control of gene expression

Question 1

what is a point mutation

Answer 1

the change of 1 base as it occurs at only 1 point in the sequence

Question 2

what are the 3 types of substitution mutation

Answer 2

1. nonsense
2. mis-sense
3. silent

Question 3

what is a nonsense mutation

Answer 3

an amino acid codon mutates to become a stop codon, stopping protein synthesis (translation)

Question 4

what is a mis-sense mutation

Answer 4

an amino acid sequence changes, possibly changing the structure and function of the protein

Question 5

what is a silent mutation

Answer 5

the genetic code is degenerate so a mutation doesnt always change the AA sequence

Question 6

what is an inversion mutation

Answer 6

a group of bases separates from dna sequence and rejoins at the same position but in inverse order. the base sequence is reversed and affects the resulting AA sequence

Question 7

what is a duplication mutation

Answer 7

can be just 1 base duplicated (has the same effect as an addition). causes a frame shift to the right. if 3 bases are duplicated, a new AA is added

Question 8

what is a translocation mutation

Answer 8

a base group separates from a dna sequence on 1 chromosome and becomes inserted into the dna sequence of a different chromosome. the mutations often have significant effects on gene expression, leading to an abnormal phenotype

Question 9

examples of mutagens

Answer 9

• high energy radiation (uv radiation and x-rays)
• carcinogens in cigarettes and car exhaust fumes
• alcohol

Question 10

effect of mutations in somatic cells (non-reproductive body cells)

Answer 10

it will only affect cells that derive from that cell, so will likely have a small local effect like a birthmark

Question 11

effect of mutations in germ-line cells (reproductive cells/gametes)

Answer 11

they will affect every single cell of the resulting organism as well as its offspring. these mutations are one source of genetic variation

Question 12

what are stem cells

Answer 12

undifferentiated cells that can divide by mitosis to form more stem cells (keep dividing and make more cells) OR cells that can differentiate into a specific cell type

Question 13

why do different cells make different proteins

Answer 13

different genes are expressed/transcribed and then translated/switched on. some genes are switched on in all cells e/g those involved in respiration while some are switched on/off as required

Question 14

how can gene expression be controlled

Answer 14

• prevention of transcription so prevention of mRNA production
• prevention of translation

Question 15

key features of stem cells

Answer 15

• potency - have the potential to differentiate into specialised cell types
• immortality - they can divide indefinitely

Question 16

ethical concerns of stem cell use

Answer 16

can be objected to on religious grounds if a person believes life begins at conception

Question 17

types of stem cell: totipotent

Answer 17

can differentiate into any cell type. found in early embryos up to the 32 cell stage

Question 18

types of stem cell: pluripotent

Answer 18

can differentiate into nearly all cell types. found in blastocytes (5-day old embryos containing ~150 cells before implantation)

Question 19

types of stem cell: multipotent

Answer 19

found in some tissues throughout life so are adult/somatic stem cells. can differentiate into a related family of cells (few cell types). used by the body to replace and repair damaged tissues. have a limited capacity to the types of cell they can change into

Question 20

types of stem cell: unipotent

Answer 20

can only differentiate into a single type of cell. derived from multipotent cells and are made in adult tissues

Question 21

sources of animal stem cells

Answer 21

• embryos - unused from IVF (any cell type) (licensed research is only allowed on embryo cells created outside the body)
• adult cells (few cell types)
• umbilical cord (limited cell types)
• placental cells (limited cell types)

Question 22

what are induced pluripotent stem cells (iPS cells)

Answer 22

a type of pluripotent cell that is produced from a unipotent cell (derived from multipotent) - the unipotent cell can be any body cell

Question 23

what are iPS cells used for

Answer 23

they are genetically altered in the lab to make them acquired characteristics of embryonic stem cells. this involves inducing genes and transcription factors within the cell i.e turning on genes that were switched off

Question 24

features of iPS cells

Answer 24

• very similar to embryonic stem cells in form and function but are not exact duplicates
• capable of self-renewal so therefore could replace embryonic stem cells in medical research and treatment, thus overcoming many ethical issues

Question 25

what is regulation of protein synthesis

Answer 25

regulating the switching on or off of genes and hence determining which genes are expressed

Question 26

how can transcription be regulated

Answer 26

- for transcription to occur, the gene must be stimulated by specific transcriptional factors - these move from the cytoplasm into the nucleus via a nuclear pore and bind to a specific region (the promoter region) of dna in the nucleus stimulating it to begin transcription - these TFs are associated with a receptor and an inhibitor in the cytoplasm - if the receptor is not stimulator then the inhibitor will prevent the TF moving into the nucleus and therefore prevent transcription occurring

Question 27

regulation of transcription example: oestrogen

Answer 27

- oestrogen is a lipid soluble molecule meaning it can easily cross the phospholipid bilayer and enter the cytoplasm - it can then bind to the receptor associated with the TF. this causes a conformational change in the molecule which moves the inhibitor from the TFs binding site - the TF can now enter the nucleus through a nuclear pore and combine with the promoter region of the dna - the combination of the TF and the dna stimulates transcription of the gene that makes up that part of dna

Question 28

overview: siRNA

Answer 28

- small interfering rna - control of translation - can act as repressors - 20-25 nucleotides in length - double-stranded rna - breaks down mRNA before translation

Question 29

how does siRNA break down mRNA before translation

Answer 29

1. an enzyme (a dicer) cuts large double-stranded rna (dsRNA) into smaller bits (siRNA) 2. 1 of 2 siRNA strands (anti-sense) combines with an enzyme. this enzyme is not actually an enzyme - RISC (RNA induced silencing complex) 3. the siRNA molecule guides the RISC to an mRNA molecule by pairing its bases with the complementary bases on the mRNA. the enzyme now cuts the mRNA into smaller sections 4. the mRNA is no longer able to be used in translation so no polypeptide is made and therefore the gene is not expressed even though the dna codes for it

Question 30

what are the uses of siRNA

Answer 30

- identifying the roles of genes in a biological pathway. siRNA that blocks a gene could be added to cells and the effects observed to see the role of the blocked gene - gene-caused diseases could be prevented by using siRNA to block these genes - could be used to prevent HIV replication in cultures - could silence oncogenes (cancer treatment)

Question 31

what is epigenetics

Answer 31

the idea that parents pass on more than genes to their offspring. environmental influences can alter the inheritance of an organism's offspring. acquired characteristics can be passed on

Question 32

what is the epigenome

Answer 32

dna and histones are covered in chemicals (tags). this second layer of tags is called the epigenome. it determines the shape of the dna-histone complex

Question 33

what is methylation

Answer 33

the addition of a methyl group (CH3) (acts as a tag) (to a nerve cell) causes tighter curling of myelin protein around the nerve cell. CH3 is added to the C bases of dna

Question 34

how can an environmental signal affect genetics

Answer 34

- an environmental signal stimulates proteins to carry its message inside the cell - here the message passes to a specific protein which can be attached to a specific sequence of bases on the dna - once attached, the protein can either change: 1. acetylation of histones leading to the activation or inhibition of a gene 2. methylation of dna by attracting enzymes that can add or remove methyl groups

Question 35

what is acetylation

Answer 35

the process whereby an acetyl group is transferred to a molecule

Question 36

what is deacetylation

Answer 36

the removal of an acetyl group from a molecule

Question 37

what is the effect of decreased acetylation of associated histones

Answer 37

decreased acetylation increases the positive charges of histones and therefore increases their attraction to the phosphate groups of dna. this makes the associated between the dna and histones stronger so the dna is less accessible to TFs. the TFs cant initiate mRNA production (transcription) from the dna i.e the gene is switched off

Question 38

how can increased methylation of dna inhibit the transcription of genes

Answer 38

1. preventing the binding of TFs to the dna by causing the surrounding myelin protein to curl tighter around it, making it inaccessible to TFs 2. attracting proteins that condense the dna-histone complex (by inducing deacetylation of the histones) making the dna inaccessible to TFs