final:stemcell

Question 1

Explain how undifferentiated cells become differentiated cells (4) super asked

Answer 1

1. Undifferentiated cells become differentiated and specialised by differential gene expression
2. Chemicals or hormones may stimulate the activation of genes coding for () cells
3. Or the permanent inactivation of some genes by epigenetic modification (ie. DNA methylation) 4. Only the active genes are transcribed into active mRNA
4. The mRNA is translated to specific protein/ enzyme
5. The protein/enzyme synthesised causes structural and functional changes to the () cell
6. The cell function is permanently modified

modification to become sclerenchyma, cellulose is synthesised.

Question 2

Explain why beta cells can produce insulin but other cells in pancreas cannot

Answer 2

1. Due to differential gene expression
2. In pancreatic cells, genes coding for insulin production are switched off
3. Due to epigenetic modifications, like histone methylation
4. In beta cells, the genes coding for insulin production are activated due to chemical stimuli/ hormones.
5. Hence the active genes coding for insulin production are transcribed to produce active mRNA
6. mRNA translates to synthesise insulin protein in Beta cells

Question 3

Explain how transcription factors may cause somatic cells to be converted to iPS cells

Answer 3

1. They may bind to inhibitors to deactivate them
2. Or bind to promoter regions of DNA
3. So no genes are switched off
4. Hence RNA polymerase can bind to transcription start site to initiate transcription
5. mRNA is produced and protein is produced
6. Which allows the cells to divide

Question 4

Suggest how undifferentiated cells gave rise to genetically varied eggs (4)

Answer 4

1. Meiosis halves the chromosome number, ensuring a haploid chromosome number
2. Genetic variation occurs through crossing over of chromatids and independent assortment of chromosomes
3. mRNA is transcribed only at the active genes
4. Protein is translated
5. The proteins synthesised causes structural and functional change to the cell

Question 5

Why difficult for specialised cell to be reprogrammed to become stem cells

Answer 5

1. Genes have already been switched off
2. Cell has been permanently modified

Question 6

Totipotent

Answer 6

1. Undifferentiated cells that have the capacity to differentiate into all cell types
2. The cells can divide continuously

Question 7

Pluripotent

Answer 7

1. Undifferentiated cells that have the capacity to differentiate into most cell types excluding extra embryonic cells
2. The cells can divide continuously

Question 8

Why important for blastula cells to be totipotent

Answer 8

1. Totipotent cells are undifferentiated
2. Totipotent cells can give rise to all cell types, including extra embryonic stem cells.

Question 9

Why cells in morula are genetically identical (2)

Answer 9

1. Zygote undergoes mitotic cell division to form the morula. Mitotic cell division produces genetically identical cells.
2. DNA is replicated in interphase
3. Each cell receives one identical copy of each DNA strand

Question 10

Comparison for adding genes to iPsC

Answer 10

1. Use the same source of somatic cells
2. Measure the amount of mRNA synthesised
3. In 5 days ?
4. Run at the same temperature and pH

Question 11

How can fertilised egg be a source of human pluripotent stem cells

Answer 11

1. Allow the fertilised eggs to grow for a few days
2. A blastocyst forms
3. The inner cell mass contains pluripotent cells
4. Extract the pluripotent cells (???)
5. Use the spare embryo from fertilised egg

Question 12

How modified stem cells can prevent a disease

Answer 12

1. Copy of functioning allele in the cells
2. Hence normal mRNA is transcribed and normal protein is translated
3. Stem cells divide continuously y mitotic cell division, disease prevented/treated

Question 13

Why better to produce differentiated cells from iPS cells than from pluripotent stem cells (2)

Answer 13

1. Obtaining pluripotent stem cells involves the destruction of an embryo but iPsC doesn’t
2. Some may think that it unethical
3. Limited supply of embryo
4. iPsC can be used in the patient providing the cell, no immune response with iPsC.

Question 14

How may more than one type of protein be synthesised from RNA produced from one gene (Splicing)

Answer 14

1. By post transcriptional changes/ pre-RNA splicing
2. Introns are removed by enzymes
3. Rearrangement of exons
4. (Give example)
5. Different versions of mature mRNA are translated to polypeptides of different amino acid sequences
6. Leading to different bondings between R groups, causing different shapes and functions of the protein produced

Question 15

Histone modification passing onto embryos

Answer 15

1. In histone modification, transcription may be promoted, or genes can be silenced.
2. These modifications are passed from the modified egg cells to the fertilised egg cell, and to the zygote then to the embryo.

Question 16

How cells become specialised after epigenetic modification

Answer 16

1. After epigenetic modification, some genes may be switched on or off
2. Only the active genes are transcribed to active mRNA
3. mRNA translated into specific proteins/ enzyme
4. Proteins/ enzymes synthesised cause permanent structural and functional changes to the cells, causing them to be specialised

Question 17

Trophoblast:

Answer 17

Outer layer of cells, develops into the placenta

Question 18

Inner cell mass:

Answer 18

Pluripotent cells, develops into the foetus

Question 19

Somatic cell cloning

Answer 19

1. Remove nucleus from normal, healthy body cell to transfer to an enucleated human ovum
2. Use mild electric shock to fuse nucleus with enucleated ovum to trigger development
3. Pluripotent stem cells are harvested from the embryo formed, cultured in a suitable environment so they can differentiate into required tissue, using SCNT
4. Tissue cells transferred to the patient

Question 20

iPSC

Answer 20

1. Reprogramming human somatic cells to form induced pluripotent stem cells
2. By transdifferentiation