Chromosome Behaviour in Mitosis (Cambridge (CIE) AS Biology)

Cells divide by mitosis to form new cells.

Describe the changes that occur within the cell between the beginning of prophase and the end of metaphase.

Fig. 1 shows a diagram of a chromosome in a cell undergoing mitosis.

Fig. 1

(i) Identify structure P.

[1]

(ii) Describe the role of structure P during mitosis.

[1]

From the image in Fig. 1

(i) Identify the stage of mitosis that the chromosome could be in.

[1]

(ii) State a reason for your answer to part i).

[1]

Explain why it is important for DNA replication to take place before mitosis can occur.

Fig. 1 is a light microscope image of a section of onion (Allium cepa) root tip showing cells in various stages of mitosis.

Image courtesy of Doc. RNDr. Josef Reischig CSc. Licenced under Creative Commons Attribution-Share Alike 3.0 Unported License. Reused and distributed under conditions found at: https://creativecommons.org/licenses/by-sa/3.0/

Fig. 1

Identify the stages of mitosis taking place in cells Q, R and S.

Assuming that the cells in Fig. 1 have a chromosome number of 6, sketch and label one of the nuclei from cell S to show the appearance of the chromosomes.

The daughter cells that are produced during mitosis are genetically identical to each other and to the parent cell.

Explain the importance of this.

In a dividing cell, DNA replication occurs before mitosis.

Steps in DNA replication are outlined in Fig.1

Complete Fig.1 by filling in the gaps using the most appropriate terms.

  1    Helicase enzyme allows the DNA double helix to unwind and the hydrogen bonds between the two strands to break, exposing the four bases, A, T, C and G.      

 The names of these bases are:        
A = ..............................................................      
 T = ...............................................................        
C = ...............................................................        
G = ...............................................................    

2   An enzyme molecule attaches to each of the two separated parental strands. The two enzyme molecules move in opposite directions, each catalysing the formation of a new strand of DNA. This enzyme is known as ...............................  

 3   DNA ................................................., the monomers of DNA, are free in the nucleus for the synthesis of the new strands.    

4   The bases of the DNA monomers form hydrogen bonds with the bases on each separated parental strand of DNA, according to the rules of  ........................................ .    

5   One DNA strand is synthesised continuously and the other is synthesised in sections known as Okazaki fragments. The fragments are joined by an enzyme called ......................................... , which catalyses the formation of phosphodiester bonds.  

6   The result of replication is two DNA molecules, each one containing an original parental strand and a newly synthesised strand. This type of replication is described as ......................................... .

Fig. 1

Fig. 2 is a photomicrograph of root tip cells at different stages in the cell cycle.

A cell in interphase is labelled.

Fig. 2

(i) Complete Fig. 2 by naming the stage of mitosis shown in each of cells J, K and L in Fig. 2.

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(ii) State one feature of the cell in interphase, visible in Fig. 2, that shows this cell is not in early interphase.

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(iii) Describe the stage of mitosis shown in cell J.

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Figure 1 shows a micrograph of cells in actively dividing tissue from the tip of a plant root.

Figure 1

(i) Name the process by which many of the cells in Figure 1 are dividing.

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(ii) Using Figure 1, complete Table 1.

[3]

Table 1

Regarding Figure 1:

(i)In which of the cells (U, V, W, X, Y or Z) can vesicles now fuse to form new cell membranes across the cytoplasm in order to separate the cell into two daughter cells.

[2]

(ii) What stage of cell division are these cells in?

[1]

When the student prepared the root tip squash, the root tip was placed on a microscope slide with a stain.

Explain why a stain was used.

Describe the events that take place during mitosis, including the name of the stage of mitosis during which each event occurs.

Hepatic stem cells divide by mitosis to form new liver cells. Each time a stem cell divides it forms a replacement stem cell and a cell that will specialise to become a liver cell.

Fig. 1 shows changes in the mass of DNA in a human hepatic stem cell during four cell cycles..

Fig. 1

Identify the processes taking place at the points marked A and B in Fig. 1 that bring about changes in the mass of DNA per cell.

Describe what happens to the number of chromosomes in the hepatic stem cells of Fig. 1.

Colchicine is an alkaloid drug extracted from the corm of the autumn crocus (Colchicum autumnale). It is often used as a treatment for gout and other inflammatory diseases. Colchicine binds to a protein called tubilin and prevents the elongation of microtubules in a cell.

Explain why anaphase would not be able to occur in the presence of colchicine.

Outline what happens to a chromosome between the beginning of telophase and the start of the next round of mitosis.

Fig. 1 below shows a drawing of an electron micrograph of a cell undergoing mitosis.

Fig. 1

Deduce which phase of mitosis this cell is in.

Fig. 2 below shows a sample of root tissue collected by a researcher.

Fig. 2

The mitotic index is the proportion of cells in a tissue sample that are undergoing mitosis and can be calculated as follows:

Calculate the mitotic index of the tissue sample in Fig. 2.

A student’s research determined that the cell cycle of the growing tissue in a similar root as part (b) was 1,560 minutes in length, and that on average, cells spent 5 hours in the visible stages of mitosis.

Calculate the percentage difference between the data gathered by the student and the mean length of the mitotic stages found by the researcher in part (b).

Suggest two possible reasons why there may have been differences in the mitotic index the researcher determined and the student’s value.

Fig. 1 shows a cell undergoing the prophase stage of mitosis.

Fig. 1

Identify structures A to C and describe how they behave during prophase. Use Table 1 to construct your answer.

Table 1

Some cell biologists believe that use of the term 'cell division' should be discontinued and replaced with 'cell multiplication'.

Discuss the reasons in favour of this claim by using your knowledge of the cell cycle.

A team of researchers estimated the numbers of cells in different stages of the cell cycle in healthy, growing onion plants. They took two tissue samples, A and B, from different parts of a plant.

One tissue sample came from a root tip, the other from a large leaf near the bottom of the plant's stem.

Their results are shown in Table 2.

Table 2

In tissue sample B, a full cell cycle took 20 hours and 45 minutes.

Calculate the time, in minutes, that these cells were in telophase during one cell cycle.

Show your working.

Identify which tissue sample, A or B, was taken from the root tip.

Explain your answer using the information in Table 2.