6.1.1 Cellular Control

Question 1

Define a gene mutation

Answer 1

a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
- occurs continuously and spontaneously

Question 2

Define mutagen

Answer 2

a factor that increases the probability of a mutation occurring

Question 3

Give four examples of mutagens

Answer 3

1) ionising radiation
2) deaminating chemicals
3) addition of methyl/ethyl groups into bases
4) viruses inserting sections of viral DNA

Question 4

Why is ionising radiation a mutagen

Answer 4

it can break the DNA strands

Question 5

Why are deaminating chemicals mutagens

Answer 5

they can alter the chemical structure of bases

Question 6

Why is the addition of methyl/ethyl groups into bases a mutagen

Answer 6

it leads to incorrect base pairing

Question 7

Why do mutations usually have no effect

Answer 7

they only slightly alter the polypeptide
this means its structure/function isn’t changed as the genetic code is degenerate
many mutations occurs in non-coding sections of DNA so have no effect on the amino acid sequence at all

Question 8

What is an insertion mutation

Answer 8

a nucleotide is randomly inserted into the DNA sequence

Question 9

What effect does an insertion/deletion mutation have

Answer 9

it changes the amino acid that would of been coded by the original codon
a frameshift mutation occurs
this can dramatically change the amino acid sequence produced, and therefore the ability of the polypeptide to function

Question 10

What is a frameshift mutation

Answer 10

it shifts the reading frame of the genetic code
this changes the triplets further in the DNA sequence

Question 11

Define deletion mutation

Answer 11

a nucleotide is randomly deleted from the DNA sequence

Question 12

What is a substitution mutation

Answer 12

occurs when a base in the DNA sequence is randomly swapped for a different base

Question 13

Why is a substitution mutation potentially less harmful than a deletion/addition mutation

Answer 13

the mutation will only change the amino acid for the single codon

Question 14

Give the 3 types of substitution mutations

Answer 14

1) silent mutation
2) missense mutation
3) nonsense mutation

Question 15

What is a silent mutation

Answer 15

when the mutation doesn’t alter the amino acid sequence of the polypeptide
(due to degenerate genetic code)

Question 16

What is a missense mutation

Answer 16

when the mutation alters a single amino acid in the polypeptide chain

Question 17

What is a nonsense mutation

Answer 17

the mutation creates a premature stop codon
this causes the polypeptide chain produced to be incomplete
therefore affects the final protein structure and function

Question 18

Define beneficial mutation

Answer 18

a change in a polypeptide that results in an altered characteristic in an organism that causes them beneficial effects

Question 19

Give an example of a beneficial mutation

Answer 19

the production of melanin

Question 20

Define harmful mutation

Answer 20

a change in a polypeptide that results in an altered characteristic in an organism that causes harmful effects

Question 21

Give an example of a harmful mutation

Answer 21

deletion mutation of three nucleotides in the gene coding for the protein CFTR
- causes cystic fibrosis

Question 22

Define a neutral mutation

Answer 22

a change in a polypeptide which offers no selective advantage or disadvantage to the individual organism

Question 23

Give three reasons why a neutral mutation may occur

Answer 23

1) the mutation doesn’t alter the polypeptide
2) the mutation only slightly alters the polypeptide so that its structure/function isn’t changed
3) the mutation alters the structure or function of the polypeptide by the resulting difference in characteristic gives no advantage/disadvantage

Question 24

What is the role of regulatory mechanism

Answer 24

control which genes are expressed at different points in time

Question 25

What is the role of regulatory genes

Answer 25

to code for proteins that control the expression of structural genes

Question 26

What is the role of structural genes

Answer 26

to code for a protein that has function within the cell e.g. enzymes, membrane carriers, hormones

Question 27

Define operon

Answer 27

a group or cluster of genes controlled by the same promoter

Question 28

Why is lactase an inducible enzyme

Answer 28

it is only synthesised when lactose is present

Question 29

What is the benefit of lactase being an inducible enzyme

Answer 29

it prevents bacteria from wasting energy and materials

Question 30

What is the function of the promoter

Answer 30

region of DNA required to allow transcription of the gene to take place

Question 31

What is the role of the operator

Answer 31

segment of DNA to which a repressor binds to inhibit the transcription of a gene

Question 32

What is LacZ

Answer 32

a structural gene that codes for lactase

Question 33

What is LacY

Answer 33

a structural gene that codes for permease - a carrier protein which allows lactose into the cell

Question 34

What is LacA

Answer 34

a structural gene which codes for transacetylase

Question 34

Give the two molecules that are located upstream of the operon

Answer 34

1) promoter for regulatory gene 2) regulatory gene LacI - codes for Lac repressor protein

Question 35

What occurs when RNA polymerase binds to lactose

Answer 35

results in a conformational change in the shape of the repressor protein this means it can no longer bind to the operator

Question 36

What occurs when lactose is absent in the lac operon

Answer 36

1) the regulatory gene is transcribed and translated to produce the lac repressor protein 2) the lac repressor protein binds to the operator region upstream of lacZ 3) due to the presence of the repressor protein, RNA polymerase is unable to bind to the promoter region 4) transcription of the structural gene doesn’t take place 5) no lactase enzyme is synthesised

Question 37

What occurs when lactose is present in the lac operon

Answer 37

1) There is an uptake of lactose in the bacterium 2) the lactose binds to the second binding site on the repressor protein, resulting in a conformational change that prevents the repressor protein from binding to the operator site 3) RNA polymerase is able to bind to the promotor region and transcription takes place 4) the mRNA from all three structural genes is translated 5) enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium

Question 38

What is a repressible enzyme

Answer 38

an enzyme that is normally synthesised until an effector is present

Question 39

Describe how the production of repressible enzymes is controlled

Answer 39

1) an effector molecule binds to a repressor protein 2) this binding helps the repressor bind to the operator region and prevent transcription of the structural genes 3) therefore no enzyme is synthesised in the presence of an effector

Question 40

Define transcription factor

Answer 40

a protein that controls the transcription of genes by binding to a specific region of DNA

Question 41

Give two examples of transcription factors

Answer 41

1) production of oestrogen 2) breakdown of protein by gibberellin

Question 42

Explain why RNA analysis is important with regards to gene expression

Answer 42

the cell expresses a gene RNA is produced by transcription this RNA present in a cell can be analysed - scientists can match the RNA present in a cell to specific genes, and find which genes are being expressed in a specific cell

Question 43

Describe the process of post transcriptional modification in eukaryotes

Answer 43

1) both the exons and introns are transcribed into mRNA 2) as the introns are not translated they are removed from the pre-mRNA molecule 3) the exons are fused together to form a continuous molecule - mature mRNA - this process is called splicing

Question 44

Explain the role of splicing in producing mature mRNA from pre-mRNA

Answer 44

splicing ensures that only coding sections of mRNA are used to form proteins

Question 45

What occurs in control at the post translational level

Answer 45

post translation, polypeptides undergo modifications in the golgi apparatus or in the cytosol

Question 46

Describe the main role of cAMP

Answer 46

main role is activation of protein kinases

Question 47

Give an example of an activation of a protein kinase

Answer 47

in eukaryotic cells, cAMP activates protein kinase A (PKA) - inactive precursor enzyme once activated, it can activate other proteins e.g. enzymes

Question 48

What is a homeobox gene

Answer 48

a gene that contains a DNA sequence that codes for a protein transcription factor

Question 49

How do homeobox genes control body plan development

Answer 49

they code for a protein transcription factor the transcription factor attaches to DNA at specific locations - they regulate the transcription of genes by turning different genes on and off in the correct order

Question 50

Why are homeobox sequences highly conserved across species

Answer 50

similar as they all code for amino acid sequences that form transcription factors - the DNA binding regions must all have the same shape mutations that cause changes in the homeobox sequence lead to non-viable organisms - these are not favoured by natural selection

Question 51

What are hox genes

Answer 51

a subset of homeobox genes in animals they are involved in the correct positioning of body parts in an organism along the anterior-posterior axis

Question 52

Describe the process of apoptosis

Answer 52

1. old cells that have undergone a large number of mitotic cell divisions are systematically processed 2. DNA becomes denser and more tightly packed 3. nuclear envelop breaks down and chromatin condenses 4. vesicles form that contain hydrolytic enzymes 5. phagocytes engulf and digest the cell via phagocytosis

Question 53

What is the importance of mitosis and apoptosis in body plan development

Answer 53

both are key mechanisms controlling development as they are constantly replacing and destroying cells

Question 54

What is another role of tumour-suppressor genes

Answer 54

can stimulate apoptosis in cells with damaged DNA - destroys any genetically damaged cells (cancer)

Question 55

What are the two groups of genes which control mitosis

Answer 55

proto-oncogenes - genes that stimulate cell division tumour-suppressor genes - genes that reduce cell division

Question 56

Describe the molecular regulation of the cell cycle

Answer 56

checkpoints are regulated by two groups of proteins cyclins - act as regulators cyclin-dependent kinases (CDK's) - act as catalysts once activated by cyclins - they ensure that the cell cycle progresses from one stage to the next

Question 57

What are internal cell stimuli

Answer 57

internal factors that affect apoptosis and the cell cycle

Question 58

Give four examples of internal cell stimuli

Answer 58

1. irreparable genetic damage 2. RNA decay 3. internal biochemical changes 4. production of cyclin D these factors all initiate apoptosis in cells

Question 59

What are external cell stimuli

Answer 59

external factors that affect apoptosis and the cell cycle

Question 60

Give four examples of external cell stimuli

Answer 60

1. presence of cell signalling molecules such as cytokines, hormones and growth factor 2. viruses 3. UV light 4. bacteria can damage/destroy cells faster than they can be replaced/repaired respond to stimuli by activating pathways to initiate apoptosis