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Reproduction biology 2nd year notes

2nd Year Biology Notes

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Reproduction

The process through which organisms produce young ones of their own kind to maintain their species are called as Reproduction.
Types of Reproduction

There are two types of reproduction.
1. Asexual Reproduction
2. Sexual Reproduction


1. Asexual Reproduction

The type of reproduction in which fusion of gamets does not take place and requires only a single parental organism and the offspring produced are exact copies of their parents. This type of reproduction is called Asexual Reproduction.

Asexual Reproduction of Plants
There are two methods of asexual reproduction in plants.
1. Natural Method of Asexual Reproduction
2. Artificial Method of Asexual Reproduction

1. Natural Method of Asexual Reproduction
In nature, plants reproduce asexually by following methods.
i. By Spores or Sporulation
ii. Vegetative Propagation
iii. Apomixis

i. By Spores or Sporulation
During alternation of generation plant produce haploid cell by meiosis called Spores. Each spore can develop into new organism without fertilization. The process of formation of unicellular spores is called Sporulation.
Example
Sporulation occurs in bacteria, protozoans, algae, fungi, mosses and fern as well as plants.

ii. Vegetative Propagation
The process which involves the separation of the part of the parent plant which then develop into new plant is called as Vegetative Propagation.
OR
When a new plant develops from tissue, organs of a plant or outgrowth of a plant. This type of reproduction is called Vegetative Propagation.
Process
In this process a plants part is separated which develops into new plant such as stem, leaves roots or buds may take part in the formation of new plant.
Methods of Vegetative Propagation
There are various method of propagation of plant by vegetative reproduction for improving crops, orchads and ornamental plants are as follows
i. By Cutting
ii. By Grafting

i. By Cutting
In this method stem or branch is cut from the plant. At the cut end of the shoot a mass of dividing undifferentiated cells called a callus forms and then adventitous roots develop form the callus. If the shoot fragment includes a node, then adventitous root forms without callus stage.
Example
Sugar cane, sweet potato and rose can be propagated by cutting. In raspberry and black berries root cutting are also used for artificial vegetative propagation.

ii. By Grafting
This is a technique whereby a branch from a desired variety of plant is joined to another plant with well established root system. The plant from which the branch is taken is called Scion and the plant to which it is joined is called Stock. The two plants involved are normally the different varieties of same species.
Example
Orange, lime and mango can be propagated by grafting.

iii. Apomixis
The modified form of asexual reproduction in which seeds are formed without fertilization is called Apomixis.
Mechanism
In apomixis, a diploid cell in the ovule gives rise to the embryo without any fertilization and the ovules mature into the seeds.
Example
In Dandelions and other plants seed formation take place without fertilization.

2. Artificial Method of Asexual Reproduction
In plant vegetative reproduction is performed by artificial method, which are as follows
i. Tissue Culture or Test Tube Cloning
ii. Protoplast Fusion Technique

i. Tissue Culture or Test Tube Cloning
Tissue culture or cloning is a special technique which is used to produce varieties of plants. By this technique, a group of genetically identical offspring produced by asexual method called Clones.
Procedure
In this method, pieces of tissues are cut from the parent plant or from a single parenchymatous cell in a medium containing all the nutrients and hormones.
The culture cells divide and form an undifferentiated Callus.
The callus then produces root and shoot with fully differentiated cells.
The test tube plant can be transferred to soil where they continue their growth.
Application
In plants tissue culture is also used in genetic engineering. To introduce new genes in plant body pieces of tissue or cells are used. By this technique, we produced a new variety of plant by introducing new DNA molecule.
Example
By cloning many thousand plants are produced from one plant. This method is used in Orchards and pinus trees to obtain wood.

Advantages of Tissue Culture
The main advantages of tissue culture are as follows
i. Development of Strong Plant: By this technique plants of Agriculture and horticulture are produced. These plants are strong than other plants produced by seeds.
ii. Development of Similar Plant: By this technique plants of similar character are developed.
iii. Development of Defence System in Plant: These plants have developed defence mechanism against any disease.
iv. Production of Useful Chemicals: By this technique, many useful chemicals are obtained such as shikonin (a dye used in silk and in the treatment of injuries caused by burning.

Disadvantages of Tissue Culture
There are also some disadvantages of tissue culture where are as follows
i. Production of Sterile Plant: The plant produced by this technique may be genetically sterile, do not reproduce by sexual method.
ii. Variation in Chromosome: This technique may cause change in the structure and number of chromosome.

ii. Protoplast Fusion Technique
Another technique known as protoplast fusion technique is developed to produce new varieties of plants.
Procedure
In this technique, outer cell wall is removed around the protoplast. After protoplast of one or more cells are fused together, then their protoplast are for culture. These protoplast produce a wall around them, then they are change into new plant. Protoplast of either same or different species may used for this technique.
Example
In potato and wild night shade plant this technique is used.


2. Sexual Reproduction

The type of reproduction in which fusion of gametes (sperm and ova) take place and two parents (male and female) are involved is termed as Sexual Reproduction.

Sexual Reproduction in Plant
In plants sexual reproduction takes place by three methods.
i. Isogamy
ii. Oogamy
iii. Heterogamy

i. Isogamy
The simplest type of sexual reproduction in which two morphologically similar gametes take part in fertilization to produced zygospore which then develop into new plant is called Isogamy.
It is also known as conjugation which means marriages of equals.
Example
This process occurs in algae and lower plants.

ii. Oogamy
The type of sexual reproduction in which a flagellated motile sperm fertilizes with non motile egg to produced a diploid zygote which then develop into new individual is called Oogamy.
Example
Some species of algae undergoes Oogamy.

iii. Heterogamy
The type of sexual reproduction in which two different structure gamets fused i.e. non flagellated large size female gamete fuses with small size flagellated male gamete to produced zygote which then develop into new plant is called Heterogamy.
It is also known as anisogamy.
Example
In higher plants such as bryophyte, heterogamy is present.


Germination

The process in which dormant or sleeping embryo awakes up renews its life and develops into a seeding is called as Germination.
OR
The breaking of dormancy of seed to produce seedling is called Germination.


Kinds of Germination

Seed can germinate into three ways i.e.
1. Epigeal Germination
2. Hypogeal Germination
3. Viviparous Germination

1. Epigeal Germination
Epi => above, geo => earth
The kind of germination in which cotyledons came above the soil due to rapid growth of hypocotyl is called Epigeal Germination.
Example
Caster oil seed, tomato, cotton etc.

2. Hypogeal Germination
Hypo => below, geo => earth
The kind of germination in which cotyledons remain under the soil due to rapid growth of epicotyl is called Hypogeal Germination.
Example
Maize-grain, Pea-gram etc.

3. Viviparous Germination
The special of germination in which seed germinates within fruit is called Viviparous Germination.
Process
The fruit is still attached to parent plant. Redicle comes out of the fruit which becomes swollen and heavy due to increasing weight the seedling gets detached and falls vertically into the soft mud gets embeded and starts growing.
Example
Rhizophora, coconut, date palm etc.


Seed

Seed may be defined as
A ripened ovule or a part of a plant body in which embryo lives in dormant condition is called Seed.


Structure of Seed

Structure of seed can be divided into two parts
1. External Structure
2. Internal Structure

1. External Structure
Externally seed consists of following parts
Seed Coat
The seed is covered from outside by a coat called Seed Coat.
The seed coat is formed by integuments. It is made up of two layers.
Testa
The outer thicker layer is called Testa.
Tegmen
The inner thin layer is called Tegmen.


Chromosomes as Carrier of Genes

Genes are small bodies found in the chromosome.
Chromosome are considered as the carrier of genes.
The chromosomes can be separately identified visually but the genes are very small units. And so far have not been seen even with the best microscope.
The chromosome and gene behave as hereditary units but the genes can not be considered outside the chromosome.
At the time of meiosis, the separation of homologous chromosomes takes place which result in the segregation of gene pairs.
In the genotype of every individual one member of each pair of genes is contributed by one parent and the other by the other parent.


Chromosomal Theory of Heredity

Introduction
The chromosomal theory of inheritance was first formulated by the American Biologist "Walter Sutton" in 1902.


Postulates

The main postulates of this theory are as under

1. Hereditary Materials
Reproduction involves the initial union of only two cells, egg and sperm. If Mendel's model is correct then these two gametes must make equal hereditary contributions. Sperm, however contain little cytoplasm, therefore the hereditary material must reside within the nuclei of the gametes.

2. Segregation of Chromosomes
Chromosomes segregated during meiosis in a manner similar to that exhibited by the elements of Mendel's model.

3. Number of Chromosome
Gametes have one copy of each pair of homologous chromosomes, diploid individuals have two copies.

4. Independent Assortment
During meiosis each pair of homologous chromosomes orients on the metaphase plate independent of any other pair.

Objection
The objection on chromosomal theory of hereditary is that when there is independent assortment of chromosomes in meiosis, the number of factors (genes) is more than the number of chromosomes. This is considered as a fatal objection about Sutton's theory.


Evidence

The material which transmits the parental characters into the coming generation is called Hereditary Material.


Fredrick Griffith's Experiment

Introduction
Fred Griffith in 1928 provided the evidence of hereditary material in bacteria.

Experimental Material
He was working on strains of steptococcus pneumoniae, which occurs in two distinct different forms.

R-Type
Rough surfaced, non-capsulated bacteria, without the capability of producing pneumonia.
i.e. non-virulent

S-Type
Smooth surfaced, capsulated bacteria, with the capability of producing pneumonia i.e. virulent.


Steps of Experiment

He observed that when the injected R-type bacteria in the mice, there was no ill effect.
When he injected the S-type, they proved to be fatal.
He also observed, when he injected both the bacteria separately after killing them by heating under high temperature, the mice did not develop the disease.
He also observed that, when the injected the living R-type with heat-killed S-type, there was a high morality among the mice.


Conclusion

Fred Griffith concluded that the R-type bacteria gained genetic property of S-type inactive bacteria when they kept together, so R-type bacteria converted into virulent S-type by the activity of DNA. Hence by this experiment, it can be proved that DNA is a genetic material.


A Very, Macleod and McCarty's Experiment

Introduction
In 1944, after a decade of research, Oswald Avery, Maclyn McCarty and Colin Macleod discovered that the transforming agent had to be DNA.

Experiment
They performed various experiments and found out that the only substance, which carried the transforming capability, was DNA because if the enzyme deoxyriba-nuclease was added to the bacteria, the transforming capability was lot.


Hershey and Chase's Experiment

Introduction
In 1952, Hershey and chase performed experiment to proof that DNA is a hereditary material.

Experience at Material
Hershey and chase labeled protein coat and DNA of Bacteriophage separately. Protein coat labeled with radioactive sulphur and DNA with radioactive phosphorus. These two viruses use to attack bacterial cells.


Steps Experiment

Hershey and chase observed that if cultures of bacteriophage are labeled with radioactive phosphorus [P32 labeling DNA] or with sulphur [S35 for labeling protein coat].
bacteriophage is ruptured, the DNA is released and treated with deoxyribsonucleas, the DNA breaks up into fragments in the solution.
The empty protein coats of the ruptured membrane appear as coats all the P32 or S35 were made to inject bacteria and multiply by the help of special technique, all the S35 labeled protein were removed.
The new phage formed contained only P32 indicating the presence of DNA molecule.


Conclusion

The conclusion appears similar to the transforming principle in bacteria, showing that DNA is the genetic material in phage, transmitted from one generation to the next.


Watson and Crick's Model of DNA

Introduction
James Watson and Francis crick, in 1953 proposed structure of the DNA molecule.


Structure of DNA

Watson and Crick suggested a ladder like organization of DNA.
1. Double Helix
Each molecule of DNA is made up of two polynucleotide chains which twisted around each other and form a double helix.

2. Backbone of DNA
The uprights of the ladder are made up of sugar and phosphate parts of nucleotide and the rungs are made up of a paired nitrogenous bases.

3. Pairing of Bases
The pairs are always as follows
Adenine always pairs with thymine and cytosine with Guanine.
The two polynucleotide chains are complimentary to each other and held together by hydrogen bonds.


Hydrogen Bonding

There are two hydrogen bonds between Adenine and Thymine (A=T) and three between Cytosine and Guanine (C≡G).

Distance
Both polynucleotide strands remain separated by 20 Aº distance.
The coiling of double helix is right handed and complete turn occurs after 34 Aº. In each turn 10 nucleotide pairs are present, therefore the distance between two pairs is about 3.4 Aº.


Genes - The Unit of Hereditary Information

Introduction
Archibald Garrod discovered in 1902, that certain diseases were more prevalent among some families and were inherited as a recessive Mendelian trait.

Alkaptonuria
Alkaptonuria is a disease in which the urine contained a substance called "Alkapton" now known as "Homogentisic acid" which was immediately oxidizes to black when exposed to the air.

Causes
He suggested that this disease occurred due to absence of an enzyme, which could break the "Alkapton" down to other products so it would not build up in the urine.
He proposed that the condition was "An inborn error of metabolism" which is occurring due to changes in the hereditary information, which must have occurred in one of the ancestors of the affected families.


Conclusion

He concluded that the inherited disorders might reflect enzyme deficiencies.


Genome

Definition
The total genomic constitution of an individual is known as Genome.
Example
In a bacterial cell, a single circular chromosome along with plasmid is genome of bacteria, while in a human being all twenty two pairs of autosome along with a pair of sex-chromosomes constitute genome.


Replication of DNA

Definition
The mechanism in which DNA prepares its copies is called DNA replication.
OR
When the formation of new DNA molecule takes place in the cells without any change, it is known as Replication of DNA.


Semi Conservative Replication

Definition
The type of replication in which new daughter double helical duplex contain one stand old and another newly synthesized is called Semi Conservative Replication.


The Meselson Stahl Experiment

Introduction
Mathew meselson and Frank Stahl performed experiments to test the semi-conservative method of DNA replication.

Experiment
They grew bacteria in a medium containing Nitrogen-15 (N15), a heavy isotope of the nitrogen.
The DNA after several generations became denser than normal because the entire bacterial DNA now contained Nitrogen-15 (N15).
They then transferred the bacteria into a new medium containing lighter isotope Nitrogen 14 (N14) and analyzed the cultures for changes in the DNA.
At first DNA, which the bacteria synthesized, was all heavy.
After one round the density of the DNA fell exactly to the value one half between the all heavy isotope DNA and all light isotope DNA.

Result
This showed that after one round of replication, each of the daughter DNA duplex contained one strand of heavy isotope, after two rounds half contained none of the heavy isotope strand to form light duplex and half contained one of the heavy strand isotope.
It was now confirmed that the semi conservative method of the replication of DNA replication was true.


One Gene One Enzyme Hypothesis

Introduction
George Beadle and Coworker Edward L. Tatum proved that the information coded within the DNA of a chromosome, is used to specify particular enzymes.


Method of Study

Beadle and Tatum created Mandelian mutation in the chromosomes of the fungus called Neurospora by the use of the x-rays.
They studied the effect of the mutations caused by them and suggested "One Gene One Enzyme Hypothesis".


Choice of Material

They choose the bread mold, neurospora crassa as an experimental organism. It had a short life cycle and was easily grown on a defined medium, containing known substances, such as glucose and NaCl.
The nutrition of Neurospora could be studied by its ability to metabolize sugars and other chemicals the scientist could add or delete from the mixture of the medium.


Production of Mutations

They induced mutations in Neurospora spores by using x-rays.
The mutated spores were placed on complete growth media enriched with all necessary metabolites, so keeping the strains alive because the strains were deficient in producing certain compounds necessary for fungus growth due to damaged DNA by earlier irradiation, hence called Mutants.


Identification of Mutant Strains

To test the mutations, they grew the mutated strains on the animal media containing sugar, ammonia, salt, a few vitamins and water.
A strain that had lost the ability to make a necessary metabolite, failed to grow on such media.
Using this approach, they succeeded in identifying and isolating the different mutants.


Identification of Specific Mutations

To determine the specific nature of each mutation, they added various chemicals to minimize media, to make the strains grow.
Using this technique, they were able to pinpoint the biochemical problem and thus the genetic deficiency of the mutants.
Many of the mutants were unable to synthesize a single amino acid or a specific vitamin.
If a spore lacked the ability to synthesize a particular amino acid, such as Arginine, it only grew if the Arginine was added in the growth medium. Such mutants were called as arg mutants.
Chromosome mapping studies on the organism facilitated their work and they mapped three areas clusters of mutant Arginine genes.
For each enzyme in the arginine biosynthetic pathway, they were able to isolate a mutant strain with a defective form of that enzyme and mutation always proved to be located at one of a few specific chromosomal sites, different for each enzyme.


Conclusion

They concluded that genes produced effects by specifying the structure of enzymes and that each gene encodes the structure of a single enzyme. This was called "One Gene One Enzyme Hypothesis".


RNA

Definition
The single stranded helical polynucleotide contain ribose sugar and uracil instead of thymine is called RNA.

Location
RNA is formed in the nucleus (in nucleolus 10%) as well as in the cytoplasm (90%).


Types of RNA

There are three types of RNA.

1. Ribosomal RNA (rRNA)
The class of RNA found in ribosome is called ribosomal RNA.
Function
During polypeptide synthesis it provides the site on the ribosome where the polypeptide is assembled.

2. Transfer RNA (tRNA)
A second class of RNA is called transfer RNA is much smaller. Human cell contains more than 40 different kinds of tRNA molecules.
Functions
During polypeptide synthesis tRNA molecules transport the amino acid into the ribosome for the synthesis of polypeptide chain.