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S294/K

Module Examination 2017

CELL BIOLOGY

PART 1

Short answer questions Answer all SIX questions in this first part of the exam. Write your answers in the answer book provided, limiting your answer for each part of a question to a few sentences or more. You are advised to spend no more than 1 hour 15 minutes on this Part A of the paper, which carries 30 of the total marks for the examination. Each question is worth 5 marks.

Question 1

(a) Identify the structures indicated in this electron micrograph of a neuron.  

Ans

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(b) Lysosomes play an important part in endocytosis and in autophagy.

(i) Briefly describe the structure and content of lysosomes, and their function in endocytosis.

(ii) What is autophagy? Identify at least three cellular processes that can involve autophagy.

Question 2

(a) DNA replication involves a number of different proteins:  DNA helicase, DNA topoisomerase, primase, DNA polymerase and DNA ligase.

(i) For four of these proteins, indicate what its particular role is in DNA replication.

(ii) Which of these proteins also has a proof-reading function and what does this function involve?

(b) Consider a eukaryotic gene in which a single base pair change has occurred within an open reading frame (ORF). With reference to the genetic code and gene expression, explain how, in some cases, this may alter the amino acid sequence of the encoded protein and why, in other cases, the protein will be unchanged.

(c) What kind of mutation is caused by deletion of a single base pair in the ORF and what are the consequences of such a mutation for the encoded protein?

Question 3

Chemiosmotic coupling describes the coupling of electron transport activity to ATP synthesis by means of an electrochemical gradient.

(a) Name the processes that involve chemiosmotic coupling in (i) mitochondria, and (ii) chloroplasts?

(b) What is the nature of the electrochemical gradient in these processes?

(c) Draw and clearly label a diagram to compare how and where chemiosmotic coupling operates in mitochondria and chloroplasts. You do not need to represent the respective electron transport chains in detail.

(d) Where does the energy come from for the electron transport chain to generate the electrochemical gradient in (i) mitochondria, and (ii) chloroplasts?               

Question 4

Molecules can pass the cell membrane either in an active or a passive manner, depending on their characteristics.

(a) Briefly describe, in one or two sentences each, what is meant by active and passive transport.

(b) What are the characteristics of molecules that can cross the cell membrane by passive diffusion? Give two examples of different types of such solutes.

(c) What are the characteristics of molecules that cannot cross the cell membrane by passive diffusion? Give two examples of different types of such solutes.

(d) How does the structure and conformation of carrier proteins and ion channels allow them to transport polar or charged solutes across the cell membrane?

(e) How do the cytosolic and extracellular concentrations of Na+ and K+ differ? Describe how the Na+/K+ATPase (sodium pump) helps to maintain these differences.

Question 5

(a) Name the germ layers that form in the vertebrate gastrula and, for each, identify one tissue / organ that it gives rise to in the mature animal.

(b) How do cell fate determinants and asymmetric cell division help to generate cell diversity in early development?

(c) How do gradients of extracellular signalling factors affect the fate of cells in developing tissues?

(d) Sequential induction is another mechanism that influences cell fate and in particular determines the fine structure of tissues. Briefly describe what happens during sequential induction.

Question 6

(a) Some bacteria are described as antibiotic-resistant.

(i) Define the term antibiotic resistance.

(ii) Describe the normal mode of action of β-lactam antibiotics such as penicillin in sensitive bacterial cells and explain why these drugs are ineffective in resistant cells.

(iii) What implications does antibiotic resistance have for the future treatment of bacterial infections?

(b) Virulence is a measure of the ease with which an infectious agent such as a bacterium may cause disease within a host organism. Describe two examples of different types of virulence factor (other than antibiotic resistance) that are characteristic of Staphylococcus aureus.

PART 2

Data handling and interpretation question

Answer ALL parts of the question in this Part. Write all your answers to this Part in the answer book provided. You are advised to spend 45 minutes on this Part of the paper. Your answers should be written concisely, but in proper sentences. You should include diagrams and calculations in your answer where appropriate.

This Part carries 25% of the marks for the examination.

Question 7

You are supplied with a copy of an article entitled ‘A niche role for cancer exosomes in metastasis’ by Yun Zhang and Xiao-Fan Wang. [Zhang Y. and Wang X. (2015), Nature Cell Biology, vol. 17, pp. 709-711].

This article discusses the findings of a study from Costa-Silva and co-workers [Costa-Silva, B et al. (2015), 'Pancreatic cancer exosomes initiate pre-metastatic niche formation in the liver', Nature Cell Biology, vol. 17, pp. 816-825], which explored a pathway by which liver metastasis was promoted by exosomes secreted by pancreatic tumour cells [pancreatic ductal adenocarcinoma (PDAC) cells].

Metastasis is the stage of tumour development when tumour cells from the primary tumour gain the ability to invade surrounding tissue, and to travel to and proliferate in different tissues around the body. At this stage the cancer is described as malignant. The secondary locations in which the tumour cells establish themselves and proliferate are known as metastatic sites.

In S294 you learned about different types of intercellular communication including cell-cell contact-dependent signalling, and signalling via secreted molecules. The article refers to intercellular communication mediated by exosomes, a relatively recently discovered mode of communication. Exosomes are extracellular membrane-bound vesicles which, when released from one cell, can then fuse with another cell and thereby transfer their contents.

The following questions are based largely on this article but in some cases draw on relevant material from S294.

(a) Based on the information provided above and your studies of S294, in what fundamental respects would intercellular communication via exosomes differ from that mediated via secreted soluble factors?

(b) In your own words, briefly describe what is meant by the term ‘premetastatic niche’, as described in the article.

(c) The article describes a study of the role of exosomes in liver metastasis. According to the authors, in which other type of cancer has a role for exosomes in metastasis been previously reported, and what paper do they cite to support this statement?

(d) What, according to the article, is carried within exosomes?.

(e) The article describes how in the Costa-Silva study, the investigators set out to determine whether exosomes released by pancreatic ductal adenocarcinoma (PDAC) cells promoted formation of a pre-metastatic niche in the liver. From the article, outline how the investigators did this. What control did they use in this experiment and what did they observe?

(f) Which cells in the liver were targeted by the PDAC-derived exosomes and how did the authors of the original study demonstrate this?

(g) How did the authors of the original study demonstrate that an increase in levels of fibronectin in the livers of mice that were treated with PDAC derived exosomes was caused by TGFβ?

(h) Outline two pieces of evidence mentioned in the article that indicate that fibronectin deposition in the liver occurs before infiltration of the tissue by bone marrow-derived cells (BMDCs).

(i) From your studies in S294, what kind of protein is fibronectin and what is its role in tissues?

(j) As stated in the article, there is evidence that BMDCs are recruited to metastatic sites during the establishment of pre-metastatic niches. The authors of the original study performed an experiment to determine if recruitment of BMDCs to the liver was a feature of the pre-metastatic niche that formed in response to PDAC-derived exosomes. For this experiment they used mice that were irradiated to kill bone marrow cells.

 What was the experiment that they performed and why did they need to irradiate the mice that they used? What did they conclude from this experiment?

(k) Based on the account of the investigation described in the article, outline in your own words the sequence of cellular and tissue-level events (in the order in which they occur) that culminate in the formation of the premetastatic niche in the liver.

PART 3

Answer ONE of the questions in this Part. You are advised to spend about 1 hour on this question. You should follow any specific guidance in the question. Note that some questions specifically request inclusion of diagrams but in all cases you may include diagrams where appropriate. This Part carries 30% of the marks for the examination.

Question 8

Write an essay entitled: ‘Prokaryotic and eukaryotic genomes: differences and similarities. Your essay should include at least:

• an explanation of what a genome is
• an explanation of what an open reading frame is
• a brief description of prokaryotic genome
• a brief description of eukaryotic genomes
• discussion of similarities and dissimilarities between the two types of genome.

You may use diagrams to illustrate key concepts, but note that inclusion of a diagram is not compulsory.

Question 9

Write an essay entitled ‘The structure and function of motor proteins in eukaryotic cells’.

Your essay should include:

• an account of general motor protein structure;
• a description of the general mechanism by which motor proteins function;
• a brief mention of the different types of motor proteins and which processes they are involved in;
• a more detailed description of the structure and function of the specific motor proteins involved in either muscle contraction or movement of eukaryotic cilia/flagella.

Include at least one relevant diagram.

Question 10

Some inherited human diseases illustrate variable expressivity; that is, the severity of the symptoms presented by different affected individuals varies significantly. Marfan syndrome and Huntington’s disease are examples of inherited diseases which show variable expressivity. Write an essay entitled ‘Diseases showing variable expressivity’ in which you compare and contrast these two diseases.

Your essay should include:

• a description of the symptoms of these inherited diseases and their underlying cause;
• a description of the pattern of inheritance of the disease genotype in each case, and an explanation for the variable expressivity of the disease genotypes in the phenotypes of the affected individual in both diseases.

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Last updated: Sep 02, 2021 10:06 AM

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