Plant Movements

2. Nyctinasty Movement (Sleep Movement)

It is the movement of the plant in the dark. For eg: leaves of clover and Oxalis close in the evening and open in the morning.

3. Thermonasty

Movement in response to temperature change. Eg: tulip flowers.

4. Seismonasty or Thigmonasty

Movement in response to touch or mechanical stimulus. Mimosa pudica (touch-me-not plant) leaves fold.

Plant Growth Hormones

Growth is an irreversible increase in mass either by volume which is outcome of division, enlargement and differentiation of cells into different types of tissues.

This growth of plants is regulated by internal chemical substances which are required in few quantity known as hormone.

Plant growth hormone are the chemicals present in few quantity that helps in the growth and development of plants. Plant hormones are also known as plant growth regulators or phytohormones.

Hormones are two types: growth promoters (Auxin, Gibberellin and Cytokinin) and growth inhibitors (Abscisic acid and ethylene).

Auxins

Auxin was first discovered plant hormone by F.W. Went (1928) from coleoptiles of canary grass (tip of oat plant).

Types of Auxins

a. Natural Auxins

Eg: Indole-3-acetic acid (IAA), Indole-3-butyric acid (IBA), Indole-3-pyruvic acid.

b. Synthetic Auxins

Eg: Naphthalene acetic acid (NAA), Indole-3-butyric acid (IBA), 2,4-dichlorophenoxyacetic acid.

Physiological Effects of Auxins

• It promotes apical dominance. The growing apical bud to higher points inhibits the growth of lateral buds. This phenomenon is apical dominance.
• It helps in cell division and xylem differentiation.
• It helps in cell elongation.
• It prevents the premature fall of leaves, flowers and fruits.
• It induces parthenocarpy, i.e. development of fruit without seeds (without fertilization).
• It promotes flowering in pineapple, litchi.
• 2,4-dichlorophenoxyacetic acid (2,4-D) is widely used as herbicide to kill dicot weeds.

Gibberellins

It is weak acidic plant hormone which helps in cell elongation and cell differentiation. It was discovered by Japanese scientist E. Kurosawa (1926) from a fungus called Gibberella fujikuroi (Fusarium moniliforme). This fungus causes foolish seedling disease in rice which is also known as bakanae in Japanese.

About 125 gibberellins are known till now. Hopkins and Henders found out of which most common gibberellins is Gibberellic acid or GA₃. Eg: GA₁, GA₂, GA₃ etc.

They are abundantly found in root and shoot apex, buds, young leaves and young embryos. They are found in higher concentration in immature seeds.

Physiological Effects of Gibberellins

• Helps in seed germination. Some light sensitive seeds like lettuce and tobacco show poor germination in dark. These seeds grow vigorously after treatment of gibberellic acid even in dark.
• Plays important role in breaking dormancy of buds and seeds.
• It helps in the elongation of the stem.
• It helps in the removal of genetic dwarfism.
• It delays senescence (the process of growing old).
• It induces parthenocarpy, i.e. development of fruits without seeds (without fertilization).

Cytokinins or Kinetins

It is mild basic growth hormone that promotes cell division. First cytokinin was discovered by Miller et al. (1955) known as kinetin which can be obtained from coconut milk and yeast-DNA. First naturally occurring cytokinin was obtained from young maize grain by Letham et al. (1964) called zeatin.

All cytokinins are derivatives of adenine or purine. It occurs largely in embryo sac, roots, flowers, fruits and germinating seeds.

Physiological Effects of Cytokinins

• It promotes the cell division.
• It helps in cell enlargement. It enhances cell enlargement of cells in leaf, cotyledon and tobacco plants in combination with auxins.
• It plays important role in morphogenesis (cell differentiation).
• Kinetin along with auxin has an ability to differentiate root and shoot from undifferentiated mass of cells called callus.

• It counteracts the apical dominance: It helps in overcoming apical dominance induced by auxins as it promotes lateral and adventitious shoot growth.
• Cytokinin delay senescence: The ageing of leaves usually due to loss of chlorophyll (i.e. yellowing) and rapid breakdown proteins is called senescence. Treatment of cytokinin delays senescence.
• It stimulates the formation of chloroplast in leaves.

A + T / G + C is constant for species. Eg: Human = 1.55; Pea = 1.62.

DNA (Deoxyribonucleic Acid)

• DNA molecules are double-stranded where both the strands are spirally coiled forming a double helix.
• The two strands are anti-parallel. If one is in 3′→5′ direction, other is 5′→3′ direction.
• These helix strands have sugar phosphate chain on the outside and nitrogen bases on the inner side.
• Both the strands are joined together by hydrogen bond.

DNA Replication

The process of synthesis of new DNA molecule from pre-existing DNA is called DNA replication. It is semi-conservative, i.e. from one DNA molecule two new DNA molecules are produced and in each DNA one is parental strand and another is new strand.

This mechanism was first demonstrated by Meselson and Stahl with the help of E. coli.

Mechanism of DNA Replication

1. Recognition of origin: The site from where the replication process starts is called origin of replication (ori). The nick is produced by an enzyme endonuclease at the ori site.
2. Activation of nucleotides: The monophosphate nucleotides are activated into triphosphate nucleotides. AMP → ATP, GMP → GTP, CMP → CTP, TMP → TTP. Energy required for replication is provided by triphosphate.
3. Unwinding / unzipping of DNA helix: Helicase enzyme breaks hydrogen bonds between the two DNA strands, separating them. A Y-shaped replication fork is formed. Topoisomerase relieves tension produced during unwinding and SSB proteins prevent recoiling.
4. Formation of RNA primer: RNA primase enzyme forms RNA primer which directs replication process. The small fragment of RNA synthesized by primase is called RNA primer.
5. Elongation of new strand: DNA polymerase III adds complementary nucleotides. The parental strand acts as template for newly synthesizing daughter strand. On 3′→5′ template, continuous leading strand is formed. On 5′→3′ template, discontinuous lagging strand is formed as Okazaki fragments. DNA polymerase I removes RNA primer and replaces it with DNA nucleotide. DNA ligase joins Okazaki fragments.
6. Proof reading and DNA repair: Wrongly introduced nitrogen bases are rechecked and corrected by DNA polymerase III and proof reading.

Genetic Code

Genetic code is the language of nucleic acid that translate the language of nucleic acid into the language of protein. The term genetic code was given by George Gamow.

Codon is a sequence of three nucleotide which together form a unit of genetic code on DNA or RNA.

Properties of Genetic Code

1. Triplet: A codon consists of three nitrogen bases. 64 codons are formed which are sufficient to code 20 amino acids.
2. Degenerate: Same amino acid is coded by more than one codon.
3. Non-ambiguous: One codon specifies only one amino acid.
4. Universal: A codon specifies the same amino acid from lower to higher organisms.
5. Non-overlapping: The same letter is not used for different codons.
6. Comma-less: The code is continuous without comma.
7. Initiation codon: AUG is the initiation codon and in some case GUG is also initiation codon.
8. Sense and non-sense codon: Out of 64 codons, 61 give 20 amino acids and 3 do not give any amino acid. UAA, UAG and UGA are stop codons.

General Terminology

Gene

The hereditary unit; small fragment of DNA.

Allele

Alternative form of gene.

Homozygous individual

An individual having two identical alleles of a trait. Also called pure line. Eg: TT, tt.

Heterozygous individual

An individual which contains two different alleles of a trait. Also called hybrid. Eg: Tt.

Character

Height.

Trait

Tall, short.

Phenotype

Physical appearance.

Genotype

Genetic makeup.

Filial generation

The offspring obtained by crossing two parents is first filial generation (F₁). Offspring obtained by crossing individuals of F₁ among themselves is second filial generation (F₂).

Sibcross

The cross between any two offspring of same parent and same generation.

Backcross

The cross between offspring of F₁ generation with either of the parents.

Testcross

The cross between offspring of F₁ generation and recessive parent.

Monohybrid cross

Cross involving only one character.

Dihybrid cross

Cross involving two different characters.

Dominant allele

One of two alleles capable of expressing itself by suppressing the contrasting allele.

Recessive allele

One which is being suppressed by its alternative allele.

Punnett square

Square board in which Mendelian crosses are presented.

Mendelian Genetics

Gregor John Mendel conducted hybridization experiment on pea plant. Mendel’s experiments are carried out under the following headings.

Selection of Materials

Mendel selected garden pea plant (Pisum sativum) because:

1. The pea plant can be easily grown and maintained.
2. They are naturally self-pollinating but can also be cross-pollinated.
3. It is an annual plant, therefore many generations can be studied in short period of time.
4. It has several contrasting characters.
5. The hybrids of pea plant are perfectly fertile.

Seven Contrasting Traits

Hybridization and Monohybrid Cross

Hybridization is a process of formation of heterozygous hybrid by crossing two contrasting parents. First he studied the inheritance of single pair of contrasting trait (monohybrid cross) and then two pairs of contrasting trait (dihybrid cross).

Mendel’s Monohybrid Cross

Mendel took two pea plants of opposite trait, one tall and one dwarf, and crossed them. All the F₁ generation were tall. Then he continued his experiment by selfing F₁ generation.

Phenotype ratio: Tall : Dwarf = 3 : 1

Genotype ratio: TT : Tt : tt = 1 : 2 : 1

From this monohybrid cross Mendel formulated two laws: Law of dominance and Law of segregation or law of purity of gametes.

Mendel’s Dihybrid Cross

In dihybrid cross Mendel considered two traits. He crossed round yellow seed and wrinkled green seed. All F₁ offspring have round yellow seed.

Phenotypic ratio: Round yellow : Round green : Wrinkled yellow : Wrinkled green = 9 : 3 : 3 : 1.

From this experiment, law of independent assortment is obtained.

Mendel’s Law of Inheritance

Gregor John Mendel who is known as father of genetics postulates three laws of inheritance on the basis of monohybrid and dihybrid cross:

1. Law of dominance.
2. Law of segregation or purity of gametes.
3. Law of independent assortment.

1. Law of Dominance

In hybrid single character is controlled by two contrasting factors called alleles. One is dominant over the other. This law states that one factor (allele) in a heterozygous pair dominates and expresses itself while the other remains recessive.

2. Law of Segregation

This law is also known as law of purity of gametes. This law states that the two alleles of a gene segregate or separate during gamete formation such that each gamete receives only one allele.

It states that two different alleles, dominant and recessive, may remain together for a longer time, do not mix with each other but keep their identity distinct and later segregate at the time of gamete formation, so gametes possess only one out of two alleles.

3. Law of Independent Assortment

The two alleles of one factor (alleles) of each trait segregate independent of the two alleles of other trait at the time of gamete formation and get randomly rearranged in the offspring.

It means that inheritance of one trait is independent to the inheritance of other trait. Eg: Mendel’s dihybrid cross.

Gene Interaction

It is the influence of allelic or non-allelic gene on normal phenotypic expression. It is of two types:

1. Intragenic
2. Intergenic

Intragenic Interaction

Interaction between alleles of the same gene at same locus, resulting in modified expression of a character. Eg: incomplete dominance, co-dominance, multiple alleles.

The incomplete dominance, co-dominance and multiple alleles are deviation from the Mendel’s law of inheritance.

Incomplete Dominance

Incomplete dominance is a phenomenon where none of the two contrasting factor (allele) of the parent is capable of expressing itself in hybrid condition.

The expression of the character in hybrid is intermediate of two parental character.

For example, monohybrid experiment is performed in two pure parents of Mirabilis jalapa (4 o’clock plant). A homozygous red flower plant (RR) is crossed with a white flower plant (rr), then F₁ hybrid has pink flower plant (Rr).

In incomplete dominance, phenotype and genotype ratio are same, i.e. 1:2:1. This is the characteristic of incomplete dominance.

Co-dominance

Co-dominance is the phenomenon in which both contrasting alleles of hybrid have equal contribution for expression of characters.

For example, the best example is different types of RBC that determine ABO blood group in human. ABO blood groups are controlled by the gene I that has three alleles I^A, I^B, I^O.

The alleles I^A and I^B are dominant over I^O. When I^AI^B are present, both express their own character and determine blood group AB because both I^A and I^B are co-dominant to each other.

Phenotype ratio: Blood A : Blood AB : Blood B = 1 : 2 : 1

Genotype ratio: 1 : 2 : 1

Multiple Allele

When more than two alleles are responsible for a single character, they are known as multiple allele. Example: ABO blood group in human (3 alleles), eye colour in Drosophila (15 alleles).

Linkage

The genes which are closer to each other on a same chromosome have tendency to stay together during inheritance through successive generation without any change or separation. This phenomenon is called linkage.

All the genes which are closer to each other and pass together are called link genes.

Types of Linkage

1. Complete linkage
2. Incomplete linkage

Complete Linkage

It is the phenomenon in which genes located in the same chromosome do not separate and are inherited together in many generations. It produces only parental characters.

It is rare type but has been reported in Drosophila by T.H. Morgan.

Example: complete linkage is shown by Drosophila having grey body-long wings and black body-vestigial wings.

The result indicates the grey body character is inherited together with long wings. It suggests both genes are linked together. Similar in case of black body with vestigial wings. Since new or non-parental combinations are not formed due to lack of crossing over at meiosis in hybrid.

Incomplete Linkage

It is the phenomenon in which linked genes present in the same chromosome have tendency to separate due to crossing over causing parental as well as non-parental combination.

It produces more parental character and some non-parental character. Example: incomplete linkage in maize plant.

Hutchinson made a cross between maize plant with coloured, full seed and the plant with colourless, shrunken seed.

Parental as well as non-parental characters are formed. The parental combinations are 96.4% and non-parental combinations are 3.6%.

Crossing Over

Crossing over is process of exchange of genetic materials or segments between non-sister chromatids of homologous chromosomes.

As a result of crossing over, new or non-parental combinations are formed. Crossing over occurs in pachytene stage of prophase I of meiosis during a process called synapsis.

Mechanism of Crossing Over

1. Synapsis: Takes place during prophase I of meiosis division. Homologous chromosome (paternal and maternal) come together and stay side by side forming a pair at the sub-stage zygotene. The protein and nucleic acid structure formed is called synaptonemal complex.
2. Tetrad formation: After synapsis, each bivalent longitudinally form four chromatids. This stage is called tetrad.
3. Crossing over: In pachytene stage paired chromosomes start separating and chromatids remain attached at one or more points. This establishes exchange of genetic material. Point of attachment between chromatids is called chiasma.
4. Terminalization: Movement of chiasma towards the end of chromosome is known as terminalization.

Significance of Crossing Over

• It produces new combinations of genes.
• Due to crossing over useful recombinations can be formed which might be used by plant breeders.
• Various organisms have been created through recombination of genes.

Sex Linked Inheritance

There are two types of chromosome: somatic chromosome (autosome) and sex chromosome (heterosome). Autosome contains gene that determine somatic or vegetative character while sex chromosome contain gene that determine sex and sex related character in an individual.

Human have 23 pairs of chromosome where 22 pairs are autosome and 1 pair is sex chromosome. In male sex chromosome is heteromorphic having one X and another Y chromosome, while in female it is homomorphic having both XX.

The sex chromosome of some animals and human comprises genes for somatic or vegetative character along with sexual character. The inheritance of vegetative or somatic character through sex chromosome is known as sex-linked inheritance.

The sex-linked inheritance first reported by T.H. Morgan is Drosophila.

Types of Sex-linked Inheritance

1. X-linked inheritance: Inheritance of somatic character located on X chromosome. Eg: colour blindness in human, eye colour in Drosophila.
2. Y-linked inheritance: Inheritance of somatic character located on Y chromosome. Eg: hypertrichosis.
3. XY-linked inheritance: Inheritance of somatic character located on both X and Y chromosome. Eg: skin cancer.

Note: Sex-linked inheritance shows criss-cross pattern of inheritance. Father passes traits to his grandson through his daughter, or mother passes traits to her granddaughter through son.

Colour Blindness (Red-Green Blindness) in Human

Nearly 110 sex-linked genes are identified in human beings like colour blindness, haemophilia etc.

It is an inability of a person to differentiate red and green colour. It is found more common in male than female.

Colour blindness is caused by affected recessive gene attached to X-sex chromosome. It shows criss-cross inheritance. It was discovered by Horner.

Symbols

• XX → Normal vision female
• XY → Normal vision male
• X^CY → Colourblind male
• XX^C → Normal but carrier female
• X^CX^C → Colourblind female

Case I: Colourblind Male × Normal Vision Female

Carrier Female × Normal Male

Case II: Colourblind Female × Normal Vision Male

Colourblind Male × Normal but Carrier Female

Conclusion: X-linked inheritance occurs both in male and female. It is more common in male than in female. Colour blindness shows criss-cross pattern of inheritance.

Sex Linked Inheritance in Drosophila

Sex linked inheritance is discovered by T.H. Morgan while working in Drosophila. He noticed the sudden appearance of one white-eyed male in the culture of red-eyed Drosophila. The eye colour is found in X gene and white eye colour is the mutant form.

Symbols

• X⁺Y → Red-eyed male
• X⁺X⁺ → Red-eyed female
• X^wY → White-eyed male
• X⁺X^w → Red-eyed but carrier female
• X^wX^w → White-eyed female

Case I: White-eyed Male × Red-eyed Female

F₁ are red-eyed. On self cross F₂ produces 25% red-eyed female, 25% red-eyed male, 25% red-eyed but carrier female and 25% white-eyed male.

Case II: White-eyed Female × Red-eyed Male

On self cross, F₂ gives 25% white-eyed female, 25% red-eyed but carrier female, 25% red-eyed male and 25% white-eyed male.

Conclusion: X-linked inheritance occurs both in male and female. It is more common in male than in female. White and red eye show criss-cross pattern in inheritance.

Reasons for Selecting Drosophila

1. It can easily grow in laboratory.
2. It completes life cycle in about 2 weeks.
3. Male and female are phenotypically differentiated.
4. It has only four pairs of chromosomes.
5. It is single mating and produces a large number of progeny.
6. It can express the characters very quickly.

Mutation

Mutation is the sudden change in the gene or chromosome. Mutation was first studied by Hugo de Vries on plant Oenothera lamarckiana.

Types of Mutation

1. Gene mutation
2. Chromosomal mutation

A) Gene Mutation (Micro Mutation / Point Mutation)

It occurs in the gene and causes change in the sequence of nitrogen bases of gene.

i) Frame Shift Mutation

The addition or deletion of nitrogen base in DNA that alters reading frame is called frame-shift mutation.

ii) Substitution Mutation

One nitrogen base is replaced by another nitrogen base.

• Transition: One purine is replaced by another purine or one pyrimidine is replaced by another pyrimidine.
• Transversion: Purine is replaced by pyrimidine or vice-versa.

Chromosomal Mutation (Macro Mutation)

Also called chromosomal aberration. The change that occurs in structure or number of chromosome is called chromosomal mutation.

1. Structural Mutation

The mutation that changes the structure of chromosome.

• Deletion: Loss of certain fragment of chromosome.
• Inversion: A piece of chromosome is broken and reinserted in opposite orientation.
• Duplication: During duplication certain segment of chromosome gets repeated.

2. Numeric Mutation

Mutation which takes place due to change in number of chromosome is called numerical mutation. It is also known as ploidy.

Aneuploidy

Change in chromosome number either due to addition or deletion of one or more chromosome is known as aneuploidy.

• Trisomy: Results due to addition of one chromosome to diploid chromosome, 2n+1. It causes Down’s syndrome and Klinefelter syndrome.
• Tetrasomy: 2n+2.
• Monosomy: 2n−1. It causes Turner’s syndrome.
• Nullisomy: 2n−2.

Euploidy

It is a type of variation in which number of set of chromosome increases or decreases in the diploid chromosome.

Types

1. Haploidy: Loss of complete one set of chromosome, i.e. 2n − n = n.
2. Polyploidy: Addition of one or more set of chromosome in diploid chromosome. It is the phenomenon of having more than two sets of chromosome. Eg: 3n, 4n, 5n etc.

Significance of Polyploidy

• It is important source of variation and helps in evolution.
• Seedless, large-sized fruits of banana, watermelon, tomato are due to polyploidy.
• Disease-resistant and high-yielding plants can be produced by polyploidy.
• It may also increase immunity of organisms.

Positive and Negative Effect of Mutation

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Biofertilizer: Why is Green Manure Superior to Chemical Fertilizer?

Biofertilizer is a natural fertilizer that contains living microorganism like bacteria, algae or fungi. These microorganism help plants grow by increasing the availability of nutrients in the soil.

Green manure is superior because it is a natural, sustainable method that improves soil health and does not cause pollution, while chemical fertilizers may harm soil in the long run.

Reasons

1. Improve soil structure: Adding organic matter makes soil soft and fertile.
2. Eco-friendly: No pollution unlike chemical fertilizer which can harm soil and water.
3. Increase soil fertility naturally: Supplies nutrients slowly and continuously.
4. Enhance microbial activity: Supports beneficial soil organisms.

Biotechnology and Food Scarcity

Biotechnology is an emerging discipline in this era. Would you think can it be used to mitigate the problem of food scarcity in our country and to help us live?

Yes, biotechnology can be used to mitigate the problem of food scarcity in Nepal.

Applications

• Improved crop yields: Developing high yielding crop varieties that are better suited to local condition.
• Disease and pest resistance: Creating crops that are resistant to common local diseases and pests, reducing crop losses.
• Nutritional value: Enhancing the nutritional content of staple crops to combat malnutrition.

Short Questions from the Source

What are plant growth hormones?

Plant growth hormones are chemical compounds produced naturally within plants that regulate their growth, development and response to environmental stimuli.

Physiological effects of cytokinin on plants

Cytokinins primarily promote cell division, regulate lateral shoot growth, prevent apical dominance and delay the aging process of leaves.

Double Fertilization in Angiosperms

Double fertilization is a unique mechanism in flowering plants involving two fusion events:

1. Syngamy: One male gamete fuses with an egg to form a diploid zygote (2n), which eventually develops into the embryo.
2. Triple fusion: The second male gamete fuses with two polar nuclei in the central cell. This results in the formation of a triploid primary endosperm nucleus (PEN).

Note: It becomes triploid (3n) because it is formed by the fusion of three haploid nuclei—one male gamete and two polar nuclei.