6.1.1 - Cellular control

Question 1

Define ‘gene mutation’ ?

Answer 1

A change in the DNA base sequence that may result in a change in the AMINO ACID SEQUENCE forming an altered polypeptide with a different tertiary structure and shape

Question 2

When do mutations occur ?

Answer 2

Mutations can occur spontaneously during DNA replication

Question 3

What increases the probability of a mutation occurring ?

Answer 3

Mutagens

Question 4

Name examples of mutagens ?

Answer 4

• Ionising radiation such as X-rays can break the DNA strands which can then be altered during the repair process
• Deaminating chemicals can alter the chemical structure of bases, converting one base into another
• Methyl or ethyl groups can be added to bases, leading to incorrect base pairing
• Viruses can insert sections of viral DNA into the DNA of cells

Question 5

What are the different effects of gene mutations on polypeptides ?

Answer 5

• Harmful mutations
• Neutral mutations
• Beneficial mutations

Question 6

What are beneficial mutations ?

Answer 6

• This produces a polypeptide with a different tertiary structure and shape meaning protein may no longer function
• This results in altered characteristics in an organism that causes beneficial effects for the organism

Question 7

What are neutral mutations ?

Answer 7

A mutation that offers no selective advantage or disadvantage to the individual organism

Question 8

What are harmful mutations mutations?

Answer 8

• This produces a polypeptide with a different tertiary structure and shape meaning protein may no longer function
• This results in altered characteristics in an organism that causes harmful effects for the organism

Question 9

What are the three ways in which a mutation in the DNA base sequence can occur ?

Answer 9

• Insertion of one or mole nucleotides
• Deletion of one or more nucleotides
• Substitution of one or more nucleotides

Question 10

Explain the process by which insertion of nucleotides occurs ?

Answer 10

• A nucleotide/ base is randomly inserted into the DNA sequence
• Frameshift mutation
• This changes all triplets/ codons downstream resulting in a different amino acid sequence
• This forms a polypeptide with. different tertiary structure/ shape

Question 11

What is frameshift mutation ?

Answer 11

• An insertion/ deletion mutation
• A mutation that has a knock-on effect by changing all triplets/ codons further on in the DNA sequence/ downstream

Question 12

Explain the process by which the deletion of nucleotides occurs ?

Answer 12

• A nucleotide/ base is deleted from the DNA sequence
• Frameshift mutation
• This changes all triplets/ codons downstream resulting in a different amino acid sequence
• This forms a polypeptide with. different tertiary structure/ shape

Question 13

Explain the process by which the substitution of nucleotides occurs ?

Answer 13

• A nucleotide/ base in the DNA sequence is randomly swapped for a different base
• It only change the amino acid coded by the triplet/codon in which the mutation occurs; it will not have a knock-on effect

Question 14

Explain the effect of automation on transcription / translation ?

Answer 14

• Substation or deletion/insertion/ frameshift mutations occurs
• This results in different DNA base sequence
• This results in different mRNA codons/ triplets being produced during transcription
• This results in tRNA molecules with different anticodons binding to mRNA during translation
• This results in a different amino acid sequence which alters the primary structure of the polypeptide

Question 15

What are the three types of substitution mutations ?

Answer 15

• Silent mutations
• Missense mutations
• Nonsense mutations

Question 16

What are silent mutations ?

Answer 16

A mutation that does not alter the amino acid sequence of a polypeptide / codes for the same amino acid

Question 17

What are missense mutations ?

Answer 17

A mutation that alters a single amino acid in the polypeptide chain

Question 18

What are nonsense mutations ?

Answer 18

• A mutation that creates a premature stop codon
• This produces an incomplete polypeptide affecting the final protein structure and function

Question 19

Define ‘regulatory mechanisms’ ?

Answer 19

Regulatory mechanisms : Mechanisms that exist within cells to make sure the correct genes are expressed in the correct cell at the correct time

Question 20

What are the three main types of regulatory mechanisms ?

Answer 20

• Transcriptional level
• Post-transcriptional level
• Post-translational level

Question 21

What are the two different types of genes ?

Answer 21

• Structural genes
• Regulatory genes

Question 22

What is the role of structural genes ?

Answer 22

A structural gene codes for a protein that has a function within a cell

Question 23

What is the role of regulatory genes ?

Answer 23

Regulatory genes code for proteins that control the expression of structural genes

Question 24

What is an operon ?

Answer 24

• Structural genes in prokaryotes can form an operon
• It is a group of genes that are controlled by the same promoter and are expressed simultaneously

Question 25

Explain how the lac operon is an example of a transcriptional level regulatory mechanism in bacteria ?

Answer 25

- Lac operon controls the production of the enzyme lactase, permease and transacetylase - Lactase breaks down/ respires lactose so it can be used as an energy source in the bacterial cell - An inducible enzyme ( only synthesized when lactose is present) - This helps prevent the bacteria from wasting energy and materials

Question 26

What are the components of the lac operon ?

Answer 26

- Promoter for structural genes - Promoter for regulatory gene - Regulatory gene lacI ( codes for the lac repressor protein ) - Operator - Structural gene lacZ - Structural gene lacY - Structural gene lacA

Question 27

What is a promoter ?

Answer 27

Region of DNA that stimulates transcription of a particular gene

Question 28

What is an operator ?

Answer 28

Region of DNA that acts as an on/off switch for gene expression

Question 29

What do Lac Z, lac A and Lac Y code for ?

Answer 29

- Lac Z : codes for lactase Lac Y : codes for permease which allows lactose to enter the cell - Lac A : codes for transacetylase

Question 30

What is the structure of the lac operon ?

Answer 30

Question 31

Explain what happens to the lac operon when lactose is absent ?

Answer 31

- The regulatory gene is transcribed and translated to produce lac repressor protein - The lac repressor protein binds to the operator region upstream of lacZ - Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region - Transcription of the structural genes does not take place - No lactase enzyme is synthesized

Question 32

Explain what happens to the lac operon when lactose is present ?

Answer 32

- There is an uptake of lactose by the bacterium - The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site - RNA polymerase is then able to bind to the promoter region and transcription takes place - The mRNA from all three structural genes is translated - Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium

Question 33

Define ‘Transcription factors’ ?

Answer 33

Transcription factors : Proteins that bind to specific regions of DNA to control the transcription of genes

Question 34

How do transcription factors work ?

Answer 34

- Some transcription factors bind to the promoter region of a gene which either allow or prevent the transcription of the gene from taking place - The presence of a transcription factor will either increase or decrease the rate of transcription of a gene

Question 35

Explain how oestrogen is involved in gene control ?

Answer 35

- Oestrogen is a lipid-soluble molecule, so can diffuse across the partially permeable membrane - It then moves into the nucleus and binds to an oestrogen receptor inside a protein complex causing a change in the shape of the receptor - Oestrogen receptor can now move to and bind to the promoter region of target genes - The oestrogen receptor attracts other transcription cofactors to bind to - This allows RNA polymerase to bind and to begin transcribing that gene

Question 36

What are the components involved in gene control with regards to gibberellins ?

Answer 36

- Repressor protein DELLA - Transcription factor (the one involved is called PIF) - Promoter of amylase gene - Amylase gene - Gibberellin - Gibberellin receptor and enzyme

Question 37

Explain how giberellin is involved in gene control ?

Answer 37

- DELLA repress or protein is bound to the transcription factor - This prevents transcription factor from binding to the promoter of the amylase gene so no transcription can occur - Gibberellin binds to a gibberellin receptor and enzyme which starts the breakdown of DELLA - The transcription factor is no longer bound to DELLA protein and so it binds to the promoter of the amylase gene - Transcription of amylase gene begins - Amylase is produced

Question 38

What are the two types of DNA sequences into eukaryotic organisms ?

Answer 38

- Coding - Non-coding

Question 39

What is the coding sequence ?

Answer 39

- The coding sequences are called exons - They are the sequences that will eventually be translated into the amino acids that will form the final polypeptide

Question 40

What is the non-coding sequence ?

Answer 40

- The non-coding sequences are called introns - They are not translated (they do not code for any amino acids)

Question 41

Explain how post-transcriptional modification occurs ?

Answer 41

- When transcription of a gene occurs, both the exons and introns are transcribed - Primary mRNA/ pre-mRNA formed also contains exons and introns - As the introns are not to be translated, splicing occurs and they are removed from the pre-mRNA molecule - The exons are then all fused together to form a continuous mRNA molecule ( mature mRNA ) that is ready to be translated

Question 42

Explain why splicing occurs during post-transcriptional modification ?

Answer 42

Splicing ensures that only the coding sections of mRNA are used to form proteins by translation

Question 43

Explain the control/ role of cAMP in the post-transcriptional level ?

Answer 43

- Protein is activated by cAMP - cAMp binds to protein and changes its shape / activates protein via phosphorylation

Question 44

What is an important role of cAMP ( protein kinase A enzyme ) ?

Answer 44

- In eukaryotic cells, cAMP activates protein kinase A (also known as PKA) - PKA is an inactive precursor enzyme - Once it is activated, it can activate other proteins (e.g. other enzymes)

Question 45

Define ‘body plan’ ?

Answer 45

Body plan : the basic pattern of the body of an organism

Question 46

What are homeobox genes ?

Answer 46

A family/ group of genes that determine the overall body plan of an organisms at the embryo stage of development

Question 47

Explain how homeobox genes function ?

Answer 47

- Homeobox genes code for transcription factors - Switches genes on and off in different cells/ tissues - Determine cell identity - Expressed in a set order during development - Regulate patterning and positioning of structures - Determine polarity - Regulate levels of apoptosis and mitosis

Question 48

Explain how homeobox genes are highly conserved ?

Answer 48

- Mutations that cause changes in homeobox sequences can lead to organisms that are unable to survive, so their mutated alleles are not passed on - This strong negative selection pressure explains why homeobox sequences are conserved

Question 49

Explain why fruit flies are used to investigate genes controlled body plan ?

Answer 49

- Low cost - Rapid reproduction rate/ short life cycles - Genetics of fruit fly is well understood - Simple genetics/ body plan - Mutations/ structures observable with light microscope

Question 50

What are hox genes ?

Answer 50

A subset of homeobox genes found in most animals that determine the identity and position of embryonic body regions along the head-tail axis

Question 51

Explain how hox genes are organised ?

Answer 51

- Organised into Hox clusters - There is a linear order to the Hox genes in each Hox cluster and this order is directly related to the order of the regions of the body that they affect

Question 52

What is apoptosis?

Answer 52

Programmed cell death

Question 53

What are the stages involved in apoptosis?

Answer 53

- Enzymes digest cell contents - Cell breaks apart into small fragments - Phagocytes engulf any remains

Question 54

Explain how mitosis and apoptosis involved in body planning ?

Answer 54

- Mitosis and apoptosis control the development of body form - Mitosis occurs in regions where more cells are needed - Apoptosis occurs in regions where cells need to be removed

Question 55

Name examples of genes that control the cell cycle ?

Answer 55

- Proto-oncogenes stimulate cell division - Tumour-suppressor genes reduce cell division/ stimulate apoptosis

Question 56

Why type of stimuli do genes controlling cell cycle/ apoptosis respond to ?

Answer 56

- External stimuli - Internal stimuli

Question 57

Name examples of external stimuli which genes involved in cell cycle / apoptosis respond to ?

Answer 57

- Cell signalling molecules such as cytokines and hormones - Presence of pathogens - Chemicals in the environment

Question 58

Name examples of internal stimuli which genes involved in cell cycle / apoptosis respond to ?

Answer 58

- DNA damage that cannot be repaired - Activation of genes that stimulate cell division - Metabolic stress, e.g. ATP depletion