Apomixis – Flowering Plants

APOMIXIS IN FLOWERING PLANTS

Reproduction in plants is of two types:

  1. Sexual or Amphimixis
  2. Asexual or Apomixis

Angiosperms have a Diplontic life cycle. The main part of the lifecycle is Sporophyte & the Gametophyte is reduced.

 

APOMIXIS

Apomixis mimics sexual reproduction but produces seeds without fertilization. Seen in some species of Family Asteraceae & Grasses.

It is of 3 types:

  1. Apogamy: Formation of sporophyte/embryo from any cell of the gametophyte/embryo sac without fertilization. Cells of the embryo sac i.e., antipodal cells or the synergids. The embryo formed is haploid.
  2. Apospory: Formation of gametophyte i.e., the embryo sac from any cell of the sporophyte without meiosis. Cells of the sporophyte are cells of nucellus & integuments. The embryo sac formed has diploid cells. The egg is also diploid. Diploid egg either undergo fertilization or undergoes Parthenogenesis.
  3. Parthenogenesis: Formation of the embryo from the egg without fertilization.

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Meristems – Plant Anatomy

MERISTEMS:

Term Meristems was given by Nageli, it means juvenile/young tissues.

Cells of these tissues have the ability to divide. These cells are thin walled, living, having dense protoplasm, conspicuous nucleus, and are in an active state of metabolism.

ON THE BASIS OF ORIGIN:

Meristems are 3 types:

  1. PROMERISTEM: Also referred as embryonic meristem. Present at the apices of the shoot and root tip. It divides to form primary meristem.
  2. PRIMARY MERISTEM: Found below the pro-meristem in the shoot and root tip & intercalary meristem. They divide to produce primary permanent tissue that forms the primary plant body.
  3. SECONDARY MERISTEM: It is formed later in the plant body. It develops from primary permanent tissue due to dedifferentiation. They divide to produce secondary permanent tissue that forms the secondary plant body. They are interfascicular cambium & cork cambium.

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PLEIOTROPY

Pleiotropy

Pleiotropy is a condition in which single gene affects many characters or single gene affects many phenotypes.

  • It occurs due to inter-relationship between metabolic pathways that contribute to the formation of the different phenotype.
  • Effect of the gene is more evident in one trait and less evident in other traits.

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Polygenic Inheritance

Polygenic Inheritance:

Polygenic inheritance is controlled by two or more pairs of nonallelic genes. In which the dominant alleles have a cumulative effect. Each dominant gene added will add to the character and the result will be an additive effect of all dominant genes. These genes are Polygenes and such type of inheritance is Polygenic inheritance. It is also referred as a Quantitative inheritance & Multiple genes interaction.

These genes don’t follow the Mendelian pattern of inheritance.

These characters are affected by environment.

Continuous variation occurs. The offsprings can be divided into a range of classes.

  • Kolreuter first studied polygenic inheritance.
  • Galton studied it in man.
  • Nelson-Ehle gave the experimental proof.

Examples:

  • kernel color in wheat.
  • cob length in Maize.
  • skin color in man
  • height in man.
  • IQ in man.
  • crop yield, milk yield etc.

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Restriction Endonuclease

Restriction Endonuclease:

Discovered by Werner Arber, Daniel Nathans, and Hamilton Smith.

  • They are known as “molecular scissors” in genetic engineering.
  • They are capable of breaking DNA at specific sites called the restricted sites.

These restricted sites have Palindromic sequences.

Palindromic sequences are sequences that have the same meaning when read from both the sides just like the word ‘MADAM’ & ‘MALAYALAM’.

And sequences like:

5‘  →  3

GTAC

CATG

3‘  →  5

  • They break the phosphodiester bonds.
  • They are found in bacteria & they protect the bacterium from foreign DNA.
  • Due to methylation of its nucleotide, they don’t act on bacteria’s own DNA.
  • The 1st restriction endonuclease isolated was Hind II.
  • More than 900 R.E have been isolated, from 230 strains of bacteria.
  • There are 3 types of R.E.Type I, II, III.

Type III is used in genetic engineering.

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Sex Linked Inheritance

Sex-Linked Inheritance:

T H Morgan while working on eye color in Drosophila, found that the gene for eye color did not follow the Mendelian pattern of Inheritance.

  • This was due to the gene being present on the X-chromosome.
  • Genes present on the X-chromosome are known as Sex-linked genes, the characters are sex-linked characters.
  • Sex-linked Inheritance is the study of inheritance of sex-linked characters.
  • Genes present on Y-chromosome are Holandric genes and they are restricted to the male sex only.
  • Females have XX chromosomes while males have XY chromosomes.
  • Males are hemizygous because they have only one allele of X-chromosome. Males will express the character of any gene on the X-chromosome whether it is dominant/recessive.

 

Examples of Sex-Linked Inheritance:

Haemophilia:

  • Also referred as Bleeder’s disease or the Royal disease.
  • Occurs due to a sex-linked recessive gene.
  • Clotting of blood does not occur even to minor injuries.
  • Haemophilia is of 2 types: Haemophilia A & Haemophilia B/Christmas disease.
  • Haemophilia A occurs due to the absence of Factor VIII, Haemophilia B occurs due to the absence of Factor IX.
  • It expresses in the female when both the X chromosomes have the recessive gene, XhXh, while XXh is a carrier female.
  • In males, it expresses when the single X-chromosome has the recessive gene XhY.

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DNA structure

 DNA

  • A polymer of deoxyribonucleotides.
  • Length is defined as the number of nucleotides or nucleotide pairs present. This is the characteristic of an organism.
  • Bacteriophage Ø x174 has 5386 nucleotides.
  • Bacteriophage λ has 48502 base pairs.
  • Escherichia coli has 4.6 x 106 base pairs.
  • The haploid genome of human DNA is 3.3 x 109 base pairs.

    Structure of polynucleotide chain

  • A nucleotide has 3 components:
  1. A nitrogenous base,
  2. A pentose sugar &
  3. A phosphate group.

Nitrogenous bases are of two types:

  1. Purine: adenine and guanine
  2. Pyrimidine: cytosine, thymine, and uracil.
  • Thymine is present in DNA and uracil is present in RNA. Thymine is 5-methyl uracil.
  • Pentose sugar is deoxyribose sugar in DNA and ribose sugar in RNA. Ribose sugar has an additional OH group at 2′ position.

A nucleoside is a nitrogenous base linked to pentose sugar through an N-glycosidic linkage.

  • A glycosidic bond is formed between C at 1st position of sugar and N at 1st position of pyrimidine / N at the 9th position of purine.

A nucleotide is formed by the linking of a phosphate group to the 5OH group of ribose sugar of the nucleoside by a phosphoester linkage.

Nitrogenous base
Nucleoside
Nucleotide
Adenine
Adenosine/ DeoxyadenosineAdenylic acid/ Deoxyadenylic acid
Guanine
Guanosine/ DeoxyguanosineGuanylic acid/ Deoxyguanylic acid
Cytosine
Cytidine/ DeoxycytidineCytidylic acid/ Deoxycytidylic acid
Thymine
DeoxythymidineDeoxythymidylic acid
    Uracil
UridineUridylic acid

 

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Population Interactions – Predation

PREDATION

It is a type of interaction in which members of one species kills and eats up members of the other species.

  • Species that captures is the predator and the species that is killed is the prey.
  • Most animals are predators leaving the scavengers.

  • Insectivorous plants are also predators

e.g., Nepenthes, Utricularia, Dionaea, Drosera.

  • It is nature’s way of transferring energy fixed by plants to higher trophic levels.
  • They help in keeping the prey population under control or due to high population densities they can cause instability in the ecosystem.

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