6.1.1 - CELLULAR CONTROL

Question 1

Define mutation

Answer 1

Any change to the base (nucleotide) sequence of DNA

Question 2

Name types of mutations

Answer 2

• Insertion mutation
• Deletion mutation
• Point mutation
• Nonsense mutation
• Missense mutation
• Silent mutation

Question 3

What is an insertion mutation?

Answer 3

• A nucleotide (with a new base) is randomly inserted into the DNA sequence
• Changes amino acid that would have been coded for by the original base triplet

Question 4

What is a deletion mutation?

Answer 4

• Nucleotide (+ its base) is randomly deleted from the DNA sequence

Similar to insertion mutation by changing the amino acid coded for

Question 5

What is a point mutation?

Answer 5

Random change to base sequence of DNA
OR one base in a sequence is replaced

Question 6

What is a nonsense mutation?

Answer 6

• Translation is stopped early (mutation results in stop codon being coded for)
  ^— signals the cell to stop translation of the mRNA molecule into an amino acid sequence

E.g. can cause cystic fibrosis

Question 7

What is a missense mutation?

Answer 7

• A codon change resulting in the production of a different amino acid
  ^— alters a single amino acid in the polypeptide chain

E.g. can cause sickle cell anaemia

Question 8

What is a silent mutation?

Answer 8

A change in codons that doesn’t result in the production of a different amino acid

Think about genetic code being DEGENERATE - multiple codons code for same amino acid

Question 9

What is a substitution mutation?

Answer 9

• A base in the DNA sequence is randomly swapped for a different base

Question 10

Describe the effects of an insertion mutation

Answer 10

• Has a knock-on effect by changing the triplets further on in the DNA sequence (a.k.a frameshift mutation)
• May dramatically change the amino acid sequence produced from this gene
  ^— therefore changes the ability of the polypeptide to function

Question 11

Describe the effects of a deletion mutation

Answer 11

• Similar to insertion mutation, has a knock-on effect by changing the groups of three bases (triplets) further on in the DNA sequence
  ^— a.k.a. Frameshift mutation
• May dramatic change the amino acid sequence produced
  ^— therefore changes the ability of the polypeptide to function

Question 12

Describe the effects of a substitution mutation

Answer 12

• Unlike insertion or deletion, this only changes the amino acid for the triplet in which the mutation occurs (NO KNOCK-ON EFFECT)

Question 13

Name the THREE types of substitution mutations

Answer 13

• Silent mutation
• Missense mutation
• Nonsense mutation

Question 14

What THREE categories can mutations be placed into based on their effects?

Answer 14

• Beneficial mutations
• Harmful mutations
• Neutral mutations

Question 15

Outline the effects of beneficial mutations

Answer 15

• Small number of mutations result in a significantly altered polypeptide with a different shape
  ^— may alter ability of the protein | e.g. active site on an enzyme changes + substrate can no longer bind
• e.g. the production of the pigment melanin
  ^— early humans living in Africa had dark skin due to producing a lot of melanin - provided protection from harmful UV radiation fun the sun, while still allowing vitamin D to be synthesised (due to high sunlight intensity)
  ^— but at lower sunlight intensities, pale skin synthesises vitamin D more easily - but as humans moved to cooler climates, mutations occurred that led to a decrease in melanin production
  ^— paler individuals would have had an advantage as they could synthesise more vitamin D (lack of this causes health problems)

THIS IS BENEFICIAL

Question 16

Outline the effects of harmful mutations

Answer 16

• Many genetic diseases re cause by these
• e.g. cystic fibrosis (CF)
  ^— 70% of CF sufferers due to deletion mutation of three nucleotides in the gene coding for the CFTR protein
  ^— low of function of the CFTR protein caused by this mutation results in symptoms (e.g. lung + pancreatic problems due to extremely thickened mucus)

Question 17

Outline the effects of neutral mutations

Answer 17

• No selective advantage or disadvantage to the individual
• can occur due to: mutation not altering polypeptide, mutation only alters polypeptide slightly so the structure is not changed, mutation altered the structure or function of the polypeptide but the resulting difference provides no advantage or disadvantage
• e.g. ability to taste. Bitter-tasting chemical in Brussels sprouts
  ^— chemical is not toxic so no advantage - caused by mutated allele of TAS2R38 gene
  ^— gene allows us to taste bitter things by coding for receptor proteins that can detect bitter-tasting chemicals

NEUTRAL, BUT MAY HAVE BEEN ADVANTAGEOUS IN THE PAST AS MANY BITTER SUBSTANCES COULD BE HARMFUL

Question 18

How does the nucleus of each cell in the body differ?

Answer 18

• Every nucleus contains the same genes
• However nit every gene is expressed in every cell
  ^— not all of these genes are expressed all the time

Question 19

What are regulatory mechanisms?

Answer 19

• They ensure the correct genes are expressed in the correct cell at the correct time
• All controlled by many different regulatory genes

Question 20

What are the THREE main types of regulatory mechanisms?

Answer 20

• Regulation at the transcriptional level (occurs during transcription)
• Regulation at the post-transcriptional level (occurs after transcription)
• Regulation at the post-translational level (occurs after translation)

Question 21

What does a structural gene code for?

Answer 21

• A protein that has a function within a cell
  ^— e.g. enzymes, membrane carriers, hormones, etc(

Question 22

What does a regulatory gene code for?

Answer 22

• Codes for proteins that control the expression of structural genes

Question 23

What is an operon?

Answer 23

A group/cluster of genes controlled by the same promoter (region of DNA which comes before the transcriptional start site)

Question 24

What is a promoter?

Answer 24

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

Question 25

What is the lac operon?

Answer 25

• Structural genes in prokaryotes can form an operon (in some bacteria - lac operon)
• Lac operon - controls the production of the enzyme lactase + 2 other structural proteins
  ^— lactase breaks down the substrate lactose so it can be used as an energy source in the bacterial cell
• a.k.a. Inducible enzyme (means it is only synthesised when lactose is present)
  ^— prevents the bacteria from wasting energy + materials

Question 26

Describe the structure of the lac operon

Answer 26

• Promoter for structural genes
• Operator
• Structural gene lack that codes for lactase
• Structural gene lacY that codes for permease (allows lactose into the cell)
• Structural gene lack that codes for transacetylase
• To the left of the operon on the bacterium’s DNA there is the promoter for eh regulator gene + regulatory gene lack that codes for the lac repressor protein

Question 27

What is an operator?

Answer 27

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

Question 28

What is the lac repressor?

Answer 28

• Has two binding sites that allow it the bind to the operator in the lac operon + to lactose (effector molecule)
• When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter
• When it binds to lactose the shape of the repressor protein distorts and it can no longer bind to the operator

Question 29

What processes take place when lactose is absent?

Answer 29

• Regulatory gene is transcribed + translated to produce lac repressor protein
• 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 synthesised

Question 30

What processes take place when lactose is present?

Answer 30

• 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 is cannot bind to the operator site
• RNA polymerase is then able to bind to the promoter T region and transcription takes place
• The mRNA polymerase is then able to bind to the promoter region and transcription takes place
• The mRNA from all three structural gene is translated
• Enzyme lactase is produced and lactose can be broken down + used for energy by the bacterium

Question 31

What are transcription factors?

Answer 31

• A protein that controls the transcription of genes by binding to a specific region of DNA
• Eukaryotes can use them to control gene expression (estimated 10% of human genes code for transcription factors)
• Allows organisms to respond to their environment
• Some hormones achieve their effect via transcription factors

Question 32

How do transcription factors work?

Answer 32

• Some transcription factors bind to the promoter region of a gene (i.e. the region of DNA ‘upstream’ of the gene that controls gene expression)
  ^— binding can either allow or prevent the transcription of the gene from taking place
• Presence of a transcriptional factor will either increase or decrease the rate of gene transcription

Question 33

Outline an example of gene control in mammals

Answer 33

• In mammals, oestrogen is involved with controlling the oestrus cycle + in sperm production
• Oestrogen is a lipid-soluble molecule + can diffuse through the plasma membrane of cells
  ^— then moves to the nucleus + binds to an oestrogen receptor
• These oestrogen receptors are actually transcription factors that are able to initiate transcription for many different genes by binding to their promoter regions
• Once bound oestrogen causes a change in the receptor shape
  ^— receptor moves away from the protein complex it is normally attached to and binds to the promoter region of one of its target genes
• Allows RNA polymerase to bind + begin transcribing that gene

Question 34

Outline an example of gene control in plants

Answer 34

• Plant cells use transcriptional factors similar to animal cells
• Gibberellin is a hormone found in plants (e.g. wheat + barley) that controls seed germination by stimulating the synthesis of the enzyme amylase
• Does so by influencing transcription of the amylase gene
  ^— when gibberellin is applied to a germinating seed there is an increased amount of the mRNA for amylase present

Question 35

Describe how mRNA is edited at a post-transcriptional level

Answer 35

• Genes in eukaryotic DNA contains sections that don’t code for amino acids (these are called introns)
• Sections of DNA that do code for amino acids are called exons
• During transcription the introns + exons are both copied into mRNA (mRNA strands containing introns + exons are called primary mRNA transcripts - pre-mRNA)
• Introns are removed from primary mRNA strands by splicing - introns are removed and exons joined, forming mature mRNA strands (occurs in nucleus)
• Mature mRNA then leaves the nucleus for the next stage of protein synthesis (translation)

Question 36

Describe how cAMP activates proteins at the post-translational level

Answer 36

• Some proteins aren’t functional straight after synthesis (so they must be activated)
• Protein activation is controlled by molecules (e.g. hormones + sugars)
• Some of these molecules work by binding to cell membranes + triggering the production of cyclic AMP (cAMP) inside the cell
• cAMP then activates proteins inside the cell by altering their 3D structure
  ^— e.g. altering the 3D structure can change the active site of an enzyme, making it become more or less active

Question 37

Explain how cAMP activates protein kinase A (PKA)

Answer 37

• PKA is an enzyme made of 4 subunits
• When cAMP isn’t bound, the 4 units are bound together + are inactive
• When cAMP binds, it causes a change in the enzyme’s 3D structure, releasing the active subunits - PKS IS ACTIVE!!

Question 38

What is a body plan?

Answer 38

The general structure of an organism

e.g. the Drosophila fruit fly has various body parts (head, abdomen, etc) that are arranged in a particular way - body plan

Question 39

How is body plan development controlled?

Answer 39

• By proteins that help set up the basic body plan so that everything is in the right place - coded for by Hox genes
  ^— e.g. 2 Hox gene clusters control the development of the Drosophilia body plan

Question 40

Describe the roles of Hox gene across different organisms

Answer 40

• Similar Hox genes are found in animals, plants + fungi, which means that body plan development is controlled in a similar way in flies, mice, humans, etc
• Hox genes have regions called homeobox sequences, which are highly conserved - meaning that these sequences have changed very little during the evolution of different organisms that possess these homeobox genes

Question 41

Describe how Hox genes control development

Answer 41

• Homeobox sequences code for a part of the proteins called the homeodomain
• The homeodomain binds to specific sites on DNA, enabling the protein to work as a transcription factor
• The proteins bind to DNA at the start of developmental genes, activating or repressing transcription and so altering the production of proteins involved in the development of the body plan

Question 42

What is apoptosis?

Answer 42

• Controlled cell death and breakdown (programmed cell death)

Question 43

State the steps of apoptosis once triggered

Answer 43

• Enzymes inside the cell beak down important cell components such as proteins in the cytoplasm and DNA in the nucleus
• As the cell’s contents are broken down it begins to shrink and breaks down into fragments
• The cell fragments are engulfed by phagocytes and digested

Question 44

Outline the importance of mitosis + apoptosis as mechanisms controlling the development of body form

Answer 44

• Mitosis + differentiation creat the bulk of the body parts
  = Meanwhile apoptosis refines the parts by removing the unwanted structures

E.g. as tadpoles develop into frogs, their tail cells are removed by apoptosis + when hands + feet first develop in humans, the digits are connected (only separated when cells in connecting tissue undergo apoptosis)

Question 45

Explain how genes that regulate apoptosis + the cell cycle can respond to stimuli

Answer 45

INTERNAL STIMULUS
^— e.g. DNA damage - if detected, this can result in the expression of genes which cause the cycle to be paused + can trigger apoptosis

EXTERNAL STIMULUS
^— e.g. stress caused by lack of nutrients - can result in gene expression that prevents cells from undergoing mitosis | gene expression which leads to apoptosis being triggered can also be caused by an external stimulus such as attack by a pathogen