gene expression is controlled by a number of factors

Question 1

define a stem cell

Answer 1

an undifferentiated cell that can divide indefinitely and turn into other specific cell types

Question 2

what are the 3 types of stem cells?

Answer 2

1. totipotent
2. pluripotent
3. multipotent

Question 3

what are totipotent stem cells

Answer 3

• can develop into any body cell type including the placenta and embryo
• all genes in the nucleus must be activated

Question 4

explain what happens to totipotent cells during embryonic development?

Answer 4

• certain parts of the DNA are selectively translated so that only some genes are switched on
• in order to differentiate the cell into a specific type and form the tissues that make up the foetus

Question 5

explain how stem cells become specialised via differentiation

Answer 5

• not all of the genes are transcribed and translated
• under the right conditions, some genes are expressed and others are switched off
• mRNA is transcribed from specific genes
• the mRNA is translated into proteins
• these proteins modify the cell, they determine the cell structure and control cell processes
• changes to the cell produced by the protein cause the cell to become specialised

Question 6

what are pluripotent stem cells

Answer 6

• can develop into any cell type including the 3 germ layers (any tissue in the body) excluding the placenta and embryo
• majority of the genes are still able to be activated

Question 7

what are multi-potent stem cells

Answer 7

• can divide to form different cell types with a limited range (closely related cell types)
• some genes have been switched off but they have the genes for some different cels still available

Question 8

what are unipotent stem cells

Answer 8

• cells that can only form one other type of cell
• they can self-renew
• lots of genes have been switched off via regulation of translation factors

Question 9

what is a unique feature of pluripotent cells and the use of this feature

Answer 9

they can divide in unlimited numbers, and can therefore be used to repair or replace damaged tissue

Question 10

what is an example of a unipotent cell?

Answer 10

cardiomyocytes

Question 11

how does our heart recover from a myocardial infraction

Answer 11

• unipotent stem cells differentiate into new cardiomyocyte cells
• so cardiomyocytes are generated and replace the old damaged cells

Question 12

which types of stem cells are found in embryos

Answer 12

• totipotent and pluripotent
• multi-potent and unipotent cells are only found in mature mammals

Question 13

what are some uses of stem cells

Answer 13

1. medical therapies for example bone marrow transplants
2. drug testing on artificially grown tissues
3. research on formation of organs and embryos

Question 14

evaluate the use of stem cells in medicine

Answer 14

1. unused IVF embryos can be donated to research facilities instead of being destroyed, some people object to this as it is the destruction of a potential foetus
2. egg cells frozen from IVF which have not been fertilised can be stimulated to divide, causes less objection as they would not survive after a few days if implanted into a womb and would not create a foetus
3. it is possible to use adult stem cells for some treatments which people feel is more ethical, however they cannot develop into all the specialised cell types that embryonic stem cells can

Question 15

explain how are induced pluripotent stem cells produced

Answer 15

• from mature, fully specialised (somatic cells)
• that are reprogrammed into specialised adult body cells to become pluripotent
• the cell regains the capacity to differentiate (express genes) through the use of transcription factors

Question 16

explain how transcription factors can be introduced to the iPS cells

Answer 16

• infecting the iPS cells with a specially-modified virus
• the virus has genes coding for the transcription factor within its DNA
• when it infects the adult cell, these genes are passed into the iPS cell so the cell is able to produce the transpiration factors

Question 17

what are the advantages of using iPS cells over stem cells?

Answer 17

• iPS cells do not require the destruction of an embryo, unlike stem cells, so they are more ethical
• iPS cells can be obtained from adult tissue unlike stem cells which are obtained from the foetus’ so more ethical
• iPS cells can be made from the patient’s own cells so they are genetically identical to the patient’s cells and the body would not reject them since the immune system doesn’t recognise the antigens as foreign

Question 18

what is a transcription factor?

Answer 18

a protein that controls the rate of transcription of genes so only certain parts of the DNA are expressed

Question 19

outline how transcription factors work

Answer 19

1. transcription factors move from the cytoplasm into the nucleus
2. in the nucleus they bind to specific DNA sites (the promoter region) near the start of their target genes
3. makes it easier/more difficult for RNA polymerase to bind to a gene, this increases/decreases the rate of transcription

Question 20

what are activators

Answer 20

transcription factors that stimulate or increase the rate of transcription by helping RNA polymerase to bind to the start of the target gene and activate transcription

Question 21

what are repressors

Answer 21

transcription factors that inhibit or decrease the rate of transcription by binding to the promoter region of the target gene, preventing RNA polymerase from binding which stops transcription

Question 22

give an example of a hormone that affects transcription and explain how it works

Answer 22

• steroid hormone oestrogen binds to an oestrogen receptor, forming an oestrogen-oestrogen receptor complex
• the complex moves from the cytoplasm into the nucleus where it binds to promoter regions near the start of the target gene
• the complex can act as an activator of transcription which helps RNA polymerase to bind to the start of the target gene

Question 23

what is RNAi

Answer 23

• small double stranded RNA molecule that stops target genes from being translated into proteins
• found in eukaryotes

Question 24

explain how siRNA and miRNA work in plants

Answer 24

• once transcribed mRNA leaves the nucleus for the cytoplasm
• double-stranded siRNA associates with several proteins and unwinds
• a single strand binds to the target mRNA
• base sequence of the siRNA is complementary to the base sequence is sections of the target mRNA
• the proteins associated with the siRNA cut mRNA into fragments so it can no longer be translated
• fragments move into a processing body

Question 25

explain how miRNA work in mammals

Answer 25

- miRNA isn’t fully complementary to the target mRNA, so it can target more than 1 mRNA molecule - **it associates with proteins** and binds to target mRNA in the cytoplasm - **miRNA-protein complex** physically blocks the translation of the target mRNA - mRNA is moved into a processing body where it can be stored or degraded - when it’s stored it can be returned and translated at another time

Question 26

what is meant by epigenetics?

Answer 26

a heritable change in gene function without change to the base sequence of DNA

Question 27

how does increased methylation of DNA affect gene transcription

Answer 27

- involves addiction of CH3 group to cytosine bases which are next to guanine - causes DNA to be more tightly coiled around histone protein - prevents transcription factors from binding - therefore gene transcription is supressed

Question 28

how does **decreased acytelation of histones** affect gene transcription

Answer 28

- positively charged histones bind to negatively charged DNA - decreasing acytelation increases positive charge of histones - binding becomes too tight and prevents transcription factors from accessing the DNA - therefore gene transcription is suppressed

Question 29

how might epigenetic changes affect humans

Answer 29

they can cause disease, either by over activating a gene’s function or by suppressing it

Question 30

give an application of epigenetics

Answer 30

- treatments of various diseases - development of ways to reverse epigenetic changes

Question 31

describe the process of RNA interference, including the organism in which it occurs

Answer 31

- rna molecules act to inhibit gene expression - usually by destroying mrna so that it cannot be translated - occurs in eukaryotes and some prokaryotes

Question 32

give some characteristics of benign tumours

Answer 32

- slow growth - defined by a clear boundry due to the cell adhesion molecules - cells retain function and normal shape - don’t spread easily - easy to treat

Question 33

give some characteristics of malignant tumours

Answer 33

- rapid, uncontrollable growth - ill-defined boundary with finger like projections - cells do not retain function and often die - spreads quickly and easily via metastasis - difficult to treat

Question 34

describe the role of tumour-suppressor genes

Answer 34

- code for proteins that control cell division - stop the cell cycle when damage is detected - involved in programming apoptosis (self-destruction) of the cell

Question 35

explain how tumour-suppressor genes can be involved in developing cancer

Answer 35

- a mutation in the gene could code for a non-functional protein - increased methylation of decreased acetylation could prevent transcription - cells will divide uncontrollably resulting in a tumour

Question 36

describe the role of proto-oncogenes

Answer 36

- control cell division - code for protein that stimulate cell division

Question 37

explain how proto-oncogenes can be involved in developing cancer

Answer 37

- mutation in the gene could turn it into a permanently activated oncogene - decreased methylation or increased acetylation could cause excess transcription - this results in uncontrolled cell division and formation of a tumour

Question 38

explain how abnormal methylation of genes can cause cancer

Answer 38

- hyper-methylation of tumour supressor genes or oncogenes can impact their function - and cause the cell to divide uncontrollably