Revision: Organisation of Tissues Science SSLC (English Medium) Class 9 Tamil Nadu Board of Secondary Education

A tissue, in biology, is defined as a group of cells that have a similar structure and perform a specific function. The word tissue originates from French, which means "to weave."

• Cells that perform a particular function always live in a group. This group of cells is called a tissue.
• For example, blood, phloem, muscle, etc. are examples of tissues.

A group of similar cells which perform a specific function.
example: Muscular tissue in animals.

A tissue is a group of cells that are similar in structure and are organized together to perform a specific task.

The tissue is a group of cells of similar structure and function.

Vascular tissue is the complex plant tissue in higher plants that are composed of xylem and phloem and is concerned with conducting water, minerals, and organic food throughout the plant body.

Meristematic tissue is a group of cells that constantly divide and produce cells indefinitely throughout the life of the plant.

Permanent tissue refers to a group of cells which temporarily or permanently cease to divide and thus assume permanent form and function

A group of living organisms that can breed among themselves constitute a species.

The population of all plant and animal species living in a particular area constitutes a biotic community.

All the members or individuals of a particular species living in a particular area constitute its population.

The zone on the earth in which all living beings exist is termed as the biosphere.

The epithelial cells are connected to each other laterally as well as to the basement membrane by junctional complexes called cell junctions.

The tissue which connects various tissues together in any organ is called connective tissue.

An organ is a group of tissues that work together to perform a specific function or set of functions in the body.

The heart, lungs, liver, and kidneys are examples, each playing a vital role in maintaining the body's overall health and function.

The cells that constitute nervous tissue are called neurons or nerve cells.

The centrosome (in animal cell) splits into two along with the simultaneous duplication of the centrioles contained in it. The daughter centrioles move apart and occupy opposite "poles" of the cell. Each centriole is surrounded by radiating rays and is termed an aster (aster : star).

Cell division is one of the most fundamental characteristics of life. This is the method which enables life to perpetuate generation after generation.

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

The two sister chromatids remain attached to each other at a small region called centromere.

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

The two sister chromatids remain attached to each other at a small region called centromere.

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

• Cell division is a vital process for growth, repair, and the formation of new organisms, helping maintain life in all living beings.
• It occurs in two forms: mitosis (in somatic and stem cells) for producing diploid identical cells, and meiosis (in germ cells) for forming haploid gametes.
• Mitosis supports body growth and tissue repair, while meiosis ensures genetic variation and maintains chromosome number in reproduction.
• Before division, the cell’s chromosome number doubles (e.g., from 2n to 4n) to ensure accurate distribution during mitosis or meiosis.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

• Cytokinesis is the division of the cytoplasm that follows nuclear division (karyokinesis), resulting in the formation of two separate daughter cells.
• In animal cells, it occurs by the formation of a cleavage furrow, while in plant cells, a cell plate forms at the centre to divide the cytoplasm.

• Meiosis is a reduction division that results in haploid gametes (sex cells) with half the chromosome number.
• In humans, it occurs in the testes (to form sperm) and ovaries (to form ova).
• In flowering plants, it occurs in the anthers (to form pollen grains) and ovaries (to form ovules).
• Fertilisation restores the diploid (2n) number, maintaining chromosome count across generations.

• Meiosis is a two-stage process: Meiosis I and Meiosis II, responsible for forming haploid gametes from diploid germ cells.
• Prophase I is a complex phase subdivided into five stages where homologous chromosomes pair up and undergo crossing over, enabling genetic recombination.
• In Metaphase I, homologous pairs (not sister chromatids) align along the equatorial plate, attached to spindle fibers from opposite poles.
• During Anaphase I, homologous chromosomes are separated and pulled to opposite poles, reducing the chromosome number by half.
• Telophase I results in the formation of two haploid daughter cells, each with half the chromosome number, setting the stage for Meiosis II.

• Meiosis II is similar to mitosis and occurs in both haploid cells formed after Meiosis I, dividing the recombined sister chromatids.
• It includes four stages: Prophase II, Metaphase II, Anaphase II, and Telophase II, resulting in four haploid daughter cells.
• The process ensures genetic variation, as each resulting cell is genetically different from the parent and from each other.
• Meiosis II plays a vital role in sexual reproduction, producing gametes (in animals) or spores (in plants) with half the chromosome number.