Secondary Biology, Reproduction in Plants and Animals Cell division; Mitosis By Mr. Kanyoro Peter
Secondary Biology, Reproduction in Plants and
AnimalsCell division; Mitosis
By Mr. Kanyoro Peter
Introduction Reproduction is the process by which mature
living organisms produce young ones of their kind/offspring.
It’s of two types;
Sexual reproduction – involves fusion of sex cells (male and female gametes) in a process called fertilization.
Asexual reproduction – where parts of mature organisms develop into new individuals in several mechanisms.
Sexual (a) and Asexual (b) reproduction
Importance of reproduction Leads to procreation hence
perpetuation/survival/sustenance of species so that it dose not become extinct
Helps in improvement of quality of species through exchange of genetic material during crossing over hence variation and adaptability.
Helps to maintain all the characteristics of the parent as in asexual reproduction.
Cell division
Organisms start as a single cell; spore or zygote
Cells of multicellular organisms divides into two and the division continues in multiples of two.
Cells of unicellular organisms divide into separate daughter cells which develop into adult individuals.
Cell division starts with division of nucleus (karyogamy) – chromosomes and then cytoplasm (cytogamy)
There are two types of cell division; mitosis and meiosis
Chromosomes Are long thread-like structures found in the nucleus
which become visible with microscope only during cell
division.
Each somatic cell (body cell) has a fixed diploid (double
number) of chromosomes (also referred to as “2n”
constitution) e.g. 23 pairs for human, 4 pairs for
Drosophila, 20 pairs for mouse and 7 pairs for garden
pea.
Each chromosome is made of two parallel strands called
chromatids. Each chromatid pair is connected at a
point by centromere.
Structure of Chromosomes
Homologous chromosomes
This is a pair of chromosomes with a
characteristic length i.e. have a similar
appearance though their genetic
composition may be different.
Each parent gives one pair of such
chromosomes during sexual reproduction.
Homologous chromosomes
Genes
Located along the length of chromosome.
Are basic units of heredity that are transferred from a
parent to offspring.
Are made of protein chemical substance called
Deoxyribonucleic acid (DNA), which contain coded
information determine the characteristics of the
offspring e.g. whether male or female.
Mitosis The type of reproduction in which a cell divides into
two daughter cells each with the same number of chromosomes as the parent cell.
Importance of Mitosis
Functions of mitosis
For simple organisms, it is a means of asexual reproduction. (For example cells of fungus)
In multicellular organisms, it allows for entire cell to grow by forming new cells and replacing older cells.
In certain species, mitosis is used to heal wounds or regenerate body parts.
Phases of mitosis It is a continuous process, divided into four
phases;
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
In order to remember the phases easily, we can use the acronym P-MAT
However, prior to cell division (P-MAT), the cell spends a lot of time in INTERPHASE where it undergoes intense activities in preparation to division.
Interphase
During interphase;
The genetic material multiplies so that
daughter cells have the same number of
chromosomes as the parent cell.
New cell organelles are synthesized e.g. golgi
apparatus, centrioles, mitochondria and
ribosomes.
Enough energy stores in form of ATP
(adenosine triphosphate) is build up
Phases of the Cell Cycle
Interphase- phase in which cells spends most time performing functions that makes it unique.
Mitosis- phase of cell during which cell divides into two daughter cells.
Stages
Prophase
In animal cells, duplicated
microscopic bodies called
centrioles separate and move
to opposite sides of the cell.
These are not present in most
plant or fungal cells.
Spindle fibres begin to form.
Nuclear membrane begins to
break down and nucleolus
disappear.
Chromosomes thicken and
shorten and become visible
through staining.
Metaphase
Nuclear membrane disappears hence chromosomes are found free in cytoplasm
Spindle fibres lengthen (they attach to centrioles by at both poles in animal)
Chromosomes align themselves at equatorial plate (equator of spindle).
Each pair is connected at the centromere where the spindle fiber is attached.
Anaphase
Spindle fibres shorten, chromatids move apart (separate) from each other and move to the opposite poles.
The spindle apparatus begin to disappear.
The cell membrane in animal cells begins to constrict towards the end.
Telophase Chromosomes
finally arrive/collect together at the opposite poles of the cell.
Nuclear membrane forms around set of chromosomes
Cytoplasm divides into two leading to the formation of two daughter cells.
Chromosomes become less distinct.
Cytokinesis
Process in which cytoplasm divides into two separate cell forms.
In animal cells, it begins with the formation of a furrow in the center of the cell which constricts the cell membrane.
The membrane begins to pinch into cytoplasm and formation of two cells begins. (Process referred to as cell cleavage).
Plant cell Final Stage Cleavage doesn’t
take place.
New cell wall is assembled at center of cell by formation of middle lamellae and separation of two daughter cells occurs.
The End;thanks for
walking with me.