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RELEVANT LEARNING OUTCOME:
(a) Explain what is meant by the term chromosomal aberration. For chromosomal aberration, knowledge of numerical aberration (including aneuploidy, as in the case of trisomy 21, i.e. Down syndrome) and structural aberration (including translocation, duplication, inversion and deletion) is required.
💡Introduction
💡Introduction
Chromosomes are condensed structures of DNA, wound around proteins.
A single chromosome consists of many different genes.
Each species has a unique chromosome number, with specific genes found on specific region of specific chromosomes (gene locus).
It is analogous to each gene having a unique postal code
As compared to gene mutations, which focusses on a single gene, chromosomal mutations looks at changes beyond a single gene.
Chromosomal aberration (mutation) can be either changes to the:
structure of chromosomes
number of chromosomes
Definition:
Chromosomal mutations/aberrations are changes in the structure or number of chromosome.
A pair of homologous with 5 different genes shown (D - H). Each gene has a specific position (locus) on the chromosome.
👁️Structural Mutations
👁️Structural Mutations
Mechanisms
Watch the video which will cover the 4 types of structural mutations:
Deletion
Duplication
Inversion
Translocation
note that there are 2 types of translocation:
Translocation: no exchange, only involve one segment from one chromosome translocating to another chromosome
Reciprocal translocation: involves exchange of chromosome segments between 2 chromosomes
Take note that for each labelled segments, it may contain multiple genes.
Effects
The effects of chromosomal structural mutation revolves around expression level of the genes involved:
Deletion - loss of genes would result in a decrease in expression of the genes.
Duplication - additional copies of the genes would result in an increase in expression of the genes.
Inversion and Translocation - changing the position of the genes may affect (increase/decrease) the expression level of the genes.
Q: What is the difference between crossing over during meiosis I and reciprocal translocation?
A: Crossing over is between homologous chromosome that exchanges alleles (alternate form of the same gene), whereas reciprocal translocation is the exchange of segments of chromosomes between non-homologous chromosomes, these segments contains different genes.
👁️Numerical Mutation - Aneuploidy
👁️Numerical Mutation - Aneuploidy
There are two types of numerical mutations, aneuploidy and polyploidy.
Aneuploidy is defined as the gain or loss of a single or several chromosomes.
Mechanism
Aneuploidy is caused by non-disjunction during meiosis I, meiosis II or mitosis.
Non-disjunction is the failure of chromatids or chromosomes to separate evenly during mitosis or meiosis.
Watch the video to see the impact of non-disjunction during meiosis I and meiosis II.
Take note of the following:
What is being separated during meiosis I and meiosis II?
What is the nomenclature (naming) for cells that are aneuploid?
Which results in a more severe outcome, in terms of proportion of normal to aneuploid gametes produced?
During:
Meiosis I, homologous chromosomes are separated
Meiosis II, (non-identical) sister chromatids are separated
Aneuploid cells are denoted with components:
n or 2n: this identifies if the cell is haploid (n) or diploid (2n), and this indicates the 'normal' number of chromosome
+/- X: this identifies if the cell has gain (+) or loss (+) of how many chromosomes (X).
For example,
a diploid cell with 2 extra chromosomes will be labelled as: 2n + 2
a haploid cell with a loss of 3 chromosomes will be labelled as n - 3
Non-disjunction during meiosis I is more severe, as the outcome are all aneuploid daughter cells (n+1, n-1), whereas non-disjunction during meiosis II results in 50% normal daughter cells (n), and 50% aneuploid cells (n+1, n-1) [see picture on the right]
What about mitosis?
Non-disjunction can also occur during mitosis, where sister chromatids are not separated evenly.
The picture on the right shows non-disjunction during mitosis of a diploid cell, where one pair of sister chromatid did not separate evenly.
Consider the following:
What is the diploid number of the cell? (number of chromosomes in a diploid cell)
Why is this diploid number not found naturally?
How many chromosomes are there in each daughter cell?
There are 3 chromosomes in the diploid cell, thus the diploid number, 2n = 3
A diploid organism consists of 2 sets of chromosomes (2 x n), one from each parent.
This should be an even number, as 'n' must be a whole number, a parent cannot contribute half a chromosome.
Therefore it is not natural to have 2n = 3, an odd number, as that would mean each parent contributed 1.5 chromosomes, which is not normal.
One daughter cell has 4 chromosomes, while the other has 2. This tallies with their label (recall that 2n = 3, for this context):
2n + 1 = 3 + 1 = 4
2n - 1 = 3 - 1 = 2
🖊️Check your understanding!
🖊️Check your understanding!
The picture on the right shows an aneuploid egg (n+1) being fertilised by a normal sperm (n) to give rise to an aneuploid zygote (2n + 1).
How would the zygote look like, if the egg cell is now an aneuploid of (n - 1), fusing with a normal sperm (n)?
👁️Aneuploid - Trisomy 21 (Down's Syndrome)
👁️Aneuploid - Trisomy 21 (Down's Syndrome)
A condition caused by aneuploidy is Down's syndrome, or trisomy 21.
The term 'trisomy 21' describes the aneuploid condition of having 3 copies of chromosome 21, instead of the normal 2.
Thus individuals who have Down's syndrome have 2n + 1 chromosomes, due to the extra chromosome 21.
Watch the video to learn more about Down's syndrome.
Take note of the following:
An individual with Down's syndrome can be formed from the fusion of one aneuploid gamete and one normal gamete. How many chromosomes are found in (diploid number of humans is 46):
aneuploid gamete
zygote formed from the fertilisation
What are your thoughts on parents choosing to abort Down's syndrome pregnancy?
Chromosomes in:
aneuploid gamete = 24 (having 1 extra chromosome 21, so n + 1)
zygote formed from fertilisation = 47 (24 + 23, or 2n + 1)
Keep your thought on this! This ethical dilemma will be explored in the last section!
👁️Numerical Mutation - Polyploidy
👁️Numerical Mutation - Polyploidy
Polyploidy the term derives from:
Poly: many
ploidy: sets of chromosome, e.g., haploid (1) ,diploid (2)
So puting the term together, polyploidy refers to having many/extra sets of chromosomes, more specifically, more than the normal 2 sets. For example, 3n, 4n, 5n and so on.
Polyploids are cells with extra sets of chromosomes - e.g., triploid (3 sets, 3n), tetraploid (4 sets, 4n) and hexaploid (6 sets, 6n)
Mechanism
Polyploids are usually formed from fusion of abnormal gametes.
For example, to get a triploid (3n) offspring, it involves the fusion of a diploid (2n) gamete and a haploid gamete: 2n + n = 3n
The abnormal diploid gamete can arise from non-disjunction of all the chromosomes during meiosis
Non-disjunction of all chromosomes during mitosis can also result in a polyploid daughter cell (the other daughter cell would have no chromosomes and dies off)
Effect
Polyploidy are generally not well tolerated in animals, and usually results in a miscarriage.
However, polyploidy are much more common in plants as it is often associated with advantageous features such as increased size, hardiness, and resistance to disease. This is called hybrid vigour.
For example, strawberries have 8 sets of chromosomes! Octoploidy (8n)!
FYI: Farmers tend to use a drug called colchicine (prevents spindle fibre formation) to induce formation of diploid gametes in plants, so as to produce polyploid offspring!
A - formation of normal gametes, fusion to form normal offspring (2n)
B - formation of 1 abnormal (diploid) gamete, fusion to form triploid offspring (3n)
C- formation of 2 abnormal (diploid) gametes, fusion to form tetraploid offspring (4n)
🖊️Learning Goals
🖊️Learning Goals
Check your understanding by attempting Qn 14 - 28 of the DNA Mutations Learning Goals.
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