Meiosis Life Sciences Grade 12 Notes and Study Guides

CHAPTER 2: MEIOSIS

2.1 What is meiosis?

Meiosis is a type of cell division whereby a diploid cell (somatic cell) undergoes two cell divisions, and divides to form four dissimilar haploid cells (sex cells). Diploid cells have two sets of chromosomes, where each chromosome has a homologous partner. Haploid cells only have one set of chromosomes. Chromosomes in haploid cells have no homologous
partners.
Before meiosis begins (during interphase), DNA replication takes place. The result is two sets of chromosomes consisting of two identical chromatids joined together with a centromere. This is shown in Figure 2.1 (below).

2.2 The process of meiosis in animal cells

Meiosis is the type of cell division used to produce gametes or sex cells (sperm and egg cells). A cell undergoing meiosis will divide twice – the first division is meiosis I and the second is meiosis II. In the first meiotic division, the number of cells is doubled, but the number of chromosomes is not. This results in half as many chromosomes per cell. In the second meiotic division, the number of chromosomes does not get reduced. The  diagram  alongside  shows  how  meiosis starts with a diploid cell and divides twice (meiosis I and II), resulting in four haploid cells.

2.2.1 First meiotic division

	Prophase 1 Chromosomes shorten and become visible as two chromatids joined by a centromere. Homologous pairs of chromosomes are now visible. The nuclear membrane and nucleolus disappear. The spindle starts to form. Chromatids from each homologous pair touch. The point where they touch is called a chiasma. DNA is crossed over (swopped) at the chiasma. The spindle continues to form.
	Metaphase 1 The spindle extends across the whole cell. The homologous chromosomes line up along the equator of the spindle in their homologous pairs. One chromosome of each pair lies on either side of the equator. The centromere of each chromosome attaches to the spindle fibres.
	Anaphase 1 The spindle fibres shorten and pull each chromosome of each chromosome pair to opposite poles of the cell.
	Telophase 1 The chromosomes reach the poles of the cell. Each pole has half the number of chromosomes present in the original cell. The cell membrane constricts and divides the cytoplasm in half to form two cells.

2.2.2 Second meiotic division

	Prophase 2 Each cell formed during meiosis I now divides again. A spindle forms in each of the new cells.
	Metaphase 2 Individual chromosomes line up at the equator of each cell, with the centromeres attached to the spindle fibres.
	Anaphase 2 The spindle fibres start to contract. The centromeres split and daughter chromosomes/ chromatids are pulled to the opposite poles of each cell.
	Telophase 2 The daughter chromosomes/chromatids reach the poles and a new nucleus forms. The cell membrane of each cell constricts and the cytoplasm divides into two cells. Four haploid daughter cells are formed. Each daughter cell has half the number of chromosomes of the original cell. The daughter cells are genetically different from each other.

An easy way to remember the events of meiosis is to use the word mnemonic IPMAT.

2.3 The significance of meiosis

There are two reasons why meiosis is important.

1. It reduces the number of chromosomes by half, in other words from diploid to haploid. This ensures that sex cells have half the number of chromosomes of other somatic cells so that when fertilisation occurs the zygote formed has the correct number of chromosomes. It balances the doubling effect of fertilisation.
2. Crossing over introduces genetic variation. Genetic variation results in offspring that are better adapted to a particular environment and ensures that they will have a better chance of survival.

2.4 Abnormal meiosis

• Sometimes mistakes occur during the process of meiosis.
• This can happen in Anaphase 1 where the homologous chromosomes may not separate. Also called non-disjunction. (see above)
• It can also happen in Anaphase 2 when there is non-disjunction of the sister chromatids.
• If there is non-disjunction of chromosome pair 21 in humans it leads to the formation of an abnormal gamete with an extra copy of chromosome 21.
• If there is fusion between a normal gamete and an abnormal gamete (with extra copy of chromosome 21) it leads to Down Syndrome.

2.5 Differences between meiosis I and meiosis II

Table 2.1 The differences between meiosis I and meiosis II

Worked example

Study the diagrams below of two stages of meiosis then answer the questions that follow.

Figure 2.11 Two stages of meiosis

1. State ONE visible reason in Diagram I which indicates that meiosis is taking place. (1)
2. How many chromosomes would be present in each daughter cell at the end of meiosis in this cell? (1)
3. Describe what takes place in the cell after the phase shown in Diagram I. (3)
4. Tabulate TWO visible differences between the phases of meiosis shown in Diagrams I and II. (5) [10]

Answers to worked example

1. The chromosomes are lined up at the equator of the cell in their homologous pairs.✔
   OR
   The chromosomes show evidence of crossing over.✔ (1)
2. Two ✔ chromosomes. (1)
3. The next phase is Anaphase 1. The spindle fibres contract.✔ (shorten) and pull each chromosome ✔ of each chromosome pair to opposite poles ✔ of the cell.

4. Diagram I (metaphase 1)	Diagram II (metaphase 2)
   1.   Chromosomes are lined up at the equator in homologous pairs.✔	1.   Chromosomes are lined up at the equator individually.✔
   2.   Four chromosomes are present✔	2.   Two chromosomes are present✔

   (5)  [10]

Activity 1

Question 1
Give the correct word or term for each of the statements or definitions provided below.

Answers to question 1
1.1 Centromere✔(1)
1.2 Homologous chromosomes✔ (1)
1.3 Daughter chromosome/chromatid✔ (1)
1.4 Chiasma✔/chiasmata✔ (1)
1.5 Chromatid✔ (1)
1.6 Prophase 1✔ (1) [6]

Question 2
Figure 2.12 (right) represents a process taking place during meiosis. Study the diagram and answer the questions that follow.

2.1 Provide labels for parts A, B, C and D. (4)
2.2 Name the process in meiosis that is illustrated in Figure 2.12. (1)
2.3 State ONE importance of the process you named in question 2.2. (2)
2.4 Draw a diagram of the structure labelled A to show its appearance immediately after the process you named in question 2.2. (2) [9]

Answers to question 2
2.1

A – Chromosome✔
B – Centromere✔
C – Chromatid✔
D – Chiasma✔/chiasmata (4)

2.2 Crossing over✔ (1)
2.3 It introduces genetic✔ variation✔ (2)
2.4

• A double-stranded chromosome with the strands joined by a centromere✔
• There is evidence of crossing over.✔ (2)
  

Question 3
Figure 2.13 (below) represents an animal cell in a phase of meiosis. Study the diagram and answer the questions that follow.

3.1 State whether the phase of meiosis shown in Figure 2.13 is meiosis I or meiosis II. (1)
3.2 Give ONE visible reason for your answer in question 3.1. (1)
3.3 Identify the parts labelled A and B. (2)
3.4 How many chromosomes:

1. were present in the parent cell before meiosis began? (1)
2. will be present in each cell at the end of meiosis? (1)

3.5 State ONE place in a human female where meiosis would take place. (1)
3.6 Could the cell represented in Figure 2.13 be that of a human? (1)
3.7 Explain your answer to question 3.6. (2)
3.8 Give TWO reasons why meiosis is biologically important. (2)
3.9 Give the term for the situation when some of the chromosomes do not separate correctly during the phase shown in Figure 2.13. (1) [13]

Answers to question 3
3.1 Meiosis II✔ (1)
3.2 Daughter chromosomes/chromatids are being pulled to the opposite poles✔ (1)
3.3

A – Spindle fibre✔
B – Cell membrane✔ (2)

3.4

a) 8✔
b) 4✔ (2)

3.5 Ovaries✔ (1)
3.6 No✔ (1)
3.7 There are only 4 chromosomes present3 instead of 23.✔ (2)
3.8

• It introduces genetic variation.✔
• It balances the doubling effect of fertilisation as it halves the number of chromosomes in the sex cells.✔ (2)

3.9 Non-disjunction✔ (1) [13]

Question 4
The diagram below shows the nuclei of the four cells that resulted from meiosis of chromosome pair 21 in a woman.

Figure 2.14: Diagram that shows the nuclei of four cells resulted from meiosis
4.1 Explain why nucleus N does NOT have a chromosome pair 21. (2)
4.2 Name and explain the disorder that will result if diagram M represents an egg cell that fuses with a normal sperm cell. (3) [5]

Answers to question 4
4.1 During Anaphase 1 the chromosome pair 21 does not separate✔/ non-disjunction. Gamete M will have an extra copy of chromosome number 21 and therefore gamete N does not have a copy of chromosome 21✔ (2)
4.2 Down syndrome✔/Trisomy 21 if gamete M fuses with normal sperm having 1 copy of chromosome 21 ✔ the resulting zygote will have 3 copies of chromosome 21✔ (3) [5]