StemUp: AQA A level Biology 3.8.2 Gene expression is controlled by a number of features

What are the features of totipotent cells? (3)

- Can divide and differentiate into every cell type in body

- Including cells that support embryo e.g. placenta

- Occur for a limited time in very early embryos

What are the features of pluripotent cells? (3)

- Can divide and differentiate into most cell types

- Excluding cells from placenta

- Found in embryos

What are the features of multi-potent cells? (2)

- Can divide and differentiate into a limited number of cell types

- Found in mature mammals

What are the features of uni-potent cells? (3)

- Can divide and differentiate into just one cell type

- Found in mature mammals

- E.g. cardiomyocytes (cardiac muscle cells) can be made from unipotent stem cells

How are induced pluripotent stem cells produced? (3)

- From adult somatic cells (non-pluripotent cells)

- And specific protein transcription factors

- Which cause the cell to express the genes associated with pluripotency

Why are induced pluripotent stem cells used over embryonic cells in medical treatment? (4)

- There is no immune rejection

- As they can be made using patient's own cells

- Overcomes some ethical issues with using embryonic stem cells

- Such as no destruction of embryo and the fact that adults can give permission

What are some supporting arguments for the use of embryonic stem cells? (2)

- They are seen as tiny balls of cells, incapable of feeling pain and therefore, not equivalent to humans

- They would otherwise be destroyed (for methods such as IVF which produce more than needed)

What are some arguments against the use of embryonic stem cells? (4)

- Embryo is a potential human

- Which is an ethical consideration

- Induced pluripotent cells can not yet reliably be reprogrammed to produce stem cells

- Can cause tumours if divide uncontrollably

What kind of biological molecules are transcription factors? (1)

Proteins

How do transcription factors regulate transcription? (4)

1. They move from cytoplasm to nucleus

2. To bind to DNA at a specific DNA base sequence on a promotor region

3. This stimulates or inhibits transcription of target genes

4. By helping / preventing RNA polymerase (and therefore, promoting or demoting gene expression)

What is the role of oestrogen in initiating transcription? (5)

1. Oestrogen diffuse across phospholipid bilayer of cell surface membrane

2. In cytoplasm, oestrogen binds to a receptor of an inactive transcription factor

3. Inactive transcription factor changes shape, resulting in the activation of a transcription factor

4. This diffuses from the cytoplasm into the nucleus and binds to specific DNA base sequence on a promotor region

5. Which stimulates transcription of genes by helping RNA polymerase to bind

What is meant by epigenetics? (3)

- Heritable changes in gene expression

- Without changes to the base ‎‏‏‎sequence of DNA

- Caused by changes in the environment

Describe how increased methylation of DNA can inhibit transcription (4)

1. Methyl groups are added to cytosine bases in DNA

2. Nucleosomes pack more tightly together

3. This prevents transcription factors binding and genes will not be transcribed

4. As RNA polymerase cannot bind

Is increased methylation of DNA reversible? (2)

- No

- It is irreversible

Describe how decreased acetylation of histones can inhibit transcription (4)

1. This causes the chromatin to become highly condensed

2. Histones will bind DNA more tightly

3. Which prevents transcription factors binding

4. Genes will not be transcribed

Is decreased acetylation of associated histones reversible? (1)

Yes

How can epigenetic changes contribute to disease development? (2)

- Epigenetic changes that increase the expression of an oncogene or silence a tumour suppressor gene

- Can lead to tumour development

What role do tests for abnormal levels of methylation and acetylation play in cancer detection? (2)

- Serve as early indicators of cancer

- Used as biomarkers for detecting the disease

What is RNA interference (RNAi)? (1)

Process by which RNA molecules inhibit the translation of mRNA produced by transcription

How is RNA interference (RNAi) achieved? (2)

Can be achieved through:

- siRNA (single RNA strand)

- miRNA (double RNA strand)

How do miRNA and siRNA function in RNA interference? (3)

1. miRNA and siRNA are contained within a RISC (RNA-induced silencing complex)

2. They bind to a molecule of mRNA with a complementary sequence

3. This binding leads to the hydrolysis of the mRNA or stops its translation

What is the role of the RNA-induced silencing complex (RISC) in RNA interference? (3)

- Facilitates the binding of miRNA or siRNA to complementary mRNA

- This leads to the degradation of the mRNA

- Or inhibition of its translation

What are the two types of tumours? (2)

- Benign

- Malignant

Compare the main characteristics of benign vs malignant tumours (10)

What is the role of proto-oncogenes in cell division? (2)

- Code for the proteins involved in the control of cell division

- They help regulate normal cell division

How can mutations in proto-oncogenes lead to tumour development? (3)

- Can stimulate cells to divide too quickly

- This results in rapid and uncontrollable cell division

- Which can lead to the development of tumours

How does decreased methylation affect proto-oncogenes? (3)

- Can cause proto-oncogenes to be continually transcribed

- This leads to increased production of proteins that stimulate cell division

- Which contributes to rapid, uncontrolled cell division and tumour development

What is the role of tumour suppressor genes in cell division? (2)

- Genes that code for proteins

- That help slow down cell division and regulate the cell cycle

How can mutations in the tumour suppressor genes lead to tumour development? (4)

- Mutations can inactivate these genes

- Which prevents them from being transcribed

- This results in a lack of proteins that slow down cell division

- Leading to rapid and uncontrollable cell division and tumour development

How does increased methylation affect tumour suppressor genes? (2)

- Prevents the transcription of tumour suppressor genes

- This leads to the absence of proteins that normally slow down cell division

- Which causes uncontrolled cell division and the development of tumours

What is the role of increased oestrogen concentrations in the development of some breast cancers? (4)

1. Areas of high oestrogen concentration (such as body fat tissues in breasts)

2. Cause uncontrollable cell division

3. Growth of cancer is minimised with drugs that block production / action of oestrogens in the breasts

4. E.g. Tamoxifen prevents oestrogen from binding to receptor