MITOSIS & MEIOSIS
Introduction
Growth is a characteristic of all living things. It is a permanent increase in size. It is accompanied by cell division and differentiation to form tissues and organs.
Growth in plants is restricted to the tips of shoots and roots, also known as growing points. The shoots and roots can continue to grow throughout the life of the plant. In animals, growth occurs throughout the body.
For a multicellular organism to grow normally, its cells must first divide. The cell division that takes place during growth is called mitosis.
Mitosis
Mitosis is a form of nuclear division that produces daughter nuclei containing the same number of chromosomes as the parent nucleus. The daughter nuclei are genetically identical.
Chromosomes are made up of the molecule deoxyribonucleic acid (DNA). DNA stores all the information that a cell needs in order to grow and to carry out vital activities. This information is stored as sections of DNA or genes.
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Why is it important to produce genetically identical daughter cells?
- A zygote (cell) divides to form an embryo. The cells formed as the zygote divides must be genetically identical for the embryo to develop normally. If an error occurs during DNA replication or mitosis, it will be transmitted to the daughter cells. This may lead to harmful changes to the genes and affect how the cells function. The embryo may not develop normally.
- Changes in gene during DNA replication may also cause abnormal proteins to be produced. Such cells (containing abnormal proteins) may be rejected or destroyed by the body's immune system.
- Mistakes made in DNA replication or mitosis may cause the uncontrolled division of cells (cancer).
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THE CELL CYCLE
The order of events that occurs between one cell division and the next is called the cell cycle. The cell cycle consists of three stages:
1. Interphase (resting stage)
2. Mitosis (nuclear division)
- Prophase
- Metaphase
- Anaphase
- Telophase
3. Cell division (division of cytoplasm AKA cytokinesis)
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INTERPHASE
- Resting or non-dividing stage of a cell
- Most time of cell life spent in interphase
- Chromosomes appear as long thin threads called chromatin.
- Centrioles divide in an animal cell
-Just before the cell divides, each chromatin thread replicates, producing two identical chromatin threads joined at a point called the centromere. Since the two chromatin threads are attached to the same centromere, they are sister chromatids.
PROPHASE
- Chromatin threads condense, coil and shorten to become chromosomes. Each chromosome consists of two sister chromatids attached at the centromere.
- Structures called asters (made of microtubules) form around the centrioles
- The two pairs of centrioles divide and move apart to opposite ends of the cell
- Nucleus and nuclear envelope disappear
- A spindle forms with spindle fibres extending from one pole of the cell to the other.
METAPHASE
- Chromosomes line up along the equatorial plane of the spindle.
- The centromere of each chromosome is attached on both sides to a spindle fibre.
ANAPHASE
- Each centromere divides.
- The spindle fibres shorten and pull the chromatids apart to opposite poles of the cell.
- Once the chromatids are separated, they are called daughter chromosomes.
TELOPHASE
- Spindle fibres break down.
- A nuclear envelope forms around the chromosomes at each pole of the cell
- A nucleolus reforms in each nucleus and the chromosomes uncoil and lengthen to become thin chromatin threads.
CYTOKINESIS
- Division of cytoplasm
- Cleavage or furrows appear in the cytoplasm between two nuclei.
- Furrows deepen and two identical daughter cells are finally produced.
IMPORTANCE OF MITOSIS
- Mitosis helps constantly replace dead cells
- Helps heal wounds
- New cells can be created to replace worn-out cells
- Allows asexual reproduction to occur (e.g. vegetative propogation and binary/multiple fission)
Meiosis
No organism can live forever. Organisms ultimately die of old age, disease, in accidents, or are killed for food. To ensure continuity of the species, organisms must produce new organisms like themselves. This production of new organisms is called reproduction.
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Gametes are reproductive cells that contain half the number of chromosomes as the normal body cells.
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In meiosis, the daughter cells produced contain half the number of chromosomes as the parent cell. For example a cell with 2n chromosomes divides to produce 4 cells, each contains n chromosomes. The daughter cells are called gametes.
nucleus
2n
Parent cell with 2n chromosomes
meiosis
n
n
n
Four gametes formed, each with n chromosomes
n
In humans, the sperm is the male gamete, and the egg (ovum) is the female gamete.
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The number of chromosomes in the normal body cell is called diploid number (2n).
Half diploid number is called haploid number (n). In humans, the diploid number is 46, and the haploid number is 23.
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Sexual reproduction involves the fusion of two gametes during fertilization. Fertilization occurs when the nucleus of the male gamete fuses with the nucleus of the female gamete to form a zygote. The zygote is the fertilized egg. In this way, the normal diploid number of chromosomes is restored in the zygote. The zygote then divides by mitosis to form an embryo.
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Sperm (n) + Ovum (n) --> Zygote (2n)
23 + 23 ----> 46
PROCESS OF MEIOSIS
For convenience, meiosis is considered to consist of two divisions: meiosis I and meiosis II. Each division in turn consisits of prophase, metaphase, anaphase and telophase (PMAT).
INTERPHASE
- Chromatin threads replicate, producing two sister chromatids attached at the centromere.
- Pair of centrioles divide
Figure 2. Crossing over of homologous chromosomes
METAPHASE I
- Pairs of homologous chromosomes arrange themselves along the equatorial plane of the spindle.
- Two chromosomes of each pair face opposite poles of cell.
- Each chromosome is attached to a spindle fibre.
TELOPHASE I
- A nuclear envelope forms around the chromosomes at each pole.
PROPHASE II
- The two pairs of centrioles move to opposite poles of the cell.
- Nuclear envelope disappears
- Spindle fibres appear
ANAPHASE II
- The centromeres divide.
- Sister chromatids seperate to become daugther chromosomes, which are pulled towards opposite poles of the cell.
CYTOKINESIS II
- Cleavage of cytoplasm results in four daughter cells being produced, each with half the number of chromosomes as the parent cell (23).
PROPHASE I
- Chromatin threads condense, coil and shorten to become chromosomes
- Homologous chromosomes pair along their whole length (this is called synapsis). Homologous chromosomes have the same shape, same genes and same length. One homologous chromosome comes from the male parent, and the other comes from the female parent.
- Homologous chromosomes appear to repel each other. Chromatids of homologous chromosomes may cross and twist around each other, causing them to break and exchange parts in a process called crossing over. Crossing over produces new combinations of alleles along the chromosomes.
- Asters form around centrioles. Centrioles move to opposite ends of the cell.
- Nuclear envelope and nucleolus disappear.
- Spindle fibres form.
ANAPHASE I
- Homologous chromosomes separate and are pulled to opposite poles of the cell as the spindle fibres shorten.
CYTOKINESIS I
- The cytoplasm furrows into two, producing two daughter cells, each with the haploid number of chromosomes.
- Centrioles divide.
METAPHASE II
- Chromosomes arrange themselves along the equatorial plane of the spindle.
TELOPHASE II
- Spindle fibres disappear
- Nuclear envelope forms around the two daughter chromosomes at each pole
- A nucleolus reforms (chromosomes reform to chromatin)
Figure 3. Process of meiosis
IMPORTANCE OF MEIOSIS
- Variations occur due to crossing over and independent assortment of chromosomes in gametes. Variations increase the chance of survival of the species during changes in the environment. Those that survive changes in the environment pass on their favorable genes to their offspring.
How does mitosis differ from meiosis?
