Cell Division, Cell Diversity & Cellular Organisation (OCR AS Biology)

Mitosis and meiosis are important in the life cycles of organisms.

Hydra is a small animal that lives in fresh water. When environmental conditions are favourable, Hydra reproduces asexually. Large numbers of offspring can be produced in this way.

Asexual reproduction in Hydra is shown in Fig. 19.1.

Fig. 19.1

Asexual reproduction involves mitosis.

Name the stages of mitosis in the correct order.

 [2]

(ii)

Suggest why Hydra reproduces asexually when conditions are favourable.

 [2]

When conditions are not favourable, Hydra reproduces sexually. This often happens in the winter.

Cells in the body wall produce sperms and eggs by meiosis.

Large numbers of sperms are released into the water. These sperms can fertilise eggs from different individuals. Each egg forms a tough outer coat, and can lie dormant at the bottom of the water until conditions improve.

(i)

Explain how sexual reproduction in Hydra leads to genetic variation in the offspring.

[6]

(ii)

Suggest why sexual reproduction in Hydra usually occurs in the winter.

[1]

Mosses are small plants that live in damp conditions.

The life cycle of many mosses involves two stages: a gametophyte and a sporophyte.

The gametophyte contains haploid cells and produces sperms and eggs.

The sporophyte contains diploid cells and produces spores which can be spread easily through the air.

The spores germinate and grow into a gametophyte.

Fig. 19.2 shows the life cycle of the moss Funaria.

Fig. 19.2

(i)

The zygote grows into the sporophyte by mitosis.

The haploid gametophyte of one species of Funaria contains 28 chromosomes.

A single DNA molecule contains two strands.

Calculate the number of strands of DNA present in the nucleus of the zygote immediately before mitosis.

[1]

Mark an X on Fig. 19.2 at the point at which meiosis occurs.

[1]

(iii)

A diagram of a moss sperm is shown in Fig. 19.3.

The flagella allow the sperm to move towards an egg.

Suggest and explain another adaptation that is likely to be present in these sperm cells.

[2]

Fig. 16 below shows 6 onion cells at various stages of mitosis.

Fig. 16

Name the stage of mitosis shown in cell A.

[1]

(ii)

Draw cell A. Label your drawing to show visible features.

The volume of cell A is 5.4 × 10⁴ μm³.

Assume that cell B is spherical.

Calculate the volume of cell B. A ruler has been added to the image to help with your calculations.
Use the formula: volume of sphere = 

Give your answer in standard form in μm³.

[3]

(ii)

State the type of microscope that was used to view these images. Justify your answer.

[2]

(iii)

Mitosis is involved in growth and repair of tissues.

State two other roles of mitosis in multicellular organisms.

Explain why a plant leaf is described as an organ.

Sperm cells in animals are formed by a process known as spermatogenesis.

Fig. 19.1 is a summary of the process of spermatogenesis.

Fig. 19.1

Three phases of meiosis are listed below.

Match each phase of meiosis to a letter on Fig. 19.1.

Metaphase 1 occurs during the stage labelled ....................

Telophase 2 occurs during the stage labelled .....................

Anaphase 1 occurs during the stage labelled ......................

The chromosomes carried by sperm are made of DNA.

The following passage about nucleic acids has four words missing. Choose the correct missing words from the list below and complete the passage by writing them in the gaps.

Nucleic acids are made from ........................................ monomers.

Phosphodiester bonds form between the monomers. They consist of a ........................................ group between the ........................................ molecules, forming the ‘backbone’ of the molecule.

In DNA, hydrogen bonding between the two antiparallel ........................................ causes the characteristic double helix shape.

Fig. 19.2 is a transverse section of a sperm cell. The mitochondria of sperm cells form a spiral around the central flagellum.

Fig. 19.2

Identify the structures labelled U, W and Z

[3]

ATP, FADH₂ and hexose 1,6-bisphosphate are three organic products of respiration in sperm cells.

Table 19 shows how the production of ATP, FADH₂ and hexose 1,6-bisphosphate in sperm cells is affected by three different substances.

Table 19

What can be concluded about the difference between the effects of cyanide and fluoride on respiration in sperm?

Fig. 1 shows how stem cells from bone marrow can differentiate into the different blood cell types required by the body.

Fig. 1

A student studying stem cells refers to bone marrow stem cells as pluripotent.

[3]

Explain the significance of process X in production of functional blood cells.

A full blood count test is used to check the number of different blood cells in a patients blood.

The patient in Table 1 is suffering from blood cancer known as leukaemia. Leukaemia affects white blood cells and bone marrow leading to uncontrolled cell division of faulty cells. This blood count for the patient was taken after completion of a course of chemotherapy, a treatment used to kill cancer cells, which are more sensitive to the chemotherapy drugs than healthy cells.  

Table 1

This patient has been experiencing the following symptoms:

• • Fatigue
  • High heart rate
  • Dizziness

Using the data in Table 1, suggest a reason for the symptoms they have been experiencing.

[4]

A doctor recommends that the patient self-isolates during their recovery period to reduce exposure to pathogens.

Explain why the doctor advised this.

[2]

An alternative treatment for leukaemia patients is stem cell transplants.

Describe the features of stem cells which make them useful in treating medical issues, such as leukaemia.

[2]

To maximise the chances of a successful stem cell transplant, it is important that the following procedures are followed:

• Selection of a donor with the same blood type and similar tissue type.
• A high dose of chemotherapy is given prior to the transplant.
• Immunosuppressants should be taken after the transplant has taken place.

Explain why these step are important to maximise the chances of a successful stem cell transplant.

[2]

A researcher prepared a stained squash sample of cells from the tip of a garlic root. The first 3 mm of a root tip was cut off and stained on a microscope slide, before placing a coverslip on top of the sample and stain. She then examined the sample under a light microscope. 

During the sample preparation, the researcher pressed down on the sample tissue by applying pressure to the coverslip. Explain why this step was taken. 

The researcher drew a picture of what she could see in the microscope's field of view. This is shown in Figure. 1.1.

Figure. 1.1

Using Figure. 1.1, calculate the percentage of cells undergoing mitosis in the garlic root tip tissue. 

This researcher’s colleagues calculated that the cell cycle of garlic root tips in growth is 1,320 minutes in length, and that on average, cells spend 4 hours in the visible stages of mitosis. 

Calculate the percentage difference between the data gathered by this researcher and the mean length of the mitotic stages found by her colleagues.

Compare and contrast the process of cytokinesis in animal and plant cells. 

The cell pictured in Figure. 2.1 is undergoing the prophase stage of mitosis. Identify structures A, B and C and describe how they behave during prophase. 

Figure. 2.1

Figure. 2.2 shows the mortality rates in men of various age groups from prostate cancer in Europe from 1980 to 2015.

Figure. 2.2

Describe and explain the overalls trend shown in Figure. 2.2.

Using data from Figure. 2.2, compare the percentage improvements in death rates between 1995 and 2015 in men of age groups 60-69 and men aged 80+.

Suggest an explanation for the difference you have calculated. 

A team of biologists estimated the number of cells in different phases of the cell cycle in Saccharomyces cerevisiae (brewer's yeast). They took two samples, A and B, from different environmental conditions. One sample came from a nutrient-rich environment, the other from a nutrient-poor environment.

Their results are shown in the table below.

In sample A, a full cell cycle took 1 hour and 35 minutes, whereas, in sample B, a full cell cycle took 60 minutes. 

Calculate the time, in minutes, that the cells in sample A were in S phase during one cycle. Show your working.

The biologists studying the Saccharomyces cerevisiae (brewer's yeast) from part (a) hypothesised that when the yeast was exposed to stressful conditions, the growth rates were low. 

Suggest, with a reason, which sample came from the nutrient-rich conditions.

The availability of nutrients is also a key factor in regulating the cell cycle of Schizosaccharomyces pombe (fission yeast).  Below is a table containing data scientists collected for two sites which were deficient in nitrogen and phosphate. 

Deduce, giving a reason, the point at which a nutrient-poor environment would stop the cell cycle of the Schizosaccharomyces pombe.

The Saccharomyces cerevisiae (brewer's yeast) nuclei are, on average, 2 µm in diameter, but the DNA molecules packed into them have been measured up to 355 µm in length. 

Describe the process that enables the DNA molecules, that comprise the 16 chromosomes of yeast, to be packed into the nuclei.

Distinguish between totipotent, multipotent and pluripotent cells.

Pluripotent stem cells extracted from the early embryo can be used to treat a variety of diseases including muscular dystrophy, osteoporosis and macular degeneration. However, in 2006, scientists in Japan found that adult stem cells could become induced pluripotent stem cells. Some believe that the use of induced pluripotent stem cells is a more appropriate method for treatment of patients than pluripotent stem cells.

Evaluate the use of pluripotent and induced pluripotent stem cells in treatment of life changing diseases such as those mentioned above.

In plants, totipotent cells are located in the growing shoot and root tips. Scientists used a plant culture technique to investigate the effect of the plant age on the level of totipotency.  Young plants were divided into five groups with 5 in each group. Each group was grown for a specific time period:

Group A was grown for 5 days

Group B was grown for 10 days

Group C was grown for 15 days

Group D was grown for 20 days

Group E was grown for 25 days

A plant tissue culture technique was used, the method was carried out as follows:

1. Cut off a 5 mm shoot tip from each of the 5 plants in groups A-E, this is the explant.
2. Place each explant on a nutrient agar medium inoculated with growth hormones and glucose.
3. Cover the explant samples to prevent contamination and grow for 2 weeks in a suitable environment.   

The results are shown in the table below.

The scientists concluded that explants grown from older plants exhibited less totipotency than those from younger plants.

Evaluate this statement.

Describe the observations that would be made by the scientists to determine the percentage totipotency of the plants in this investigation.

Explain why it is important that one chromosome from each homologous pair ends up in each new cell during meiosis. 

Meiosis is important in the life cycles of many organisms. One example of this is for organisms that reproduce sexually.

Explain why it is important for these organisms. 

A moss is a flowerless spore-producing plant. The spores are produced in capsules and under favourable conditions can germinate into a branching green thread. Figure. 5.1 shows some of the stages of the life cycle of a moss plant.

Mark on the diagram with the letter X where meiosis occurs.

Figure. 5.1

The processes in meiosis can lead to genetic variation among the haploid cells produced.

Explain these processes and how they lead to genetic variation. 

Figure. 6.1 shows how the mass of DNA changes over time during two different types of cell division of a diploid organism. 

Figure. 6.1

Explain what is happening at stage X and Y. 

Identify which of the division types shown in Figure. 6.1 represents meiosis and explain your answer. 

Figure. 6.2 shows a diploid cell containing different chromosomes. 

Figure. 6.2

Complete Figure. 6.2 showing the appearance of the four different types of gametes produced by this diploid cell. 

A diploid organism contains 38 chromosomes.

Calculate the possible number of different combinations of chromosomes following meiosis and explain why this value may not be the actual number.