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Specialisation: how one genome becomes two hundred kinds of cell questions

Differentiation and the control of gene expression in outline; totipotent, pluripotent and multipotent stem cells with their sources; and the levels of organisation from cell to tissue to organ to system, using squamous and ciliated epithelium, xylem and phloem as the named examples.

5 original questions · 16 marks · the specialisation: how one genome becomes two hundred kinds of cell notes · Cells, microscopy and biological organisation

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• 4 marks · Explain

  A stem cell in the bone marrow and a mature sperm cell carry exactly the same DNA. Explain how the two cells come to have such different structures.

  Mark scheme

  1. B1 every cell holds a complete copy of the genome, so the genes present in the two cells are the same
  2. B1 only a fraction of those genes is transcribed in any one cell, so different proteins are made and different structures develop
  3. B1 which genes are expressed is set by the signals a cell receives, such as hormones, growth factors, its neighbours and its position in the embryo
  4. B1 once established the pattern of expression is maintained through subsequent divisions, so a cell's daughters stay the same type

• 4 marks · Explain

  A phloem sieve tube element has lost its nucleus and almost all of its organelles, yet it is still alive. Explain how this suits it to its function, and explain why it cannot work without a companion cell.

  Mark scheme

  1. B1 losing the nucleus and most organelles clears the centre of the tube, so sap flows through with little obstruction
  2. B1 the end walls survive as sieve plates perforated with pores, through which the contents pass from one element to the next
  3. B1 with no nucleus and no ribosomes the element cannot make its own proteins, so the companion cell makes them and passes them through plasmodesmata
  4. B1 the companion cell has dense cytoplasm and many mitochondria, and supplies the ATP for loading sucrose actively into the sieve tube

• 3 marks · Compare

  Compare a totipotent cell, a pluripotent cell and a multipotent cell, giving one place each type is found.

  Mark scheme

  1. B1 a totipotent cell can produce any cell type in the organism and the extra-embryonic tissues such as the placenta, and is found in the zygote and the first few divisions
  2. B1 a pluripotent cell can produce any cell type in the body but not the placenta, and is found in the embryonic stem cells of the blastocyst
  3. B1 a multipotent cell can produce only a limited range of cell types, and is found in adult tissues such as bone marrow

• 3 marks · Suggest

  Long-term smokers secrete more mucus than usual and the cilia on their airway epithelium are damaged. Suggest why such a smoker develops a persistent cough and gets more chest infections.

• 2 marks · State

  State what is meant by a tissue, and state what is meant by an organ.

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