Eden Schools DurbanLIFE SCIENCE GRADE 12

CELL DIVISION (MITOSIS & MEIOSIS)

During sexual reproduction, a sperm cell fuses with an egg cell (ovum) to form a unicellular zygote . How does this unicellular structure develop into the multicellular structure which consists of millions of cells?It is the result of simple cell division called MITOSIS which results in the production of new cells – 1 parent cell divides into 2 daughter cells, 2 into 4, 4 into 8, 8 into 16 and so on. In this way, the unicellular zygote can form a multicellular organism.

REMEMBER : According to the cell theory: “Every cell comes from another cell.”

Mitosis is a process of simple cell division which occurs in somatic (or body) cells during which the parent cell divides into 2 daughter cells which are identical to each other and to the parent cell – they are identical in that they have the same number and type of chromosomes.There are two types of cells in the body : SOMATIC or BODY CELLS (which are genetically identical) and SEX CELLS (sperm cells and egg cells).

PARENT CELL

2 DAUGHTER CELLS IDENTICAL TO EACHOTHER AND TO THE PARENT CELL

WHY IS MITOSIS IMPORTANT?1. It is a form of asexual reproduction during which, for example, a

unicellular Amoeba can split into two daughter cells by a process called binary fission. Mitosis is also a form of vegetative reproduction in fungi, mosses and ferns.

2. It results in growth of the body.3. It repairs and replaces damaged and worn out cells.

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A REVIEW OF THE STRUCTURE OF CHROMOSOMESA chromosome is made up of two identical chromatids joined by a region called the centromere. Each chromatid is made up of a number of genes. Each gene is made up of DNA molecules, each responsible for a particular characteristic.

The diagrams below simply show the structure of chromosomes.

Photograph of human chromosomes

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Look for a pair of similar looking (homologous) chromosomes

A NORMAL HUMAN KARYOTYPE(Karyotype refers to all the chromosomes in the nucleus of an individual

Each species of organisms has its characteristic number of chromosomes, for example, in human beings, every somatic cell has 46 chromosomes and every sex cell has 23 chromosomes.

The chromatin is composed of DNA which becomes visible as chromosomes during cell division.

The DNA becomes wrapped around nucleosomes, histosomes and other proteins forming the chromosome

OUTLINE OF THE PROCESS OF MITOSIS

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The process of mitosis can take place very quickly (20 minutes in certain bacteria) or very slowly (hours in other cells). It is a continuous process which, for convenience of study, is divided into phases.The nucleus first divides (called karyokinesis). This is followed by division of the cytoplasm (called cytokinesis).

The diagrams below show the cell cycle:

PROPHASE METAPHASE ANAPHASE TELEPHASE CYTOKNESIS

A brief description of the events that occur during mitosis in an animal cell follows :

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INTERPHASE

CYTOKNESI

INTERPHASEThe cell is undergoing its normal activities. The chromatin material unwinds & the chromosomes become visible as single strands. DNA replication takes place & the chromosomes appear as double strands each having two chromatids joined by a centromere.

PROPHASEThe chromosomes become more clearly visible. The nucleolus, nuclear membrane and larger organelles disappear. The centrosome splits and each centriole moves to an opposite pole of the cell. Spindle fibres arise from each centriole.

METAPHASEChromosomes arrange themselves along the middle (equatorial plane) of the cell. Each is attached to a spindle fibre at the centromere.

ANAPHASEThe chromosomes split and each half (chromatid) is pulled to an opposite pole. An invagination (constriction) begins to form in the middle of the cell.

TELOPHASEA nucleus is formed at each pole. The invagination deepens and divides the cytoplasm (cytokinesis) into two. Two daughter cells identical to each other and to the parent cell are formed. The chromatin network re-forms and the cells then go into interphase.

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There are some differences between mitosis in plant and animal cells (tabulated as follows):

ANIMAL CELLS PLANT CELLS

Spindle fibres radiate from the centriole.

Spindle fibres radiate from the poles.

An invagination divides the cytoplasm into two.

A cell plate, which becomes a cross wall, divides the cytoplasm into two.

Now look at the series of drawings below, and then read the notes which follow:Notes: Sperm and egg cells have half the number of chromosomes (23) than in normal

body cells (23 pairs, or 46). These are called male and female gametes When the sperm and egg fuse during fertilisation, the zygote formed now has

the correct number of chromosomes (23 pairs, or 46) As you can see, 23 chromosomes came from the father (paternal

chromosomes) and 23 from the mother (maternal chromosomes). Thus you

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inherit half of your chromosomes from your father, half from your mother. This does not mean you are half like your father and half like your mother (see genetics later)

Male and female gametes (sperm and egg cells) are formed by a reduction division called MEIOSIS. During this process, the number of chromosomes is halved to form the gametes.

Note: I did not use 46 chromosomes, it would not have fitted in. I used 4 pairs to fit them in the drawing, you must imagine there are 23 pairs in humans

Imagine if meiosis did not take place? 46 + 46 = 92, 92 + 92 = 184. The number of chromosomes will just keep increasing. No! All humans must have the same genetic information (in 46 chromosomes).

So meiosis maintains a constant chromosome number from one generation to the next.

THE DIFFERENCE BETWEEN MITOSIS AND MEIOSISMITOSIS (In humans) MEIOSIS (In humans)

Nucleus 46 46

Chromosomes both diploid (2n) Chromosomes (23 prs)

23 23 Chrom Chrom

46 46 Chrom Chrom (23 prs) (23 prs)

23 23 Chrom Chrom

Table of differences between mitosis and meiosis

MITOSIS MEIOSIS

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MITOSIS MEIOSIS

1. Nucleus divides once (replication of cell)

Nucleus divides twice (Meiosis I = Reduction; Meiosis II = Replication)

2. Chromatids are pulled to opposite cells (daughter chromosomes, in daughter cells)

Chromosomes are pulled to opposite cells

3 Two daughter cells formed Four daughter cells formed4 Each daughter cell has an identical

nucleus to the parentNone of the daughter cells have the same number of chromosomes as the parent cell had

5 Each daughter cell has the same number of chromosomes as the parent cell (i.e., diploid, two sets of chromosomes)

Each daughter cell has half the number of chromosomes present in the parent cell (i.e., haploid)

6 Takes place to grow tissues, or replace worn or damaged cells

Takes place to make male and female sex cells (gametes) for fertilisation during sexual reproduction

7 Ensures that all the daughter cells have the same genetic complement as the parent cell

Ensures that the number of chromosomes remains constant from one generation (parent) to the next (children)

The drawing below shows the relationship between mitosis and meiosis.

THE LIFE CYCLE INVOLVING SEXUAL REPRODUCTION THE LIFE CYCLE INVOLVING SEXUAL REPRODUCTION (e.g., in humans)(e.g., in humans)

ADULT MALE (2n, diploid)

Testes SPERM (n, haploid)

ADULT FEMALE (2n, diploid)

Ovary OVUM (egg; n, haploid)

MEIOSIS

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EMBRYO

ZYGOTE (2n, diploid)THE PROCESS OF MEIOSIS (see drawings on page 11)

MEIOSIS I1.PROPHASE I1.1 The

chromosomes condense and migrate towards the nuclear envelope.1.2 Each chromosome is made up of two identical chromatids, known as

sister chromatids. 1.3 Formation of spindle fibers.

MITOSIS

FERTILISATION

MITOSIS

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MEIOSIS II

MEIOSIS I

Prophase I

Prophase II Anaphase IIMetaphase II Telophase II

Metaphase IAnaphase I Telophase I

1.4 Pairing of homologous chromosomes takes place (also known as bivalents).

1.5 The homologous chromosomes interchange equivalent sections of chromatids, which is a process known as crossing over (The point at which the chromosomes break is called the chiasmata). See page 13.

1.6 The chromosomes undergo thickening and move away from the nuclear envelope.

1.7 The nuclear envelope and nucleoli dissolves.2. METAPHASE I 2.1 The paired bivalents or homologous chromosomes line up on the

equatorial plane, that lies in the center of the cell. 2.2 The centromeres, a region in the chromosome where the chromatids are

held together, are located in the opposite poles. 3. ANAPHASE I 3.1 The homologous chromosomes migrate to the opposite poles of the

cell. 3.2 The separation is random, i.e., the paternal and maternal chromosomes

separate at random. 3.3 The sister chromatids are not separated, but remain together (each

chromosome still has 2 chromatids at this stage). 4. TELOPHASE I 4.1 The chromosomes continue to migrate towards the poles. 4.2 Both the poles have haploid (n) number of chromosomes. 4.3 Condensation of the chromosomes and cytokinesis (division of

cytoplasm) take place. 4.4 Nuclear envelope starts forming. 4.5 Two daughter cells with haploid chromosome number are formed.MEIOSIS II (Simlar to mitosis)Note: The chromosomes have evidence of crossing over (this distinguishes them from mitosis) 5. PROPHASE II 5.1 The nuclei and nuclear membrane are separated. 5.2 The chromosomes start moving towards the equatorial plane. 5.3 The two sister chromatids are still held by the centromere.6. METAPHASE II 6.1 The chromosomes are aligned singly on the equator. 6.2 The centromeres are oriented towards the opposite poles. 7. ANAPHASE II 7.1 The sister chromatids held at the centromere are separated by the

spindle fibers. 8. TELOPHASE II

8.1 Four nuclei (two each in a daughter cell) are formed, along with the process of cytokinesis.

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8.2 Each of the four nuclei develops nuclear envelope.8.3 Four haploid daughter cells or gametes are formed, each with

dissimilar sets of chromosomes (due to the random separation or independent assortment in Meiosis I).

Note: the number of possible different gametes formed will be 223 due to random separation or independent assortment in Meiosis is 8 388 608! Remember this is without crossing over and just in a sperm or egg cell!! All this (crossing over, random assortment and an egg and sperm fusing) mixes genetic material and brings variety.

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Use two colours to show that the homologous chromosomes separate randomly in meiosis I, i.e., the paternal and maternal chromosomes separate randomly.

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What is crossing over? A process called crossing over occurs in late Prophase I of meioses.The homologous pairs of chromosomes (bivalents) swap pieces of their inner chromatids by breaking and reforming their DNA while they are paired up.In this way so genes from a maternal chromosome change places with some genes from a paternal chromosome.This increases variation among the daughter cells as there will be new combinations of genetic material. The point of crossing over where the chromatids break is called the chiasma.Why is crossing over important?The exchange of genetic material produces chromatids with a unique combination of genes.During the cutting process some mistakes may occur which lead to mutations. This introduces new genes into the genetic make-up of a species. Why are the four gametes made during meiosis different? Crossing over causes gametes to inherit chromatids with unique gene

combinations. The chromosomes from each parent are distributed in the new gametes

completely at random.Which chromosome of a given homologous pair goes to which pole is unaffected by the behavior of the other chromosome pairs. This is called the random assortment of chromosomes.When these pairs separate into a human gamete a single set of chromosomes in a gamete could consists of 17 maternal and 6 paternal chromosomes, or 14 paternal and 9 maternal chromosomes, or maybe all 23 maternal chromosomes could go into 1gamete. Usually however, the gamete contains a mixture of maternal and paternal chromosomes.

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REVISION QUESTIONS1 The diagrams below represent two different phases in meiosis of two

different cells.

1.1 Give the names of the parts labelled:(a) A (1)

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Meiosis I: Anaphase I:The homologous chromosomes are separating.

Meiosis II: Metaphase II:The chromosomes will split into 2 chromatids. Note that crossing over occurred in Meiosis I (tells you that this is NOT mitosis)

(b) B (1)1.2 Identify the phase represented in:

(a) Diagram 1 (1)(b) Diagram 2 (1)

1.3 Name the process during meiosis which is responsible for the appearance of the chromosomes illustrated in Diagram 1. (1)

1.4 How many chromosomes would be found in each of the resulting cells at the end of the division of the cell shown in Diagram 1? (1)

1.5 Explain TWO ways in which meiosis is important. (4)

[10]2 The diagram below represents an animal cell in a phase of meiosis.

2.1 State which phase of meiosis is represented in the diagram above. (1)

2.2 Give a reason for your answer to QUESTION 2.1. (2)

2.3 Identify parts A and B. (2)

2.4 How many chromosomes …(a) were present in the parent cell before it underwent meiosis?

(1)(b) will be present in each cell at the end of the meiotic division? (1)

2.5 State ONE place in the body of a human female where meiosis would take place. (1)

2.6 Could the cell represented in the diagram be that of a human? (1)2.7 Explain your answer to QUESTION 2.6. (1)

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[10]

Total: 20 marks

MODEL ANSWERS

1 The diagrams below represent two different phases in meiosis of two different cells.

1.1 Give the names of the parts labelled:(a) A – Spindle fibres√ (1)(b) B - Centromere√ (1)

1.2 Identify the phase represented in:(a) Diagram 1 - Meiosis II: Metaphase II√ (1)(b) Diagram 2 - Meiosis I: Anaphase I√ (1)

1.3 Name the process during meiosis which is responsible for the appearance of the chromosomes illustrated in Diagram 1. (1)Crossing over√

1.4 How many chromosomes would be found in each of the resulting cells at the end of the division of the cell shown in Diagram 1? (1)2√

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Meiosis I: Anaphase I:The homologous chromosomes are separating.

Meiosis II: Metaphase II:The chromosomes will split into 2 chromatids. Note that crossing over occurred in Meiosis I (tells you that this is NOT mitosis)

1.5 Explain TWO ways in which meiosis is important. (4)1. Meiosis occurs during gamete formation to make male and

female sex cells (gametes) √ for fertilisation during sexual reproduction to ensure that the number of chromosomes remains constant from one generation (parent) to the next (children) √.

2. Crossing over√ and random assortment√ during meiosis ensures greater genetic variety in the formation of gametes.

[10]

2 The diagram below represents an animal cell in a phase of meiosis.

2.1 State which phase of meiosis is represented in the diagram above. (1)Meiosis II Anaphase II√

2.2 Give a reason for your answer to QUESTION 2.1. (2)Single chromosomes are separated into two chromatids√ and are moving to opposite poles.

2.3 Identify parts A and B. (2)A - Spindle fibres√B – Cell membrane√

2.4 How many chromosomes …(a) were present in the parent cell before it underwent meiosis?

(1)8√

(b) will be present in each cell at the end of the meiotic division? (1)4√

2.5 State ONE place in the body of a human female where meiosis would take place. (1)

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Ovary√2.6 Could the cell represented in the diagram be that of a human? (1)

No√2.7 Explain your answer to QUESTION 2.6. (1)

Humans will have 23 chromosomes after meiosis√ [10]

Total: 20 mark

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