REPRODUCTION IN PLANTS AND ANIMALS - KCSE BIOLOGY NOTES

​Concept of reproduction

​The process by which mature individuals produce offspring is called reproduction. Reproduction is a characteristic of all living organisms and prevents extinction of a species. 

Monkeys Mating

There are two types of reproduction:

1. Sexual
   this reproduction involves the fusion of male and female gametes to form a zygote.
2. Asexual reproduction.
   This is a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it does not involve the fusion of gametes, and almost never changes the number of chromosomes. [Source: Wikipedia.org]

​Importance of reproduction

​The main importance to reproduction is to give rise to young ones of same ensuring continuity of the group.

​Chromosomes, mitosis and meiosis (mention gamete formation)

Cell Division
Cell division starts with division of nucleus. In the nucleus are a number of thread-like structures called chromosomes, which occur in pairs known as homologous chromosomes.
Each chromosome contains-genes that determine the characteristics of an organism. The cells in each organism contains a specific number of chromosomes.
There are two types of cell division:

1. Mitosis
2. Meiosis

These stages of cell division occur in a smooth and continuous pattern.
a) Mitosis
This takes place in all body cells of an organism to bring about increase in number of cells, resulting in growth and repair. The number of chromosomes in daughter cells remain the same as that in the mother cell.
Mitosis is divided into five main stages:

1. Interface
   The term interphase is used to describe the state of the nucleus when the cell is just about to divide. During this time the following take place:
   1. Replication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell.
   2. Division of cell organelles such as mitochondria, ribosomes and centrioles.
   3. Energy for cell division is synthesised and stored in form of Adenosine Triphosphate (ATP) to drive the cell through the entire process.
      During interphase, the following observations can be made:
      - Chromosomes are seen as long, thin, coiled thread-like structures.
      - Nuclear membrane and nucleolus are intact.
2. Prophase
   the chromosomes shorten and thicken. Each chromosome is seen to consist of a pair of chromatids joined at a point called centromere.
   Centrioles (in animal cells) separate and move to opposite poles of the cell. The centre of the nucleus is referred to as the equator.       Spindle fibres begin to form, and connect the centriole pairs to the opposite poles. The nucleolus and nuclear membrane disintegrate and disappear.
3. Metaphase
   Spindle fibres lengthen, in animal cells they attach to the centrioles at both poles. Each chromosome moves to the equatorial plane and is attached to the spindle fibres by the centromeres. Chromatids begin to separate at the centromere.
4. Anaphase
   Chromatids separate and migrate to the opposite poles due to the shortening of spindle fibres. Chromatids becomes a chromosome. In animal cell, the cell membrane starts to constrict.
5. Telophase.
   The cell divides into two. In animal cells it occurs through cleavage of cell membrane. In plants cells, it is due to deposition of cellulose along the equator of the cell.(Cell plate formation). A nuclear membrane forms around each set of chromosome. Chromosomes later become less distinct.

Significance of Mitosis

• It brings about the growth of an organism.
• It brings about asexual reproduction.
• Ensures that the chromosome number is retained.
• Ensures that the chromosomal constitution of the offspring is the same as the parents. 

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b) Meiosis
This type of cell division takes place in reproductive organs (gonads) to produce gametes. The number of chromosomes in the gamete is half that in the mother cell.
Meiosis involves two divisions of the parental cell resulting into four daughter cells. The mother cell has the diploid number of chromosomes. The four cells (gametes) have half the number of chromosomes (haploid) that the mother cell had, in the first meiotic division there is a reduction in the chromosome number because homologous chromosomes and not chromatids separate.
Homologue pairs separate during a first round of cell division, called meiosis I. Sister chromatids separate during a second round, called meiosis II. Since cell division occurs twice during meiosis, one starting cell can produce four gametes (eggs or sperm). In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase. [additional information by the Khan Academy]
Meiosis I (First Meiotic division)
Before entering meiosis I, a cell must first go through interphase. As in mitosis the cell prepares for division, this involves replication of chromosomes, organelles and buildup of energy to be used during the meiotic division.

1. Prophase I
   Homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents. Chromosomes shorten and thicken hence become more visible. Chromosomes may become coiled around each other and the chromatids may remain in contact at points called chiasmata (singular chiasma). Chromatids cross-over at the chiasmata exchanging chromatid portions. Important genetic changes usually result.
2. Metaphase I
   Spindle fibres are fully formed and attached to the centromeres, the bivalents move to the equator of the spindles.
3. Anaphase I
   Homologous chromosomes separate and migrate to opposite poles, this is brought about by shortening of spindle fibres hence pulling the chromosomes, the number of chromosomes at each pole is half the number in the mother cell.
4. Telophase I
   Cytoplasm divides to separate the two daughter cells.

Meiosis II (Second Meiotic Division)
Usually the two daughter cells go into a short resting stage (interphase) but sometimes the chromosomes remain condensed and the daughter cells go straight into metaphase of second meiotic division. The second meiotic division takes place just like mitosis.

1. Prophase II
   each chromosome is seen as a pair of chromatids.
2. Metaphase II
   Spindle forms and are attached to the chromatids at the centromeres, chromatids move to the equator.
3. Anaphase II
   Sister Chromatids separate from each other, then move to opposite poles, pulled by the shortening of the spindle fibres.
4. Telophase II
   The spindle apparatus disappears, the nucleolus reappears and nuclear membrane is formed around each set of chromatids. The chromatids become chromosomes, cytoplasm divides and four daughter cells are formed, each has a haploid number of chromosomes.

Significance of Meiosis

1. Meiosis brings about formation of gametes that contain half the number of chromosomes as the parent cells.
2. It helps to restore the diploid chromosomal constitution in a species at fertilisation.
3. It brings about new gene combinations that lead to genetic variation in the offspring.

​Asexual reproduction

Asexual reproduction is the formation of offspring from a single parent, the offspring are identical to the parent.
Types of asexual reproduction.

1. Binary fission in amoeba.
2. Spore formation in Rhizopus.
3. Budding in yeast.

Binary fission in amoeba

​This involves the division of the parent organism into two daughter cells, the nucleus first divides into two and then the cytoplasm separates into two portions. Binary fission also occurs in bacteria, Paramecium, Trypanosoma and Euglena.

​Spore formation/reproduction in mucor / Rhizopus

Rhizopus is a saprophytic fungus which grows on various substrate such as bread, rotting fruits or other decaying organic matter. The vegetative body is called mycelium which has many branched threads called hyphae. Horizontal hyphae are called stolons, vertical hyphae are called sporangiophore.
The tips of sporangiophore become swollen to form sporangia, the spore bearing structure, each sporangium contains many spores, as it matures and ripens, it turns black in colour.
When fully mature the sporangium wall burst and release spores which are dispersed by wind or insects. When spores land on moist substratum, they germinate and grow into a new Rhizopus and start another generation.
Spore formation in ferns
The fern plant is called a sporophyte, on the lower side of the mature leaves are sari (Singular: sorus) which bear spores. 

​Budding in yeast

​Budding involves the formation of a protrusion called a bud from the body of the organism, the bud separates from the parent cell, in yeast budding goes on so fast and the first bud starts to form another bud before the separation. A short chain or mass of cells is formed.

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