How do organisms reproduce?

Introduction

Reproduction is basically creating others of one's own kind. It's not required to maintain the life of an individual organism, but it is required to keep the species alive.

At the most basic level, reproduction involves the copying of one organism's DNA i.e DeoxyriboNucleic Acid, they are just some molecules that have instructions to build up an organism. The DNA molecules are embedded in structures called chromosomes which are present in the nucleus of the cell.

Types of Reproduction

1 Asexual Reproduction

This is the kind of Reproduction in which only one parent is involved. The DNA of the parent is passed on to the offspring without any change in it.

Let's look at some examples.

1.1 Fission

This is the type of asexual reproduction in which an organism simply splits into its offsprings.

It's usually seen in unicellular/single-celled organisms.

Binary Fission

In binary fission, the parent organism divides into two parts through mitosis.

(Mitosis is the type of cell division in which the chromosome number in the offspring is the same as in the parent, this is the kind of division that happens to our body cells when we grow)

Eg:- Amoeba, Leishmania (kala-azar causing pathogen)

Multiple Fission

In multiple fission, the parent cell divides into numerous daughter cells.

Eg:- Plasmodium (This is the pathogen that causes malaria).

1.2 Fragmentation

This kind of reproduction is seen in algae like organisms, or some other multicellular organisms with relatively simple body organisation without any tissue or organ specialization.

Eg:- Spirogyra

Spirogyra (aka water silk) is an algae found in freshwater sources, they grow into long strands of cells which break apart by themselves or by collision with objects. The pieces that get separated grow into new individuals.

1.3 Regeneration

This kind of reproduction is seen in multicellular organisms like hydra and planaria. When they get cut into segments by some accidents, the segments grow into new individuals. Don't confuse this with fragmentation, in fragmentation, the organisms are not specialized into organs. In regeneration the organisms are specialized. This is not exactly reproduction, because this requires the organism to be cut or differentiated by an external force.

1.4 Budding

When a hydra gets mature, small buds start to grow from it's main body, these buds grow and separate from the parent and develop into a new individual, this process is called budding. This can also be seen in unicellular yeast.

1.5 Vegetative Propagation

This is the kind of reproduction that occurs when Vegetative parts of a plant ( parts other than flowers) like stems, roots, leafs etc. develop into new plants. This can be used for plants who have lost their capacity to form seeds, or if we want to replicate a plant with desirable characters. Eg: Rose, Jasmine etc.

In Bryophyllum buds appear on the leaves which fall on the ground to produce new plants.

1.6 Spore Formation

Some simple multicellular organisms like fungi (eg:- Rhizopus) use the method of spore formation to reproduce. They have sacs called sporangium which contain spores. WHen the sporangium matures, it breaks and spores travel through air. These spores are well protected by a thick wall. When the spores enter a favourable condition, they break and develop into a new individual.

2 Sexual Reproduction

In this type of reproduction, two parents will be involved in the process of reproduction. During this type of reproduction, the DNA from both the parents get fused to develop the offspring's DNA.

2.1 Germ cells

Now here comes a problem, if we fuse the DNA from both parents, won't there be double the number of DNA present in the offspring?

No, the DNA that merges is not from just another cell on the body, it's from germ-cells of parents. Germ-cells are the cells specialized to perform reproduction. They only have half the number of chromosomes (haploid n). A human somatic cell has 46 chromosomes (diploid 2n), but a germ-cell only has 23. So 23 from one parent, 23 from the other, this makes up the offspring.

2.2 Meiosis

This is the kind of cell division that happens to germ cells. The germ mother cell contains diploid (2n) cells which undergo meiosis to form the germ cells(n). In meiosis the chromosome number gets halved to produce the daughter cells.

2.3 Variation

Variation is the small tweaks that happen to the DNA of an organism during the process of reproduction. This happens because of the imperfections of the DNA copying process. Most often the variations are desirable and help the organism to thrive, if that's the case then the organism with the variation will pass on its character and slowly make way to "evolution".

2.4 Reproduction in Flowering Plants

In some cases reproduction implants can occur through its vegetative parts, Vegetative propagation - but in vegetative propagation, only one parent is involved and the offspring is the exact copy of the parent. But in angiosperms (flowering plants) the reproductive parts i.e flowers (usually) can perform sexual reproduction.

The petals and sepals are not really required to perform the reproduction technically, but it helps in pollination by attracting insects or birds or whoever is interested (coughs {george mendel}). We will look at pollination later.

There are male and female reproductive parts in a flower, they are stamen and pistil respectively.

The stamen consists of anther and filament, the anther contains pollen sacs which contain pollen grains, they are the male gametes of a flower. The filament just helps the anther to stay up.

The pistil consists of a stigma, style and an ovary. The stigma is the pollen receiving part of a flower, the style is the part through which the pollen enters the ovary. The ovary contains the ovule, which contains the egg i.e the female gamete.

Pollination: This is the process of transfer of pollen grains from the anther of a flower to the stigma.

Pollination can happen through many means like - air, water, insects, mammals etc.

Self pollination - When the pollen from a plant is transferred to the stigma of the same flower or of the same plant.

Cross pollination - When two plants are involved in pollination.

Unisexual flowers : Contain either stamen or pistil, not both. Eg:- papaya, watermelon

Bisexual flowers : Contain both stamen and pistil on the same flower, this makes self pollination possible in the same flower. Eg:- Hibiscus, mustard.

When the pollen lands on the stigma, a tube grows from the pollen towards the ovary, then the tube enters the ovule and fertilisation occurs. The fertilised egg is called zygote/ The zygote undergoes mitosis and the ovule turns into a seed and the ovary turns into a fruit.

2.5 Reproduction in humans

By the age of 10-12 in girls and 12 - 14 in boys, the reproductive organs mature and become ready for reproduction. This age is known as puberty. Production of gametes begins (gametogenesis).

The formation of male gametes takes place in testes, the testes are situated outside in the scrotum, this is because it requires 2- 5℃ less than the body temp. to perform gametogenesis.

Testes also helps in the production of testosterone, the male sex hormone, which bring the secondary sexual characters to a male body.

The sperm mainly contains genetic material in it's head and a tail to help it move towards the ovum, the female gamete. There are other glands called the prostate gland, seminal vesicles and bulbourethral glands that secrete substances to help the sperm on it's way.

Sperm lifespan - 72 hours.

The female germ cells are formed in ovaries ( by ovulation) , which also secrete female sex hormones.When a female is born, the ovaries already contain immature partially developed eggs, they start to mature only when they reach puberty, this stage is called menarche. The sperm fertilises the egg in the oviduct/fallopian tube to form a zygote. The zygote undergoes mitosis several times and it gets attached to the wall of the uterus, this is called implantation. The zygote develops into an embryo in the uterus. The embryo receives nutrition from the mothers blood through placenta, a tissue between the mother and the baby. In humans the embryo takes nine months to fully form.

Ovum lifespan - 1 day

What if the egg is not fertilised?

Every month the uterus prepares itself for fertilisation by developing a blood-rich lining along the uterus . If fertilisation does not happen, the lining breaks and is released through the vagine, this is called menstruation.

2.6 Contraception

Contraception is the prevention of fertilisation through physical barriers (such as condoms, copper - T etc. or surgically, cutting the vas deferens or the oviduct or by the use of pills that can cause hormonal inhibition and thus preventing pregnancy.