cellular control Flashcards
1
Q
what are the two main classes of DNA mutation
A
- Substitution (point mutation)
- Insertion and deletion(ideal) this is where one or more nucleotides are inserted or deleted from a length of DNA this causes a frameshift
2
Q
describe point mutations
A
- Genetic code consists of nucleotide base triplets within the DNA, during transcription of a gene the code is copied to a length of MRNA as codons, the sequence is therefore a copy of the sequence of base triplets on the gene
3
Q
what are the three types of point mutations
A
- silent
- missense
- non sense
4
Q
describe silent
A
- All amino acids involved in protein synthesis apart from methionine have more than one base triplet code, this reduces the effect of point mutations as they do not cause a change in the sequence of amino acids in a protein this is called the redundancy or degeneracy of the genetic code
- A point mutation involving the change to the base triplet where the triplet still codes for the same amino acids is a silent mutation, this means that the primary structure does not change so therefore the secondary and tertiary structure does not change
5
Q
describe missense mutations
A
- A change in the base triplet that leads to a change in the amino acid sequence in a protein is a missense mutation
- Have a significant effect on the protein produced, there is alteration to the primary structure that leads to change in the tertiary structure so it can not carry out its function
- Sickle cell anaemia results from a missense mutation on the 6th base triplet of the gene for the Bpolypeptide chains of haemoglobin, the amino acid valine instead of glutamic acid is inserted in this position resulting in deoxygenated haemoglobin crystallising within the erythrocytes causing them to become sickle shaped, blocking capillaries and depriving tissues of oxygen
6
Q
describe non sense mutations
A
- Point mutation may alter a base triplet so that it becomes a termination triplet, this is the disruptive point mutation results in truncated protein that will not function and this protein will be degraded within the cell
- Genetic disease of Duchenne muscular dystrophy is the result of nonsense mutation
7
Q
describe indel mutations
A
Insertions and deletions
- If nucleotide base pairs not in multiples of three are inserted in the gene or deleted from the gene, because code is non overlapping and read in groups of three bases, all subsequent base pairs are altered and this is a frameshift
- When MRNA from a mutated gene is translated the amino acid sequence after the frameshift is disrupted therefore the primary structure and secondary structure is altered so protein cannot function
- Thalassaemia – haemoglobin disorder results from the frameshifts due to deletions of nucleotide bases
- Insertions of deletions of a triplet base pair results in the addition or loss of an amino acid and not in a frame shift
Expanding triple nucleotide repeats
- In expanding triple nucleotide repeat the number of CAG triplets increase at meiosis and again from generation to generation
- Huntingtons disease results from an expanding triple nucleotide repeat, if the number of repeating CAG sequences goes above a certain critical number then the person with that genotype will develop the symptoms of Huntington disease later in life
8
Q
what are the beneficial effects of mutations
A
- Driven evolution through natural selection
- Have rise to blue eyes – may be harmful in areas where light intensity is high as the lack of iris pigmentation could lead to lens cataracts however in more temperate zone sit could enable people to see better in les slight
- Early humans in Africa had black skin – high concentration of melanin protecting them from sunburn and skin cancer – when they migrated to temperate regions a paler skin would be an advatages and enabled vitamin D to be made with lower intensity of sunlight – protects from rickets, heart disease and cancer
- Some are unharmful such as the inability to smell certain flowers and differently shaped ear lobes
9
Q
describe the regulatory mechanisms in prokaryotic cells
A
- Enzymes that catalyse the metabolic reactions involved in basic cellular functions are synthesised at a constant rate, enzymes that may only be needed under specific conditions are synthesised at varying rates according to the need of the cell
10
Q
describe transcriptional level Lac Operon
A
- The bacterium E.coli normally metabolises glucose as a respiratory substrate, however if glcose is absent and lactose is present, lactose induces the production of two enzymes, these are lactose permease which allows lactose to enter the bacterial cell and B-galactosidase which hydrolyses lactose to glucose and galactose
- The lack operon consist of a length of DNA containing an operator region lacO next to the structural genes lacZ and lacY that code for the enzyme B-galactosidase and lactose permase respectively
- Next to the operator region lacO is the promoter region P to which the enzyme RNA polymerase binds to begin transcription of the structural genes lacZ and lacY
- Operator region and promoter region are the control sites
- A small distacen away from the operon is the regulatory gene I that codes for a repressor protein LacI when the regulatory gene is expressed the repressor protein produced binds to the operator preventing RNA polymerase from binding to the promoter region
- The repressor protein therefore prevents the genes lacZ and lacY from being transcribed so the enzymes for lactose metabolism are not made and the genes are off
- When lactose is added to the culture medium, once all the glucose has been used, molecules of lactose bind to the lacI repressor protein molecules this alters the shape of the LacI repressor protein and prevents it from binding to the operator, the RNA polymerase enzyme can then bidn to the promoter region and begin transcribing the structural genes into MRNA that will then be translated into the two enzymes, thus lactose induces the enzymes needed to break it down
11
Q
describe transcription factors
A
- Every cell in a eukaryotic organism has the same genome but because different cells use it differently and function differently, this is the basis of cell differentiation
- Transcription factors are proteins or short non-coding pieces of RNA that act within the cells nucleus to control which genes in a cell are being turned off or on, transcription factors slide along a part of the DNA molecule seeking and binding to their specific promoter regions, they may then aid or inhibit the attachment of RNA polymerase to the DNA and activate or suppress transcription of gene
- Make sure that different genes are expressed or suppressed and are involved in regulating the cell cycle
- Tumour suppressor genes and proto-oncogenes help regulate cell division via transcription factors – mutations to these genes can lead to uncontrolled cell division or cancer
12
Q
what are the post transcriptional gene regulations
A
introns and exons
13
Q
describe interons and exons
A
- Within a gene there are non-coding regions of DNA called introns which are not expressed. They separate the coding or expressed regions which are called exons
- All DNA of a gene both introns or exons is transcribed, the resulting mrna is called primary mrna is then edited and the RNA introns (lengths corresponding to the DNA introns) are removed, the remaining Mrna exons corresponding to the DNA exons are joined together
- Endonuclease enzymes may be involved in the editing and splicing processes
- Some introns may themselves encode proteins and some may become short non-coding lengths of RNA involved in gene regulation
- Genes can be spliced in different ways, a length of DNA with its introns and exons can encode more than one protein according to how its spliced
14
Q
describe post translation level of gene regulation
A
- Post-translation regulation of gene expression involves the activation of proteins
- Many enzymes are activated by being phosphorylated
- Camp is important second messgender
1. Signalling molecule such as a protein hormone binds to a receptor on the plasma membrane of the target cell
2. This activates a transmembrane protein which then activates a G protein
3. The activated G protein activates adenyl cyclase enzymes
4. Activated adenyl cyclase enzymes catalyse the formation of many molecules of camp from ATP
5. Camp activates protein kinase A (PKA)
6. Activated PKA catalyses the phosphorylation of various proteins hydrolysing ATP in the process, this phosphorylation activates many enzymes in the cytoplasm for example those that convert glycogen to glucose
7. PKA may phosphorylate another protein CREB, camp response element binding
8. This then enters the nucleus and acts as a transcription factor to regulate transcription
15
Q
what is a hoemobox gene sequence
A
- A Hoemobox gene sequence is a sequence of 180 base pairs (excluding introns) found within genes that are involved in regulating patterns of anatomical development in animals, fungi and plants,
