Lesson 6: Reproduction

Reproduction is the process by which Living organism produce new individuals of their kinds. It means the proliferation and production of new offspring from existing ones.

Two major types or forms of reproduction are known:
(a) Asexual Reproduction
(b) Sexual Reproduction

Unit 1: Asexual Reproduction
Asexual reproduction is a form of reproduction that does not require the union of male and female gamete, and it does not involve special sex cells or gametes. It is a reproduction where one or more off springs/cells are produced from a part of a single parent (individual).

Types of Asexual reproduction
(i) Fission
Here the parent organism or cell divides into two. Fission is mitotic division of parent cell to yield daughter cells. This form is found mainly in unicellular organisms like amoeba, paramecium, chlamydomous and bacteria.

(ii) Budding
This is a process where a portion of parent cell forms an out-growth or bud which detaches itself to form a new cell i.e. separates from the parent cell. This form of reproduction is found in yeast and hydra.

(iii) Fragmentation
This involves the breaking of parent or cell into pieces while the end of each of the develops individual. Spirogyra is an example.

(iv) Spore Formation
Spores are dust-like gamete found in lower organisms like mucor and rhizopus. Spores are borne in sporangia which burst and disperse by the wind. The spores’ germinate when it falls on suitable substrate.

(v) Vegetative Propagation
This is a form of asexual reproduction where vegetative organs (leaf, stem and root) are involved as distinct from their special reproductive structures like flowers.

Organs of Vegetative Reproduction includes
(a) Bulb
(b) Stem Tubers.
(c) Rhizomes
(d) Runners

Unit 2: Sexual Reproduction
These involve the union of two gametes. Sexual production is the production of new off spring by the fission or union of the sex cells or gametes from different parents.

Examples of Sexual Reproduction.
(i) Conjugation
This process is common in protozoans. Two cell strains Lie side by side with protuberances formed from the walls of the cells of the two strains. These protuberances meet to form conjugation tube while the gamete of one filament passes through conjugation tube and fusses with the gamete of other filament to form a zygote. The zygote forms a zygospore. The zygospore burst to germinate new filament.

(ii) Fusion of Male and Female gametes
This is a form of cell division that takes place in reproductive tissues. It is a reduction division which takes place in the formation of gametes (sex cells) i.e. the sperms and eggs. In meiosis, the chromo some number of the adult is reduced by half from diploid number (2n) to haploid number (n).

Stages of Meiosis
There are two stages; the first and second meiotic division. Each division have four phases – Prophase, metaphase, anaphase and telophase.

(a) First Meiotic Division

It is the resting stage. Cell is normal and not dividing. Chromosomes are long and thread like.

Prophase I
Chromosomes are visibly seen i.e., each homologue is seen as a single structure. Homologous chromosomes pair up or lie side by side forming a bivalent. Each chromosome consists of two strands or chromatids to make up four bivalent in all. There is a crossing over and each chromatid exchange genetic content by breaking and joining to a different strand.

Metaphase I
The nuclear membrane disappears and a spindle is formed while the bivalent is arranged on the equator of the spindle.

Anaphase I
The spindle attaching the chromosomes shortens while a pair of chromatids moves to the opposite poles of the cell.

Telophase I
Chromosomes arrive at the two poles of dividing cells. Cytoplasm divides to form two daughter cells. Each has help number of chromosomes in the cells that divided.

(b) Second Meiotic Division

Prophase II
The chromosomes are visible, each with chromatids joined at centromere. Centriole divide into two each migrates to different poles of the cell with the spindle fibre of 2nd division laid down.

Metaphase II
The chromosomes with their centromeres attached to the spindle arrange themselves at the equator of the cell and each centromere divides into two.

Anaphase II
The chromatids split and separate by the division of the centromere. Single chromatids move to the opposite poles of the cell.

Telophase II
As the chromatids arrive at the two poles of the cell, walls develop around each new nucleus and spindle the disappear then the cytoplasm divides into two. Each daughter cell contains a number of chromatids equal to the number of chromosome cell as it was at the end of the first division.

A sum of four cells is produced with each containing one member of each homologous pair of chromosomes present in the original cell.

Functions of the Male and Female Gonads
Gonads imply male and female reproductive organs in animals.

Human Male Reproductive Organs
The human male gonads are called testes. The male reproductive system is made up of testes, epididymis, vas deferens, seminal vesicles, prostate gland, cowpers gland and penis.

The sex organs are two testes located in the scrotum outside abdominal cavity. The testes have seminiferous tubules where sperms are produced. After production, sperm cells are passed through tiny, ducts and stored in epididymis. Sperms are then released through the sperm duct (vas deferens) into urethra which passes through the penis and emptied outside.

Human Female Reproductive Organs
The organs are called ovaries, held in position by ligaments. When ovulation takes place, the ovum is released from ovary wall into the large funnel shaped oviduct (fallopian tube). While the oviducts opens into uterus located behind the ladder, the uterus opens into vagina through a narrow muscular neck (cervix).

Unit 3: Plant Reproductive Organ
Pollination: is the term commonly used for reproduction in flowering plants. It is the transfer of the pollen grains by wind, insect and man to the stigma of the gynoecium.

Structure and Functions of Male and Female Parts of Flower
(a) Androecium
: is the male reproductive part that consists of individual unit called stamen. A stamen has two parts;

(i) Stalk popularly called filament
(ii) Another consisting of two lobes. Each lobe contains pollen sacs which houses the pollen grain.

(b) Function: It produces male gametes that bring about sexual reproduction. Gynoecium is also called female whorl and made up of individual unit called carpel or pistil.

The gynoecium may have 1 carpel (monocarp pellary) 2 carpels (bicapellary), 3 or more carpels (polycarpellary) when carpel are fused, gynoecium is called syncaipous but when free, it is called apocarpous. Carpel has three parts; stigma, style and ovary which houses ovule.

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