DNA structure


  • 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
Adenosine/ DeoxyadenosineAdenylic acid/ Deoxyadenylic acid
Guanosine/ DeoxyguanosineGuanylic acid/ Deoxyguanylic acid
Cytidine/ DeoxycytidineCytidylic acid/ Deoxycytidylic acid
DeoxythymidineDeoxythymidylic acid
UridineUridylic acid



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Population Interactions – 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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DNA Fingerprinting

DNA fingerprinting

DNA fingerprinting is a technique to identify an individual by looking at unique patterns in their DNA.

  •  In human’s 99.9%  of the base sequences of DNA are same & are referred as Bulk genomic DNA.
  • The difference lies in remaining 0.1%. It is these differences which make every individual unique in their phenotypic appearance. This DNA has small stretches of repetitive sequences. They are referred as Repetitive DNA.
  • The only exception is monozygotic twins.

Repetitive DNA can be separated from bulk genomic DNA by density gradient centrifugation.

  • Bulk DNA forms a major peak while repetitive DNA forms several small peaks and are referred as satellite DNA.
  • Satellite DNA contains short nucleotide sequences repeated thousands of time tandemly.
  • Satellite DNA is divided into different categories according to their size i.e., micro-satellites, mini-satellites etc.
  • Micro-satellite consists of 2 – 6 bp repeats.
  • Mini-satellite consists of 10 – 100 bp repeats, they are referred as VNTRs.

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Regulation of gene expression

Regulation of gene expression

It is the metabolic, physiological or environmental conditions that regulate the expression of genes.

Housekeeping or constitutive genes: genes whose products are continuously required by the cell hence they continuously transcribe and translate.

Non-constitutive genes: genes whose products are not continuously required by the cell & if they continuously transcribe and translate there will be a lot of wastage.

  • Regulation of expression of such genes is a must.

In eukaryotes:

Genes can be regulated at

  • Transcriptional level.
  • During processing i.e., splicing.
  • Transport of mRNA from the nucleus to the cytoplasm.
  • Translational level.

In prokaryotes:

Genes are regulated at the transcriptional level.

There are 2 systems of regulations:

  • An inducible system and a repressible system.
  • In the inducible system, an addition of a substance induces transcription of certain genes. It produces catabolic enzymes. It is always OFF. E.g., lac operon.
  • In the repressible system, an addition of a substance stops transcription of certain genes. It produces anabolic enzymes. It is always ON. E.g., Tryptophan operon.

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Cell Division – Mitosis & Meiosis

CELL DIVISION: Mitosis & Meiosis

Cell Cycle

Can be divided into 2 stages:


  • G1 Growth phase 1.
  • S  Synthetic phase.
  • G2 Growth phase 2.
  • During Synthetic phase, DNA duplicates. The genetic material doubles mean from 2N it becomes 4N.


  • M Mitosis/Meiosis.
  • C Cytokinesis.

Mnemonic:Go Sally Go! Make Children!”


It is an equational division. Occurs in somatic cells.

Prophase, Metaphase, Anaphase, Telophase

Mnemonic:People Meet And Talk”



  • Coiling of chromatin occurs, forming thin long threads.
  • By the end, chromosomes start forming,
  • Nucleolus & nuclear membrane starts disappearing by the end.
  • Spindle fiber formation starts.
  • Centriole in animal cells starts moving towards the poles.


  • Nuclear membrane and nucleolus has disappeared,
  • Spindle fibers have formed,
  • 2 types of spindle fibers occur chromosomal fibers that are attached to chromosomes at the centromere & continuous fibers that join the 2 poles.
  • Chromosomes having two chromatids are seen,
  • Chromosomes align themselves on the equatorial plate due to contraction of spindle fibers.
  • Amphiastral mitosis occurs in animal cells & anastral mitosis occurs in plant cells.

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