Chapter 4 - Reproduction
2nd Year Biology Notes
Chapter 4 - Reproduction
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.
Post a Comment