stem cells and gene expression

Question 1

Explain what is meant by the terms totipotent and pluripotent.

Answer 1

totipotent cells can give rise to a complete human/all cell types including zygote + early embryo
pluripotent can differentiate into any cell types; excluding placenta

Question 2

Explain how cells produced from stem cells can have the same genes yet be of different types.

Answer 2

{not all / different} genes are switched {on / off} /active / activated ;
correct and appropriate reference to factors /mechanisms for gene switching ;
e.g. reference to promoters / transcription factors

Question 3

Describe the mechanism by which a signal protein causes the synthesis of mRNA.

Answer 3

signal protein {binds to / joins to / interacts with / activates}
receptor on surface membrane;
messenger molecule moves from cytoplasm and enters nucleus;
{produces / activates} transcription factor;
binds to promoter region;
RNA polymerase transcribes target gene

Question 4

Explain how oestrogen enables RNA polymerase to transcribe its target gene.

Answer 4

Oestrogen diffuses through the cell membrane;
attaches to complementary ERα receptor of TF
ERα receptor changes shape;
ERα oestrogen receptor leaves protein complex which inhibited it’s action;
ERα oestrogen receptor binds to promoter region;
enables RNA polymerase to transcribe target gene
mRNA created
Or

Oestrogen diffuses through the cell surface membrane into the cytoplasm
Oestrogen diffuses through a nuclear pore into the nucleus
Within the nucleus, oestrogen attaches to an ERα oestrogen receptor that is held within a protein complex, this causes the ERα oestrogen receptor to undergo a conformational change
The new shape of the ERα oestrogen receptor allows it to detach from the protein complex and diffuse towards the gene to be expressed
The ERα oestrogen receptor binds to a cofactor which enables it to bind to the promoter region of the gene, this stimulates RNA polymerase binding and gene transcription

Question 5

Compare the structure of dsRNA and DNA.

Answer 5

Similarities; 2 max
Polynucleotides/polymer of nucleotides;
Contain Adenine, Guanine, Cytosine;
Have pentose sugar/5 carbon sugar;
Double stranded/hydrogen bonds/base pairs.

Differences; 2 max
dsRNA contains uracil, DNA contains thymine;
dsRNA contains ribose DNA contains Deoxyribose;
dsRNA is Shorter than DNA; fewer base pairs in length;

Question 6

Explain how the methylation of tumour suppressor genes can lead to cancer.

Answer 6

Methylation prevents transcription of gene;
Protein not produced that prevents cell division / causes cell death / apoptosis;
No control of mitosis.

Question 7

Describe how alterations to tumour suppressor genes can lead to the development of tumours.

Answer 7

(Increased) methylation (of tumour suppressor genes);
Mutation (in tumour suppressor genes);
Tumour suppressor genes are not transcribed/expressed
OR
Amino acid sequence/primary/ tertiary structure altered;
(Results in) rapid/uncontrollable cell division;

Question 8

Describe what is meant by a malignant tumour

Answer 8

mass of undifferentiated / unspecialised / totipotent cells;
uncontrolled cell division;
(not ‘repeated’)
metastasis / (cells break off and) form new tumours /
spread to other parts of body;

Question 9

Describe how altered DNA may lead to cancer.

Answer 9

(DNA altered by) mutation;
(mutation) changes base sequence;
of gene controlling cell growth / oncogene / that monitors cell division;
of tumour suppressor gene;
change protein structure / non-functional protein / protein not formed;
(tumour suppressor genes) produce proteins that inhibit cell division;
mitosis;
uncontrolled / rapid / abnormal (cell division);
malignant tumour;

Question 10

Define epigenetics

Answer 10

Heritable phenotype changes (gene function) that do not involve alterations in the DNA sequence/mutation.

Question 11

What is the proteome of a cell?

Answer 11

The proteome is the full range of / number of different proteins that a cell is able
to produce (at a given time);
OR
The proteome is the full range of / number of different proteins the genome /
DNA is able to code for

Question 12

Give two structural differences between a molecule of messenger RNA (mRNA) and a molecule of transfer RNA (tRNA).

Answer 12

mRNA does not have hydrogen bonds / base pairing but tRNA does;
OR
mRNA is linear / straight chain but tRNA is cloverleaf;
2. mRNA does not have an amino acid binding site but tRNA does;
Accept mRNA cannot carry an amino acid, tRNA can
3. mRNA has more nucleotides;
Accept mRNA is longer or converse
4. (Different) mRNAs have different lengths but all tRNAs are similar / same
length;
5. mRNA has codons but tRNA has an anticodon;

Question 13

Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.

Do not include descriptions of transcription and splicing in your answer 5

Answer 13

mRNA associates with a ribosome / ribosome attaches to mRNA;

Idea of association is required

2. Ribosome moves to / finds the start codon / AUG on the mRNA;
3. tRNA brings / carries (appropriate / specific) amino acid;

Must be explicitly stated and not inferred.

4. Anticodon (on tRNA complementary) to codon (on mRNA) by complementary base pairing / anticodon-codon complex
5. Ribosome ‘fits’ around two codons / can fit two tRNAs;

Must be explicitly stated and not inferred.

6. Amino acids join by peptide bonds / condensation reaction to form polypeptide (using the energy from ATP hydrolysis);
7. Ribosome moves along to next codon on mRNA and process repeated

Question 14

Describe the role of two named enzymes in the process of semi-conservative replication of DNA.

Answer 14

(DNA) helicase causes breaking of hydrogen/H bonds (between DNA strands);

Reject ‘helicase hydrolyses hydrogen bonds’.

2. DNA polymerase joins the (DNA) nucleotides;

Reject if suggestion that DNA polymerase joins the complementary nucleotides or forms H bonds.

Reject if joining RNA nucleotides or forming RNA.

3. Forming phosphodiester bonds;

Question 15

Scientists have investigated the use of different types of stem cell to treat damage to the heart after a myocardial infarction. During a myocardial infarction, a number of different cell types in the heart die. This includes cardiomyocytes which are heart-muscle cells.

Embryonic pluripotent stem cells (ESCs) can divide and differentiate into a wide range of different cell types.

(a) Using the information given, suggest one reason why ESCs might be suitable to treat damage to the heart.

Answer 15

(ESCs) can replace any type of (heart) cell;

Accept named type of cell, e.g. heart muscle cell

Question 16

Scientists have investigated the use of different types of stem cell to treat damage to the heart after a myocardial infarction. During a myocardial infarction, a number of different cell types in the heart die. This includes cardiomyocytes which are heart-muscle cells.

Embryonic pluripotent stem cells (ESCs) can divide and differentiate into a wide range of different cell types.
ESCs have not yet been used to treat people who have had a myocardial infarction. This is because of concern that the use of ESCs might lead to more harm to the person. One way that ESCs might lead to more harm is by differentiating into the wrong types of cells.

Suggest one other way that putting ESCs into a person’s heart might lead to more harm to the person.

Answer 16

Might divide out of control;

2. Leading to tumour / cancer;

Question 17

Define what is meant by epigenetics.

Answer 17

Heritable changes in gene function;

2. Without changes to the base sequence of DNA;

Question 18

Explain how increased methylation could lead to cancer

Answer 18

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

Question 19

Give one way in which benign tumours differ from malignant tumours

Answer 19

Cells of benign tumours cannot spread to other parts of the body / metastasise;

OR

Cells of benign tumours cannot invade neighbouring tissues.

Accept answers clearly in the context of malignant tumours.

Question 20

Explain how the methylation of tumour suppressor genes can lead to cancer.

Answer 20

Methylation prevents transcription of gene;

2. Protein not produced that prevents cell division / causes cell death / apoptosis;
3. No control of mitosis.

Question 21

i)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)

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

Answer 21

(Receptor / transcription factor) binds to promoter which stimulates RNA polymerase / enzyme X;

Transcribes gene / increase transcription;

ii)Other cells do not have the / oestrogen / ERα receptors;

But do not accept receptors in general

Question 22

Describe two ways in which both types of tumour may cause harm to the body

Answer 22

may damage organ concerned; may cause blockages / obstructions; may damage / exert pressure on other organs;

Question 23

i)Explain the link between sunbathing and skin cancer. 2

ii)Suggest why fair-skinned people are at a greater risk of skin cancer than dark-skinned people when sunbathing.

iii)Suggest why people with a family history of cancer are at a greater risk of cancer than those with no family history of cancer. 1

Answer 23

i)because sun’s radiation contains ultra violet radiation; this causes mutation of genes which control division;
ii)because fair skin has little melanin which protects against u.v. radiation;

iii)because cancer has genetic component / may have inherited (onco)gene / gene which gives predisposition to / causes cancer;

Question 24

Scientists found a correlation between prostate cancer and exposure to cadmium ions.

The scientists investigated the effects of cadmium ions on cells from a human prostate gland. They grew a culture of these cells in liquid growth medium and removed samples at intervals.

For each sample they measured

• how much DNA was not methylated, * the activity of the enzyme methyltransferase.

Methyltransferase is an enzyme that adds methyl groups to some of the bases in DNA. The addition of a methyl group is called methylation.

(a) The scientists set up another culture as a control.

Describe how the scientists would have set up a control experiment for this investigation (2)
b)

Answer 24

a)No cadmium; Other conditions same as cadmium-treated group;