17.3 Flashcards
What is a mutation? (1)
a change in the DNA sequence/base sequence
OR
a change in the quantity or structure of the DNA of an organism
How can a mutation cause discontinuous variation? (2)
(1) if the mutation occurs during the formation of the gametes
(2) the mutation may be inherited
Any change to 1 or more bases in the DNA triplets could result in a change in the amino acid sequence in the polypeptide.
What are the 3 types of mutation? (3)
(1) substitution
(2) deletion
(3) insertion
Substitution is a type of mutation. Describe the process of the substitution of bases. (1)
A nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
The substitution of a base in a DNA molecule can lead to a nonsense mutation.
Describe what a nonsense mutation is & how it can cause the protein to not perform its normal function. (3)
Use the example of the DNA triplet code GTC, which codes for glutamine.
(1) it occurs if the base change results in the formation of ** a stop codon**
(2) for example, if GTC changed to ATC, ATC is transcribed to UAG, which is a stop codon
(3) so the production of the polypeptide would be stopped prematurely due to the stop codon
The substitution of a base in a DNA molecule can lead to a missense mutation.
Describe what a missense mutation is & how it can cause the protein to not perform its normal function. (4)
Use the example of the DNA triplet code GTC, which codes for glutamine.
(1) It occurs when the base change results in a different amino acid being coded for
(2) If GTC becomes GTG, the amino acid histidine replaces the original amino acid, which is glutamine
(3) If this amino acid is important in forming bonds that determine the tertiary structure of the final protein, then the replacement amino acid may not form the same bonds
(4) The protein will then be a different shape and will not function properly (e.g. enzymes won’t catalyse reactions)
The substitution of a base in a DNA molecule can lead to a silent mutation.
Describe what a silent mutation is & how it WILL cause the protein to perform its normal function. (4)
Use the example of the DNA triplet code GTC, which codes for glutamine.
(1) when the substituted base, although different, still codes for the same amino acid as before
(2) This is due to the DEGENERATE NATURE of the genetic code, in which most amino acids have more than one codon
(3) If GTC becomes GTT, the codon will still code for glutamine
(4) There is no change in the polypeptide produced, and the mutation will have no effect
NOTE: to make the 3 types of mutation consequences clear, check the table on p.242
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The deletion of a base in a DNA molecule is when a nucleotide is lost from the normal DNA sequence.
Describe how the deletion of a base can cause the protein to not perform its normal function. (3)
(1) 1 deleted nucleotide creates a FRAME SHIFT because the reading frame that contains each three letters of the code has been shifted to the left by one letter
(2) So the gene is now read in the wrong 3-base groups & the genetic message is altered
(3) The position of the deleted base is important because it determines how significant the effect was (e.g. if it was at the end, only 1 amino acid changes. If it was at the beginning, the whole sequence changes)
Since mutations are spontaneous, what’s the average mutation rate for all species? (1)
HINT: the unit is mutations per genes per generation
1 or 2 mutations per 100,000 genes per generation
What are the 2 main mutagenic agents? What does each agent do? (2)
(1) High energy radiation that disrupts the DNA molecule
(2) Chemicals that alter the DNA structure OR interfere with the transcription
Mutations have both costs and benefits. What are they? (2+1)
Costs:
- they often produce an organism that’s less well suited to its environment
- Mutations that occur in body cells can disrupt normal cellular activities such as cell division
Benefit:
- It can produce the genetic diversity necessary for natural selection & speciation
Cell division is controlled by __(1)__. Most cells divide at a fairly ___(2)___ rate to ensure that __(3)__ or worn-out cells are ____(4)____. In normal cells, these genes control the rate:
1. proto-oncogenes _____(5)_____ cell division
2. tumour suppressor genes ____(6)____ cell division.
(1) genes
(2) constant
(3) dead
(4) replaced
(5) stimulate / promote
(6) slow down / inhibit
Describe how proto-oncogenes work. (3)
(1) In a normal cell, growth factors attach to the receptor protein on the cell surface membrane
(2) and, via relay proteins in the cytoplasm,
(3) switch on the genes necessary for DNA replication
A gene mutation can cause proto-oncogenes to mutate into oncogenes. These oncogenes can affect the cell division in 2 ways. Describe each way. (4)
Way 1:
(1) The receptor protein on the cell membrane can be permanently activated
(2) so that cell division is switched on even in the absence of growth factors
Way 2:
(3) The oncogene may code for a growth factor that is then produced in excessive amounts
(4) This stimulates excessive cell division
When cells divide too rapidly, a tumour or cancer develops. What are the two types of tumors? Describe each. (6)
(1) malignant:
(2) has cancerous cells that can invade nearby tissues
(3) it can spread to other parts of the body, where they continue to grow, forming metastases
(4) benign:
(5) They do not spread to other parts of the body
(6) they can be removed relatively easily and generally don’t come back
If a tumour suppressor gene becomes mutated, it is inactivated. How does that affect the cell division and the type of cells that are produced? (3)
(1) The gene will stop inhibiting cell division.
(2) The rate of cell division increases
(3) this produces mutant cells, which are structurally & functionally different from normal cells.
NOTE: Most mutated cells die. However, any that survive are capable of making clones of themselves, forming tumours. The tumour can either be benign or malignant.
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ANALOGY for proto-oncogenes & tumour suppressor genes:
The car represents cell division.
The proto-oncogene is like the accelerator pedal that increases the speed of the car.
The tumour suppressor gene is like the brake pedal that reduces the speed of the car.
remember this for an easier life bae.
“Suggest one environmental factor that might increase the rate of mutation in the p53 gene and cause lung cancer.” (1)
Cigarette smoke
OR
ionising radiation - e.g. UV / x-rays / gamma rays
OR
chemical substances - e.g. formaldehyde / benzene / mustard gas
“Describe how a deletion mutation alters the structure of a gene.” (2)
(1) Removal of one or more bases / nucleotide
(2) frameshift / (from point of mutation) base sequence change
” Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of 1 base for another.” (3)
(1) Deletion causes frame shift / alters the base sequence (from point of mutation)
(2) changes many amino acids / sequence of amino acids (from this point)
(3) substitution alters one codon / triplet / 1 amino acid altered / code degenerate / same amino acid coded for
NOTE: when they talk about gene mutation in an ENZYME, and they ask how the mutation causes it to be non-functional, talk about how the enzyme won’t be able to form E-S complexes because of the change in active site & tertiary structure.
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NOTE: whenever they ask about silent mutations, mention the words degenerate code or the degenerate nature of the genetic code
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