8.2 - Gene expression is controlled by a number of features

Question 1

Give two characteristic features of stem cells. (2)

Answer 1

• Will replace themselves / keep dividing / replicate
• Undifferentiated / can differentiate / develop into other cells / totipotent / multipotent / pluripotent

Question 2

Give one reason why the use of embryonic stem cells is more controversial than the use of adult stem cells. (1)

Answer 2

• Adults can consent to the use of their stem cells (and embryos cannot)
• Many believe that an embryo is a potential human and therefore has human rights (and therefore should not be experimented on/destroyed)
• There are questions around who ‘owns’ embryos produced during IVF/therapeutic cloning and therefore who gives consent to their being used/destroyed
• Embryonic stem cells collected before birth (from the amniotic fluid) can be stored, but this is expensive and not everyone can afford it

Question 3

Define the term induced pluripotent stem cell. (1)

Answer 3

• An adult/somatic cell that has the properties of a pluripotent stem cell / can differentiate into many cell types (due to being treated with transcription factors that switch on genes associated with differentiation)

Question 4

Distinguish between totipotent, multipotent and pluripotent cells. (3)

Answer 4

• Totipotent cells can differentiate into any type of cell in the body / totipotent cells are found in the zygote
• Multipotent cells can differentiate into a limited number of cell types / multipotent cells are found in the umbilical cord/some adult tissues/named example
• Pluripotent cells can differentiate into almost any/most cell type (but not all) / pluripotent cells are found in the embryo

Question 5

Define what is meant by epigenetics. (2)

Answer 5

• Heritable changes in gene function
• Without changes to the base sequence of DNA

Question 6

Explain how increased methylation could lead to cancer. (3)

Answer 6

• Methyl groups (could be) added to (both copies of) a tumour suppressor gene
• The transcription of tumour suppressor genes is inhibited
• Leading to uncontrolled cell division

Question 7

Give one way in which benign tumours differ from malignant tumours. (1)

Answer 7

• Cells of benign tumours cannot spread to other parts of the body / metastasise
• Cells of benign tumours cannot invade neighbouring tissues

Question 8

Oestrogen is a hormone that affects transcription. It forms a complex with a receptor in the cytoplasm of target cells. Explain how an activated oestrogen receptor affects the target cell. (2)

Answer 8

• (Receptor / transcription factor) binds to promoter which stimulates RNA
  polymerase / enzyme X
• Transcribes gene / increase transcription

Question 9

Oestrogen only affects target cells. Explain why oestrogen does not affect other cells in the body. (2)

Answer 9

• Similar shape to oestrogen;
  Binds receptor / prevents oestrogen binding
• Receptor not activated / will not attach to promoter / no transcription

Question 10

Epigenetic changes can affect the expression of genes. Explain what is meant by the term epigenetic in this context. (1)

Answer 10

• Factors that affect gene expression/function without altering the DNA base sequence (e.g. the addition of chemical/acetyl/methyl groups)

Question 11

Give two examples of environmental factors that can lead to epigenetic change. (2)

Answer 11

• Diet
• Stress
• Smoking
• Pollution
• Exercise

Question 12

Acetylation of histones is one form of epigenetic change in which acetyl groups are added to histones resulting in increased gene expression. Explain how acetylation affects gene expression in this way. (2)

Answer 12

• The DNA winds/wraps less tightly around acetylated histones
• Transcription factors/RNA polymerase/transcriptional machinery can bind more easily (leading to transcription and therefore increased expression of the gene)

Question 13

State and explain the effect of methylation on gene expression. (2)

Answer 13

• Reduced/no/inhibits gene expression
• Methyl groups/methylation prevents RNA polymerase/transcription factors/transcriptional machinery from binding/attaching to the DNA

Question 14

siRNAs only affect specific genes, or parts of genes. Explain why this is the case. (1)

Answer 14

• The siRNA base sequence is complementary to the (target) mRNA

Question 15

Explain how siRNAs prevent the expression of target proteins. (2)

Answer 15

• They/their associated proteins break mRNA into fragments/small pieces
• The mRNA cannot be translated / cannot attach to a ribosome / the mRNA triplet code cannot be read/translated

Question 16

A mutation occurs in a gene that codes for a transcription factor. This transcription factor activates a tumour suppressor gene. Explain how this mutation could prevent the expression of the transcription factor and therefore lead to tumour growth. (2)

Answer 16

• The tertiary structure/3D structure of the transcription factor changes / is no longer complementary to the promoter region (due to the mutation)
• The transcription factor can no longer bind to the promoter region (of the tumour suppressor gene)
• RNA polymerase can no longer bind to the start of the tumour suppressor gene / initiate transcription

Question 17

Define what is meant by the term epigenetics. (2)

Answer 17

• Heritable/inherited changes in DNA/gene function
• Without changes to the base sequence of DNA

Question 18

What is methylation and how does it make genes inactive? (4)

Answer 18

• The addition of methyl/CH3 groups
• To cytosine bases (of DNA/gene)
• (This) changes the DNA structure / makes DNA/gene(s) less accessible (to transcriptional machinery)
• (This) represses transcription OR stops gene(s) being expressed

Question 19

In certain cases, increased methylation could lead to cancer. Explain how. (4)

Answer 19

• Methylation of a tumour suppressor gene (could occur) OR methyl groups (could be) added to a tumour suppressor gene
• (This means) transcription of the tumour suppressor gene is repressed/inhibited
• (This leads to) protein that prevents cell division not being produced OR a protein that causes cell death/apoptosis not being produced
• (This leads to) uncontrolled cell division / no control of mitosis

Question 20

What are siRNAs? (2)

Answer 20

• (Small) double-stranded (non-coding) RNA molecules
• (That) break down mRNA OR stop mRNA being translated (into proteins)

Question 21

Explain how transcription factors stimulate the expression of a gene. (3)

Answer 21

• Transcription factors move from the cytoplasm into the nucleus
• In the nucleus, they bind to a specific base sequence/promoter region on the DNA
• This initiates transcription of the gene / transcription can then begin / mRNA can then join by complementary base pairing to the gene sequence
• mRNA is then translated OR a polypeptide is formed using the mRNA (meaning the gene is expressed);

Question 22

State and explain three reasons why malignant tumours are considered more harmful than benign tumours. (3)

Answer 22

• Malignant tumours can metastasize because the cells don’t stick together in the same way as they do in benign tumours
• Malignant tumours extend into the tissue around them OR grow finger-like projections in to the tissue around them because they aren’t surrounded by a capsule like benign tumours
• Malignant tumours are more likely to be life threatening than benign tumours because tumour tissue replaces normal tissue
• Benign tumours are more easy to remove by simple surgery because they do not metastasize / malignant tumours require a combination of radiotherapy
• Chemotherapy and surgery are required to remove malignant tumours because of metastasis

Question 23

Explain how an increase in oestrogen levels may lead to the development of breast cancer. (3)

Answer 23

• Oestrogen binds to the transcription factor of a gene controlling cell division
• This transcription factor then binds to/activates the gene to initiate transcription
• Increased transcription of the gene leads to increased cell division and tumour formation;
  or
• Oestrogen triggers mutation of proto-oncogenes into oncogenes
• The oncogene is permanently activated/switched on
• This leads to uncontrolled cell division