Sexual Reproduction – Meiosis

Sexual reproduction is the type of reproduction which involves the fusion of the male gametes and the female gametes to form a zygote which later develop to a young offspring.

Types of Sexual Reproduction

1. Conjugation: is a simple types of sexual reproduction which occurs in some lower organisms such as mucor, rhizopus, paramecium and spirogyra. Conjugation is the process by which nuclear material is passed from one cell to another.

In spirogyra, for example, conjugation starts with two filaments called conjugants coming together to lie side by side. lateral outgrowth start to develop opposite each other from opposite cells in the two filaments. The outgrowth meet and the separating well dissolve to form a continuous channel called conjugation tube. The cytoplasm of each cell shrinks away from the cell walls and round up to form gametes.

One of the gametes regarded as migratory move through conjugation tube into the other cell resulting to the fussion of cytoplasm and nucleir of the cells to form zygote. After sometime the zygote develop into zygospore which will germinate to a new filament

FERTILIZATION

Fertilization (fusion of gametes) is defined as the fusion of a minute haplow male gamete (sperm) with a haphoid female gamete (egg) to form a diploid zygote.

In higher animal the gametes are formed in special organ called gonads. The male gamete is called spermatozoa (sperm) which are produced in the testes by a meotic cell division called spermatogenesis. The female gamete is called ovum or egg which are produced in the ovary by meotic cell division called oogenesis. The process of gamete formation either male or female is called gametogenesis. These two type of gametes (male&female) are brought together during mating and they unite by process of fertilization to form a diploid zygote which later develop to a new organism.

In higer plants, male and female gametes are also produced. The flowers are the reproductive units in plants. The male gamete is called pollen grain which are produce by the anther while the female gametes called ovules are produced by the ovary of the flower

MEIOSIS

Meiosis is the form of division which bring about a halving of the chromosome number as well as an interchange of genetic material between homologous chromosomes. In animal, meiosis takes place at the formation of the gametes and in diploid plants at the formation of spores.

The formation of spores

Meiosis consists of two successive divisions

1. 1^st Meiotic division: when the parent cell split into two
2. 2^nd meiotic division: when the products then divide again giving a total of four daughter cells

1^st MEIOTIC DIVISION (REDUCTION DIVISION)

The 1^st meiotic division is divided into five stages. These are Interphase, Prophase, Metaphase, Anaphase and Telophase.

INTERPHASE: This is the resting stage of the cell. At this stage chromosome are not seen

Middle Prophase: Homologous (marching) chromosome become thicker, shorter and clearly visible. Spindle is formed.

Late Prophase: Each chromosome reproduce replica of itself (double stranded). Nuclear membrane starts disappearing. Cross-links called chiasmata (chiasma) are formed between chromosome. Chiasmata help to hold two homologous chromosomes together and also exchange of genetic material (or gene) take place between homologous chromosome which leads to variation or mutation

METAPHASE 1

Nuclear membrane gets dissolved in the cytoplasm. Bivalent chromosomes are arranged at the equator and attached to the spindle by their centromeres.

ANAPHASE 1

Each member of the bivalent chromosomes moves apart and moves toward the poles of the cell by the shortening of the spindle

TELOPHASE 1

Bivalent chromosomes arrive at the poles of the cell. A new nuclear membrane is formed around each group of chromosomes. Chromosomes uncoil. Nucleolus appear on each nucleus. Nucleus looks granular again. The two nucleic have half the number of chromosomes of the parent cell. This is the reduction division of meiosis. No cleavage of cytoplasm takes place

Chromosome become distinct in the two nucleic centrioles divide, separate and move to the opposite poles of the cell. Spindles are formed.

METAPHASE II

Nuclear membrane gets dissolved in the cytoplasm chromosomes are attached to the spindle by their centromeres. The two chromatids of the chromosome become more distinct.

ANAPHASE II

The chromosomes get detached at the centromeres. The chromosomes separate and more to the pole.

TELOPHASE II

The chromatids arrive the polls of cells. Nuclear membranes are formed round the four cells. A nucleolus appears in each cell. Spindles get dissolved in the cytoplasm. Cleavage of cytoplasm follows. Four haploid cells are formed at the end of the second meiotic division. Each cell has half the number of chromosomes.

IMPORTANCE OF MEIOSIS

1.Meiosis ensures that the fertilized egg (zygote) has a diploid number of chromosome. Therefore, meiosis brings about the reduction of chromosome to half the original number both in the male (sperm) and the female (egg) gametes. For example, every human being has 46 chromosome. During the sperm or egg formation in a process known as gametogenesis, the 46 chromosomes will reduce to 23 chromosomes. When the sperm fuses with the egg, a zygote with 46 chromosomes result.

2.Random assortment of genes during meiosis lead to genetic variation among offspring of the same parents. This is why no two individuals can look exactly alike unless identical twins.

Meiosis can lead to a marked gene change (mutation) which can also bring about variation among individuals.

Life processes involving meiosis

Formation of spermatozoa

Formation of eggs

Formation of pollen grains in flowering plants

Formation of ovules in flowering plants

ASSIGNMENT

1. Describe two methods of reproduction in unicellular animals you have