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TOPIC 6: REPRODUCTION
Reproduction is the ability of living organisms to form new individuals of the same species from those already in existence. Here, the new organisms replace those that have died and so life continues. It can also be defined as the process whereby organisms produce new individuals of the same species. It is one of the important features of living things.
The Merits and Demerits of Sexual and Asexual Reproduction
Explain the merits and demerits of sexual and asexual reproduction
Sexual Reproduction
This is a type of reproduction in which new organism is produced when a male gamete fuses with a female gamete. Sexual reproduction involves the fusion of two gametes. The process of fusion of gametes is called fertilization.
These two gametes differ in form and function and each is produced from a different organ. In animals the gametes producing organs are called gonads. These include the ovaries and testes. In flowering plant structures concerned with the production of gametes are the ovaries and anthers. The testes and anthers produce the male gametes while ovaries produce the female gametes.
Merits of Sexual Reproduction
- It ensures genetic stability
- It ensures perpetuation of life
- It brings variation
- Leads to the interaction among organisms
Demerits of Sexual Reproduction
- Offspring have a great chance of inheriting diseases from the parent
- The reproduction takes long time
- It needs energy
- The sexual reproduction produces few numbers of offspring
- It depends on presences of two parents
- It leads to great chance of spreading diseases
- It takes a long time until offspring are produced
Asexual Reproduction
This is the type of reproduction whereby production of offspring is from single organism without the production of gametes. There is no fusion of gametes.
Examples of organisms who produce asexually are amoeba and bacteria. Asexual reproduction involves only one individual organism. That means no change of genetic material is passed from a parent to an offspring. The offspring are also identical to their parents.
In this kind of reproduction, in same organisms, body part such as roots, stems or leaves may become reproductive body organs.
Depending on the type of organisms asexual reproduction may be of different forms or ways such as:
- Fission
- Sporulation/Spore formation
- Budding
- Fragmentation
- Vegetative propagation
- Binnary Fission (Splitting)
- Suckers
- Bulbils
Binary Fission (Splitting)
This is an asexual reproduction in which an organism divides into two equal parts, which are identical to each other.
Each part then grows to attain the original size of the parent cell and hence become a separate and independent organism.
Fragmentation
This is a form of asexual reproduction in which organisms (parent) breaks into two or more parts. Fragments grow and develop into a new organism with identical features as the parent. Example worms such as Nematodes and flat worms.
Sporulation/Spore formation
Sporulation is asexual reproduction by the use of spores. The spore develops from single cell as a result of mitosis, forming a structure known as sporangium. When the sporangium is truly developed, the wall bursts to release the spores which when placed in suitable area they germinate into new organisms.
Other organisms, which reproduce by sporulation, are ferns and mosses.
Budding
Budding is a type of asexual reproduction in which a new organism arises as an outgrowth (bud) of the older organism (parent). The bud later separates from the parent and grows to become an independent organism to attain the size of the parent. Examples: yeast and hydra
Some flowering plants reproduce through the formation of structure called buds. Roots may form such buds, leaves or underground stems and such buds sprout to form new independent plants.
Vegetative propagation
Vegetative propagation is a form of asexual reproduction found in plants in which a bud grows and develops into a new plant. The detached plant, root, stem or leaves at some stages grows and develops into an independent plant.
Artificial vegetables propagation: This is vegetative propagation, which occurs through man’s manipulation. Man can learn from plants’ natural vegetative propagation and can intervene and make propagation of plants artificially.
Natural vegetative propagation: Vegetative propagation involves different parts of plants as discussed below. Such as:
Bulb
In this type each bud grows to form a shoot, which produces a new bulb at the end of the growing season.
Bulb is a modified underground shoot having fresh strong leaves
Tubers
These are short swollen underground storage organs formed from a stem or a root. New tubers are made at the end of the growing season but do not arise from old tubers.
Stem tubers
These are short swollen underground stems, which store food, such as starch. Normally, yam plants form a number of tubers each of which can rise to a new plant. Such new plant continues to live after the death of the parent.
Root tubers
These are swollen adventitious underground roots. Roots tubers such as sweet potatoes and cassava store their food in root tubers and do not bear leaves or bud.
Rhizomes
These are horizontally growing underground stems, bearing leaves, buds and adventitious roots. Examples are lilies, ferns and grass.
Stolons
These are slender stems, creeping horizontally as they grow along the ground surface. Examples: strawberries, black currant and oxalis.
Suckers
These are short horizontal branches, arising from the main stem or just below ground level. Suckers contain food reserves. Examples: Bananas, sisal and pineapples
Tap root
A taproot is the main root that arises from a radical. Tap roots may become swollen and act as storage organs.
Tillers
These are collection of shoots. Grass plants consist of a number of tillers. Each tiller has a number of leaves, which arise from the stem of nodes at the base of the leaves.
Leaves
Some plants such as cactus propagate vegetative using leaves. When the leaves fall off from a plant they develop adventitious roots and buds, which later may grow and develop into mature plants.
Merits of Asexual Reproduction
- Asexual reproduction results into an individual with the same genetic constitution as their parent.
- Its offspring matures faster than sexually reproduced organisms
- It does not depend on processes of pollination, seed or fruit dispersal
Demerits of Asexual Reproduction
- Asexually reproducing organisms are at a great risk to perish or get destructed when environmental conditions are unfavorable
- The parents may pass undesirable characteristics to the offspring since only one individual organism is involved in asexual reproduction
- Competition for resources such as food and shelter may occur due to large number of organisms being produced
The Meaning of Meiosis
Give the meaning of meiosis
Reproduction involves the transmission of genetic materials from one generation to the next insuring that species survive. The process of reproduction involves meiosis.
Meiosis is the type of cell division, which occurs in the reproductive organs to produce sex cells known as gametes.
In this type of cell division the parent cells has diploid number of chromosomes. However the daughter cell arising from the cell division has the half number of chromosomes a condition known as haploid state.
Therefore meiosis reduces the chromosomes number into half means from 2n to n.
The Significance of Meiosis in Relation to Reproduction
Explain the significance of meiosis in relation to reproduction
Meiosis leads to the formation of reproductive cells (Gametes) such as ova and sperms each with half number of chromosomes of the parent cells in organisms reproducing through sexual reproduction.
It involves the possibility of exchange of pieces of genetic information between the paternal and maternal chromosomes of each pair leading to new combination of characteristics in the gametes.
It brings about variation when the members of each pair of chromosomes are separated from each other independently (Random assortment)
Meiosis leads to new combination of genes through the process of independent assortment of chromosomes occurring during meiosis I
Meiosis involves number of processes from prophase, metaphase, anaphase and telephase. There are two meiotic divisions, the first and the second division. All the above named processes occur in both the first meiotic division and the second meiotic division.
First Meiotic Division
The first division of meiosis consists of:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
Telophase I
Prophase I
During prophase I the following events occur.
- The nucleus disappears and the centrioles if present migrate to the poles
- Chromosomes condenses and the spindle is formed
- As prophase proceed homologous chromosomes come to lie side by side and there after become intertwined by the process called synapsis
Through synapsis homologous chromosomes is referred to as crossing over. The point at which homologous chromosomes exchange genetic materials is known as chiasmata
Early Prophase I
The following events occur during early prophase:
- Chromosomes contract, thickening, shortening and become more visible
- Nucleus disintegrate and disappear
Mid Prophase I
- Homologous chromosomes come together (synapses) forming a bivalent
Late Prophase I
- Chromatids cross over by chiasmata which results into exchange of genetic materials
Metaphase I
- Bivalent homologous chromosomes moves to the equator of the spindle
Anaphase I
- The two homologous chromosomes part company and migrate to opposite poles of the spindle
- The centromeres of the homologous pairs migrate towards the opposite poles where they are attracted.
Telophase
- The chromosomes reach their destination
- The spindle apparatuses breaks down and disintegrates
- Then the nucleus membrane reforms around each set of chromosomes
- The cell constricts across the membrane and divides into two
Telephase I make the end of the first meiotic division.
- At the end of this prophase the number of chromosomes in each cell is half the chromosomes number cell.
- This is the reason why first meiotic division is referred to as Reduction division
Second meiotic division
Prophase II
Centrioles replicates and a new spindle apparatuses formed
Metaphase II
Chromosomes migrate to the equator f the spindle
Anaphase II
- Sister chromatids part company and migrate to opposite poles of the cell
Telophase II
- The spindle apparatuses disappears
- The nucleus repairs and a new nucleus membrane is formed around each set of chromatids
- The chromatids uncoil and the cell divides into two
- Chromosomes regain their thread like structure and the cell enter interphase. Meiosis results into the formation of four daughter cells each with haploid set of chromosomes
- It should be stressed that the four daughter cells formed has the half number of chromosomes present in the original parent cell.
Experiments to Show Stages of Meiosis Process
Carry out experiments to show stages of meiosis process
Meiosis Vedeo
Some plants use flowers to reproduce. The flowers contain all the parts needed for the reproduction process. Sexual reproduction in flowering plants takes place in the flower. Within a flower, there are usually structures that produce both male gametes and female gametes.
The Structure of the Flower
Describe the structure of the flower
A flowering plant is an angiosperm, which is any plant that produces a flower or fruit. The sole purpose of the flower is to allow the plant to reproduce. Each part of the flower plays a role in the steps of reproduction. There are male gametes and female gametes. They are both directly involved with sexual reproduction. Pollen may spread from plant to plant but can only reproduce with the same species of plants. Let's take a look at the different parts of the flower.
Structure of a flower
STERILE PARTS
Let's start with the sterile parts of the flower, or those parts that are neither male nor female. These sterile parts are not directly involved in the fertilization process. Technically, a flowering plant can reproduce without them; however, they do play important roles in helping with reproduction.
- Pedicel:The pedicel is a small stalk or stalk-like part bearing a single flower in an inflorescence. The Some flowers have no stalk and are directly attached to the stem of the plant.
- Receptacle:The receptacle is the place on the stem where floral organs originate and attach.It is the axis (stem) to which the floral organs are attached. In most angiosperms, floral organs are attached in alternating successive whorls. A whorl is an arrangement of sepals, petals, leaves, stipules or branches that radiate from a single point and surround or wrap around the stem.
- Sepals:Sepals are the parts that look like little leaves that cover the flower before it blooms. They cup the flower to protect it while it grows. Sepals are mostly green and in most flowers they resemble leaves. A flower bud is protected by sepals. Collectively, all of the sepals form the calyx.Sepals may be hairy, for example in roses, or smooth, for example in hibiscus.Some flowers have sepal-like structures beneath the calyx known as epicalyx, for example the hibiscus flower.
- Petals:Petals are the delicate and usually brightly coloured part that gives the flower its character. Flowers have more than one petal, and the flower petals are collectively called the corolla. The colour and scent attracts the agents of pollination such as birds and insects. Fused petals form a corolla that can be tubular or funnel-shaped as in the flowers of sweet potato and pumpkin plants. Some of the petals of leguminous plants are keel-like and they enclose stamens and carpels.Sepals and petals are collectively referred to as the perianth.
Reproductive Parts of the Flower
Identify reproductive parts of the flower
The flower is the reproductive unit of some plants (angiosperms). Parts of the flower include petals, sepals, one or more carpels (the female reproductive organs), and stamens (the male reproductive organs).
The female reproductive organs
The pistil is the collective term for the carpel(s). A carpel is actually three parts fused into one: stigma, style, and ovary. The stigma is at the top of the flower. It is sticky to catch the pollen. Each carpel includes an ovary (where the ovules are produced; ovules are the female reproductive cells, the eggs), a style (a tube on top of the ovary), and a stigma (which receives the pollen during fertilization).
The ovary is at the base of the flower. From the ovary, extends a tubular structure called the style and on the top of the style is a surface receptive to pollen called the stigma. The stigma can take many different forms, most of them designed to help trap pollen.
Female parts of a flower
The female parts of the flower are discussed in detail below:
- Ovary:The ovary contains ovules. Ovules are unfertilized female gametes. The position of the ovary in relation to the receptacle varies from one flower to another. The ovary could be either superior or inferior in relation to the position of the receptacle. A superior ovary is one that is positioned above the receptacle, for example the ovary of a bean flower. An inferior ovary is one that is positioned below or enclosed within the receptacle. Examples are the ovaries of a rose and a black jack flower.
- Style:The style is a long tube that attaches the stigma to the ovary. The length of the style varies from one flower to another. This length determines the position of the stigma. In the maize plants, the style and the stigma hand outside the flower.
- Stigma:The stigma is a glandular sticky structure at the tip of the carpel. The stigma is the tissue into which the pollen grains are deposited. The branches of the stigma correspond to the number of carpels. Five branches of the stigma indicate the presence of five carpels.
The male reproductive organs
The male parts of a flower consist of one or more stamens. Stamens are the male reproductive parts of flowers. A stamen consists of an anther (which produces pollen) and a filament. The pollen consists of the male reproductive cells; they fertilize ovules.
Male parts of a flower
The following is a detailed description of the female parts of the flower.
- Filament:The filament is a slender stalk that supports the anther. The filament may arise from the receptacle or the petals. It supports the anther.
- Anther:The anther is attached to the apex of the filament. It produces the pollen grains that contain the sperm needed for fertilization.
Types of flowers
Flowers are classified either as complete or incomplete. Complete flowers have sepals, petals stamens and carpels. A flower with both stamens and carpel is said to be bisexual. The hibiscus flower is bisexual.
Incomplete flowers lack some of the floral parts. Flowers that have carpels only are referred to as pistillate flowers. Flowers that have stamens only are referred to as staminate flowers.
The Term Pollination
Explain the term pollination
Pollination is the transfer of pollen grains from the anthers to the stigma of a flower. Pollen grains from the anthers are carried to the stigma by wind, insects or birds. These are referred to as the agents of pollination.Pollination, an important step in the reproduction of seed plants, is the transfer of pollen grains (male gametes) from the male reproductive organ to the female reproductive organ that contains the ovule (female gamete) or transfers it to the ovule itself.
Pollination is a very important part of the life cycle of a flowering plant. It is part of the sexual reproduction process of flowering plants, which results in seeds that will grow into new plants.
Types of Pollination
Identify types of pollination
Pollination can either be self or cross pollination:
- Self pollination:This is the transfer of pollen grains from the anthers to the stigma of the same flower.
- Cross pollination:This is the transfer of pollen grains from the anthers of the flower to the stigma of another flower of the same species.
Disadvantages of self pollination:A potential drawback is that both gametes come from the same parent. If the plant is well adapted to a stable environment, the production of uniform offspring may be advantageous. However, inbreeding will result and if there are disadvantageous recessive characteristics in the parent, they are much more likely to be exposed than if the plant cross-pollinates.
Advantages of cross pollination:Cross-pollination is less reliable and more wasteful than self-pollination, but it is genetically favourable because genes are transferred and variation increases
Factors that favour cross pollination and hinder self pollination
- Dioecious plants: Some plants have flowers that are only male - they have onlystamen. Other plants of the same species have flowers that are only female - they have only carpels.
- Monoecious plants: Some flowers on a plant are only male; other flowers on the same plant are only female. So, self pollination is avoided by a difference in the timing of their development.
- Protandry: Anthers on some plants mature first. Pollination of immature stigma on the same plant is therefore not possible.
- Protogyny: The stigmas mature first.
- Self-incompatibility: Pollination can occur but the pollen tube doesn't grow well, if at all, so no fertilisation takes place.
Agents of Pollination
Outline agents of pollination
These are organisms or physical conditions that facilitate transfer of pollen grains from the anthers to stigmas. Plants, being immobile, normally require agents for the transport of pollen, which are commonly wind, insects, birds, mammals (bats, rodents, primates), and water. Insects are the most common animals that will pollinate a carpel. The main agents of pollination are wind, insects and birds.
Wind pollinated flowers
Wind-pollinated flowers are also those flowers whose pollen is transferred by wind from anthers to stigmas. Wind picks pollen grains from the anthers and transfers them to the stigma.
Structure of a wind-pollinated flower:The anthers and stigma of wind-pollinated flowers are exposed. This makes sit easy for wind to blow the pollen that can then easily land on the stigma. Flowers of grasses are a representative of wind pollinated flowers.
Adaptations of wind-pollinated flowers
Flowers that depend on wind for pollination are adapted in various ways. Plants that are pollinated by wind have the following characteristics:
- Small petals with exposed anthers and stigma. In some plants the petals are often absent or have dull-coloured petals that do not attract insects or birds. The petals and sepals are very similar in shape and size.
- Large anthers which produce large amounts of pollen grains. Very large quantities of pollen are produced to increase chances of pollination as much will be lost while blown about.
- Anthers are loosely attached to the filaments and hang freely to allow the anthers to be easily shaken by the wind.
- The pollen grains are small, smooth, dry and light in weight and therefore easily carried in the air by wind. Some pollen grains have bladder-like structures that contain air, thus, increasing their buoyancy.
- Feathery stigmas with a large sticky surface so they are more likely to catch pollen from the air.
- Large and feathery stigmas, freely hanging out of the flowers, which provide a large surface area on which the pollen grains can land. The stigma may be branched or hairy to increase the surface area.
- Long, hairy style to expose the stigma outside the flower.
- No nectar produced because they is no need to attract pollinators to the flower.
- Not scented as the flowers do not attract insects.
- Filaments grow long so stamens hang out of the flower and shake in the wind to disperse pollen.
Wind-pollinated flower
Insect-pollinated flowers (bees, moths, butterflies)
Insect-pollinated flowers are also referred to as entomophilous flower. The term entomophilous is derived from the word entomophily- which means to be carried by insects. Features of insect pollinated flowers include the following:
- Large, brightly-coloured petals which attract insects. Flower structure may be adapted for one particular of insect, allowing them to land and feed.
- Usually scented; therefore they attract insects to the flower.
- Nectarines which contain nectar, for example, mango flowers have nectarines from which bees collect nectar for making honey, and while doing so transfer the pollen. The insects are guided to the nectarines by the nectar guides.
- Sticky stigma that insects come into contact with and deposit pollen while collecting nectar. The pollen grains picked by insects from other flowers stick onto the stigma.
- The stigma and anthers are held firmly in position within the flower. This ensures that when an insect lands on a flower, the stigma is not broken. The stigma and anthers are located inside the flower where pollinators are more likely to pick up pollen.
- The anthers are small in size and produce few but large pollen grains.
- The pollen grains are fairly large, heavy, sticky and with small spines. This enables them to adhere to bodies of pollinators (insects or birds).
A bee collects pollen on its body as it feeds
Flower pollinated by birds:Birds pollinate flowers when they search for nectar. Birds such as sunbirds have long slender and slightly curved beaks that they use to probe into the flower. Pollen grains stick on the beak. The pollen grains are deposited on the stigma of another flower of the same species by the bird as the bird feeds.
A bird feeding on nectar
The Concept of Fertilization
Explain the concept of fertilization
Fertilisation is the union of the male and female gametes to form a zygote. Pollen must fertilise an ovule to produce a viable seed. Fertilisation starts when a pollen grain lands on the stigma. Only after pollination, when pollen has landed on the stigma of a suitable flower of the same species, can a chain of events happen that ends in the making of seeds.
After a male's pollen grains have landed on the stigma during fertilization, pollen tubes develop within the style, burrowing down to the ovary, where the sperm fertilizes an ovum (an egg cell), in the ovule. A pollen tube emerges from the grain, its growth being controlled by the tube nucleus at the tip of the tube. It may grow downwards in response to chemicals made by the ovary (a response known as chemotropism).
In a process called fertilisation, the two gametes join and their chromosomes combine, so that the fertilised cell contains a normal complement of chromosomes, with some from each parent flower. The fertilised ovule goes on to form a seed, which contains a food store and an embryo that will later grow into a new plant. The ovary develops into a fruit to protect the seed. Some flowers, such as avocados, only have one ovule in their ovary, so their fruit only has one seed. Many flowers have lots of ovules in their ovary, so their fruit contains many seeds.
There are 2 types of seeds. Some are endospermic while others are non-endospermic. In endospermic seeds the food reserve is the endosperm, which is outside the plant embryo. Examples of this type of seed are maize and wheat. Non-endospermic seeds have food reserve within the cotyledon(s) of the plant embryo. This occurs in broad beans.
Process of Fertilization in Flowering Plants
Explain process of fertilization in flowering plants
During the growth and extension of the tube, the generative nucleus, behind the tube nucleus, divides by mitosis to produce 2 male haploid sperm nuclei. The pollen tube enters the ovule through the micropyle and penetrates the embryo sac wall. Then, the tip of the tube bursts open, the tube nucleus disintegrates, creating a passage for the male nuclei and what follows is called double fertilisation
- 1 male gamete fuses with the egg cell to produce a diploid zygote which undergoes mitosis to form a diploid embryo.
- 1 male gamete fuses with both the polar nuclei to produce the triploid primary endosperm nucleus. The triploid nucleus undergoes mitosis to form the endosperm. The endosperm stores food materials that the embryo utilizes for growth and during germination.
The process of fertilisation
Immediately after fertilization the ovule becomes the seed and the ovary becomes the fruit.
This is what happens:
- The zygote divides many times by mitosis to produce an embryo. It differentiates to become a plumule (young shoot), radicle (young root) and either 1 or 2 cotyledons (seed leaves). It is attached to the wall of the embryo sac by a suspensor.
- The primary endosperm nucleus divides many times by mitosis to produce endosperm tissue. In some seeds this endosperm is a food store for later use by the seed. In others it may gradually disappear as the cotyledons develop.
- To accommodate all this growth the embryo sac expands and the nucellus is crushed out of existence, giving its nutrients to the embryo and endosperm.
- The integuments surrounding the embryo sac become the tough and protectivetesta (seed coat). The micropyle remains though so that oxygen and water can be taken in during seed germination.
- The water content of the seed decreases drastically so the seed is prepared for dormancy.
- The ovary wall becomes the pericarp- the fruit wall, the whole ovary now being the fruit. The function of the fruit is to protect the seeds and to aid in their dispersal, e.g. by an animal. That is why they can be brightly coloured and sweet; animals will eat them and scatter the seeds either at the time of eating or when they are passed out of the gut in defecation, unharmed.
A detailed account of what happens after fertilisation is given below:
- Formation of the testa: The testa is also referred to as the seed coat. The two integuments of the embryo sac fuse to form one seed coat. The seed coat thickens and hardens. Sometimes the outer integument forms the hard thick testa while the inner integument remains thin and transparent. The thin transparent inner integument is referred to as the tegmen. The testa protects the seed against dehydration, physical damage and invasion by microorganisms. The microphyle permits oxygen and water to enter the seed during germination. The hilum is a scar on the testa that marks the point of attachment to the fruit.
- The formation of the embryo:The egg cell nucleus fuses with the male nucleus to form a zygote. The zygote undergoes mitosis to form the embryo. An embryo is a rudimentary plant comprising the plumule, radicle and cotyledons.
- Formation of the pericarp: The ovary wall becomes thick and swells up with food substances. The pericarp develops from the ovary wall. The pericarp is often the edible layer in fruits. In mature fruit it may dry up or remain fleshy.
- Disintegration of the floral parts:The sepals, petals, stigma and style wither, dry up and fall off. In some cases, some of the floral parts may become fleshy and form part of the fruit. The fruit retains scars at the points of attachment to the pedicel and style.
The Male and Female Reproductive Systems
Describe the male and female reproductive systems
Female Reproductive System
In humans, like animals, female reproductive system is composed of:
- Ovaries: Ovary is situated near each kidney. Ovary produces ova, estrogen and progesterone as female sex hormones.
- Fallopian tube: It is also known as egg tube/oviduct, it is a funnel shaped opening. Fertilization normally takes place within this tube.
- Uterus:The two fallopian tubes unite to form an expanded tubular organ called uterus womb. It is there that fertilized ova implant and develop into an embryo. N.B. Placenta is formed as an embryo develops, so as to allow penetration of nutrients, gases (oxygen and carbon dioxide) and waste products of metabolism An embryo receives nutrient and oxygen gases from maternal blood circulation and give out waste product through placenta into maternal blood system.
- Vagina: This is the posterior part of the female reproductive duct connecting the uterus with the exterior. It is in this region that sperms are deposited. N.B. Placenta with an endocrine at the last period of pregnancy, it secretes progesterone like ovaries, which prevent production of ova and contractions of the uterine walls.
Male Reproductive System
The male mammal reproductive system is composed of:
- Testicles: These are situated in a pouch of skin called scrotum. They produce sperms as endocrine gland it produces secretes male sex hormone called testosterone. Testosterone influences male secondary characteristics during puberty in males.
- Sperm duct: It Is attached to each testicle, it acts as a temporary store for sperms. Each epididymis leads into a duct called sperm duct or vas di deferens.
- Vas deferens: Is the structure, which carries sperms away from the epididymis to penis
- Seminal vesicles and prostate glands:These are situated just below the urinary bladder and two structures called seminal vesicles. Each seminal vesicle has a tube which leads to the sperm duct. Around the junction of urinary bladder and urethra is a gland called prostate gland
The Process of Gamete Formation in Mammals
Outline the process of gamete formation in mammals
Gamete formation: Puberty
On average, female attain puberty (Sexual Maturity) when they are 14 years old and male when they are 16 years old. In both sexes, attainment of puberty is accompanied by certain behavioral changes as well as development of certain structures.
Those changes are known as secondary sexual characteristics.
NB: Puberty can be defined as the period when male/female changes from childhood to adulthood. These changes are influenced by sex hormones that are testosterone in males, progesterone and estrogen in females.
Secondary characteristics in males:
- Hairs on their chins and pubic region
- Shoulders widen
- Voice deepens
- Pay more attention to female sex
Secondary characteristics in females:
- Enlargement of the mammary gland and hips
- Deposition of fat which gives them more round appearance
- Development of pubic hair
- Menstruation cycle
- Pay more attention to males (young men)
NB: At puberty sex organs become fully functional in males and females.
Gamete formation:During puberty stage is when both males and females are able to produce fertile cells which united (of male and female) can cause pregnancy.
It is believed that female gametes are produced before puberty but after puberty is when they are fully matured and fertile while male gamete at large are produced during and after puberty that is when can cause pregnancy when united with female gamete.
The Processes of Ovulation and Menstruation
Explain the processes of ovulation and menstruation
Ovulation: It refers to the release of ova/ovum from ovaries to the uterus. It is expected to occur at the middle of the menstruation cycle.
Menstruation Cycle: This can be defined as the period between one and next menstrual cycle (28 days)
Menstruation: This is the discharge of mucus, epithelial cells and blood through vagina (3-5 days)
NB: The menstruation and liberation of ova every 28 days alternates between the two ovaries whereby in humans is called menstrual cycle and in non-human mammals is called oestrus cycle.
The Process of Fertilization Pregnancy and Child Birth
Explain the process of fertilization pregnancy and child birth
The Menstrual Cycle
At the beginning of the cycle, the menstruation can take place between 3-5 or 7th day from the first day. At this period the secretion of female sex hormones are very low but increase after menstruation stops.
After menstruation oestrogen secretion increases, follicle stimulating hormone secreted to encourage production of ovarian follicle.
Luteinizing hormone is for maturation and ovulation of follicle while oestrogen prepares (thickening) the uterine for implantation when fertilization takes place.
At the 14th (middle) day is when ovulation can take place and secretion of progesterone increases so as to thicken the uterine wall ready for pregnancy.
NB: If no fertilization occurs, the cycle starts again.
Placenta:This is the portion of uterus, which is invaded by the villi and the thickened portion of the chorion. The chorion like amnion is a thin membrane but it has a thick portion called villi, finger-like projection.
Umbilical Cord
- It connects the developing embryo to placenta to the maternal blood system
- It carries two arteries and a vein of blood circulation of an embryo
- AN embryo uses umbilical cord for gas exchange, receiving nutrients and removal of waste products via placenta into maternal blood system
Birth
- It starts by a sudden fall in the level of oestrogen and progesterone resulting in periodic contractions of muscular walls of the uterus which cause pains called labour pains
- Under the influence of hormones a child is given out through vagina
Copulation
- When a male is sexually stimulated, the spongy penis is filled with blood and becomes erect. The erect penis is inserted into the vagina and moved back and forth, this movement stimulates sense organs in the penis and ejaculation occurs.
- Ejaculation refers to the release of sperm into the vagina; it can cause pregnancy when the fertile sperm unites with fertile ova.
Fertilization:It can be defined as the process of uniting male and female gametes to form a zygote. The sperms remain alive for up to 48 hours while ova remains alive for up to 36 hours.
Pregnancy:It refers to the situation when a female conceives. That is fertilization takes place to form zygote. Soon after zygote is formed its cell starts to divide into many cells called embryo. It takes 3-5 days for the zygote to reach to uterus for implantation.
Implantation:
- Refers to the process whereby an embryo attaches itself to the uterine wall
- It takes 3 – 5 days to implant fully
- After implantation envelope is formed, the outer chorion, inner amnion
- Between these membranes of envelop there are fluids called chorionic fluids and amniotic fluids respectively, both fluids act as shock absorbers, protecting embryo from physical damage
Factors Which May Hinder Fertilization
Outline factors which may hinder fertilization
A normal couple that is trying to start a family will usually be successful after a few months. However, at least one in ten couples do not conceive after a year or more of trying.
There are many reasons why couples can’t produce children, some of them are:
- Ova are not released in normal monthly cycle
- The fallopian tubes may be blocked/twisted
- The women may make antibodies that destroy the sperm
- The vas deferens may be blocked
- A high proportion of the sperm produced are abnormal
- Very few sperms are produced in one ejaculation
Ways of overcoming these problems
- In – Vitro Fertilization (IVF)
- Fertility Drugs
- Artificial Insemination
The Concept of Artificial Insemination and its Importance
Explain the concept of artificial insemination and its importance
Artificial insemination, also known as AI, is a procedure used to treatinfertilitythat involves direct insertion of semen into a woman's womb. It is a process originally used on livestock that has been adapted for human use. In human use, the sperm could originate from the woman's male partner, unless the male is infertile or there is no male partner (i.e. single woman or woman in same sex relationship).
The most commonly used method of artificial insemination, is IUI (Intrauterine Insemination), as it has the best success rate. Other types of AI are:
- IUTPI (Intrauterine tuboperitoneal insemination
- ICI (Intracervical insemination)
- ITI (Intratubal insemination)
Artificial insemination is beneficial to couples or individuals in many circumstances. For example a couple may be producing healthy sperm and eggs but not necessarily be able to have intercourse (maybe due to a medical condition). Some other scenarios where artificial insemination could be beneficial are listed below.
- A woman may want to raise a child alone - in this case she would request a sperm donor to be artificially inseminated.
- The female may be infertile due tocervical factor infertility- the cervix is supposed to produce a mucus that helps sperm travel to the womb. With cervical factor infertility, the cervix is either not producing enough of this mucus, or it is producing mucus containing sperm killing substances.
- The woman may be sufferingEndometriosis- this is when cells from the womb lining start to grow in places they should not within the woman's reproductive system e.g. ovaries or fallopian tubes. One of the possible results of this condition is infertility. Artificial insemination can be successful in mild to moderate cases of this.
- The female could have semen allergy - this is rare but can still happen, due to certain proteins in the sperm the woman may suffer an allergic reaction when the sperm makes contact. With IUI most of the proteins would be removed before sperm insertion.
- The male is unable to produce enough sperm for successful fertilization.
- The man is impotent(erectile dysfunction)- and would therefore be unable to perform sexual intercourse.
- The male could be infertile as a result of a medical treatment - some treatments carry the risk of infertility e.g.radiotherapy. Before the treatment the male would have been given the chance to freeze some of his sperm.
- The couple could be in same sex relationship - two women who want to raise a child together would use sperm from a donor, which one of the women would receive via artificial insemination.
- Reason for infertility cannot be determined - IUI may be recommended to a couple that cannot conceive even if no underlying reason for the infertility can be identified.
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