19- genetics of living systems

Question 1

What is a mutation, what are the different types?

Answer 1

• A mutation is a change in the sequence of bases in DNA.
• Protein synthesis can be disrupted if the mutation occurs within a gene.
• The change is caused by the substitution, deletion or insertion of one or more nucleotides within a gene.

Question 2

How can the mutation of a gene not have any effect?

Answer 2

• The substitution of a singular nucleotide changes the codon in which it occurs.
• The degenerate nature of the genetic code may mean that the new codon still codes for the same amino acid, leading to no change in the protein synthesised.

Question 3

What is a frame shift mutation?

Answer 3

• A deletion or insertion of a nucleotide, or nucleotides, leads to a frameshift mutation. The triplet code means that sequences of bases are transcribed consecutively in non-overlapping groups of 3. This is the reading frame of a sequence of bases.
• Each group of 3 bases corresponds to one amino acid.
• The addition or deletion of a nucleotide moves, or shifts the reading frame of the sequence of bases. This will change every successive codon from the point of mutation.

Question 4

What are the 3 different affects of mutations?

Answer 4

• No effect- there is no effect on the phenotype of an organism because normally functioning proteins are still synthesised.
• Damaging- the phenotype of an organism is affected in a negative way because the proteins are no longer synthesised or proteins synthesised are non- functional. This can interfere with one or more essential processes.
• Beneficial- very rarely a protein is synthesised that results in a new and useful characteristic in the phenotype.

Question 5

What are mutagens?

Answer 5

A chemical, physical or biological agent which causes mutations.

Question 6

Give 5 examples of mutagens and what they do.

Answer 6

• Physical mutagens; ionising radiations e.g. Xrays- break one or both DNA strands- some breaks can be repaired but mutations can occur in the process
• Chemical mutagens; deaminating agents- chemically alter bases in DNA such as converting C to U in DNA, changing the base sequence.
• Biological agents; alkylating agents- methyl or ethyl groups attached to bases, incorrect pairing of bases during replication.
  Base analogs- incorporated into DNA in place of the usual bases resulting in the incorrect pairing of bases during replication.
  viruses- viral DNA may insert itself into a genome, changing the base sequence.

Question 7

What are chromosomal mutations?

Answer 7

• they affect the whole chromosome or number of chromosomes within a cell. They can also be caused by mutagens and normally occur during meiosis.
• They can be silent, but often lead to developmental difficulties.

Question 8

Give examples of chromosome mutations.

Answer 8

• Deletion- a section of the chromosome breaks off and is lost within the cell
• Duplication- sections get duplicated on a chromosome
• Translocation- a section of one chromosome breaks off and joins another non0homologous chromosome
• inversion- a section of chromosome breaks off, is reversed, and then joins back onto the chromosome.

Question 9

What are the different ways that genes can be regulated?

Answer 9

• Transcriptional; genes can be turned on and off
• Post-transcriptional control; mRNA can be modified which regulates translation and the types of proteins produced.
• Translational control; translation can be stopped or started.
• Post-translational control; proteins can be modified after synthesis which changes their function.

Question 10

Explain how chromatin remodelling is transcriptional control.

Answer 10

• Heterochromatin is tightly wound DNA causing chromosomes to be visible during cell division whereas euchromatin is loosely wound DNA present during interphase.
• The transcription of genes is not possible when DNA is tightly wound because RNA polymerase cannot access the genes.
• The genes in euchromatin can be freely transcribed.
• Protein synthesis doesn’t occur during cell division by during the interphase between cell division.
• The is a form of regulation that ensures the proteins necessary for cell division are synthesised in time. It also prevents the complex and energy-consuming process of protein synthesis from occurring when cells are actually dividing.

Question 11

Explain how histone modification is an example of transcriptional control

Answer 11

• DNA coils around histones because they are positively charged and DNA is negatively charged. Histones can be modified to increase or decrease the degree of packing.
• The addition of acetyl groups or phosphate groups reduces the + charge on the histones and this causes DNA to coil less tightly, allowing certain genes to be transcribed.
• The addition of a methyl group makes histones more hydrophobic so they bind more tightly to each other causing DNA to coil more tightly and preventing the transcription of genes.

Question 12

What is epigenetics?

Answer 12

• A term that is increasingly used to describe the control of gene expression by the modification of DNA.
• It is sometimes used to include all of the different ways in which gene expression is regulated.

Question 13

What is an operon?

Answer 13

It is a group of genes that are under the control of the same regulatory mechanism and are expressed at the same time.
- They are more common in prokaryotes.
- They are an efficient way of saving resources because if certain gene products are not needed, then all the genes involved in their production can be switched off.

Question 14

What is a lac operon? How does it work?

Answer 14

• A group of 3 genes, lacZ, lacY and lacA, involved in the metabolism of lactose.
• They are structural genes as they code for 3 enzymes and they are transcribed into a single long molecule of mRNA.
• A regulatory gene, lacI, is located near to the operon and codes for a repressor protein that prevents the transcription of the structural gene in the absence of lactose.
• The repressor protein is constantly produced and binds to the operator, which is close to the structural genes.
• The binding of this protein prevents RNA polymerase binding to DNA and beginning transcription. This is down regulation.
• The section of DNA that is the binding site for RNA is called the promotor.
  -When lactose is present, it binds to the repressor protein causing it to change shape so it can no longer bind to the operator. As a result, RNA polymerase can bind to the promoter, the 3 structural genes are transcribed, and the enzymes are synthesised.

Question 15

Explain the role of cAMP in the metabolism of lactose.

Answer 15

• The binding of RNA polymerase still only results in a relatively slow rate of transcription that needs to be increased or up-regulated to produce the required quantity of enzymes to metabolise lactose efficiently.
• This is achieved by the binding of cAMP repressor protein (CRP), that is only possible wen CRP is bound to cAMP.
• The transport of glucose into an E.coli cell decreases the levels of cAMP, reducing the transcription of the genes responsible for the metabolism of lactose. If both glucose and lactose are present, glucose is the preferred respiratory substrate, so is metabolised.

Question 16

What is a post-transcriptional control method?

Answer 16

• The product of transcription is a precursor molecule, pre-mRNA.
• This is modified forming mature mRNA before it can bind to a ribosome and code for the synthesis of the required protein.
• A cap is added to the 5’ end and a tail is added to the 3’ end. They both help to stabilise mRNA and delay degradation in the cytoplasm. The cap also aids binding of mRNA to ribosomes.
• Splicing occurs where the RNA is cut at specific points, the introns are removed and the exons are joined together.
• Both of these occur in the nucleus.

Question 17

Explain the post-transcriptional control RNA editing.

Answer 17

• The nucleotide sequence of some mRNA molecules can also be changed through base addition, deletion or substitution.
• They have the same effect as point mutation and result in the synthesis of different proteins which may have different functions. This increases the range of proteins that can be produced from a single mRNA molecule or gene.

Question 18

What are 3 mechanisms that regulate the process of protein synthesis? - translational control.

Answer 18

• Degradation of mRNA- the more resistant the molecule, the longer it will last in the cytoplasm, that is, a greater quantity of protein is synthesised.
• Binding of inhibitory proteins to mRNA prevents it binding to ribosomes and the synthesis of proteins
• activation of initiation factors which aid the binding of mRNA to ribosomes.

Question 19

What are protein kinases and how do they impact protein synthesis?

Answer 19

• They are enzymes that catalyse the addition of phosphate groups to proteins .
• The addition of a phosphate group changes the tertiary structure and so the function of a protein.
• Many enzymes are activated by phosphorylation.
• Protein kinases are therefore important regulators of cell activity. Protein kinases are themselves often activated by cAMP.

Question 20

Give examples of post-translational control.

Answer 20

• Addition of non-protein groups such as carbohydrate chains, lipids or phosphates
• Modifying aas and the formation of bonds such as disulphide bridges.
• Folding or shortening of proteins
• Modification by cAMP- e.g. in the lac operon, cAMP binds to the CRP, increasing the rate of transcription of the structural genes.

Question 21

What are homeobox genes?

Answer 21

• They are a group of genes which all contain a homeobox.
• A homeobox is a section of DNA 180 base pairs long that is highly conserved in plants, animals and fungi. This part of the protein binds to DNA and switches other genes on or off. They are regulatory genes.

Question 22

What are hox genes?

Answer 22

• They are a group of homeobox genes which are only present in animals.
• They are responsible for the correct positioning of body parts.
• In animals the Hox genes are found in gene clusters- mammals have 4 clusters on different chromosomes.

Question 23

What are somites and what do they do?

Answer 23

• The individual vertebrae and associated structures have all developed from segments in the embryo called somites.
• The somites are directed by Hox genes to develop in a particular way depending on their position in the sequence.

Question 24

How can apoptosis be used in body plans?

Answer 24

• Apoptosis can be used to shape different body parts by removing unwanted cells and tissues.
• cells undergoing apoptosis can also release chemical signals which stimulate mitosis and cell proliferation leading to the remodelling of tissues.
• Hox genes regulate both mitosis and apoptosis.

Question 25

What factors affect the expression of regulatory genes?

Answer 25

- It can be influenced by the environment, both internal and external. - Stress can be defined as the condition produced when the homeostatic balance within an organism is upset. - This can be due to external factors such as a change in temp or intensity of light. - Internal factors can change due to release of hormones or psychological stress. These factors will have a greater impact during the growth and development of an organism. - Drugs can also affect the activity of regulatory genes e.g. thalidomide.