Classwork Series and Exercises {Biology- SS2}: Growth

Introduction

Growth is a permanent increase in size and complexity of an organism during development from embryo to maturity, as a result of cell division. Growth is an anabolic process. For it to occur, the cell needs plenty of food to provide the necessary energy and materials for building up new protoplasm.

In a unicellular organism, the young organism grows until it reaches its maximum size, then it reproduces to give rise to young individual. This usually happens by simple division of the adult cell into two daughter cells as in the amoeba. All multicellular organisms begin life as a single fertilized cell. This cell divides into two, then into four and so on.

Related Article: Organisation of Life

BASIS OF GROWTH

• Cell Division: For an embryo to grow or a tissue to regenerate, its cells must divide. There are two types of cell division – mitosis and meiosis.

1. Meiosis: It is a form of division which brings about a halving of the chromosome number as well as interchange of genetic material between homologous chromosomes. In animals meiosis takes place at the formation of gametes and in diploid plants at the formation of pores.
2. Mitosis(karyokinesis): is the process in cell division by which the nucleus divides, typically consisting of four stages and normally resulting in two new identical daughter cell, each of which contains a complete copy of the parental chromosomes. Tissue repair and binary fission (asexual reproduction) are all mitotic divisions. The period between mitotic divisions is called interphase (resting period), and mitosis itself occurs in four phases: prophase, metaphase, anaphase, and telophase.During interphase the chromosomes are extended long threads that cannot be visibly identified. The DNA of the chromosomes is replicated during this phase, resulting in duplication of the genetic material.During prophase the chromosomes coil up and contract, becoming short rods. Each chromosome consists of a pair of strands, called chromatids, held together at the centromere. At the same time the nuclear envelope disappears, and the centriole divides and the two daughter centrioles move toward opposite poles of the cell.During metaphase the chromosomes move so that their centromeres are aligned in the equatorial plane of the cell (the metaphase plate), and the mitotic spindle forms. The mitotic spindle is formed of fibers composed of microtubules, which extend from the centrioles to the metaphase plate and to the centromeres of the chromosomes.During anaphase the chromatids of each chromosome separate, becoming new daughter chromosomes, which are drawn to opposite poles of the cell by the spindle fibers.During telophase the daughter chromosomes arrive at the poles of the cell, where they are surrounded by two new nuclear envelopes as they begin to uncoil and extend. During this phase, cytokinesis, division of the cytoplasm, occurs. A furrow forms around the cell in the equatorial plane and deepens until the two daughter cells are separated.

DIFFERENCES BETWEEN MEIOSIS AND MITOSIS

MEIOSIS	MITOSIS
Haploid number of chromosomes result after division	Diploid number of chromosomes are formed after division
Two stages of division	Only a stage of division
Four daughter cells are formed	Two daughter cells are formed
Occurs only in sex cells	Occurs in body or somatic cells
Exchange of genetic materials is due to crossing over	No crossing over as well as exchange of materials

Cell Enlargement: After the formation of new daughter cells, there is tendency for them to increase in size.

Cell Differentiation: In the process of growth, the formerly undifferentiated cells begin to grow to the extent that they start becoming specialized for a particular function due to structural changes. Examples are organs like xylem which initial function is to transport mineral salts and water, it start to grow lignin on its walls. Making it to play the role of strengthening of the stem.

REGULATION OF GROWTH BY HORMONE

Organisms produce growth hormones in small quantities which have profound effects on other parts of their body where they are needed for some form of growth.

Examples are plant hormones like auxins, gibberellins, cytokinin, abscisic acid and florigens which help the plants in the following:

PLANT HORMONES	FUNCTION
Auxin	Cell elongation, apical dominance, tropism, lateral bud development, lateral root initiation, fruit growth, leaf abscission and control of cell differentiation.
Gibberellin	Cell elongation, apical dominance, apical division, flower buds, flower sex, fruit growth, tropism and seed germination.
Cytokinin	Apical division and protein synthesis, fruit growth and control of cell differentiation in culture.
Florigens	Flower buds growth
Abscisic acid	Apical division and protein stnthesis

Animal hormones include adrenalin, insulin, thyroxine, pituitrin, testosterone, oestrogen, progesterone and are secreted for the following:

• During fright
• For body metabolism
• In reproductive functions
• Growth of long bones etc.

EXERCISES

Lets see how much you’ve learnt, attach the following answers to the comment below

1. One of the following is not a plant hormone (a) cytochrome (b) cytokinin (c) Auxin (d) gibberellins
2. Auxin _____. (a) Promotes fruit growth (b) stimulates positive phototropism (c) causes roots to develop (d) all of the above
3. Apical dominance is caused by the production of _____ by the apical meristem. (a) ethylene (b) gibberellin (c) abscisic acid (d) Auxin
4.  The following are basis of growth except (a) cell enlargement (b) increase in size (c) cell division (d) cell differentiation
5. ______________ is referred to as the resting period between the mitotic phase. (a) anaphase (b) interphase (c) metaphase (d) prophase

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