9.1 Gene mutation Flashcards
What is a mutation
What is gene mutation
Any change to the quantity or the base sequence of DNA of an organism
Any change to one or more nucleotide bases or a change in base sequence in DNA is a gene mutations
How are sudden differences produced between individuals
Mutations occur during formation of gametes may be inherited which can produce distinct differences between individuals
Describe how substitution of bases is a mutation
What can this lead to
A nucleotide in a DNA molecule is replaced by another nucleotide that has a different base.
This means one triplet is changed which will possibly code for a different amino acid.
So there will be a different polypeptide produced.
What problems does substitution of bases make
If the role of the amino acid makes proteins that are enzymes and structural proteins
If the amino acid is changed, the polypeptide produced will differ depending on the role of the amino acid.
- Eg if it 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.
The protein may be a different shape so not function properly.
If protein is an enzyme, it may have a changed active site so substrate won’t fit into it so it will not catalyse the reaction
What is the other (more common) result of an amino acid substitution
- The new triplet may still code for the same amino acid as before
- Due to the degenerate nature of the genetic code.
- So it will not impact the polypeptide chain
Or… It will mutate an intron which doesn’t code for proteins so won’t make a difference in polypeptide produced
What is the deletion of bases
. A gene mutation by deletion occurs when a nucleotide is lost from the normal DNA sequence
This will affect the entire amino acid sequence in the polypeptide so it is unlikely to function correctly.
This is because each triplet will be changed so they will all be read differently as they have all shifted to the left
What is a chromosome
Thread structures inside nucleus of animal and plant cells
Contains 1 long and condensed molecule of DNA, and histones
What are chromosome mutations
Two types that result from non-disjunction
Changes in the structure or number of whole chromosomes
Take two forms:
- Changes in whole sets of chromosomes (polyploidy)
- Changes in number of individual chromosomes (aneuploidy)
What is another name for changes in the number of individual chromosomes as a mutation
When does it occur
Non-disjunction
Sometimes individual homologous pairs of chromosomes fail to separate properly during meiosis
Eg instead of 2 going in each cell, 3 goes in 1 and 1 goes in the other.
Or the chromosomes just don’t separate
So leads to uneven distribution of chromosomes in each daughter
This results in a gamete having either one more or one less chromosome.
After fertilisation this can result in the resultant offspring having more or fewer chromosomes
Eg leading to down syndrome
What is polyploidy
What occurs
and where?
Polyploidy is a type of non-disjunction mutation
Changes in the whole set of chromosomes
Every single chromosome in the cell is affected
When organisms have three or more sets of the same chromosomes rather than the usual two
So a triploid cell is produced instead of diploid
Occurs mostly in plants
What is aneuploidy
Changes in the number of individual chromosomes.
So only one chromosome is affected
Eg if one homologous pair of chromosomes fail to separate during meiosis
So a haploid gamete is still produced, but it will have one more or one less chromosome
Describe the process of how polyploidy occurs
How can this produce a triploid gamete
. Lets say there are 3 homologous pairs of chromosomes, as each chromosome has previously doubled in interphase
. Every pair of chromosomes fail to separate equally, so both the chromatids go into one cell
. So in the two cells produced by meiosis I, all the chromosomes are in one and none are in the other
. Then maybe meiosis II occurs normally, but the chromosomes all line up vertically, not in pairs, and then split from one cell, whilst there are still non in the other cell
. This means the process of meiosis will produce two diploid gametes , and two empty gametes
This process can also occur in meiosis II
A triploid 3N gamete is produced when one of the diploid daughter cells from this fuse with a haploid daughter cell produced in a normal functioning cell
So they fuse to make 3 of each chromosome
Describe how the process of aneuploidy can occcur
So if we start with 3 homologous pairs of chromosomes, and their centromeres join to spindle fibres which pull the two chromosomes apart
. However the spindle fibres don’t attach to one of the chromosomes, so both are pulled to the left pole, and none in that pair are pulled to the right
. Then meiosis II occurs normally, so pulls the chromatids apart in each of the two cells to produce 4 haploid daughter cells.
However in the left cell, it only pulls apart a pair of chromosomes
So in the 4 haploid cells, two has one more chromosome and two have one less
It can also occur in meiosis II, but two of the 4 daughter cells will be normal haploid, one will have one extra chromosome and one will have one less
