Growth and reproduction

Growth and reproduction are fundamental biological processes essential for the continuation of life. They involve the precise control of cell division, ensuring the accurate transmission of genetic information from one generation to the next.

The Basis of Life: Cells and DNA

All living organisms are made of cells, which are considered the smallest units of life. The biochemical basis of life is similar for all living things, suggesting a common ancestor. DNA (deoxyribonucleic acid), the molecule of heredity, is crucial for storing genetic information, copying it for daughter cells, passing it between generations, and controlling protein production.

Cell Division: Mitosis

Mitosis is a type of nuclear division that produces two genetically identical daughter cells from a parent cell. It is a precisely controlled process that occurs as part of the cell cycle.

The cell cycle consists of three main stages:

• Interphase: A period of cell growth and DNA replication. This is subdivided into:

  • G1 phase: Cell grows, and new organelles and proteins are made.

  • S phase: DNA replication occurs, so each chromosome consists of two identical chromatids.

  • G2 phase: Cell continues to grow, and proteins needed for cell division are made.

• Mitosis (M phase): The nucleus divides. It includes stages: prophase, metaphase, anaphase, and telophase.

• Cytokinesis: Division of the cytoplasm, usually occurring after nuclear division, to produce two new cells.

Functions of Mitosis:

• Growth of multicellular organisms: From a single-celled zygote to a multicellular organism, new cells are produced.

• Replacement of damaged or dead cells and tissue repair: Cells are constantly dying and are replaced by identical cells.

• Asexual reproduction: New individuals are produced from a single parent, resulting in genetically identical offspring (clones).

Stem Cells: Unspecialised cells that retain the ability to divide by mitosis and differentiate into various specialised cell types, crucial for growth and tissue repair. Uncontrolled cell division can lead to the formation of tumours and cancers.

Cell Division: Meiosis

Meiosis is a type of reductive nuclear division that occurs in reproductive organs to produce gametes (sex cells). Unlike mitosis, meiosis results in cells with half the number of chromosomes (haploid, n) compared to the parent cell (diploid, 2n).

Differences between Mitosis and Meiosis:

• Chromosome number: Mitosis maintains the diploid number; meiosis halves it to haploid.

• Genetic identity: Mitosis produces genetically identical daughter cells; meiosis produces genetically different daughter cells.

• Number of daughter cells: Mitosis produces two daughter cells; meiosis produces four daughter cells.

Sources of Genetic Variation (during meiosis):

• Crossing over: Exchange of alleles between homologous chromosomes during Prophase I.

• Independent assortment (random orientation): Homologous pairs of chromosomes and sister chromatids align and separate randomly.

• Random fertilisation: Any sperm can fertilise any egg, leading to unique combinations of chromosomes.

Reproduction: Asexual vs. Sexual

Asexual Reproduction:

• Definition: Production of genetically identical offspring from a single parent.

• Mechanism: Relies entirely on mitosis.

• Examples: Unicellular organisms (e.g., Amoeba) divide into two. Multicellular organisms can produce new individuals by budding (e.g., Hydra, yeast) or vegetative propagation in plants (e.g., runners, rhizomes, tubers, bulbs).

• Advantages: Fast, efficient, and ensures desirable characteristics are passed on.

• Disadvantages: No genetic variation, making populations vulnerable to changing environments or diseases.

Sexual Reproduction:

• Definition: Involves the fusion of haploid nuclei (fertilisation) from two gametes to form a diploid zygote, producing genetically different offspring.

• Gametes: Male (sperm, pollen grains) are typically small, mobile, and produced in large numbers. Female (egg cells, ovules) are larger, stationary, and produced in smaller numbers, often containing food reserves.

• Gametogenesis: The production of gametes by meiosis. In humans, it occurs in testes (spermatogenesis) and ovaries (oogenesis). In flowering plants, male gametes are in pollen grains, and female gametes are in embryo sacs within ovules.

• Fertilisation: The fusion of sperm and egg nuclei. In mammals, it occurs in the oviduct. In plants, it occurs in the ovary following pollen tube growth.

• Advantages: Creates genetic variation, which is crucial for adaptation to changing environments and for evolution.

Growth, Reproduction, and Evolution

The interplay of cell growth and reproduction, especially through meiosis and sexual reproduction, leads to genetic variation within populations. This variation is the raw material for natural selection, where individuals with advantageous phenotypes are more likely to survive, reproduce, and pass on their favourable alleles. Over long periods, this process of evolution can lead to the formation of new species (speciation).