Chapter 19: Genetics of Living Systems

Question 1

Define point mutation.

Answer 1

• When only one nucleotide is affected.

Question 2

Define frameshift mutation.

Answer 2

• Shift in reading frame of sequence of bases.

- Change in every successive codon from point of mutation.

Question 3

Outline substitution mutations.

Answer 3

• Change in codon in which mutation occurs.
• Different base substituted into codon.
• Degenerate code –> new codon can still code for the same amino acid.
• Different amino acid coded for –> change in primary structure of protein.

Question 4

Outline Insertion + Deletion mutations.

Answer 4

• Frameshift mutation.

- Different base inserted or deleted into base sequence.

Question 5

What is a silent mutation?

Answer 5

• No change in protein or activity of proteins synthesised.
• No effect on phenotype.
• Usually in non-coding regions or DNA or code for same amino acid –> due to degenerate code.

Question 6

What is a nonsense mutation?

Answer 6

• Codon becomes stop codon instead of amino acid.
• Shorter protein synthesised.
• Non-functional protein with negative effect on phenotype.

Question 7

What is a missense mutation?

Answer 7

• Incorrect amino acid incorporated into primary structure.
• Could be conservative or non-conservative.
• Conservative = amino acid change leads to similar amino acid being coded for –> less harmful.
• Non-conservative = amino acid has different properties + more likely to have effect on proteins –> more harmful.

Question 8

Features of transcriptional control

Answer 8

Chromatin Remodelling:

• Heterochromatin –> tightly wound DNA –> causes chromosomes to become visible in prophase.
• Euchromatin –> loosely wound DNA in interphase –> freely transcribed,
• Tightly wound DNA –> RNA polymerase cannot bind to promoter region + access genes –> transcription blocked.

Histone modification:

• Histones –> positively charged + wrapped around DNA which is negatively charge.
• Acetylation/phosphorylation = reduce positive charge on histones –> DNA coils less tightly –> certain genes transcribed.
• Methylation = makes histones more hydrophobic –> bind more tightly to each other –> DNA coils more tightly –> transcription of genes blocked.

Transcription Factors:

• Protein that moves in from cytoplasm and bind to DNA in order to switch genes on/off by increasing/decreasing rate of transcription.
• Activators = start transcription.
• Repressors = stop transcription.

cAMP:

• Secondary messenger that activates proteins by binding of CRP to cAMP –> up regulates transcription.

Question 9

How does the lac operon work when lactose is present? (transcriptional control)

Answer 9

1. Enzyme coded for by lac operon allows lactose to enter bacteria.
2. Lactose binds to repressor protein.
3. Repressor protein changes shape.
4. Transcription unblocked –> RNA polymerase binds to promoter region.
5. Enzymes needed to metabolise lactose produced.

Question 10

Difference between operator + promoter?

Answer 10

• Operator = section of DNA to which repressor protein/transcription factor binds.
• Promoter = section of DNA to which RNA polymerase binds in order to initiate transcription.

Question 11

Distinguish between structural genes and regulatory genes.

Answer 11

• Structural genes = transcribed onto mRNA + code for proteins/enzymes needed for structure or function.
• Regulatory genes = turn genes on/off; not transcribed; determine which structural genes are expressed; starts/stops transcription.

Question 12

Outline features of post-transcriptional control.

Answer 12

RNA Processing:

• Pre-mRNA –> modified to form mature mRNA before it binds to ribosomes for protein synthesis.
• Splicing takes place –> introns removed + exons joined together in nucleus.
• Cap added at 5’ end –> aid binding of mRNA to ribosome.
• Tail added to 3’ end.
• Stabilise mRNA + delay degradation in the cytoplasm.

RNA Editing:

• Insertion, deletion, substitution of bases occurs –> change in sequence of mRNA nucleotides.
• Similar to point mutations –> different proteins with different functions synthesised.
• Increases range of proteins that can be synthesised from a single mRNA molecule.

Question 13

Outline features of translational control.

Answer 13

• Degradation of mRNA in cytoplasm = the more resistant the mRNA molecule the longer it spends in the cytoplasm + the more proteins are synthesised.
• Inhibitory proteins = bind to mRNA to prevent translation.
• Initiation factors = aid binding of mRNA to ribosomes.

Protein Kinases:

• Catalyse phosphorylation of protein to activate proteins.
• Changes tertiary structure of protein + function.

Question 14

Outline features of post-translational control.

Answer 14

• Modification to proteins after protein synthesis/
• Addition of non-protein groups –> carbohydrates, lipids, phosphates.
• Modification of amino acids + formation of bonds –> disulphide bonds.
• Folding or shortening of protein/polypeptide chains.
• Binding of cAMP to CRP –> in lac operon –> increasing transcription of structural genes.

Question 15

How does lac operon work when no lactose is present?

Answer 15

1. Regulatory gene, Lac I –> codes for repressor protein.
2. Repressor binds to operator.
3. Prevents binding of RNA polymerase to promoter.
4. Prevents transcription of proteins that hydrolyse lactose.

Question 16

Outline difference between post-transcriptional + post-translational control of gene expression.

Answer 16

Post-transcriptional:

• Affects size/shape/sequence of mRNA.
• Involves editing of primary pre-mRNA –> removal of introns to form mature mRNA.

Post-translational:

• Affects conformation of proteins.
• Involves activation of proteins by cAMP.

Question 17

State what is meant by a homeobox gene.

Answer 17

• Regulatory gene.
• Homeobox sequence (180 base pairs).
• Code for a homeodomain on a protein.
• Gene products bind to DNA.
• Initiates transcription/switch genes on or off.
• Control development/body plans.

Question 18

Why is there very little change by mutation in homeobox genes?

Answer 18

• These genes are very important.
• Mutation would have big effects/alter body plan.
• Many genes affected/knock-on effects.
• Mutation most likely lethal/selected against.

Question 19

Outline the process of apoptosis (programmed cell death regulated by hox genes).

Answer 19

1. Enzymes break down cytoskeleton.
2. Cytoplasm becomes dense + organelles more tightly packed.
3. Cell surface membrane changes –> blebs form.
4. Chromatin condenses –> nuclear envelope breaks down DNA into fragments.
5. Cell breaks into vesicles taken up by phagocytes which remove the cellular debris.

Question 20

Evaluate the validity of:

All hox genes are homeobox genes, but not all homeobox genes are hox genes as hox genes are only found in animals.

Answer 20

• Hox genes are one form of homeobox gene.
• Hox genes present in vertebrates in hox clusters.
• Other forms of homeobox genes present in other clusters.
• Hox genes only found in animals but homeobox genes are found in animals, fungi and plants.
• THEREFORE THE STATEMENT IS VALID.

Question 21

Describe + explain the difference between the functions of RNA polymerase + DNA polymerase.

Answer 21

RNA Polymerase:

• Transcription –> one DNA strand acts as the template for the formation of a new DNA strand.
• Production of mRNA in nucleus.
• Binds to promoter region to initiate transcription.

DNA Polymerase:

• DNA replication.
• Semi-conservative.
• Joins sugar-phosphate backbone before cell division.

Question 22

Discuss possible effects mutation can have on the structure and function of a protein.

Answer 22

• Different primary/tertiary/secondary structure.
• Protein shorter due to deletion/stop codon or longer due to insertion/duplication.
• Protein unchanged due to silent mutation.
• Function lost/worse/better.