8.2 Gene Expression

Question 1

What is a stem cell?

Answer 1

Undifferentiated cells, that can divide indefinitely and turn into other specific cell types

Question 2

Name and define three types of stem cell

Answer 2

1. Totipotent = can develop into any cell type including placenta and embryo
2. Pluripotent = can develop into any cell type excluding placenta and embryo
3. Multipotent = can only develop into a few different types of cells

Question 3

What happens to totipotent cells during embryonic development?

Answer 3

Certain parts of the DNA are selectively translated so that only some genes are ‘switched on’, in order to differentiate the cell into a specific type and form the tissues that make up the foetus

Question 4

Give a unique feature of pluripotent cells and the use of this feature

Answer 4

They can divide in unlimited numbers, and therefore be used to repair or replace damaged tissue

Question 5

What is a unipotent cell? Give an example

Answer 5

• A cell that can only develop into one type of cell
• This happens at the end of specialisation when the cell can only propagate its own type
• Example is cardiomyocytes

Question 6

What types of stem cell are found in embryos?

Answer 6

• Totipotent and pluripotent
• Multipotent and unipotent cells are only found in mature animals

Question 7

Uses of stem cells

Answer 7

• Medical therapies e.g. bone marrow transplants, treating blood disorders
• Drug testing on artificially grown tissues
• Research e.g. on formation of organs and embryos

Question 8

How are induced pluripotent stem cells produced?

Answer 8

• From mature, fully specialised cells
• The cell regains capacity to differentiate through the use of proteins, in particular transcription factors

Question 9

What is a transcription factor?

Answer 9

• A protein that controls the transcription of genes so that only certain parts of the DNA are expressed e.g. in order to allow a cell to specialise

Question 10

How do transcription factors work?

Answer 10

1. Move from the cytoplasm into the nucleus
2. Bind to the promotor region upstream of target gene
3. Makes it easier or more difficult for RNA polymerase to bind to gene. Increases or decreases the rate of transcription

Question 11

Give an example of how a hormone that affects transcription and explain how it works

Answer 11

1. Steriod hormone oestrogen diffuses through cell membrane
2. Forms hormone-receptor complex with ER alpha receptor in the cytoplasm
3. Complex enters the nucleus and acts as a transcription factor to facilitate binding of RNA polymerase

Question 12

What is meant by epigenetics?

Answer 12

A heritable change in gene function without change to the base sequence of DNA

Question 13

How does increased methylation of DNA affect gene transcription?

Answer 13

• Involves addition of CH3 group to cytosine bases which are next to guanine
• Prevents transcription factors from binding
• Therefore gene transcription is suppressed

Question 14

How does decreased acetylation of DNA affect gene transcription?

Answer 14

• Positively charged histones bind to negatively charged DNA
• Decreasing acetylation increases positive charge of histones
• Binding becomes too tight and prevents transcription factors from accessing the DNA
• Therefore gene transcription is suppressed

Question 15

How might epigenetic changes affect humans?

Answer 15

They can cause disease, either by over activating a gene’s function or by suppressing it

Question 16

Give an application of epigenetics

Answer 16

• Treatments of various diseases
• Development of ways to reverse epigenetic changes

Question 17

Describe the process of RNA interference, including the organisms in which it occurs

Answer 17

• RNA molecules act to inhibit gene expression, usually by destroying mRNA so that it cannot be translated
• Occurs in eukaryotes and some prokaryotes

Question 18

Give some characteristics of benign tumours

Answer 18

• Slow growth
• Cells retain function and normal shape
• Don’t spread easily
• Easy to treat

Question 19

Give some characteristics of malignant tumours

Answer 19

• Rapid uncontrollable growth
• Cells do not retain function and often die
• Spreads quickly and easily (metastasis)
• Difficult to treat

Question 20

Describe the role of tumour suppressor genes

Answer 20

• Code for proteins that control cell division - in particular stopping the cell cycle when damage is detected
• They are also involved in programming apoptosis i.e. self destruction of the cell

Question 21

Explain how tumour suppressor genes can be involved in developing cancer

Answer 21

• A mutation in the gene could code for a nonfunctional protein
• Increased methylation or decreased acetylation could prevent transcription
• Cell will uncontrollably divide, resulting in a tumour

Question 22

Describe the role of proto-oncogenes

Answer 22

• Controls cell division
• In particular, code for proteins that stimulate cell division

Question 23

Explain how proto-oncogenes can be involved in developing cancer

Answer 23

• Mutation in the gene could turn it into a permanently activated oncogene
• Decreased methylation or increased acetylation can cause excess transcription
• This results in uncontrolled cell division and formation of a tumour

Question 24

Explain how abnormal methylation of genes can cause cancer

Answer 24

Hyper-methylation of tumour suppressor genes or oncogenes can impair their function and cause the cell to divide uncontrollable

Question 25

Explain how oestrogen can be involved in developing breast cancer

Answer 25

• Oestrogen is an activator of RNA polymerase
• Therefore, in areas of high oestrogen concentration, such as adipose tissue in the breasts, cell division can become uncontrolled