- 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.
- There are two types of reproduction: sexual and asexual reproduction.
- Sexual reproduction involves the fusion of male and female gametes to form a zygote.
- Asexual reproduction does not involve gametes.
- 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:
- 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.
- 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.
- Mitosis is divided into four main stages.
- Prophase, Metaphase, Anaphase and Telophase.
- These stages of cell division occur in a smooth and continuous pattern.
- 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:
- Replication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell.
- Division of cell organelles such as mitochondria, ribosomes and centrioles.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Each division has four stages Prophase, Metaphase, Anaphase and Telophase.
- As in mitosis the cell prepares for division.
- This involves replication of chromosomes, organelles and build up of energy to be used during the meiotic division.
First Meiotic division
- 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.
- Spindle fibres are fully formed and attached to the centromeres.
- The bivalents move to the equator of the spindles.
- 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.
- Cytoplasm divides to separate the two daughter cells.
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.
- Each chromosome is seen as a pair of chromatids.
- Spindle forms and are attached to the chromatids at the centromeres.
- Chromatids move to the equator.
- Sister chromatids separate from each other
- Then move to opposite poles, pulled by the shortening of the spindle fibres.
- 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
- Meiosis brings about formation of gametes that contain half the number of chromosomes as the parent cells.
- It helps to restore the diploid chromosomal constitution in a species at fertilisation.
- It brings about new gene combinations that lead to genetic variation in the offsprings.
- Asexual reproduction is the formation of offspring from a single parent.
- The offspring are identical to the parent.
Types of asexual reproduction.
- Binary fission in amoeba.
- Spore formation in Rhizopus.
- Budding in yeast.
- 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 in 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 sori (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.
Sexual Reproduction in Plants
- In flowering plants, the flower is the reproductive organ which is a specialised shoot consisting of a modified stem and leaves.
- The stem-like part is the pedicel and receptacle, while modified leaves form corolla and calyx.
Structure of a flower
- A typical flower consists of the following parts:
- made up of sepals.
- They enclose and protect the flower when it is in a bud. Some flowers have an outer whorl made of sepal-like structures called epicalyx.
- consists of petals. The petals are brightly coloured in insect - pollinated flowers.
- Is the male part of the flower. It consists of stamens.
- Each stamen consists of a filament whose end has an anther.
- Inside the anther are pollen sacs which contain pollen grains.
- Is the female part of the flower.
- It consists of one or more carpels.
- Each carpel consists of an ovary, a style and a stigma.
- The ovary contains ovules which become seeds after fertilisation.
- A monocarpous pistil has one carpel e.g. beans.
- A polycarpous pistil has many carpels.
- If the carpes are free, it is called apocarpous as in rose and Bryophyllum,
- In carpels that are fused it is called syncarpous as in Hibiscus.
- A complete flower has all the four floral parts.
- A regular flower can be divided into two halves by any vertical section passing through the centre. e.g. morning glory.
- Irregular flower can be divided into two halves in only one plane e.g. crotalaria.
- This is the transfer of pollen grains from the anther to the stigma.
Types of pollination
- Self pollination is the transfer of pollen grains from the anther of one flower to the stigma of the same flower.
- Cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of a different flower, of the same species.
Agents of pollination
- Agents of pollination include wind, insects, birds and mammals.
- Insect pollinators include bees, butterflies and mosquitoes.
Mechanisms that hinder self-pollination
- Stamens ripen early and release their pollen grains before the stigma, mature. This is called protandry e.g. in sunflower.
- The stigma matures earlier and dries before the anthers release the pollen grains. This is called protogyny and is common in grasses.
- Self sterility or incompatibility: Pollen grains are sterile to the stigma of the same flower, e.g. in maize flower.
- Shorter stamens than pistils.
Fertilisation in Plants
- The pollen grain contains the generative nucleus and a tube nucleus.
- When the pollen grain lands on the stigma, it absorbs nutrient and germinates forming a pollen tube.
- This pollen tube grows through the style pushing its way between the cells.
- It gets nourishment from these cells.
- The tube nucleus occupies the position at the tip of the growing pollen tube.
- The generative nucleus follows behind the tube nucleus, and divides to form two male gamete nuclei.
- The pollen tube enters the ovule through the micropyle.
- When the pollen tube penetrates the ovule disintegrates and the pollen tube bursts open leaving a clear way for the male nuclei.
- One male nucleus fuses with the egg cell nucleus to form a diploid zygote which develops into an embryo.
- The other male gamete nucleus fuses with the polar nucleus to form a triploid nucleus which forms the primary endosperm.
- This is called double fertilisation.
After fertilisation the following changes take place in a flower:
- The integuments develops into seed coat (testa).
- The zygote develops into an embryo.
- The triploid nucleus develops into an endosperm.
- The ovules become seeds.
- The ovary develops into a fruit.
- The ovary wall develops into pericarp.
- The style, dries up and falls off leaving a scar.
- The corolla, calyx and stamens dry up and fall off.
- In some the calyx persists.
- Fruit development without fertilisation is called parthenocarpy e.g. as in pineapples and bananas.
- Such fruits do not have seeds.
Classification of fruits
- False fruits develops from other parts such as calyx, corolla and receptacle, e.g. apple and pineapple which develops from an inflorescence.
- True fruits develop from the ovary, e.g. bean fruit (pod).
- True fruits can be divided into fleshy or succulent fruits e.g. berries and drupes and dry fruits.
- The dry ones can be divided into Dehiscent which split open to release seeds and indehiscent which do not open.
Types of fruits
|Type of fruit||Structure||Example|
|Berry Fleshy-succulent||Ovary fleshy, thin skinned juicy with many seeds||Tomato, orange, Sodom apple|
|Drupe fleshy-succulent||Outer layer fleshy, inner layer hard, endosing one or more seeds||Mango, plum|
|Pod Dehiscent (dry)||Ovary wall thin, contains many seeds. Splits when ripe||Bean, pea|
|Schizocarp Dehiscent (dry)||The ripe fruit breaks up into small one seeded parts||Castor oil|
|Caryopsil Dry||Pericarp and seed coat are fused to form thin covering||Maize grain|
|Cypsela Dry indehiscent||One seeded fruit. The calyx persists||Bidens, Tridax|
|Pome||Outer fleshy layer develops from calyx and receptacle||Pear, apple|
|Multiple fruit||Formed from several flowers in a cluster||Pineapple|
|Achene||Ovary wall separated from seed||Sunflower|
- This is the arrangement of the ovules in an ovary.
- The placenta appears as one ridge on the ovary wall e.g. bean.
- The placenta is on the ridges on ovary wall.
- Ovules are in them e.g. pawpaw.
Axile placentation :
- The placenta is in the centre.
- Ovary is divided into a number of loculi. e.g. orange.
- The placenta is formed at the base of the ovary e.g. sunflower.
Free Central placentation
- Placenta is in the centre of the ovary.
- There are no loculi e.g. in primrose.
Methods of fruit and seed dispersal
- Fleshy fruits are eaten by animals.
- Animals are attracted to the fruits by the bright colour, scent or the fact that it is edible.
- The seeds pass through the digestive tract undamaged and are passed out with faeces. E.g. tomatoes and guavas.
- Such seeds have hard, resistant seed coats.
- Others have fruits with hooks or spines that stick on animal fur or on clothes.
- Later the seeds are brushed of or fall off on their own e.g. Bidens pilosa (Black jack).
- Fruits and seeds are small and light in order to be carried by air currents.
- A fruit that is a capsule e.g. tobacco split or has pores at the top e.g. Mexican poppy.
- The capsule is attached to along stalk when swayed by wind the seeds are released and scattered.
- Some seeds have hairy or feather-like structures which increase their surface area so that they can be blown off by the wind e.g. Sonchus.
- Others have wing-like structures e.g. Jacaranda and Nandi Flame.
- These extensions increase the surface area of fruits and seeds such that they are carried by the wind.
- Fruits like coconut have fibrous mescocarp which is spongy to trap air, the trapped air make the fruit light and buoyant to float on water.
- Plants like water lily produce seeds whose seed coats trap air bubbles.
- The air bubbles make the seeds float on water and are carried away.
- The pericarp and seed coat are waterproof.
Self dispersal (explosive) Mechanism
- This is seen in pods like bean and pea.
- Pressure inside the pod forces it to open along lines of weakness throwing seeds away from parent plant.
Reproduction in Animals
- Sexual reproduction involves the fusion of gametes.
- In animals two individuals are involved, a male and a female.
- Special organs known as gonads produce gametes.
- In males testes produce sperms while in females ovaries produce ova.
- The fusion of male gamete and female gamete to form a zygote is called fertilisation.
There are two types of fertilisation. External and internal.
- Example in amphibians takes place in water.
- The male mounts the female and shed sperms on the eggs as they are laid.
- Eggs are covered by slippery jelly-like substance which provides protection.
- Many eggs are released to increase the chances of survival.
- This occurs in reptiles, birds and mammals.
- Fertilisation occurs within the body of the female.
- Fewer eggs are produced because there are higher chances of fertilisation since sperms are released into the female body.
Reproduction in Humans
Structure of female reproduction system
The female reproduction system consist of the following:
- Are two oval cream coloured structures found in lower abdomen below the kidneys.
- They produce the ova.
- Are tubes which conduct the ova produced by the ovaries to the uterus.
- Fertilisation occurs in the upper part of the oviduct.
- They are also known as fallopian tubes
- The uterus is a hollow muscular organ found in the lower abdomen.
- The embryo develops inside the uterus.
- The inner lining endometrium supplies nutrients to embryo.
- The embryo is implanted into the inner uterine wall- the endometrium which nourishes the embryo.
- The thick muscles of the uterus assist in parturition.
- Has a ring of muscles that separates the uterus from the vagina.
- It forms the opening to the uterus
- Is a tube that opens to the outside and it acts as the copulatory and birth canal through the vulva.
Structure of male reproductive system
The male reproductive system consists of the following:
- Each testis is a mass of numerous coiled tubes called semniferous tubules.
- Each is enclosed within a scrotal sac that suspends them between the thighs.
- This ensures that sperms are maintained at a temperature lower than that of the main body.
- The lining of seminiferous tubules consists of actively dividing cells which give rise to sperms.
- Between the seminiferous tubules are interstitial cells which produce the male hormones called androgens e.g. testosterone.
- The seminiferous tubules unite to form the epididymis, which is a coiled tube where sperms are stored temporarily .
- Vas deferens (sperm duct) is the tube through which sperms are carried from testis to urethra.
- Seminal vesicle produces an alkaline secretion which nourishes the spermatozoa.
- Produces an alkaline secretion to neutralise vaginal fluids.
- Secretes an alkaline fluid.
- All these fluids together with spermatozoa form semen.
- Is a long tube through which the semen is conducted during copulation.
- It also removes urine from the bladder.
- Is an intro-mittent organ which is inserted into the vagina during copulation.
Fertilisation in Animals
- Fertilisation is preceded by copulation in which the erect penis is inserted into the vagina.
- This leads to ejaculation of semen.
- The sperms swim through the female's genital tract to the upper part of the oviduct.
- The head of the sperm penetrates the egg after the acrosome_ releases lytic enzymes to dissolve the egg membrane.
- The tail is left behind.
- Sperm nucleus fuses with that of the ovum and a zygote is formed.
- A fertilisation membrane forms around the zygote which prevents other sperms from penetrating the zygote.
- After fertilisation the zygote begins to divide mitoticaly as it moves towards the uterus.
- It becomes embedded in the wall of the uterus a process called implantation.
- By this time the zygote is a hollow ball of cells called blastocyst or embryo.
- In the uterus the embryo develops villi which project into uterus for nourishment later the villi and endometrium develop into placenta.
- Embryonic membranes develop around the embryo.
- The outermost membrane is the chorion which forms the finger-like projections (chorionic villi) which supply nutrients to the embryo.
- The amnion surrounds the embryo forming a fluid filled cavity within which the embryo lies.
- Amniotic cavity is filled with amniotic fluid.
- This fluid acts as a shock absorber and protects the foetus against mechanical injury.
- It also regutates temperature.
- The chorionic villi, allantois together with the endometrium from the placenta.
- The embryo is attached to the placenta by a tube called umbilical cord which has umbilical vein and artery.
- The maternal blood in the placenta flows in the spaces lacuna and surrounds capillaries from umbilical vein and artery.
- The umbilical cord increase in length as the embryo develops.
Role of placenta
- Maternal blood and foetal blood do not mix.
- This ensures that the pathogens and toxins from maternal blood do not reach the foetus.
- The placenta allows maternal antibodies to pass into the foetus, providing the foetus with immunity.
- The placenta facilitates the transfer of nutrients from maternal blood to foetus.