each other at one point along their length, a narrow region called the centromere, until they are separated during anaphase of mitosis.

Part (b): Why S phase replication is essential

Mitosis is a process of separating existing genetic material, not creating new DNA (so the DNA must already have been doubled before mitosis begins. During S phase, every chromosome is replicated to form two identical sister chromatids, meaning that by the start of mitosis there are two complete, identical copies of the genome (one for each future daughter cell) attached together at each centromere. During anaphase, the centromeres divide and the sister chromatids of every chromosome are pulled apart to opposite poles of the cell, so that when the nucleus and cytoplasm divide, each daughter cell ends up with one full, identical set of chromosomes. If DNA replication had not occurred in S phase, mitosis would simply distribute the single existing chromosomes between the two poles, so at best only one daughter cell could receive a complete set of chromosomes while the other would receive none) the two cells produced would not be genetically identical diploid cells like the parent.

Part (c): The protective role of telomeres

Telomeres are found at each end of a linear chromosome and consist of DNA with a short sequence repeated many times, rather than genes that code for proteins. The enzyme DNA polymerase cannot completely copy right to the very end of a linear DNA molecule during replication, so a short length of DNA is inevitably lost from each end of a chromosome every time a cell divides. Because this lost DNA comes from the repetitive, non-coding telomere rather than from a functional gene, the coding sequences further along the chromosome are protected from this loss for many rounds of replication. Telomeres also mark the true ends of the chromosome, helping to stop chromosome ends from fusing together or being wrongly recognised and repaired by the cell as broken DNA. Only once telomeres have become critically short after many divisions does the loss of DNA begin to threaten nearby genes, at which point the cell typically stops dividing.

Final answers

• (a) Sister chromatids are the two identical copies of a chromosome made by replication in S phase, joined together at the centromere.
• (b) S phase replication produces two identical sister chromatids per chromosome so that, after they are separated in anaphase, each daughter cell receives one complete, identical set of chromosomes; without it, the daughter cells could not both be genetically identical to the parent.
• (c) Telomeres are non-coding, repetitive DNA at the chromosome ends; because DNA polymerase cannot fully copy the chromosome end, telomeric DNA (not genes) is lost with each replication, protecting genes from loss and helping to prevent chromosome ends fusing together.