6 - ICH - Cellular control

Question 1

State the different forms of gene mutation that can occur

Answer 1

Substitution

Addition

Deletion

Question 2

Define:

Gene mutation by:

1. Substition
2. Deletion
3. Addition

Answer 2

Substition = A mutation where ≥1 nucleotides are substituted for another in a DNA strand

Deletion = A mutation where ≥1 nucleotides are deleted and lost from the DNA strand

Addition = A mutation where ≥1 nucleotides are added into the DNA strand

Question 3

Define Mutation

Answer 3

Mutation = Spontaneous change to the sequence of nucleotides

Question 4

What can a gene mutation result in?

Answer 4

The change in the DNA base sequence may result in a difference in the amino acid sequence of the subsequent polypeptide.

• This could result in a Non-functional protein

Question 5

What are mutagenic agents? Give another name for them?

Examples (2)

Answer 5

Mutagenic agents (mutagens) = External factors that increase the basic mutation rate

E.g

• High neergy radiation that can disrupt the DNA molecule
• Chemicals that alter DNA structure or interfere with transcription

Question 6

Are mutations passed onto the next generation?

Answer 6

• Mutation in body cells are NOT passed onto the next generation
• Mutations occuring during the formation of gametes may be inherited, often producing sudden and distinct differences between indiviuals

Question 7

Effect of substitution? + Example

Answer 7

May change a single amino acid in the polypeptide chain.

• May have no effect because DNA code is degenerate - more than one trip codes for each amino acid
• May have a dramatic effect e.g. a substitution of one base in the gene coding for ß chains in haemoglobin molecules causes sickle cell anaemia

Question 8

Effect of deletion and addition?

Answer 8

Have a much more dramatic effect than substitution

• Gain or loss of a base will affect every triplet after the effected base in the sequence = Frame-shift
• Results in the alteration of the entire amino acid sequence following mutation. Tertiary structure is likely ro change and thus results in a non-functional protein

Question 9

Effect of mutations on the way a protein functions

• Neutral (2)
• Harmful (2)
• Beneficial (4)

Answer 9

Neutral:

• Might’ve occured in non-coding part of DNA
• Degenerate nature of DNA code may mean the new codon still codes for the same amino acid as more than one triplet codes for each amino acid ∴ no effect on protein shape or function

Harmful:

• Mutated gene may produce new vresion of allele of a gene
• Many mutated genes give rise to genetic diseases e.g. sickle cell anaemia

Beneficial:

• Some mutations produce beneficial alleles
  
  • e.g. Decrease in melanin production in the skin of early humans migrating temperature climates, results in paler skin allowing more Vitamin D to be made in the skin in climates with less sunshine
  • People with dark skin cannot make as much vitamin D and suffer from rickets
  • Paler skinned people would have a survival advantage over dark skinned people in areas of less sunshine

Question 10

What is a Point mutation?

What are the 3 types of point mutation?

Answer 10

Point mutation

• Mutations where only one base is affected
• 3 Types
  
  • Silent mutation - No change in amino acid sequence of polypeptide
  • Missense mutation - Mutation changes code for 1 amino acid ∴ 1 amino acid in sequence changes
  • Nonsense mutation - Mutation changes the code turning the triplet into a stop codon ∴ polypeptide is shorter than expected

Question 11

What is gene regulation?

Answer 11

Gene regulation = Controlling whether a gene is being expressed or not

Question 12

Why is gene regulation needed and who is it needed for?

Answer 12

Gene regulation is required for eukaryotic and prokaryotic cells to specialise and work in a coordinated way

Question 13

State the 4 mechanisms used to regulate gene expression (give a brief decription of what each does)

Answer 13

• Transcriptional - Genes an be turned on/off
• Post-transcriptional modification - mRNA can be modified which regulated translation and the types of polypetide/ protein produced
• Translational - Translation can be stopped/started
• Post translational modification - Proteins can be modified after synthesis which changes their functions

Question 14

What is an operon?

Answer 14

Operon = Section of DNA that contains a cluster of structural genes that are all transcribes together

Question 15

State an example of using transcription to regulate genes

Answer 15

Lac operon

Question 16

What is the Lac Operon - background

Answer 16

Lac Operon is a DNA base sequence that controls the production of enzymes needed for lactose metabolism in E.Coli bacteria.

These enzymes include:

• ß Galactoside Permease (lactose permease) - Enables the cell to take up lactose
• Lactase (ß Galactosidase) - hydrolyses lactose → glucose + galactose

Question 17

Lac Operon:

What happens if the E.Coli is grown on:

1. Medium containing only glucose
2. Medium containingonly lactose

Answer 17

• Medium containing only glucose
  
  • Doesn’t produce lactose permease or lactase
  • Genes that code for them are not expressed
• Medium containingonly lactose
  
  • Genes are switched on and both enzymes are produced

Question 18

Lac Operon:

What makes up the lac operon? (3)

Answer 18

• Structure genes - Comprises the length of DNA with base sequences needed to produce the mRNA that codes for the polyptptides that make up the 2 enzymes lactase and lactose permease
• Protomer - Portion of DNA to which the enzyme RNA polymerase attaches to in order to begin the process of transcription of mRNA
• Operator - Allows promoter to work if nothing is bound to it. A repressor switches off the operator

Question 19

What is repressor and is it a part of the lac operon?

What is it’s role in the lac operon?

Answer 19

A repressor is a protein moelecule coded for by a regulatory gene that is NOT a part of the lac operon

Role:

The repressort has 2 binding sites:

• One is complementary to operator DNA so it binds with it preventing RNA polymerase from binding to the promoter ∴ structural genes are not being expressed ∴ mRNA can’t be transcribed ∴ no enzymes produced
• One is complementary to the lactose and when it’s bound to it, the repressor protein changes shape and can’t bind to the operator ∴ meaning that RNA polymerase can bind to promoter and transcription can take place. Structural genes are expressed ad enzymes are produced

Question 20

List 3 advantages of being able to switch off genes

Answer 20

• Save energy
• Amino acids can be used to make vital proteins
• No space wasted for extra enzymes

Question 21

What happens are the lac operon?

1. Lactose is NOT present
2. Lactose is present

Answer 21

LACTOSE NOT PRESENT:

1. Regulatory gene is expressed and the lac repressor, which is a transcription factor that binds to the operator site when there’s no lactose present
2. This blocks transcription as RNA polymerase can’t bind to promoter

LACTOSE PRESENT:

1. Lactose binds to repressor gene, changing the repressor’s shape ∴ no longer complementary to operater site
2. RNA polymerase binds to promoter, transcription of structural genes begin

Question 22

Transcriptional level control:

• Eukaryotes
• Prokaryotes

Answer 22

Eukaryotes:

• Transcription factors bind to specific DNA sites near the start of their target genes
  
  1. Transcription factor binds to a specific DNA site near the start of the taget gene
  2. RNA polymerase binds to complex
  3. Transcription starts

Prokaryotes:

• Transcription factors binding to operons e.g. lac operon

Question 23

What is chromatin?

What is chromatin subsivided into + what is different about the 2 types?

Answer 23

Chromatin = Uncondensed DNA in a complex with histones

• Heterochromatin = Tightly would DNA causing chromosomes to be visible during cell division
• Euchromatin = Loosely wound DNA present during interphase

Question 24

Without any modification why can euchromatin be transcribed but not heterochromatin?

Answer 24

• Euchromatin is loosely wound DNA so can be transcribed
• Heterochromatin is tightly wound DNA so RNA polymerase can’t access the genes ∴ heterochromatin must be modified in order for their respective genes to be transcribed

Question 25

What type of gene modification is histone modification?

Purpose?

Answer 25

• Histone modification = transcriptional control
• DNA coils around histones because they’re positively charged and DNA is negatively charged
• Histones are modified to increase or decrease the degree of packing (condensation)

1. Adding acetyl groups (acetylation) or phosphate groups (phosphorylation) makes it more negative
   
   • Causes DNA to coil less tightly ∴ allows genes to be transcribed
2. Adding methyl groups (methylation) makes histones more hydrophobic
   
   • Bind more tightly to each other causing DNA to coil more tightly ∴ prevent gene transcription

Question 26

Describe the role of cyclic AMP (cAMP) in post transcriptional. pre translational control

Answer 26

• Binding of RNA polymerase only results in a relatively slow rate of transciption which needs to be increased to produce the required amount of enzyme to metabolise lactose efficiently
• Achieved by the binding of cAMP receptor protein (CRP)
  
  • CRP can only bind when it in turn is bound to cAMP

Question 27

Relationship between the concentrations of cAMP, lactose and glucose?

Answer 27

• Transport of glucose into an E.Coli decreases the levels of cAMP
• Reducing level of transcription of genes responsible for metabolism of lactose
• ∴ means that if both lactose and glucose are present it will favour metabolising glucose over lactose

Question 28

State and explain the 2 methods of post transcription/ pre translational control:

Method 1 (4)

Method 2 (2)

Answer 28

RNA Processing:

• Transcription results in the production of the precursor molecule pre-mRNA (contains introns) which needs to be modified into mature-mRNA (no introns)
• A Cap = modified nucleotide, is added onto 5’ end and a string of adenine bases onto the 3’ end in order to stabalise it and prevent degradation in cytoplasm
• Splicing occurs where introns are removed to form mature-mRNA
• Occurs in nucleus

RNA Editing:

• Nucleotide sequences of some mRNA molecules can be changed through addition, deletion and substitution
• Increases the range of proteins produced, which may have different functions that can be produced from a single mRNA molecule or gene

Question 29

Name the 3 main mechanisms involved with regulating the process of protein synthesis in translational control

Answer 29

Degradation of mRNA

• The more resistant the molecule the longer it will last in the cytoplasm and the greater the quantity pf protein will be synthesised

Binding proteins to mRNA

• Prevents it binding to ribosomes and the synthesis of proteins

Activation of initiation factors

• May aid the binding of mRNA to ribosomes
• The eggs of many organisms produce large quantities of mRNA which is not required until after fertilisation, at which point initiation factors are activated

Question 30

4 examples of post trasnplational control

Answer 30

• Addition of non-protein groups e.g. carbohydrate chains, lipids or phosphates…
• Modifying amino acids and formation of disulphide bridges
• Folding or shortening of proteins
• Modification by cAMP - cAMP is required for protein activation providing the energy required for a protein to form its tertiary structure