6.1.1 - Cellular Control (set A - Mutation + Gene Control)

Question 1

What is a mutation, how are they caused?

Answer 1

Change in the sequence of bases in DNA - can disrupt protein synthesis if mutation occurs within gene

• mutation caused by substitution, deletion or insertion of one or more nucleotides within a gene

Question 2

What is a point mutation?

Answer 2

If only one nucleotide is affected its a point mutation

Question 3

Outline the effect of deletion or insertion of a nucleotide on the reading frame of a sequence of bases?

Answer 3

Insertion or deletion of a nucleotide/nucleotides leads to a frameshift mutation or introduction of a stop codon

• triplet codes mean sequences of bases are transcribed consecutively in non-overlapping groups of 3 (reading frame of a sequence of bases) - addition or deletion of a nucleotide, shifts the reading frame of the sequence of bases - changes every successive codon from the point of mutation unless number of nucleotides changed is a multiple of 3 (full codon so no change on reading frame) - protein formed is affected due to new amino acid

Question 4

Outline the effect of substitution on the sequence of bases?

Answer 4

substitution of single nucleotide changes the codon in which it codes (if new codon codes for a different amino acid it will lead to change in primary structure of protein)

• degenerate nature of code means it’s possible that new codon still codes for same amino acid - leading to no change in the protein synthesised
• can cause introduction of stop codon

Question 5

Outline and explain the 3 types of effects of different mutations??

Answer 5

• no effect/neutral - no effect on phenotype of an organism - normally functioning proteins still synthesised
• damaging - phenotype of organisms is negatively affected - proteins no longer synthesised or synthesised and are non-functional
• beneficial - rarely a mutation can occur resulting in a protein being synthesised with useful characteristics in the phenotype (eg immunity to HIV)

Question 6

Outline the causes of mutations?

Answer 6

• mutations can occur spontaneously, often during DNA replication but the rate of mutation is increased by mutagens

Mutagen - chemical, physical or biological agent which causes mutations

Question 7

Outline and provide examples of physical, chemical and biological mutagens?

Answer 7

Physical - ionising radiation (X-ray) - breaks DNA strands which can result in mutations during repair

Chemical - deaminating agents - chemically alter DNA bases eg cytosine to uracil changing base sequence

Biological (viruses ext) - viral DNA may insert itself into genome changing base sequence

Question 8

Give the 3 types of mutations?

Answer 8

• Insertion
• deletion
• substitution

Question 9

Outline the 4 stages at which gene expression can be regulated/controlled and the function of each?

Answer 9

• transcriptional control - genes can be turned on or off
• post-transcriptional control - mRNA can be modified which regulates translation
• translation control - translation can be stopped or started
• post-translational control - proteins can be modified after synthesis

Question 10

Explain the need for gene regulation?

Answer 10

Entire genome of organisms present in every eukaryotic cell (containing a nucleus) includes genes not required by the cell so expression of genes and rate of protein synthesis has to be regulated - genes can be turned on or off and the rate of potential synthesis increased or decreased

• housekeeping genes are those that code for proteins constantly required (eg enzymes for respiration ext)

Question 11

With focus on chromatin remodelling explain how it affects and regulates the transcription of genes?

Answer 11

DNA is wound around proteins (histones) - heterochromatin is tight wound DNA causing chromosomes to be visible during cell division - transcription of genes is not possible as RNA polymerase cannot access genes

• euchromatin is loosely wound DNA present during interphase - genes can be freely transcribed

simple form of regulation - ensures protein necessary for cell division are synthesised in time and not synthesised during energy-consuming process like when cells dividing

Question 12

With focus on chromatin remodelling - specificaly histone modification explain how it affects and regulates the transcription of genes?

Answer 12

DNA coils around histones because they are + charged, DNA - charged - histones can be modified to increase or decrease degree of packing

Addition of acetyl groups (acetylation) or phosphate groups (phosphorylation) reduces + charge on histones which causes DNA to coil less tightly allowing certain genes to be transcribed

• addition of methyl groups (methylation) makes histones more hydrophobic so DNA coils more tightly preventing transcription of genes

Question 13

Outline and explain the groups involved in chromatin remodelling - specificaly histone modification as a mechanism for regulating transcription of genes?

Answer 13

• Addition of acetyl groups (acetylation) or phosphate groups (phosphorylation) reduces + charge on histones which causes DNA to coil less tightly - allowing certain genes to be transcribed
• addition of methyl groups (methylation) makes histones more hydrophobic so DNA coils more tightly preventing transcription of genes

Question 14

With focus on lac operon explain how it affects and regulates the transcription of genes?

Answer 14

Operon - group of genes under control of same regulatory mechanism/promoter and expressed at the same time (more common in prokaryotes)

• lac operon found in bacteria (E.coli) controls the production of enzymes β- galactosidase, lactose permease and transacetylase

Question 15

Outline transcription factors in eukaryotes regarding transcriptional control?

Answer 15

proteins which bind to specific DNA sequences at ‘promoter’ regions are known as transcription factors - the right complex of transcription factors is required for transcription to begin

• by increasing or decreasing production of specific transcription factors, transcription of other genes can be controlled

RNA polymerase is not considered to be a transcription factor

Question 16

Outline and explain the function of parts of lac operon - focus on Operator, Promoter and repressor molecule?

Answer 16

Operator - repressor protein binds here, prevents RNA polymerase binding to DNA (controls switching on and off of genes as RNA polymerase can not bind)

Promoter - section of DNA which is the binding site for RNA polymerase

Repressor molecule - binds to operator, preventing transcription as it partially blocks binding of RNA polymerase (coded for by a regulatory gene) in absence of lactose

Question 17

Outline 3 mechanisms involved in controlling the transcription of genes at transcriptional level?

Answer 17

• transcription factors
• chromatin remodelling (with focus on histone modification)
• lac operon

Question 18

Explain RNA processing at a post-transcriptional level?

Answer 18

• product of transcription is pre-mRNA which is modified forming mature mRNA before it can bind to a ribosome and code for proteins
• a cap (modified nucleotide - aids in binding to ribosome) and a tail (long chain of adenine nucleotides) are added to mRNA which help stabilise and protect mRNA - delaying degradation in cytoplasm

splicing occurs - RNA cut a specific points

Question 19

Explain the process splicing which occurs at a post-transcriptional level?

Answer 19

RNA is cut at specific points - the introns (non-coding DNA) are removed and the exons (coding DNA) are joined together

• both proceeds occur in nucleus

Question 20

Explain RNA editing at a post-transcriptional level?

Answer 20

Nucleotide sequence of some mRNA molecules can be changed through intentional base addition, deletion or substitution

• same effect as point mutation - result in synthesis of different proteins which may have different function - increases range of proteins produced from single mRNA molecule

Question 21

Outline and explain fully how the rate of protein synthesis is regulated at a translational level - give the 3 ways?

Answer 21

• degradation of mRNA - more resistant the molecule is, longer it lasts in cytoplasms (addition of a cap and tail to mRNA delays degradation)
• binding of inhibitory proteins to mRNA - prevents mRNA binding to ribosomes and thus inhabiting proteins synthesis
• activation of initiation factors aiding in binding to ribosome - for example eggs of organisms produce large quantities of mRNA which is not required until fertilisation at which point initiation factors are activated

Question 22

Explain the function of the enzymes protein kinases?

Answer 22

Catalyse addition of phosphate groups to proteins (phosphorylation) - which change tertiary structure and thus function of protein

• important regulators for cell activity - often activated by secondary messenger cAMP

Question 23

Outline 2 mechanisms involved in controlling the product of transcription at a post-transcriptional level?

Answer 23

• RNA splicing
• RNA editing

Question 24

Outline and explain fully how the products of protein synthesis are regulated and modified at a post-translational level - give the 4 ways?

Answer 24

• addition of non-protein groups - eg carbohydrate chains, lipids or phosphates
• modifying amino acids and formation of bonds (disulfide bridges
• folding or shortening of proteins
• modification by cAMP - example is lac operon cAMP which binds to cAMP receptor proteins increasing rate of transcription of structural genes

Question 25

Give the 2 proteins coded by the lac operon?

Answer 25

• beta-galactosidase - breaks downs lactose in 2 simple sugars (glucose and galactose)
• permease - membrane bound protein - which provides route for lactose outside of cel into cell

Question 26

Explain fully lac operon as an example of transcriptional control?

Answer 26

Group of 3 genes (beta galactosidase and lactose permease and transacetylase) which are transcribed on molecule of mRNA

• regulatory gene lacI located near operon and codes for a repressor protein (prevents transcription of structural genes in absence of lactose)
• repressor protein binds to area called operator - which prevents RNA polymerase binding to DNA and transcription (known as down regulation)
• promoter - section of DNA that is binding site for RNA polymerase
• when lactose is present - it binds to repressor protein causing it to change shape (no longer bind to operator) RNA polymerase can bind to promoter allowing transcription

Question 27

Define morphogenesis?

Answer 27

Regulation of the pattern of anatomical development

• controlled by specific group of regulatory genes (homebox genes) which can switch genes on and off which allows control of development of the body plan

Question 28

Explain briefly what homeobox genes are?

Answer 28

Ancient, highly conserved regions of DNA in animals, plants and fungi

• code for homeodomains - protein segments that act as transcription factors

Question 29

Explain what can be implied through the fact homeobox genes have rarely changed?

Answer 29

Their activity is absolutely fundamental to the development of a body that actually works - any mutation would be fatal and therefore would not be passed on through evolution

Question 30

Explain what hox genes are, and the role they play?

Answer 30

Specific group of homeobox genes only found in animials

• highly consvered across great evolutionary distances

Question 31

Explain what body plans are?

Answer 31

Usually represented as cross-sections through the organism

• common feature of animals is that they are segmented - these segments have multiplied and overtime specialised to perform different functions

Question 32

Define apoptosis?

Answer 32

Programmed cell death essential for shaping organisms

Question 33

Outline the factors affecting the expression of regulatory genes?

Answer 33

• expression of regulatory genes can be influenced by the environment both internal and external

Question 34

Outline briefly the stages of apoptosis?

Answer 34

1) cell shrinkage + DAN fragmentation
2) membrane blabbing
3) results in apoptotic bodies
4) phagocytosis of apoptotic bodies

Question 35

State what is meant by a homeobox gene?

Answer 35

Group of regulatory genes (switch genes on and off) which all contain a homeobox, a section of DNA 180 base pairs long which code for 60 amino acids