Numerical Chromosomal Abnormalities

Question 1

Critically compare mitosis and meiosis

Answer 1

Mitosis and meiosis are the two types of cell division in eukaryotic organisms

Mitosis:

Mitosis and meiosis are the two types of cell division in eukaryotic organisms

These daughter cells are genetically identical to the parent cell and to each other

This process is used for the growth of tissues, the replacement of cells, and asexual reproduction. The basic steps are:

1. Prophase: The chromosomes condense and become visible. The mitotic spindle forms and the nuclear envelope breaks down
2. Metaphase: The chromosomes align in the middle of the cell, known as the metaphase plate
3. Anaphase: The sister chromatids separate and move towards opposite poles of the cell
4. Telophase and Cytokinesis: The chromosomes de-condense, the nuclear envelope re-forms, and the cell divides into two

Meiosis:

Meiosis, on the other hand, is a process that produces four daughter cells, each of which has half the number of chromosomes as the parent cell

These daughter cells are not genetically identical to the parent cell or to each other

This process is used for sexual reproduction, and it introduces genetic diversity

Meiosis I:

1. Prophase I: The chromosomes condense, the nuclear envelope breaks down, and homologous chromosomes pair up and exchange genetic material in a process called crossing over
2. Metaphase I: The pairs of homologous chromosomes align in the middle of the cell
3. Anaphase I: The homologous chromosomes separate and move towards opposite poles. Unlike in mitosis, the sister chromatids do not separate at this stage
4. Telophase I and Cytokinesis: The cell divides into two

Then Meiosis II follows:

1. Prophase II: The chromosomes condense and the nuclear envelope breaks down
2. Metaphase II: The chromosomes align in the middle of the cell
3. Anaphase II: The sister chromatids separate and move towards opposite poles
4. Telophase II and Cytokinesis: The cell divides into two, resulting in four haploid cells

Critical comparison:

1) Number of Daughter Cells:

• Mitosis produces two daughter cells, while meiosis produces four

2) Genetic Composition:

• Daughter cells from mitosis are genetically identical to parent, while those from meiosis are genetically unique

3) Chromosome number:

• Daughter cells from mitosis maintain the diploid chromosome number, while those from meiosis are haploid

4) Role in the organism:

• Mitosis is involved in growth, tissue repair, and asexual reproduction, while meiosis is involved in sexual reproduction and provides genetic diversity

Question 2

Describe the normal human karyotype and define terms associated with abnormal chromosome number

Answer 2

The normal human karyotype consists of 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes

The autosomes are numbered from 1 to 22, with 1 being the largest and 22 the smallest

The sex chromosomes are designated as X and Y, with XX typically representing female and XY typically representing male

1) Aneuploidy:

• This term refers to a cell that doesn’t contain a multiple of 23 chromosomes (the haploid number). An aneuploid cell can have one extra or one less chromosome
• Trisomy: A specific type of aneuploidy where there is one extra chromosome. E.g. Trisomy 21
• Monosomy: This is another type of aneuploidy where there is one less chromosome. E.g. Turner Syndrome

2) Polyploidy:

• This term refers to cells that contain more than two complete sets of chromosomes
• E.g. For example, triploidy refers to cells with three complete sets of chromosomes (69 total), and tetraploidy refers to cells with four complete sets of chromosomes (92 total)

3) Nondisjunction:

• This is the failure of chromosome pairs to separate properly during cell division, which can lead to aneuploidy in the resulting cells
• Nondisjunction can occur during mitosis, meiosis I, or meiosis II

4) Mosaicism:

• This refers to the presence of two or more populations of cells with different genotypes in one individual, all of whom are derived from a single zygote
• It can occur due to a mutation during development which propagates to only a portion of the cells

5) Chimerism:

• This is similar to mosaicism, but it results from the fusion of two different zygotes
• Individuals who have received organ transplants can also be considered chimeras

Question 3

Explain how abnormal chromosome numbers arise by the process of meiotic non-disjunction

Answer 3

Nondisjunction is an error in cell division that occurs when paired chromosomes or sister chromatids fail to separate and move to opposite poles during meiosis or mitosis

This results in the production of gametes with abnormal numbers of chromosomes

1) Nondisjunction in Meiosis I:

• If nondisjunction occurs in Meiosis I, the homologous pairs of chromosomes fail to separate
• This means that both members of the homologous pair go into one daughter cell, while the other daughter cell gets none
• Consequently, at the completion of meiosis II, this leads to two gametes that have an extra chromosome (n+1) and two gametes that are missing that chromosome (n-1)

2) Nondisjunction in Meiosis II:

• If nondisjunction occurs in Meiosis II, the sister chromatids fail to separate
• This leads to one normal gamete (n), one with an extra chromosome (n+1), and one with one less (n-1), with the fourth being normal (n)

When these abnormal gametes are involved in fertilization, they lead to zygotes with abnormal chromosome numbers. For example, if a normal sperm fertilizes an egg with an extra chromosome 21 (due to nondisjunction), the resulting zygote will have three copies of chromosome 21 instead of the normal two. This is known as Trisomy 21, which leads to Down syndrome

Similarly, if a normal sperm fertilizes an egg that is missing a sex chromosome (also due to nondisjunction), the resulting zygote will have only one X chromosome instead of two sex chromosomes. This is known as Monosomy X, which leads to Turner syndrome

Question 4

Define mosaicism and explain how it can arise due to mitotic non-disjunction

Answer 4

Mosaicism is a condition where an individual’s body is made up of two or more populations of cells with different genotypes

In other words, not all cells of the body have the same genetic makeup

This can occur as a result of a mutation during development which is propagated to only a subset of the adult cells

The cells of the body that originate from the cell in which the change occurred will carry the mutation, while the rest of the cells will not

Mitotic non-disjunction refers to the failure of sister chromatids to separate during mitosis

1. After fertilization, the zygote begins to divide by mitosis to produce all of the cells in the body
2. During one of these mitotic divisions, non-disjunction may occur, causing one daughter cell to receive both sister chromatids and the other to receive none
3. The cell that receives both chromatids will have an extra copy of a chromosome (trisomy), while the cell that does not receive a chromatid will be missing a chromosome (monosomy)
4. These cells will then continue to divide and produce more cells with the same chromosome number. As the individual develops, patches of cells with the abnormal number of chromosomes will be spread throughout the body, resulting in mosaicism

The phenotype of an individual with mosaicism can be less severe than in individuals where all cells carry the chromosomal abnormality, because some cells are genetically normal