cellular control Flashcards
what is a mutation
is a random change to the genetic material
what is a gene mutation
a change to the DNA
how might mutations occur
- spontaneously during DNA replication before cell division
- certain chemicals such as tar in tabacco smoke
- ionising radiation such as UV light , x rays and gamma rays
mutations associated with mitotic divisions are ….
somatic mutations and are not passed to offspring . they may be associated with the development of cancerous tumours
mutations associated with meiosis and gamete formation may be ….
inherited by offspring
what are the two main classes of DNA mutation
- point mutation
- insertion or deletion
what is a point mutation
one base pair replaces (is substituted for ) another
what is an insertion or deletion mutation
one or more nucleotides are inserted or deleted from a length of DNA , these may cause a frame shift
what are the tree types of point mutations
silent
missense
nonsense
describe silent mutations
when a change to the base triplet, where that triplet still codes for the same amino acids. the primary structure of the protein , and therefore the secondary and tertiary structure is not altered
describe missense mutations
a change to the base triplet sequence that leads to a change in the amino acid sequence in a protein
within a gene , such a point mutation may have a significant effect on the protein produced . the alteration to the primary structure leads to a change to the tertiary structure of the protein , altering its shape and preventing it from its usual function
what does sickle cell anaemia result from
from a missense mutation on the sixth base triplet of the gene for the b-polypeptide chain of haemoglobin , the amino acid valine , instead of glutamic acid , is inserted at this position . this results in deoxygenated haemoglobin crystallising within erythrocytes , causing them to become sickle shaped , blocking capillaries and depriving tissue of oxygen
what are nonsense mutations
a point mutation may alter a base triplet, so that it becomes a termination (stop) triplet. this particularly disruptive point mutation results in a truncated protein that will ot function . this abnormal protein will most likely be degraded withn the cell.
what genetic disease can be a result of nonsense mutation
duchenne muscular dystrophy
describe insertions or deletions
if nucleotide base pairs , not in multiples of three , are inserted into the gene or deleted from the gene , because the code is non over lapping and read in groups of three bases , all the subsequent base triplets are altered. this is a frame shift . when the mRNA from such a mutated gene is translated , the amino acid sequence after the frame shift is severely disrupted . the primary sequence of the protein and subsequently the tertiary structure , is much altered . consequently the protein cannot carry out its normal function . if the protein is very abnormal , it will be rapidly degraded within the cell
insertions or deletions of a triplet of base pairs results in
the addition or loss of an amino acid , and not a frame shift
what results from frameshifts due to the deletion of nucleotide bases
some forms of thalassaemia , a haemoglobin disorder
describe expanding triplet nucleotide repeats
some genes contain a repeating triplet ( eg CAG CAG CAG) in an expanding triple nucleotide repeat , the number of CAG triplets increases at meiosis and again from generation to generation
what disease results from an expanding triple nucleotide repeat , and how
huntington disease
if the number of repeating triplets sequence goes above a certain critical number , then the person with that genotype will develop the symptoms of huntingtons disease later in life .
are mutations always harmful
no , many are beneficial and have helped to drive evolution through natural selection
describe 2 useful mutations
- a mutation that gave rise to blue eyes , could be harmful in areas where sunlight intensity is high , due to lack of pigments . however in more temperate zones it could enable people to see better in less bright light
- humans in Africa would have black skin , the high concentrations of melanin protecting them from the sunburn and skin caner. when humans migrated to temperate regions , a paler skin would be an advantage , enabling vitamin D to be made with a lower light intensity of sunlight
