SEXUAL REPRODUCTION

There are two types of sexual reproduction, this includes conjugation and fusion of gametes.

Conjugation: the simplest form of sexual reproduction, as contrasted with asexual reproduction. This is observed in some unicellular organisms e. g. paramecium, fungi e. g. rhizopus, algae e. g. spirogyra. Two similar organisms (conjugants) join together and exchange genetic material contained in their nuclei. After the exchange, the organisms separate. The zygote or zygosphere form is capable of developing into a new organism.
Fusion of Gametes: This occurs in higher organisms where gametes (male and female sex cells) are produced in special structures by a process known as gametogenesis which involves meiotic cell divisions. Fertilization, the process in which haploid male and female sex cells fuse together to producing a single diploid cell (zygote) that develops into an adult organism occurs after gamatogenesis.

EVALUATION

Describe conjugation in spirogyra.
Define spermatogenesis and oogenesis.

MEIOSIS

Meiosis is the reduction method of division that leads to the formation of four daughter cells that are haploids. Meiosis differs from mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell. Meiosis ensures the chromosome number of an individual remains the same from generation to generation.

Meiosis consists of two successive divisions:

First Meiotic Division

Interphase: resting phase, chromosomes are not seen.
Prophase I: At early prophase, chromosomes contract and become clearly visible. At middle prophase, homologous chromosomes come together and spindle is formed. At late prophase crossing over takes place between homologous chromosomes at a place called chiasma.
Metaphase: Nuclear membrane disappears, the bivalent chromosomes assemble at the equator and are attached to the spindle by their centromere.
Anaphase: Bivalent chromosomes separate completely and move to the opposite pole of the cell.
Telophase: bivalent chromosomes arrived at the pole, nuclear membrane are formed around the chromosomes at the two poles, two daughter cells result with half the number of chromosomes in the parent cell.

Second Meiotic Division

It consists of four stages similar to mitosis, no resting stage and no replication of chromosomes. At the end, four daughter cells are formed.

Importance of Meiosis

Meiosis aids the formation of

Ova or egg cells.
Pollen grains in flowering plants.
Ovules in flowering plants.

GENERAL EVALUATION/REVISIONAL QUESTIONS

Describe conjugation in Mucor.
Mention two structures each where meiosis takes place in plants and animals.
Describe sexual reproduction in hydra.
What is meiosis?
Outline four importance of meiosis to life.
List five excretory products in living organisms.
What is crossing over?
Outline four life processes involving meiosis

WEEKEND ASSIGNMENT

The process which ensures that the chromosome number for each species of organism remain constant from generation to generation is called fission B. meiosis C. mitosis D. oogenesis
The cell organelle, from which spindle fibres originate during cell division in animal cells is known centrosome B. chromosome C. lysosome D. ribosome
In animals meiosis comes A. after fertilization B. after every mitotic division C. before fertilization D. before every meiotic division
One of the ways in which body cells differ from gamete cells is in Type of centromeres they contain B. Number of chromosome pairs they contain C. Type of chromatids they contain D. Number of chromosomes they contain.
Which of these excretory structures is not found in amphibians? Gills B. Lungs C. Skin D. Flame cell

THEORY

In a tabular form, state five differences between
1. Mitosis and meiosis.
2. Sexual and asexual reproduction.
3. Explain the phases of meiosis.