Archive for category Chapter 5: Cell Division

5.3 Appreciating the Movement of Chromosomes During Mitosis & Meiosis

Failure of chromosomes to move in an orderly manner during cell division
1. Most of the time,  the mechanism that separate chromosomes in meiosis work in an orderly manner, but sometimes homologous chromosomes fail to separate. This is call non-disjunction.

2. If non-disjunction occurs, half the daughter cells produced have an extra chromosome (n+1), whilst the other half have a chromosome missing (n-1).

3. The result of non-disjunction may be genetic disorders such as Down’s syndrome, Klinefelter’s syndrome and Turner’s syndrome.

Non-disjunction in Meiosis I

Non-disjunction in Meiosis II

4. In plants, non-disjunction sometimes affects all pairs of homologous chromosomes, resulting in the formation of diploid (2n) gamete. Fertilisation involving a diploid gamete with a haploid (n) gamete produces a triploid (3n) zygote, whilst fertilisation involving two diploid gametes produces a tetraploid (4n) zygote.

5. Triploid and tetraploid plants ofter have advantageous features, such as increased size, hardness and resistance to diseases.

 

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5.2 Meiosis

A cell with two sets of chromosomes (one set from the male parent and the other from the female parent) is referred to as diploid (symbol 2n). Somatic cells or non-reproducing body cells are diploid.

A cell with a single set of chromosomes is referred to as haploid (symbol n). Gametes or sex cells are haploid.

Human somatic cell has 46 (23 pairs) chromosomes while the ovum or sperm has 23 chromosomes. In humans n= 23.

There are 23 pairs of homologous chromosomes in each humans somatic cell. Members of a homologous pair are identical in length and in the position of the centromere and can be identified by their characteristic shape.

Gametes are produces by  a process called meiosis.

Meiosis is a division of the nucleus to produce four daughter cells each containing half the chromosome number of the parent nucleus. Meiosis is associated with sexual reproduction.

Meiosis is preceded by an interphase during which the cell replicates its DNA and organelles.

Meiosis (reduction division)

1. During meiosis,  the cell undergoes DNA replication once, followed by two nuclear divisions.

First meiotic division (meiosis I):

  • The behaviour of chromosomes differs from mitosis. In meiosis I, homologous chromosomes pair up and exchange DNA whereas chromatids remain connected to each other. Meiosis I is divided into four phases: prophase I, metaphase I, anaphase I and telophase I.

Second meiotic division (meiosis II):

  • The behaviour of chromosomes are typical of mitosis. Meiosis II is divided into four phases: prophase II, metaphase II, anaphase II and telophase II.

2. Prophase I

  • Chromosomes shorten and thicken and each is seen to comprise two chromatids joined at the centromere.
  • The homologous chromosomes pair up. Each pair of homologous chromosome is called a bivalent.
  • The maternal and paternal chromatids intertwine to form crosses or chiasmata (singular: chiasma).
  • The formation of chiasmata results in exchange of DNA between maternal and paternal chromosomes, a process called crossing over.
  • Nuclear membrane breaks down and the nucleoli disappear.
  • Centrioles migrate to the poles and the spindle forms.

3. Metaphase I

  • The bivalents become arranged around the equator of the spindle, attached by their centromeres.
  • The arrangement is completely random relative to the orientation of other bivalents, leading to genetic variation in the gametes.

4. Anaphase I

  • The spindle fibres contract and pull the homologous chromosomes, centromeres first, towards the poles of the spindle.
  • One of each pair is pulled to one pole, its sister chromosome to the opposite pole.

5. Telophase I

  • The chromosomes reach their opposite poles. The chromosomes for two haploid sets, one set at each end of the spindle.
  • The nuclear membrane forms around each set of chromosomes, the spindle fibres disappear and the chromatids uncoil.
  • Cytokinesis usually occurs and two haploid cells are formed.
  • The nucleus may enter interphase but no further DNA replication occurs.

6. Prophase II

  • The nucleoli disappear and the nuclear membrane breaks down.
  • The centrioles divide and move to opposite poles.
  • Spindle fibres develop.
  • Chromosomes condense and move to the equator of the spindle.

7. Metaphase II

  • The chromosomes arrange themselves on the equator of the spindle.

8. Anaphase II

  • The centromeres divide and are pulled by the spindle fibres to opposite poles, carrying the chromatids with them.

9. Telophase II

  • The chromatids uncoil and become indistinct.
  • The spindle fibres disappear.
  • The nuclear membrane and the nucleoli reform.
  • Cytokinesis occurs and four haploid cells are formed from one parent cell.

 

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Enjoy the Mitosis Song.

Let’s enjoy the song after a long-time learning about Mitosis from the previous topic. :)

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5.1 Mitosis

Cell division starts with the division of the nucleus.

There are two forms of nuclear division: mitosis and meiosis.

Mitosis is a division of the nucleus to produce two new daughter cells containing chromosomes identical to the parent cells.

Significance of mitosis

1. Growth: Mitosis allows a zygote to produce more cells in order to grow into a multicellular organism.

2. Repair and replacement: Mitotic cell division allows the body to repair itself, or even regenerate following injury. For example, a house lizard will regenerate a tail that is lost to a predator. Mitosis also allows a multicellular organism to maintain its tissues, many of which require frequent replacement, for example skin cells and blood cells.

3. Asexual reproduction: Mitosis provides the basis of asexual reproduction, in which offsprings are formed from a single parent. the offsprings are called clones.

The cell cycle

1. Cell division is just a small part of the cycle of cell growth and asexual reproduction known as the cell cycle.

2. The cell cycle is defined as the period from the formation of a cell by division to the point when that cell divides itself. The length of a cell cycle is very variable, depending on the type of cells.

Interphase (G1, S G2 phase)

1. Interphase is not a ‘resting phase’. During interphase, the cell is metabolically active and is involved in protein and DNA synthesis.

2. Interphase may account for 90% of the total cell cycle.

3. Interphase is divided into 3 shorter phases G1, S and G2 respectively.

4. During G1 (gap or growth phase 1) phase, the cell is sensitive to internal and external signals that help it decide whether to divide or not. Once decided to divide, the cell becomes metabolically active. The cytoplasm increases in volume due to the synthesis of a new proteins and organelles.

5. During S (synthesis) phase, DNA replicates and two sister chromatids form from each chromosome. In animal cells, the centrioles duplicate.

6. During G2 (gap or growth phases 2) phase, organelles and proteins necessary for cell division are synthesised.

Mitosis (M phase)

1. Mitosis is a continuous process, but it may be subdivided into four main phases, based on the appearance and behaviour of the chromosomes:

2. Prophase

  • The chromosomes condense, that is, they shorten and thicken and finally become visible under the light microscope.
  • Each chromosome consists of sister chromatids attached at point called the centromere, The two sister chromatids correspond to identical molecules of DNA formed during the S stage.
  • The nucleoli disappear, the nuclear membrane breaks down, and the centrioles migrate to opposite poles of the cell. Centrioles are absent in plant cells.

Click on the video below to watch more detail explanation:

3. Metaphase

  • The spindle fibres are fully formed.
  • All chromosomes are arranged with their centromeres along the equator of the spindle.

Here is the video about metaphase. Enjoy it :)

4. Anaphase

  • Anaphase begins with the separation of the centromeres.
  • The sister chromatids are drawn to opposite poles of the cell. Once the sister chromatids are separated they are referred to as daughter chromosomes.
  • The poles move further apart, lengthening the cell.

Can’t get it? Watch this video! :)

5. Telophase

  • Telophase begins when the two sets of daughter chromosomes have reached the two poles of the cell.
  • The spindle fibres disintegrate, the nuclear membrane forms around each set of daughter chromosomes, and the nucleoli reappear.
  • The chromosomes uncoil and become less visible under the light microscope.

Hehe, don’t worry. This is the video:

6. Cytokinesis

  • Cytokinesis is the process of cytoplasmic division to form two daughter cells.
  • Cytokinesis usually begins before nuclear division is completed.
  • In cytokinesis the organelles become evenly distributed between the two daughter cells.
  • In animal cell, a cleavage furrow forms at the equator of the cell and deepens until the daughter cells separate.
  • In plant cell, the Golgi apparatus buds off carbohydrate-filled vesicles that line up along the cell’s equator.
  • The vesicles fuse, producing the cell plate. The cell plate extends outwards to the existing cell wall and separates the two daughter cells.

The video quality may not be good, but the process inside does happen in animals and humans.

7. Most animal cells are capable of mitosis.

8. Only specialised groups of plant cells called meristems are capable of mitosis.

9. There are 3 type of meristems.

- Apical meristem

  • These are found at the tips of shoots and roots. Apical meristems are responsible for the increase in length of plants.

- Lateral meristems

  • These are found in stem and roots. Lateral meristems contribute to an increase in girth.

- Intercalary meristems

  • These are found at nodes in monocotyledonous plants. Intercalary meristems contribute to a an increase in length of monocots.

Controlled and Uncontrolled Mitosis

Controlled Mitosis

1. Regeneration is the ability to restore lost or damaged tissues, as well organs or limbs.

2. Regeneration involves controlled mitosis.

3. Some lizards drop a jumping and twisting tail to entice a pursuing predator, and then regenerate itself a new tail ready for the next encounter.

4. Another type of regeneration is the healing of wounds. Whenever we have a cut on our skin, the healing takes place over a period of time because new cells are made to replace the destroyed and damaged cells.

5. Many plants are capable of total regeneration, that is,  the formation of a whole plant from a leaf, stem or root. For example, if a Begonia leaf together with its petiole is detached and laid on damp sand, roots develop at the end of the petiole and vegetative buds are generated on the lamina. Entire new plants develop from these buds.

Uncontrolled Mitosis

1. Cancer is a disorder of the body’s growth in which cells multiply due to uncontrolled mitosis.

2. Tumour cells undergo mitosis without cytokinesis. This process produces single cells with many nuclei.

3. The result is a population of abnormal cells called tumours.

4. Tumours of two types: benign tumour and malignant tumour.

5. The cells in a benign tumour normally grow slowly and remain constrained in one area, but the cells of a malignant tumour grow uncontrollably and destroy other tissues.

6. Cancer kills more than six million people worldwide each year.

Cloning plants by tissue culture

1. Cloning is the production of one or more individual plants or animals that are genetically identical to another plant or animal.

2. Commercial plant growers can clone plants by a technique called tissue culture.

3. Tissue culture is a technique or process of keeping tissues alive and growing in a culture medium. Scientists are able to produce whole plants using cells, tissues or organs from different parts of a plant. Tissue culture is also called cell culture or micropropagation.

4. Advantages of tissue culture

  • Can produce plants that are difficult to reproduce by traditional methods.
  • Many clones can be produced quickly in large numbers.
  • The plantlets are free from diseases.

5. An outline of plant tissue culture

  •  All apparatus and materials used in this technique must be sterilised.
  • The surface of a leaf is sterilised with ethanol or dilute sodium hypochlorite solution.
  • The leaf is then cut into small pieces. The small pieces of plant tissue are called explants.
  • The explants are then placed inside a test tube containing nutrient agar and growth hormones.
  • After six to eight weeks, the explants develop new shoots.
  • The shoots are then cut free from the explants, and placed in a flask containing a new medium that helps roots to develop.
  • The rooted plantlets are then transferred to soil and kept in a controlled environment until fully acclimatised.
  • From one original plant, hundreds of genetically identical plant could be produced.

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