Chapter 33, Problem 1Q

Summary Introduction

To review:

The decrease in the number of radio-labeled cells in the zones of cell division and cell elongation over the time.

Introduction:

The undifferentiated mass of cells that has the capability to divide and give rise to a new plant is called as meristems. The meristems located at the apex regions are referred to as the apical meristems that are found at the root apex (root apical meristem) and shoot apex (shoot apical meristem).

Explanation of Solution

Introduction:

Root apical meristem (RAM) is responsible for the generation of cells that help in the development of root for the absorption of water from the soil. The tip of the RAM contributes to the formation of the root cap, which protects the soft tissues of root from damage and also helps in the penetration into the soil. The middle portion of RAM is the quiescent center, which consists of a mass of cells that are held at the G₀ stage of the cell cycle, but can enter G₁ phase and undergo division when required.

The apical portion of the RAM gives rise to three distinct zones. The zone of cell division is present just above the quiescent center, which possesses the actively dividing cells giving rise to primary root tissues. Above this, zone of elongation is present where the cells are continuously elongating in size. The zone of cell maturation is present just above the elongation zone. Here, the cells are not dividing anymore and get matured and differentiated to perform a specialized function.

In an experiment the growing root tips of the plant were incubated in radio-labeled thymidine solution to identify the dividing cells in roots, the thymidine is incorporated in DNA and gets labeled. Later, when the labeled cells were put in the solution containing nonlabeled thymine, the number of labeled cells decreased in the zone of division and elongation of the root. This is because the cells of zone of division and elongation actively took up the labeled nucleotide during replication of DNA (deoxyribonucleic acid) as it is the part of DNA structure. However, when incubated in nonlabeled thymidine solution, the cells of these two zones took up the nonlabeled nucleotide during the replication and thus, the number of labeled cells decreased with the increase in the incubation time in nonlabeled solution. However, the number of labeled cells gradually increased in the zone of maturation as the daughter cells from the zone of division and elongation are the ones that reach the maturation zone.

Conclusion

Hence, there was a decrease in the labeled cells in the division and elongation zones and increase in the zone of maturation when incubated long in the solution of nonlabeled thymidine solution. This is so because during the DNA replication, the dividing cells took up the labeled nucleotide when incubated in the labeled solution and gradually lost them during successive replications in non-labelled solution. As there is no further division of cells in maturation zone of the root, the labeled nucleotides are more compare to zone of division and elongation.