The Control of Gene Expression

Question 1

what is a mutation?

Answer 1

any change to the base (nucleotide) sequence of DNA

Question 2

how are mutations caused?

Answer 2

by errors during DNA replication

Question 3

how can the rate of mutations be increased?

Answer 3

by mutagenic agents

Question 4

list the types of mutations that can occur

Answer 4

substitution
deletion
addition
duplication
inversion
translocation

Question 5

what is substitution?

Answer 5

one or more bases are swapped for another

Question 6

what is deletion?

Answer 6

one or more based are removed

Question 7

what is addition?

Answer 7

one or more based are added

Question 8

what is duplication?

Answer 8

one or more bases are repeated

Question 8

what is inversion?

Answer 8

a sequence of bases is reversed

Question 9

what is translocation?

Answer 9

a sequence of bases is moved from one location in the genome to another. this could be the movement within the same chromosome or movement to a different chromosome.

Question 10

what is the result of a mutation in a polypeptide?

Answer 10

a mutation in a polypeptide that makes up an enzyme may change the shape of the enzyme’s active site. this may stop substrates from being able to bind to the active site, leaving the enzyme unable to catalyse the reaction.

Question 11

why don’t all mutations affect the order of amino acids?

Answer 11

the degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet. this means not all types of mutations will always result in a change to the amino acid sequence of the polypeptide.

Question 12

what mutations do change the amino acid sequence of a polypeptide and why?

Answer 12

additions, duplications and deletions.
this is because these mutations all change the number of bases in the DNA code. this causes a frameshift in the base triplets that follow, so that the triplet code is read in a different way.

Question 13

what are some examples of mutagenic agents?

Answer 13

ultraviolet radiation, ionising radiation, some chemicals and some viruses.

Question 14

what are the ways that mutagenic agents are able to increase the rate of mutations?

Answer 14

acting as a base — chemicals called base analogs can substitute for a base during DNA replication, changing the base sequence in the new DNA.
altering bases — some chemicals can delete or alter bases.
changing the structure of DNA — some types of radiation can change the structure of DNA, which causes problems during DNA replication.

Question 15

what are stem cells?

Answer 15

stem cells are unspecialised cells that can develop into other types of cell.
stem cells divide to become new cells, which then become specialised.
all multicellular organisms have some form of stem cell.

Question 16

where are stem cells found?

Answer 16

stem cells are found in the embryo and in some adult tissues.

Question 17

what are totipotent cells?

Answer 17

these are stem cells that can mature/develop into any type of body cell in an organism.
they are only present in mammals in the first few cell divisions of an embryo.

Question 18

what are multipotent stem cells?

Answer 18

stem cells present in adult mammals. these are able to differentiate into a few different types of cell.

Question 19

what are unipotent stem cells?

Answer 19

stem cells present in adult mammals. these can only differentiate into one type of cell.

Question 20

what are pluripotent stem cells?

Answer 20

stem cells that can specialise into any cell in the body. but they don’t have the ability to become the cells that make up the placenta.

Question 21

why can stems cells become specialised?

Answer 21

because different genes are expressed.

Question 22

how do stem cells become specialised?

Answer 22

• stem cells all contain the same genes — but during development, not all of them are transcribed and translated; expressed.
• under the right conditions, some genes are expressed and others are switched off.
• mRNA is only transcribed from specific genes.
• the mRNA from these genes is then translated into proteins.
• these proteins modify the cell — they determine the cell structure and control its functions.
• changes to the cell produced by these proteins cause the cell to become specialised. these changes are difficult to reverse, so once a cell has specialised it stays specialised.

Question 23

what are cardiomyocytes?

Answer 23

they are heart muscle cells that make up a lot of the tissue in our hearts.

Question 24

what was the previous idea about cardiomyocytes from scientists?

Answer 24

that mature mammals are not able to regenerate their own heart cells at all. and so if their hearts become damaged or worn out due to old age this is a major problem.

Question 25

what do scientists now think about cardiomyocytes?

Answer 25

if old or damaged, cardiomyocytes can be replaced by new cardiomyocytes derived from a small supply on unipotent stem cells in the heart.

Question 26

how can stem cells be used to treat human disorders?

Answer 26

stem cells can be used to replace cells damaged by illness or injury.
stem cell therapies exist where bone marrow which contain stem cells can become specialised to form any type of blood cell. bone marrow transplants can be used to replace the faulty bone marrow in patients that produce abnormal blood cells. the stem cells in the transplanted bone marrow divide and specialise to produce healthy blood cells. this technique has been used to treat leukaemia and lymphoma.

Question 27

list 3 conditions with its theoretical solutions that scientists have researched, where stem cells can be used as treatment.

Answer 27

spinal cord injuries — stem cells could be used to replace damaged nerve tissue.
damage caused by heart attacks — stem cells could be used to replace damaged heart tissue.
organ transplants — organs could be grown from stem cells to provide new organs for people on donor waiting lists.

Question 28

what are the huge benefits to using stem cells in medicine?

Answer 28

it could save many lives — stem cells could be used to grow organs for those people awaiting transplants.
it could improve the quality of life for many people — it can replace the damaged cells in the eyes of people who are blind.

Question 29

what are the 3 main potential sources of human stem cells?

Answer 29

adult stem cells
embryonic stem cells
induced pluripotent stem cells

Question 30

what are adult stem cells?

Answer 30

• these are obtained from the body tissues of an adult.
• they can be obtained in a relatively simple operation with very little risk involved, but quite a lot of discomfort.
• adult stem cells aren’t as flexible as embryonic stem cells — they can only specialise into a limited range of cells, not all body cell types.
• they’re multipotent.

Question 31

what are embryonic stem cells?

Answer 31

• these are obtained from embryos at an early stage of development.
• embryos are created in a lab using IVF — egg cells are fertilised by sperm outside the womb.
• once the embryos are approximately 4 to 5 days old, stem cells are removed from them and the rest of the embryo is destroyed.
• embryonic stem cells can divide an unlimited number of times and develop into all types of body cells.
• they’re pluripotent.

Question 32

what are induced pluripotent stem cells (iPS cells)?

Answer 32

• they are created by scientists in the lab. the process involves ‘reprogramming’ specialised adult body cells so that they become pluripotent.
• the adult cells are made to express a series of transcription factors that are normally associated with pluripotent stem cells. the transcription factors cause the adult body cells to express genes that are associated with pluripotency.
• iPS Cells could become really useful in research and medicine in the future. however more research is needed on true pluripotent embryonic stem cells before they can be properly utilised.

Question 33

what are the ethical issues surrounding embryonic stem cell use?

Answer 33

• obtaining stem cells from embryos created by IVF raises ethical issues because the procedure results in the destruction of an embryo that could become a fetus if placed in the womb.
  some people believe that at the moment of fertilisation an individual is formed has the right of life so they believe it is wrong to destroy embryos.

Question 34

what are other issues surrounding stem cell use?

Answer 34

the use of iPS Cells can be used to grow a patients tissue or an organ for a transplant however there is a risk of rejection from the patients body as their immune system may recognise the tissue as foreign and attack it.

Question 35

what is the transcription of genes controlled by?

Answer 35

protein molecules called transcription factors.

Question 36

how do transcription factors operate?

Answer 36

• in eukaryotes, transcription factors move from the cytoplasm to the nucleus.
• in the nucleus they bind to specific DNA sites near the start pf their target genes — the genes they control the expression of.
• they control expression by controlling the rate of transcription.

Question 37

what are the types of transcription factors?

Answer 37

activators
repressors

Question 38

what are activators?

Answer 38

they stimulate or increase the rate of transcription — e.g. they help RNA polymerase bind to the start of the target gene and activate transcription.

Question 39

what are repressors?

Answer 39

they inhibit or decrease the rate of transcription — e.g. they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription.

Question 40

what other molecule can affect the expression of genes?

Answer 40

oestrogen

Question 41

how does oestrogen affect transcription?

Answer 41

oestrogen is a steroid hormone and it affects transcription by binding to a transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex.
the complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene.
the complex can act as an activator of transcription.

Question 42

can oestrogen act as a repressor?

Answer 42

yes, on some cells it can. it depends on the type of cell it is and the target gene.