AS Biology 9700: Stem Cells, Mitosis and Genetically Identical Daughter Cells

Stem cells in the bone marrow divide repeatedly by mitosis throughout a person's life. Some of the daughter cells remain in the bone marrow as stem cells, while others go on to differentiate into new red blood cells, replacing red blood cells that have worn out.

(a) State what is meant by the term stem cell. [2]

(b) Explain, in terms of the chromosomes present at the start of mitosis and their behaviour during the division, why the two daughter cells produced when a bone marrow stem cell divides are genetically identical to each other and to the parent stem cell. [3]

(c) Suggest why it is important that the new red blood cells produced in this way are genetically identical to the person's other body cells, rather than being genetically varied. [2]

Part (a): What a stem cell is

A stem cell is a cell that has not yet become specialised for a particular function. Its key feature is that it can continue to divide by mitosis indefinitely: when it divides, some of the daughter cells remain as stem cells (maintaining the stem cell population in the bone marrow), while others go on to differentiate, developing the specific structure and function of a particular cell type, such as a red blood cell.

Part (b): Why the daughter cells are genetically identical

By the time a bone marrow stem cell enters mitosis, every one of its chromosomes has already been copied during the S phase of the preceding interphase, so each chromosome is present as two sister chromatids, identical copies of each other, joined together at a centromere. During mitosis, the chromosomes condense and line up individually at the equator of the cell in metaphase, with a spindle fibre from each pole attached to the centromere of every chromosome. In anaphase, the centromere of every chromosome divides, separating its two sister chromatids, and the spindle fibres pull one chromatid to each pole of the cell. Because this happens for every chromosome at the same time, each pole receives one complete set of chromosomes, each one an exact copy of a chromosome in the original stem cell. When the cell then divides in two, each daughter cell therefore contains a full, identical set of genetic information, matching both the parent stem cell and each other.

Part (c): Why genetic identity matters for red blood cells

Red blood cells are part of the same individual as the bone marrow stem cells that produce them, and they need to function correctly within that person’s body. Because they are produced by mitosis, new red blood cells carry exactly the same genes as the person’s other body cells, meaning they are built according to the same genetic instructions and are properly suited to their role, for example producing the correct haemoglobin to transport oxygen. If new red blood cells were instead genetically varied, differing from one another and from the rest of the body’s cells, some could carry mutations that disrupt normal function (such as producing a faulty protein), which would make the blood cell population less reliable at carrying out its role and could be harmful to the individual.

Final answers

(a) A stem cell is an undifferentiated cell that divides by mitosis; some daughter cells remain as stem cells, others differentiate into specialised cells.
(b) Each chromosome was replicated into two identical sister chromatids before mitosis; anaphase separates these chromatids, one to each pole, so both daughter cells receive one complete, identical set of chromosomes.
(c) Red blood cells need the same genes as the rest of the body’s cells to function correctly (e.g. producing normal haemoglobin); genetic identity via mitosis ensures this, whereas variation could introduce harmful faults.