Hereditary Notes - Cell Division, Genetics, Cell Division, Mendel's Experiment and Genetics Technology

Heredity can be defined as the transfer of characters from one generation to another generation. The characters transferred from one generation to another through genes are called hereditary characters. Those characters gained after birth as a consequence of adaption or environmental effects, not through genes are called acquired characters.

Every living creature receives the genetic characters that determine its form and function from its parents. Genetic material gets reproduced and transmits from parent to child through gametes which are produced by the cell division process. The passing of genetic features from parents to Children is mediated by cells. Therefore, cell division is one of the most important mechanisms in the transfer of hereditary from parents to their offspring. In cell division, a parent cell splits up into daughter cells. In a general classification, cell division occurs in two different ways.

i. Mitosis Cell Divison

ii. Meiosis Cell Division

1.1 Mitosis Cell Divison

It is a type of cell division in which a mother cell divides to form two daughter cells. As the cells of the same types having the same numbers of chromosomes similar to that of the mother cell are formed, it is also called homotypic or equational cell division. This phenomenon generally occurs in vegetative or somatic cells.

Mitosis Cell division completes in two steps. They are karyokinesis and cytokinesis. These are discussed below.

1.1.1 Karyokinesis

It is simply the division of the nucleus. The stages of Karyokinesis are described below.

1.1.1.1 Prophase

Prophase is the initial stage of karyokinesis. At this stage the chromatic fibers condense to form visible chromosomes, the centrosome divides into two parts called asters and starts moving towards opposite poles of the cell. Similarly, spindle fibers begin to appear and nucleolus and nuclear membrane start disappearing at this stage. This marks the beginning of the metaphase. It is the longest phase of the mitosis.

1.1.1.2 Metaphase

In this stage, the nucleolus and nuclear membrane completely disappear. The chromosomes become thicker, shorter, more distant, and move freely. Spindle fibers are also completely developed. Those spindle fibers from centrioles capture the chromosomes and line them up in the equatorial plane of the cell.

1.1.1.3 Anaphase

The centromeres of each chromosome get divided into two centromeres. Each chromosome separates into two single-stranded chromosomes as shown in the figure. These chromosomes, then are pulled towards the opposite poles due to the contradiction of spindle fibers. Anaphase is the shortest phase of mitosis cell division.

1.1.1.4 Telophase

It is the final stage of Karyokinesis. At this stage, the spindle fibers disappear. The chromosomes decondensed and return to a long, uncoiled form called chromatin reticulum. Nuclear membranes and nucleoli reappear. In the end, two daughter nuclei are formed.

1.1.2 Cytokinesis

Cytokinesis refers to the division of the cytoplasm of the cell. The number of cells gets added up in cytokinesis. Animal cytokinesis occurs by the deepening of the constriction and formation of new cell membranes. Plant cytokinesis occurs through the formation of cell plates. In this way, a mother cell divides into two daughter cells. The number of chromosomes in the daughter cells is individually equal to that of the mother cell.

1.2 Meiosis Cell Divison

It is the type of cell division in which the reproductive or germinal cells divides to form four daughter cells having half the number of chromosomes in each of them. It is also known as reductional cell division.

Meiosis cell division completes in two successive stages. They are Meiosis-I and Meiosis-II.

1.2.1 Meiosis-I

The homologous chromosomes separate out and go to form two different daughter cells. This reduces the number of chromosomes to half. So meiosis-I is called reductional cell division. As the daughter cells are genetically diverse, this type of cell division is also called heterotypic cell division. It completes in two stages.

1.2.1.1 Karyokinesis-I

Karyokinesis-I completes in four stages which are described below.

1.2.1.1.1 Prophase-I

It is the initial phase of cell division. It is completed in five sub-phases as leptotene, zygotene, pachytene, diplotene, and diakinesis. Each of these steps is briefly described below.

Leptotene

At this stage, chromatin fibers condense and form elongated chromosomes. The nuclear membrane and nucleolus become distinct. At this stage, the chromosomes pair up but look like a single one.

Zygotene

At this stage, chromosomes pair up. The phenomenon of pairing up of homologous chromosomes is called synapsis. The paired chromosomes form bivalents. During synapsis, one chromosome comes from the paternal and the other comes from the maternal cell.

Pachytene

During this stage, shortening and thickening of chromosomes occur. Then those homologous chromosomes split in a way that each chromosome forms four chromatids. which is called tetrad formation. The chromatids coil around each other. In the end, crossing over takes place between them.

Diplotene

Homologous chromosomes separate out, except at the point of cross-over. The contact point between these chromosomes is called chiasmata. In the end, the chiasmata move away from the centromere. At this phase nuclear membrane is distinct but the nucleolus begins to disappear.

Diakinesis

Chiasmata shifts towards the end of the chromosomes. This movement is called the terminalization of chiasmata. The nuclear membrane begins to disappear and the nucleolus disappears completely. Spindle fibers begin to appear.

1.2.1.1.2 Metaphase-I

Nuclear membrane disappears completely. The pair of homologous chromosomes lie on the same equatorial plane. Centromere lies towards the poles and spindle fibers attach with the centromere of the chromosomes.

1.2.1.1.3 Anaphase-I

Homologous chromosomes separate from the connections and move toward spindle poles. This phenomenon is called a disjunction. The centromeres do not divide. So, chromosomes are double-stranded. Here, reduction takes place as the number of chromosomes in daughter nuclei is half that in the mother nuclei.

1.2.1.1.4 Telophase-I

The chromosomes reach towards the opposite poles and become elongated. Nucleolus and nuclear membrane reappear. There is the formation of two daughter nuclei, each at the opposite poles.

1.2.1.2 Cytokinesis-I

In Cytokinesis, two daughter cells are formed through cleavage. And, Meiosis II occurs after this stage.

1.2.2 Meiosis-II

In this type of cell division, the chromosomes are distributed equally into the cells and the number of chromosomes remains the same. So,meiosis-II is also called homotypic or equational cell division. It completes in two steps- Karyokinsis and Cytokinesis.

1.2.2.1 Karyokinesis-II

It completes in the following four phases as discussed below:

1.2.2.1.1 Prophase-II

In this phase, the chromatin fibers become shorter and the nuclear membrane and nucleolus start to disappear. Spindle fibers begin to appear.

1.2.2.1.2 Metaphase-II

At this phase, the nuclear membrane and nucleolus disappear completely. All chromosomes arrange themselves in an equatorial plane. The centromeres of all the chromosomes are attached to the spindle fibers.

1.2.2.1.3 Anaphase-II

The division of centromere takes place, along which the double-stranded chromosomes move towards the opposite poles due to the contraction of the spindle fibers.

1.2.2.1.4 Telophase-II

Chromosomes elongate and undergo decondesion forming chromatin reticulum. The nuclear membrane and nucleolus reappear and spindle fibers disappear at this stage.

1.2.2.2 Cytokinesis-II

At this stage, both daughter cells from the cytokinesis-I undergo independent cytokinesis.

In this way, four daughter cells (haploid) are formed from a single mother cell (diploid). This completes a cycle of meiosis cell division.

2. Nucleic acids

Nucleic acids are naturally occurring chemical compounds that allow organisms to transfer genetic information from one generation to another. They are found in the nucleus, chloroplasts, and mitochondria of the cell.

Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA) are the two major nucleic acids.

2.1 Deoxyribonucleic acid (DNA)

DNA is a double helix ( twisted) strand of the nucleotides. It has four nitrogenous bases. They are Adenine, Guanine, Cytosine, and Thymine. The combination of these nitrogenous bases is the code system for the messages from DNA. It is the basic genetic material of the chromosome which passes from one generation to another.

2.1.1 Functions of DNA

1. It helps in transferring hereditary characters from parents to offspring.
2. It helps in gene expression. Gene expression is the process by which the information encoded in a gene allows a cell to respond to the changing environment.

2.2 Ribonucleic acid (RNA)

RNA is a single strand of nucleotides. It has four nitrogenous bases. They are Adenine, Guanine, Cytosine, and Uracil.

2.2.1 Functions of RNA

1. It helps in protein synthesis.
2. It is also referred to as an enzyme as it helps in the process of chemical reactions in the body.

2.3 Differences Between DNA and RNA

DNA (Deoxyribonucleic acid)	RNA (Ribonucleic acid)
i. DNA is a double-stranded molecule having a long chain of nucleotides.	i. It has four nitrogenous bases - Adenine, Guanine, Cytosine, and Uracil.
ii. It has four nitrogenous bases - Adenine, Guanine, Cytosine, and Thymine.	ii. RNA consists of the ribose sugar.
iii. DNA consists of deoxyribose sugar.	iii. RNA consists of ribose sugar.
iv. RNA consists of the ribose sugar.	iv. DNA is responsible for the storage of genetic information for the long term.

3. Chromosomes

Chromosomes are thread-like structures that are visible only at the time of cell division. Chromosomes transfer genetic information to the daughter cells.

The number of chromosomes is fixed in the cell of an individual of a particular species. As discussed in the cell division, it is known that a cell having only one set of chromosome is known as a haploid cell (gamete), and a cell having two sets of chromosomes are known as diploid cells. The number of chromosomes in a haploid cell is denoted by 'n'. Similarly, the number of chromosomes in diploid cells is denoted by '2n'.

The numbers of chromosomes in a cell of some organisms are given in the table below.

S.N.	Name of Organism	Number of pair of Chromosomes	Number of chromosomes
1.	Human Being	23	46
2.	Gorilla	24	48
3.	Chimpanzee	24	48
4.	Pineapple	25	50
5.	Elephant	28	56
6.	Wheat	21	42
7.	Mango	20	40
8.	Tiger	19	38
9.	Tomato	12	24
10.	Rice	12	24
11.	Maize	10	20
12.	Pea	7	14
13.	Cucumber	7	14
14.	Aloe Vera	7	14
15.	Cat	19	38

3.1 Structure of Chromosome

Chromosomes are made up of DNA and proteins. A chromosome has different parts as centromere, chromomere, telomere, and so on. The shape of chromosomes changes with the phase of the cell division.

3.2 Types of Chromosomes

Based on the position of the centromere, the chromosomes are of the following types.

3.2.1 Metacentric Chromosome

In this type of chromosome, the centromere is located in the middle of the chromosome. The amount of chromosomes is equal.

3.2.2 Submetacentric Chromosome

In this type of chromosome, the centromere is not exactly in the middle, rather, it is shifted slightly away from the center. One of the arms is a bit longer while the other is slightly shorter in this type of chromosome.

3.2.3 Acrocentric Chromosome

This type of chromosome has a centromere located close to one of the ends of the chromosome. One arm is very long, while the other arm is very short in this type of chromosome.

3.2.4 Telocentric Chromosome

In this type of chromosome, the centromere is present at the very end of the chromosome. The arms are present on only one side as shown in the figure below.

4. Sex Determination

Each cell of a human being contains 23 pairs of chromosomes. Out of 23 pairs of chromosomes, 22 pairs of chromosomes are similar, in the case of both males and females. These chromosomes are also known as autosomes. The twenty-third pair contains both chromosomes similar in females but dissimilar in males. The chromosomes of the twenty-third pair are called sex chromosomes. Sex Chromosomes are responsible for sex determination. There are two types of sex chromosomes.They are X- Chromosome, and Y- Chromosome.

i. X-Chromosome

It is one of the two sex chromosomes that are involved in sex determination. It occurs paired in females but single in males. It contains genes for female sex determination.

ii. Y-Chromosome

It is the other chromosome out of two sex-determining chromosomes. It is found only in the cells of male individuals. It contains genes for male sex determination.

Differences Between X-Chromosome and Y-Chromosome

X-Chromosome	Y-Chromosome
i. It occurs paired in females and single in males.	i. It is found only in the cells of male individuals.
ii. It contains genes for female sex determination.	ii. It contains genes for male sex determination.
iii. It is five times larger than Y-Chromosome.	iii. It is smaller than X-Chromosome.
iv. It occurs paired in females and single in males.	iv. It contains more genes than Y- Chromosomes.

Hence, it can be concluded that among the two chromosomes of the twenty-third pair in males, one is the X-chromosome and the other is Y-chromosome, i.e., the males have the XY chromosome pair. In females, both are X-chromosomes, i.e. they have XX chromosome pair. So, it can be said that the sex of the child depends upon the sperm, containing X-chromosome or Y-chromosome, that fuses with the ovum(egg).

If the female gamete (egg) is fused by the X-chromosome of the male gamete(sperm), the resulting combination will be XX as shown in the figure above. Hence, the child will be female.

If the egg is fused with the Y- Chromosome of the sperm, the male child will be produced. In this way, the sex of the child to be born can be determined.

5. Some Terminologies in Genetics

Genetics

The branch of biology that deals with the study of the mechanism of transfer of hereditary characteristics from one generation to another is known as genetics.

Gene

A gene is a segment of DNA that codes for a particular character in an organism. The functions of genes are listed beelow.

i. Genes carry hereditary information from one generation to another.

ii. Genes control the structure and mechanism of the body.

Phenotype

Phenotype is the physical traits that are observed in an individual. The characteristics like the color of the eyes, the color of the hair, the tallness of the individual, etc are phenotype. It can be altered by genotype.

Genotype

Genotype refers to the genetic constitution of the individual. It cannot be studied by direct observation but can be known through the study of the ancestors, offspring, etc. Genotype is not affected by environmental factors.

Monohybrid Cross

Monohybrid Cross refers to the genetic cross between the gametes of two parents considering a single pair of characters. In this type of cross, parents differ by a single trait. Inheritance of the single trait is studied in this cross. So, it is also known as a single trait cross.

Dihybrid Cross

Dihybrid Cross is a genetic cross that involves two parents with two pairs of contrasting traits. In this type of cross, the inheritance of two independent traits is studied. So, it is also called two traits cross.

6. Mendel's Experiment

Greoger Johann Mendel, an Austrian scientist, who is known as the father of genetics, studied the transfer of hereditary characters, on the basis of his experiments. He explained about genetics and gave different laws.

6.1 Selection of Plant by Mendel

Mendel selected the pea (Pisum sativum) plant for his experiment, which was grown in his own garden due to the following reasons-

1. The pea plant contains a greater number of contrasting characters with pairs.
2. The life cycle of the pea plant is very short. So, the results of the experiment can be obtained very quickly.
3. Controlled breeding can be performed in them.
4. They have the sexual mode of reproduction.
5. They can produce more numbers of offspring at a time.

6.2 Contrasting Pair of Characters in Pea Plant.

The seven pairs of contrasting characters of pea plants on which Mendel experimented are illustrated below.

S.N.	Contrasting Characters	Dominant Characters	Recessive Characters
1.	Height of stem	Tall (T)	Dwarf (t)
2.	Position of flower	Axial (A)	Terminal (a)
3.	Color of pods	Green (G)	Yellow (g)
4.	Shape of pods	Inflated (I)	Constricted (i)
5.	Shape of seeds	Rounded (R)	Wrinkled (r)
6.	Colour of flowers	Violet(V)	White (v)
7.	Color of seed	Yellow (Y)	Green (y)

First, he took two plants; one pur tall and another pure dwarf. They were then artificially pollinated. He avoided self-pollination between these plants. Then, he collected the seeds and planted them. The result he observed was that all the plants were hybrid tall. He called this generation the first filial generation. In this generation, he found that the dwarfishness of the pea plant was recessive while the tallness was the dominant one.

For the f2 generation, he allowed those hybrid plants to self-pollination and he planted the seeds of this generation. As a result, what he found was that both the tall and dwarf plants were seen. The ratio of tall pea plants to dwarf ones in the f2 generation was 3:1. This ratio, he called a phenotypic ratio. But, when the genetic composition of those offspring was studied, he found that there was one tall pea plant, two hybrid tall pea plants, and one dwarf pea plant. So, the genotypic ratio was found to be 1:2:1 in the f2 generation.

The entire experiment can be represented below.



This shows the possibility of production of only hybrid pea plants, i.e. Tt. Then, the offspring of the f1 generation was self-pollinated as shown in the figure

Phenotypic ratio = 3:1 (tall: dwarf)

Genotypic ratio = 1:2:1 ( Pure tall: hybrid tall: pure dwarf)

In the same way, Mendel also observed the output on all seven pairs of contrasting characters of pea plants and found the same genotypic and phenotypic ratio in all of them.

7. Mendel's Laws of Inheritance

On the basis of his experiments, Mendel formulated some laws to explain the mechanism of inheritance. This is also called Mendelism. Here, we discuss Law of Dominance, and Law of Segregation

7.1 Law of Dominance

The law of dominance states that when two parents having contrasting pair of characters are crossed, only one form of character appears in the next generation. Those characters are called dominant characters. The characters which are not expressed are called recessive characters.

For example, let us observe the results obtained in the first filial generation. The products of the f1 generation were all hybrid tall. The Dwarfishness of those plants was suppressed and only tallness was observed.

7.2 Law of Segregation

This law states that though the two contrasting pair of characters, either dominant or recessive, remain together for a long time, they do not mix up and only segregate at the time of gamete formation so that each gamete carry only one character. This law is also called the law of purity of gametes.

In the experiment done by Mendel, the offspring of the f1 generation are found to carry both the characters: tallness and dwarfishness. But the dominant character was tallness. As a result of which dwarfishness was masked in the f1 generation. But, at the time of gametogenesis, those characters separated out and when self-pollinated, the pure tall offspring, pure dwarf offspring, and hybrid were formed. The reappearance of pure tall and pure dwarf pea plants indicated the process of segregation.

8. Genetic technology

Genes are found in all living organisms. The genes are passed from one generation to another. Genetic technology, sometimes also called genetic engineering can be defined as the branch of biotechnology that allows the modification, removal, or transfer of genes from one species to another.

Genetic technology can be used to improve the productivity of plants and animals. The sustainability of living organisms can be improved by the modification of the genes present in them. Genetic technology can also be applied in the artificial breeding of animals and plants so that a new variety can be generated.

8.1 Role of DNA Test in different investigations

The DNA test is a medical test that can identify any changes in DNA sequence or chromosome structure. DNA testing is done by collecting and analyzing the blood semen, saliva, urine, hair teeth, etc, and the things worn or touched by the person to be investigated. DNA testing can be done in newborn screening, diagnostic testing paternity testing research testing, and so on.

DNA testing is mostly used in different investigations which are given below.

• DNA testing is done to obtain the DNA profile of a particular person. DNA profiling is a technique by which an individual can be identified at the molecular level.
• DNA testing is performed to determine whether an individual is the biological parent of a particular person. This is also called DNA paternity testing.
• Medical disorders that are characterized by the alteration of genes can be identified by DNA testing.
• It is done in the genetic matching of the donor and acceptor of organs in organ transplanting.

8.2 Traditional Methods of Selection of improved seeds

seed selection is the process of selecting seeds of good quality that are healthy and disease free. Seed selection is important in order to grow healthy plants so that there will be high yielding of the products. Selection of the improved seeds is a traditional practice done by the farmers that is prevalent now too.

The different traditional methods of selection of improved seeds are:

• Broken, crushed, or wrinkled seeds are avoided.
• Seeds free from insects, infections, etc should be prioritized for sowing.
• The seeds can be selected by immersing them in water. The seeds that float on the water are avoided while the others can be selected.
• Seeds having high germination capacity should be selected.

8.3 Cross Breeding

Cross-breeding is the process of causing animals or plants to breed with another species.

Some examples of cross-breeding are the crossing of different varieties of rose flowers, the crossing of two different varieties of dogs, etc.

Some of the advantages of cross-breeding are given below:

i. The weakness of the particular species can be improved or eliminated by breeding with the species having strong characteristics.

ii. The cross-breeding of plants helps in the faster evolution of crops.

iii. Crossbreeding creates genetic variation.

iv. It helps to improve yield in breeder varieties.

Some of the disadvantages of crossbreeding are:

i. It causes a shortage of pure species in the long run.

ii. If any of the two species is infected with any disease the crossed species will be disease infected.

iii. The challenges in cultivating the new varieties increase as more cautions are to be followed.

iv. In the case of plants, pollen grains have to be produced in abundance to ensure the changes of pollination which result in the wastage of pollens.

8.4 In-Vitro fertilization

In-vitro fertilization is the process of fertilization in which an egg is combined with the sperm outside the living body and in an artificial environment. It refers to the fertilization that occurs in a laboratory vessel or other controlled experimental environment. This method is used to overcome female infertility which is caused due to problems in the fallopian tubes and male fertility to which there is a defect in sperm quality.

8.5 Artificial Insemination (AI)

Artificial insemination is the process of fertility treatment that is used to deliver sperm directly into the cervix or uterus. When an ovum is released, semen is introduced into the woman's vagina, uterus, or cervix, depending upon the method used.