Reasons: Why Mendel Chose Pea Plant (Pisum Stavium) For His Breeding Experiments

MENDEL, Gregor Johann Mendel (1822 — 1884) laid the foundation of classical genetics by formulating two laws of heredity;

• Law of Segregation 
• Law of Independent Assortment 

By profession, he was a priest but he's fond of gardening. He performed series of breeding experiments on garden pea, Pisum sativum in monastery garden for eleven years (1854 — 1865). Now the question arises, Why Mendel chose only the Pea Plant? There are lots of other plants and other species which can be chosen for performing experiments. But Mendel only chose Pea Plant (Pisum sativum). Following are the reasons:

Reasons For Selection Of Pea Plant:

Mendel can choose some other plant for experiments but he selected pea plant due to following reasons:

Easy to Cultivate: Pea plant (Pisum sativum) was easy to cultivate. It grew well in his garden.  

Hermaphrodite: Its flowers were hermaphrodite I.e. pea plant have both male and female sexual organs. 

Cross-fertilization (Cross-Pollination) Easily Controlled: It was normally self-fertilizing (self-pollinating) that is the fertilization of plants and some invertebrate animals by their own pollen or sperm rather than that of another individual, but could also be cross fertilized (cross-pollinated) that is the transfer of pollen grains from an anther of a flower of one plant to a stigma of a flower of another plant of the same species.  

Short Generation Time: As the time gap between generations was short, Mendel could raise many generations of pea within a short time. This is one of the main reason for choosing Pea plant (Pisum sativum).  

Many Distinct Traits: Pea had many sharply distinct. its each trait had two clear cut alternative forms or varieties: e.g. seed shape had a round or wrinkled phenotype, plant height was either tall or short, seed color could be yellow or green etc. Mendel called them pair of contrasting traits. He focused on seven such contrasting pair of traits.

Process Of Recombinant DNA Technology (Genetic Engineering)

Recombinant DNA Technology Definition:

Recombinant DNA is the technology used for making artificial DNA (genetic modification) by combining different genetic materials (DNA) from different sources. It is the laboratory methods of genetic recombination to get genetic material (DNA) from different sources which would create a genomic sequence that would not otherwise be found in the genome. 

Gene Cloning Process:

The production of exact identical copies of a particular gene (DNA sequence) extracted from an organism. Identical twins are the perfect example of Gene Cloning.

Gene cloning produces many identical copies of a particular extracted gene. Recombinant DNA technology (genetic engineering) is used when large number of genes is required. The polymerase chain reaction (PCR) is used to create a lesser number of copies within a laboratory test tube.

Process of Recombinant DNA Technology:

Recombinant DNA technology popularly known as genetic engineering aims at synthesizing recombinant DNA also known as recombinant deoxsyribonucleicacid which contains DNA from two different sources. In order to produce recombinant DNA, following are required:

• Gene of interest, which is to be cloned
• Molecular scissors, to cut out the gene of interest
• Molecular carrier (vector), on which gene of interest could be placed
• An expression system, on which the gene of interest along with the vector is then introduced as a result of which a specific product is made

How to get a gene:

In the process of recombinant DNA technology, first of all we’ll have to get a gene which is to be cloned. There are two ways to get the gene of interest.

• To Isolate It from The Chromosome: Genes can be isolated from the chromosomes by cutting the chromosomes on the flanking sites of the gene using special enzymes known as restriction endonucleases.
• Chemical Gene Synthesis by Messenger RNA (mRNA): If the genes are small or for some reason we can’t isolate from chromosomes, they can also be synthesized chemically in the laboratory. To make a gene chemically in laboratory, messenger RNA (mRNA) is used. We’ll use the reverse transcriptase enzyme method. Reverse transcriptase is the enzyme which forms DNA from an RNA template in reverse transcription. This DNA molecule is called complementary DNA (cDNA) and is mainly associated with retro viruses.  

Molecular Scissors (Restriction Endonucleases):

Using of Molecular Scissors is the second step in making recombinant DNA. Molecular Scissors also known as Restriction Endonuclease is the enzyme which cleaves DNA molecules at specific base sequences producing small gene fragments by breaking internal covalent bonds linking nucleotides, used in recombinant DNA technology and chromosome mapping. They are natural enzymes of bacteria, which they use for their own protection against viruses. The restriction enzyme (Restriction Endonuclease) cuts down the viral DNA, but does no harm to the bacterial chromosome. They are called restriction enzymes or molecular scissors because they restrict the growth of viruses. In 1970, Hamilton D. Smith, at Johns Hopkins University, isolated the first restriction enzyme.

Palindromic Sequences:

A palindromic sequence is the nucleic acid sequence (DNA or RNA) which reads the same in both directions. These sequences are recognition sites for enzymes. Bacteria produce a variety of such restriction enzymes, which cut the DNA at very specific sites characterized by specific sequence of four to six nucleotides arranged symmetrically in the reverse order. Such sequences are known as palindromic sequences. So far more than 400 such enzymes have been isolated out of which about 20 are frequently used in recombinant DNA technology.

EcoR1 (A Restriction Enzyme): EcoR1, a commonly used restriction enzyme (endonuclease), cuts double stranded DNA when it has this sequence of bases at the cleavage site. Notice there is now a gap into which a piece of foreign DNA can be placed, if it ends in bases complementary to those exposed by restriction enzyme.

Sticky Ends:

The single stranded but complementary ends of the two DNA molecules are called sticky ends because they can now bind by the complementary base pairing. They therefore facilitate the insertion of foreign DNA into vector DNA.

Molecular Carrier (Vector):

A vector is the mean by which recombinant DNA is introduced into a host cell. One common type of vector is plasmid. Plasmids were discovered by scientists studying the sex life of the intestinal bacterium Escherichia coli.

Plasmids are natural extra chromosomal circular DNA molecules which carry genes for antibiotic resistance and fertility etc. One of the plasmids discovered earlier is PSC 101 has antibiotic resistance gene for tetracycline, whereas PSR 322, has antibiotic resistance gene for tetracycline as well as ampicillin. Inserting gene of interest in tetracycline gene of plasmid (PSR 322) would enable separating out colonies of bacteria in a medium containing ampicillin and vice versa.

Recombinant DNA Preparation:

For preparation of recombinant DNA, the plasmids are cut with the same molecular scissors (restriction enzymes), which was used for isolation of the gene. The gene of interest for instance insulin is then joined with the sticky ends produced after cutting the plasmids with the help of another special enzyme called DNA ligase. DNA ligase is a very important enzyme in recombinant DNA technique that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. It seals the foreign piece of DNA into vector. Now the two different pieces of DNA have joined together which is now known as Recombinant DNA or Chimaeric DNA.

Expression of Recombinant DNA:

A clone can be a large number of molecules (i.e. cloned genes) or cells (i.e. cloned bacteria) or organisms that are identical to an original specimen. Bacterial cells take up recombinant plasmid, especially if they are treated with calcium chloride to make them more permeable. Thereafter, as the cell reproduces, a bacterial clone forms and each new cell contains at least one plasmid. Therefore, each of the bacteria contains the gene of interest, which will express itself and make a product. From this bacterial clone, the cloned gene can be isolated for further analysis or protein product can be separated. Besides plasmids, the DNA of bacterial viruses (for example lambda phage) can also be used as a vector. After lambda phage it attaches to a host bacterium, recombinant DNA is released from the virus and enters the bacterium. Here it will direct the reproduction of many more viruses. Each virus in bacteriophage clone contains a copy of the gene being cloned. 

How It Was Proved That Deoxyribonucleic Acid (DNA) Is The Heredity Material

Deoxyribonucleic Acid (DNA)

Deoxyribonucleic acid is a molecule that carries most of the genetic instructions used in the development, functioning and reproduction of all known living organisms and many viruses.

DNA as Hereditary Material

Work of Griffith: The first evidence of hereditary nature of DNA was provided by a British microbiologist Frederick Griffith who made some unexpected observations while experimenting with pathogenic bacteria. His observations are:

• When he injected mice with a virulent strain of Streptococus pneumoniae bacteria (then known as Pneumococcus), the mice died of blood poisoning.
• When he injected similar mice with a mutant strain of S. pneumoniae that lacked the virulent polysaccharide coat, the mice showed no ill effects. This shows that coat was apparently necessary for virulence.

S forms: The normal pathogenic form of this bacterium is referred to as the S form because it forms smooth colonies on a culture disk.

R forms: The mutant forms, which lack an enzyme needed to manufacture the polysaccharide coat, is called the R form because it forms rough colonies.

Decisive Experiment

Griffith Experiment of Transformation

Purpose of Experiment: To determine whether the polysaccharide coat that lacked in mice which showed no ill effect, had a toxic effect.

Actual Experiment: Griffith injected heat killed dead bacteria of the virulent S strain into the mice, the mice remained perfectly healthy.

Control Experiment: As a control experiment (a test where the person conducting the test only changes one variable at a time in order to isolate the results. An experiment where all subjects involved in the experiment are treated exactly the same except for one deviation is an example of a control experiment.), he injected mice with a mixture containing dead S bacteria of the virulent strain and live coatless R bacteria, each of which by itself did not harm the mice.

Result: Unexpectedly, the mice developed the disease symptoms and many of them died.

Further Observations: The blood of the dead mice was found to contain high levels of live, virulent streptococcus type S bacteria, which had surface proteins characteristic of the live (previously R) strain. Somehow, the information specifying the polysaccharide coat had passed from the dead, virulent S bacteria to the live, coatless R bacteria in the mixture, transforming permanently the coatless R bacteria into the virulent S variety.

Transformation: Transformation is the transfer of genetic material from one cell to another and can alter the genetic makeup of the recipient cell.

Work of Avery, Macleod AND McCarty

Determining that DNA is Heredity Material

Transforming Principle

In 1944, in a classic series of experiments Oswald Avery along with Colin Macleod and Maclyn McCarty repeated Griffith's experiments and characterized what they referred to as the transforming principle. They first prepared mixture of dead S Streptococcus and live R Streptococcus that Griffith had used. Then they removed as much of the protein as they could from their preparation, eventually achieving 99.98% purity. Despite removal of nearly all the protein, the transforming activity was not reduced. Moreover the properties of transforming principle resembled there of DNA. The protein digesting enzymes or RNA digesting enzymes did not affect the transforming activity, but the DNA digesting enzyme DNase destroyed all the transforming activity.

Hershey and Chase Experiment

Hershey and Chase Experiment To Determine DNA is Heredity Material

In 1952 by Alfred Hershey and Martha Chase experimented with bacteriophages T₂ and provided additional evidence supporting Avery's conclusion.

³²P Labeled Viruses: In some experiments they labeled viruses with radio (radioactive) isotope ³²P, which was incorporated into the newly synthesized DNA of growing phage.

³⁵S Labeled Viruses: In other experiments the viruses were grown on a medium containing ³⁵S an isotope (radioactive) of sulphur which is incorporated into the amino acids of newly synthesized protein coats.

Exposure of Bacteria to Labeled Viruses: Now one bacterial culture was exposed to ³²P labeled viruses and other culture was exposed to ³⁵S labeled viruses.

Removal of Protein Coats: The bacterial cells were agitated violently in a blender to remove the protein coats of the infecting viruses from the surfaces of the bacteria.

Test for the Presence of Label: When bacteria were tested for the presence of label it was found that bacteria had lost nearly all of ³⁵S label. However, 32P label was present in bacteria of the other group.

Reason: It is because ³⁵S was only present in protein coat while ³²P was present in DNA that had transferred to the interior of the bacteria.

Confirmation: It was confirmed when viruses were released from the infected bacteria. The viruses released from ³²P culture were labeled while viruses released from ³⁵S culture were unlabelled.

DNA — the Hereditary Material

Thus it was proved that the hereditary information injected into the bacteria that specified the new generation of viruses was DNA and not protein. 

INTRODUCTION TO GENETICS

Genetics

Introduction to Genetics:

A person's eye can be blue, brown, green, gray, or hazel; a person’s hair can be different shades of brown, red, blond, or black; a parakeet’s feathers can be green, blue, or yellow, with black or gray markings. What causes these biological spectra of colours? We can frame the question in more general terms: What is genetic basis of variation among a population's individuals? Or, what principles account for the transmission of these variations from parents to offspring?

One possible explanation of heredity is a “blending hypothesis”, the idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green. The hypothesis predicts that mating a blue parakeet with a yellow one would result in green offspring, and once blended, the hereditary material of the two parents would be as inspirable as the colours of mixed paint. If the blended hypothesis were correct, over many generations a freely mating population of blue and yellow parakeets would give rise to a uniform population of green birds. The actual results of parakeet breeding, however, contradict such a prediction. The blending hypothesis also fails to explain other phenomenon of inheritance, such as traits skipping a generation.

An alternative to the blending model is a "particulate model" of Inheritance: the gene idea. According to this model, parents pass on discrete heritable units – genes – that retain their separate identities in offspring. An organism's collection of genes is more like a bucket of marbles than a pail of paint. Like marbles, genes can be sorted and passed along, generation after generation, in undiluted form.

The first scientific advance in the study of inheritance was made by the Austrian monk Gregor Mendel who published a paper in 1866 which laid the foundations for the present-day science of genetics. He demonstrated that characteristics do not blend but pass from parents to offspring as discrete units. These units, which appear in the offspring in pairs, remain discrete and are passed on to subsequent generations by the male and female gametes which each contain a single unit. The Danish botanist Johannsen called these units GENES in 1990 and the American geneticist Morgan in 1912 demonstrated that they are carried on the chromosomes. Since the early 1900s the study of genetics has made great advances in explaining the nature of inheritance at both the level of the organism and at the level of gene.

Definition of Genetics:

Genetics is the study of genes, heredity, and variation in living organisms. 

A Chromosome

ROLE OF GENES IN INHERITANCE

Human Gene

Role of Genes in Inheritance:

Did you ever have such an interesting experience, that an outsider had recognized you at a momentary look, that whose youngster you are or whose sib you are? How was he ready to make such an exact estimate? Is it accurate to say that it was your enchanting grin that looked like that of your mother or your face line was similar to that of your father? Have you ever observed which of your traits; your hair or eye shade, your earlobes or nose, your teeth and your lips, your hands or your feet are similar to those of your parents or grandparents? Your perception is superbly right. You take after your parents, and they look like their parents. It is exceptional phenomenon of life. A cub resembles a lion, and a kitten resembles cat and a puppy take after a dog. This "resemblance" generation after generation is because of heredity. Heredity is passing on of traits from parents to offspring.

Life in every living being; each character of his structure, metabolic system, functions and functions, at all levels of biological association is programmed and controlled by a set of guidelines. These guidelines are as coded biological data called genes. Your grandparents passed on genes to your parents and your parents passed on genes to you. Subsequently you and your sibs have numerous genes/qualities of your parents and even of your grandparents. Inherited characters are controlled by genes. Thus Heredity is the way by which genes transmit traits and attributes from parents to progeny.

Genetics is branch of science which deals with the exploration or study of heredity. Genes are the fundamental units of biological data.

So, genes can be defined as, “a unit of heredity that is transferred from parents to progeny and is held responsible for determining characteristics of offspring.”

Gamete – A Vehicle of Inheritance:

Your life has begun as a single cell – the zygote. All the cells of your body emerged from it through mitosis. It was not simply a common cell; rather it was the pioneer illustrative of your personality. Its core (nucleus) had a complete project of coded genetic data for forming you into a one of a kind individual. Each one cell of your body originates from it (zygote), has gotten the same set of genes. How was your zygote structured? Yes! It was produced by the fertilization of your fatherly and motherly gametes. Those two miniature cells – a sperm and an egg provided the main physical connection between you and your parents. This is the heredity bridge. For transmitting of traits between parents and progeny, offspring must pass through this heredity bridge.