Session 11 ILOs - Mitosis and Meiosis

Question 1

Describe the relationship between DNA molecules, chromosomes and chromatids

Answer 1

A chromosome consists of a single, double-stranded DNA molecule. Chromatids are two molecules of double-stranded DNA joined together in the centre by a centromere.

Question 2

Outline and describe chromosome and chromatid structure

Answer 2

A chromatid is one of two identical halves of a replicated chromosome
A chromosome is a form of condensed DNA (chromatids are less condensed)

Question 3

Explain the difference between ‘gene’ and ‘allele’

Answer 3

Gene is a functional piece of DNA that determines a specific trait while alleles are a different variation of a gene (a gene is comprised of 2 alleles, whereas an allele can be a dominant or recessive allele)

Question 4

Describe the general outline of each phase within the process of mitosis

Answer 4

1. Prophase - Chromsomes condense and the nuclear envelope disintegrates
2. Prometaphase - Kinetochores attach to the chromatids and the spindles form
3. Metaphase - Spindles contract and the chromatids align along the centre of the cell (metaphase alignment) - each chromatid attached to spindle fibre originating from opposing poles
4. Anaphase - Chromatids are pulled to opposite ends of the cell
5. Telophase - The chromosomes (newly formed) appear at either pole of the cell and begin to decondense and the nuclear envelope re-forms ready for cytokinesis (the physical process of cell division)

Question 5

Explain how genetically identical daughter cells are created during mitosis

Answer 5

The cell copies or ‘replicates’ its chromosomes, and then splits the copied chromosomes equally to make sure that each daughter cell has a full set

Question 6

Explain how mitotic nondisjunction can lead to trisomy and mosaicism (mitosis)

Answer 6

Mitotic non-disjunction can occur when sister chromatids are not distributed evenly during anaphase (uneven distribution).

Trisomy is when non-disjunction occurs upon the very first post-zygotic cell division, where there are 2 cells (one with an extra chromosome and one with one less chromosome) - the cell line of the extra copy continues = trisomy

Mosaicism occurs when non-disjunction happens later in post-zygotic division, which means 2 cell lines for (normal cell line and then the cell line outlined above) - so this means the normal cell line AND the cell line of the extra copy continues = mosaicism

Question 7

Describe the general outline of each phase within the process of meiosis

Answer 7

Meiosis I
1. Prophase - Centromeres separate and the nuclear membrane disintegrates - homologous pairs ‘find’ each other’ AND there is crossover of genetic material
2. Metaphase - Homologous pairs line up along the metaphase plate
3. Anaphase - Homologous pairs separate to each pole
4. Telophase - nuclear membrane reappears (but only 23 chromosomes). End of telophase, the centrosomes have doubled ready for Meiosis II
END WITH 2 DAUGHTER CELLS

Meiosis II
1. Prophase - Again, the nuclear membrane disintegrates
2. Metaphase - Again, the chromosomes line up along the metaphase plate
3. Anaphase - Chromatids are separating
4. Telophase - Reformation of nuclear membrane
END WITH CREATION OF 2 CELLS FOR EACH OF THE 2 ORIGINAL DAUGHTER CELLS (haploid)

Question 8

Compare and contrast meiosis I and meiosis II

Answer 8

In meiosis I, homologous chromosomes separate, while in meiosis II, sister chromatids separate

Meiosis I produces 2 diploid daughter cells, whereas meiosis II produces 4 haploid daughter cells

Genetic recombination (crossing over) only occurs in meiosis I

Question 9

Compare and contrast spermatogenesis and oogenesis

Answer 9

Spermatogenesis is a much quicker process (60 days), compared to oogenesis (12-50 years)

1 spermatocyte produces 4 gametes (haploid), whereas oogenesis produces 1 egg and 3 polar bodies (haploid)

Each spermatogonium precedes to a spermatid, which continues to mature sperm without delay. However in oogenesis, a oogonium is converted to a primary oocyte which is arrested in prophase I AND only 1 full egg is produced per oocyte

Question 10

Compare and contrast mitosis and meiosis

Answer 10

Mitosis results in two identical daughter cells, whereas meiosis results in four gamete cells

Mitosis gives identical cells to each other and to the mother cell, while meiosis leads to genetic variation due to crossing over and independent assortment

Mitosis results in diploid cells, whereas meiosis results in haploid cells

Mitosis includes one division , while meiosis includes two

Question 11

Explain how genetic variation is created during meiosis

Answer 11

Genetic variation occurs in 2 ways:

1. During Prophase 1, where recombination occurs (ds-DNA breaks)
2. During Metaphase 1 where there are random assortment of chromosomes

Question 12

Explain how meiotic nondisjunction leads to monosomy and trisomy (meiosis)

Answer 12

In meiosis, non-disjunction occur in either Meiosis I or Meiosis II (either would occur during Anaphase)

Non-disjunction during Meiosis I:
Non-disjunction results in 1 cell having both copies of the chromosome and the other cell having no copies (one cell both copies, one cell no copies). These cells then enter meiosis II where they produce 2 cells each (2 cells with 2 copies, 2 cells with no copies). When they combine with normal gametes, then 2 cells have 3 copies (trisomy) and 2 cells have 1 copy (monosomy)

Non-disjunction during Meiosis II:
Meiosis I produces 2 normal cells. These cells then enter meiosis II, however if 1 cell undergoes non-disjunction, then it produces one cell with 2 copies and one cell with no copies) BUT also another normal cell line (2 normal cells). When these cell lines combine with normal gametes, then one cell has 3 copies (trisomy) and one cell has 1 copy (monosomy) and there are 2 normal cells.