Cell Division

MITOSIS:

→ 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: made up of deoxyribonucleic acid (DNA) that stores all the information that a cell needs in order to grow and carry out vital activities by a section called genes

WHY IS IT IMPORTANT TO PRODUCE GENETICALLY IDENTICAL DAUGHTER CELLS?

  • DNA replication copies all the information stored within the chromosomes
  • Ensures that daughter cells produced by mitosis contain all the sections of DNA needed for subsequent cell division and differentiation so that the resulting daughter cells are genetically identical (error carry forward)

Interphase:

  • Cells are resting/non-dividing, carrying out activities such as absorbing nutrients and building up protoplasm
  • chromosomes appear as long thin threads called chromatin, and cannot be distinguished at this stage
  • Centrioles divide in an animal cell at interphase
  • Chromatin threads replicate, producing two identical chromatin threads joined at a point called the centromere, called sister chromatids

STAGES OF MITOSIS:

  1. Prophase
  • Early Prophase: Chromatin threads condense, coil and shorten to become chromosomes and chromosomes are now visible under a microscope as x-shaped structures
  • In animal cells, asters, which are made of microtubules, form around the centrioles. The two pairs of centrioles move apart to the opposite ends of the cell
  • Late prophase: The nucleolus and nuclear envelope disintegrate and breakdowns
  • A spindle forms with the spindle fibres extending from one pole of the cell to the other
  1. 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
  1. Anaphase
  • Each centromere divides, and the spindle fibres shorten and pull the chromatids apart to the opposite poles of the cell. Once the chromatids are separated, they are called daughter chromosomes
  1. Telophase
  • Spindle fibres break down. A nuclear envelope forms around the chromosomes at each pole of the cell
  • The nucleolus reforms in each nucleus and the chromosomes uncoil and lengthen to become thin chromatin threads
  1. Cytokinesis
  • The division of Cytoplasm
  • In animal cells, cleavage or furrows appear in the cytoplasm between the two nuclei. The furrows deepen and two identical daughter cells are finally produced

In plant cells:

  • centrioles are absent
  • cleavage of cytoplasm does not occur during cytokinesis, instead, a cell plate is formed between two daughter nuclei, dividing the cell into two. The cell plate is formed by the fusion of small fluid-filled vesicles produced by the Golgi apparatus

Why is mitosis important?

  • Mitosis enables the growth of an organism
    • For a multicellular organism to grow, new cells must be produced by mitosis
  • Mitosis is needed for the repair of worn-out parts of the body
    • replaces dead cells and heals wounds e.g when you get cut, your skin produces new cells to help seal the cut. cells in the Malpighian layer of the skin divides constantly by mitosis, replacing worn-out cells from the upper layers of the skin
  • Allows asexual reproduction to occur
    • Many flowering plants are capable of vegetative reproduction. Mitosis can cause shoots and roots to develop in storage organs such as rhizomes and bulbs. These shoots then grow into new daughter plants that are identical to their parent plants

MEIOSIS:

Meiosis: Form of nuclear division that produces daughter nuclei containing half the number of chromosomes as the parent nucleus

→ number of chromosomes in the normal body cell is called a diploid number, half is a haploid number

Gametes: Reproductive cells that contain half the number of chromosomes as normal body cells, sex organs go through meiosis

Fertilisation: when two gametes fuse to form a zygote, which is a fertilised egg

Interphase:

  • Chromatin threads replicate, producing two identical sister chromatids attached to the centromere that is not visible under the microscope. the pair of centrioles also divides

STAGES OF MEIOSIS I

  1. Prophase I
  • Chromatin threads condense, coil and shorten to become chromosomes
  • Each chromosome consists of two sister chromatids attached to the centromere and is now visible under the light microscope
  • Homologous chromosomes that have the same length, shape and genes pair along the length called a synapsis
  • one homologous series comes from the male parent, and one from the female parent
  • Chromatids of homologous chromosomes may cross and twist around one another which is called a chiasma
  • the strength of their coiling may break and exchange parts, called crossing over
  • as the chromosomes shorten further, homologous chromosomes appear to repel each other. Asters form around the centrioles which move apart to opposite poles of the cell. the nucleolus and nuclear envelop break down and disintegrate, the spindle fibres form
  1. Metaphase I:
  • Pairs of homologous chromosomes arrange themself along the equatorial plane of the spindle
  • the two chromosomes of each pair face opposite poles of the cell and each chromosome are attached to a spindle on both sides
  1. Anaphase I:
  • Homologous chromosomes separate and are pulled to opposite poles of the cell as the spindle fibres shorten
  1. Telophase I:
  • A nuclear envelope forms around the chromosomes at each pole, followed by the division of the cytoplasm
  1. Cytokinesis I:
  • The cytoplasm cleaves into two, producing two daughter cells, each with a haploid number of chromosomes, the centrioles divide

STAGES OF MEIOSIS II:

  1. Prophase II:
  • Two pairs of centrioles move to opposite poles of the cell, the nuclear envelope and nucleolus disappear, and spindle fibres appear
  1. Metaphase II:
  • Chromosomes arrange themselves on the equatorial plane of the spindle
  1. Anaphase II:
  • The centromeres divide, and sister chromatids separate and become daughter chromosomes, which are pulled towards opposite poles of the cells
  1. Telophase II:
  • Spindle fibres disappear, nuclear envelope form around the two daughter chromosomes at each pole. A nucleolus reforms
  1. Cytokinesis II:
  • Cleavage of cytoplasm results in four daughter cells being produced, each with half the number of chromosomes as the parent cell

WHY IS MEIOSIS IMPORTANT:

  1. Meiosis produces haploid gametes

→ When the nucleus of a male gamete fuses with the nucleus of the female gamete, the diploid number of chromosomes is restored in the zygote. This maintains the normal diploid number of chromosomes in the species

  1. Meiosis results in variations in the gametes produced

→ Variations due to the crossing over and independent assortment of chromosomes. one chromosome from each pair can combine with either chromosome of the other pair. This results in four different gametes being produced from two chromosomes. Since fertilisation is random, such variations in the gametes produce variations in the offspring

→ variation increases the chances of survival of species during changes in the environment. When the environmental conditions change, the organisms with favourable characteristics will survive, while others die, those that survive will pass on their favourable genes to their offspring