gene expression

Question 1

what are stem cells?

Answer 1

undifferentiated cells which can divide indefinitely by mitosis. Stem cells first divide and then differentiate into specialised cells by expressing (transcribing and translating) certain genes in their genome.

Question 2

what is a totipotent cell?

Answer 2

Totipotent cells can divide and differentiate into any type of body cell, e.g. zygotes divide into totipotent cells. Occur in early mammalian embryos.

Question 3

what is a pluripotent cell?

Answer 3

divide + differentiate into most types of body cells e.g. later embryos cells

Question 4

what is a multipotent cell?

Answer 4

divide + differentiate into limited types of body cells e.g. mature mammals after birth. blood MP stem cells- red, white, platelets

Question 5

what is a unipotent cell?

Answer 5

divide + differentiate into one type of cell e.g. cardiomyocytes (heart muscle cells)

Question 6

how can stem cells help replace damaged nerves in spinal cords after injury?

Answer 6

stem cells can divide and differentiate into healthy neurones to replace the damaged neurones

Question 7

why are embryonic stem cells more useful than adult?

Answer 7

they are pluripotent and can differentiate into most cell types. Adult stem cells are only multipotent.

Question 8

give an example of use of adult stem cells

Answer 8

treating leukemia (a white blood cell cancer). Bone marrow, which contains blood stem cells, is taken from a close relative to reduce chances of rejection due to similar genetics. The bone marrow is transplanted to the patient and the stem cells divide and differentiate into healthy white blood cells giving a long term treatment.

Question 9

ethics embryonic stem cells

Answer 9

• Some people consider that embryos have human status from conception.
• Others believe that the embryo is a potential human being but at the early stages it is not a true human being and the removal of cells from spare embryos for medicine is fine
• Others believe that embryos have no moral rights whatsoever.

Question 10

describe iPs + advantages

Answer 10

• Specialised cells are removed from a patient.
• Transcription factors associated with genes causing pluripotency are added to the somatic cells causing them to become pluiripotent. They are now known as iPS cells (similar to embryonic stem cells in form and function).
• Factors are added to iPS cells to cause them to divide and differentiate into the specialised cell required e.g. neurone.
• The specialised cells are then transplanted back into the patient
  Advantages of iPS cells:
• They can differentiate into any cell type for medicine.
• Divide indefinitely
• No ethical issues as embryonic cells are not being used.
• Less chance of rejection due to being the patient’s own cells

Question 11

describe plant cell differentiation with tissue culture

Answer 11

• Within plant tissue culture totipotent plant cells are grown in sterile conditions to produce a callus – a mass of undifferentiated plant cells.
• The callus is placed in a growth medium and subjected to specific conditions.
• Under these condition the callus differentiate into specific plant tissues or tiny plantlets which grown into whole plants.

Question 12

what is a transcription factor?

Answer 12

• Transcription factors are proteins.
• Transcription factors move from the cytoplasm to the nucleus.
• They bind a promotor (specific base sequence) of a gene.
• This stimulates or inhibits RNA polymerase to transcribe the gene and produce mRNA.

Question 13

how does oestrogen initiate transcription?

Answer 13

1. Oestrogen is lipid soluble and passes through the phospholipid bilayer by diffusion
2. Oestrogen binds to its complementary receptor in the cytoplasm forming a complex.
3. The receptor/oestrogen complex is a transcription factor which moves to the nucleus and binds to a promoter on the DNA.
4. This stimulates RNA polymerase to transcribe the target gene and produce mRNA.

Question 14

what is siRNA?

Answer 14

• siRNA is short, single stranded and bound to a RISC protein which acts as an enzyme.
• The single strand has a specific complementary base sequence to the mRNA that is to be destroyed.
• The siRNA binds to the mRNA and the RISC enzyme hydrolyses a phosphodiester bond in the mRNA in the sugar-phosphate backbone, destroying the mRNA.
• The mRNA can no longer be used in translation, and is broken down into individual nucleotides by other enzymes.

Question 15

how can siRNA be used?

Answer 15

can be added via viruses into cells to prevent the expression of a target gene by destroying its mRNA, to allow us to study what the gene does

Question 16

how is miRNA different to siRNA?

Answer 16

The base sequence on miRNA is not fully complementary to the mRNA and therefore is not specific to one mRNA and therefore targets more than one type of mRNA. siRNA is only specific to one mRNA.

Question 17

what is epigenetics?

Answer 17

Epigenetics involves heritable changes in gene function without changing the base sequence of DNA

Question 18

how does DNA methylation prevent transcription?

Answer 18

INCREASED methylation- Methylation is when a methyl group (CH3) is attached to a cytosine base on DNA. This prevents transcription factors from binding the promotor on DNA. Therefore RNA polymerase is not activated and transcription cannot happen.

Question 19

how does histone acetylation prevent transcription?

Answer 19

DECREASED acetylation- causes chromatin to
condense so transcription factors can’t bind to the
promotor on DNA, RNA polymerase is not activated and
transcription can’t be initiated. Increased acetylation causes
chromatin to uncondense, allowing the transcription factor to bind the promotor, activate RNA polymerase and cause
transcription.

Question 20

how can epigenetic marks be inherited?

Answer 20

if they are present on the chromatin of gametes

Question 21

what is a tumour suppressor gene + how do mutations result in uncontrolled cell division?

Answer 21

genes that slow cell division by producing proteins that stop mitosis. Mutations to the tumour suppressor genes, changes the DNA base sequence which changes the amino acid sequence of the protein
* Therefore the protein is non-functional and no longer prevents cell division
* Leading to uncontrolled cell division, forming tumours.

Question 22

what is a proto-oncogene + how do mutations lead to uncontrolled cell division?

Answer 22

genes that stimulate cell division by producing proteins that cause mitosis.
Mutation of proto-oncogenes to oncogenes causes the production of more proteins which cause cell division.
* Leading to uncontrolled cell division, forming tumours.

Question 23

what is a malignant tumour?

Answer 23

cancerous tumours that grow rapidly and cause damage to other tissues. Cells may break off and spread to other parts of the body (metastasis)

Question 24

what is a benign tumour?

Answer 24

non cancerous tumours that grow slowly. They don’t cause damage to other tissues. Cells do not break off and spread to other parts of the body (they don’t metastasise)

Question 25

how does increased methylation of a TS gene lead to uncontrolled cell division?

Answer 25

the transcription factor cannot bind the promotor, RNA polymerase is not activated and transcription is prevented. The proteins which prevent cell division are not translated leading to uncontrolled cell division and formation of a tumour.

Question 26

how does decreased methylation of an oncogene lead to uncontrolled cell division?

Answer 26

the transcription factor can bind the promotor, activating RNA polymerase and causing transcription. Therefore the proteins which cause cell division are translated leading to uncontrolled cell division and formation of a tumour.

Question 27

how can increased oestrogen increase risk of breast cancer?

Answer 27

due to its role with the receptor as a transcription factor
1. Breast cells to divide more often.
2. Therefore more likely that mutations occur when DNA replicates.
3. Faster division of cancerous cells will produce tumours more quickly.