Biology Exams 4 U

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Chargaff’s Experiment and Why is Chargaff's Rule Important?

Let’s begin with Chargaff’s rule.

Chargaff’s rule states that

Rule 1: The amount of Adenine ~equals the amount of Thymine

The amount of Guanine ~equals the amount of Cytosine

The amount of purine = the amount of pyramidine

Chargaffs Rule Summary

Rule 2: The amount of A+T ≠ amount of G+C. This ratio varies among different organisms but same in different tissues of the same organism 

Chargaff’s Experiment

He developed a paper chromatography technique to separate and quantify nucleotide bases

Chargaff's experiment

Let us divide his experiment into two parts

Experiment. 1: Determination of base composition of DNA from different species

First he isolated DNA from different organisms like Human tissues, E.coli, rat etc. separated and quantified nucleotide bases

Experiment 2: Determination of base composition from different sources of same organism

First he isolated DNA from different tissues of the same organisms like DNA from Human thumus, spleen, sperm etc and quantified nucleotide bases


His procedure consisted of three steps.

  • The first was the separation of the DNA mixture into individual components by paper chromatography.
  • Next,the separated compounds were converted into mercury salts.
  • And finally, the purines and pyrimidines were identified via their ultraviolet absorption spectra.

From the result he derived Chargaff's Rule

Watch this video for better understanding

Why is Chargaff's Rule Important?

Explained base-pairing regularities or "complementarity relationships" of DNA thus providing the most important clue for the Watson and Crick for deducing the structure of DNA. At the beginning, Watson and Crick proposed “Like with like” base pairing where purine pairs with purine and pyramidines with pyramidines; A=A, G=G etc.  Complementary base pairing provided the most stable orientation of base pairs in the DNA double helix. This orientation (purine to pyramidine base pairing) best matched with the X-ray crystallographic data. Thus Chargaff’s Rule provided the solution for solving this puzzle.

DNA copying mechanism

Watson and Crick could explain how DNA makes copies of itself using this base complementarity relationship. One DNA strand serves as a template for the synthesis of other strand.


  • Tetra nucleotide Hypothesis by Phoebus Levene
  • Triple helical structure of DNA  proposed by Linus Pauling
Learn More: How to calculate the percentage of bases  in a DNA strand using Chargaff’s rule? (Chargaff's Rule Questions)

                       Watson and Crick 1953 paper
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How to calculate the number of different possible gametes produced by the diploid genotype AaBbCCdd? Simple Formula

 1. How many unique gametes could be produced through independent assortment by an individual with the genotype AaBbCCdd?

Possible no. of gametes= 2n  ’n’ is the no. of heterozygous gene pairs

How to calculate the number of different possible gametes are produced by the diploid genotype


No. of heterozygous gene pairs n=2

Possible no. of gametes= 2n

Answer: 2n  =22=2x2=4

You can watch this video for better understanding

2. How many different possible gametes are produced by the diploid genotype aaBbCC ?


Possible no. of gametes= 2n  ’n’ is the no. of heterozygous gene pairs

No. of heterozygous gene pairs n=1

Answer: 21  =2

3. How many different possible gametes are produced by the diploid genotype (AaBbCcDdEe)?



Answer: 2n  =25= 2 x 2 x 2 x 2 x 2 = 32

4.For the following genotypes, How many gametes will be produced?

a) AA





Possible no. of gametes= 2n  ’n’ is the no. of heterozygous gene pairs

a) AA     2n =20 =1; the only gamete is A

b)Bb      2n =21 =2; the gametes are B and b

c)BBCc  2n =21 =2; the gametes are BC and Bc

d)cCDd 2n =22 =4; the gametes are cD,CD,Cd and Cd

Thank you so much:)

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Milestones in Ethology

 Ethology, literally means "character study" (Gk. ethos = character, custom, habitat; logos =Study). It is simply the scientific study of the different types of animal behavior. 

Milestones in  Ethology

Milestone in Ethology

1859 : Charles Darwin published his monumental and epoch:making book "Origin of Species by Means of Natural Selection", setting the stage to consider ethology in evolutionary terms. Later, Konard Lorenz, Tinbergen and Karl von Frisch built up ethology on this foundation.

1872 : Darwin published "The Expression of the Emotions in Man and Animals".

1873 : Spalding carried out investigatory studies on imprinting.

1898 : Charles Whitman studied the behaviour of pigeons and doves and published the book "Animal Behaviour",

1898 : Thorndike reported trial and error learning.

1904 : Pavlov demonstrated conditioned reflexes.

1910 : Heinroth studied the behaviour of ducks and gees.

1913:'37 : Kohler made studies on insight learning.

1937 : Konrad Lorenz made studies on instinct.

1950 : Karl von Frisch, Kiramer and Pittendrigh independently presented convincing evidences in support of the internal clocks in animals.

1952 : Niko Tinbergen and his associates detected the social releaser.

1952 : Kramer made serious studies on the navigation and orientation of birds.

1960 : Goodall, for the first time, studied the behaviour of chimpanzees, living with them in the Chimpanzee Reserve in Tanzania.

1963 : Tinbergen published a landmark paper regarding the aims and methods of ethology.

1966 : Reynolds introduced the concept of breeding system.

1970 : Robert Andrey published the much acclaimed book "The Social Contract: A Personal  Inquiry Into the Evolutionary Sources of Order and Disorder".

1972 : Tinbergen, Lorenz and Von Frisch jointly won the Nobel Prize. 

Konard Lorenz and Nikolaas Tinbergen, Karl von Frisch shared the Nobel prize in physiology and Medicine. They were the first ethologists to receive Nobel Prize and their contributions were cited as discoveries on the organization and elicitation of individual and social behavior patters.

1977 : Emlon and Orien advanced the concept of matting system.

1989 : Clogan made authentic studies on motivation.

1994 : Anderson made detailed studies on sexual selection.

1997 : Bonys made detailed studies on biomimicry.

2001 : Sokolowski masterly reviewed the influence of cloned genes on complex animal behaviour.

2003 : Emmons and Lipton presented a comprehensive description of the role of various gene products in behaviour.

2017 : Jeffrey C. Hall, Michael Rosbash and Michael W. Young were awarded Nobel Prize in Physiology and Medicine for their discoveries of molecular mechanisms controlling the circadian rhythm.

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What are Waxes? Structure, Beeswax formation & Function

Definition of Waxes: Waxes are simple lipids made up of fatty acid chains esterified to long-chain alcohols. The alcohol may contain 12-32 C

How is ester bond formed in wax?

How are waxes formed? ester linkage in wax
The OH of -COOH group of fatty acid reacts with OH of alcohol releasing a water molecule and thus forms the ester linkage.
Please watch this 4 minute video for better understanding on ester bond formation in wax

Where are the waxes found in nature?

common waxes found in nature
  • Bees wax
  • The term wax derived from “weax "meaning substance made by bees
  • spermaceti wax in sperm whale
  • Carnauba palm wax: a Brazilian Palm

Waxes: Properties and Function

  • Waxes are simple lipids; hydrophobic and insoluble in water
  • Chemically inert with high melting points


  • Bees used to construct combs; for food storing during winter
  • In birds, aquatic plants and animals; offers water proofing
  • Waxes also serve as energy-storage substances in plankton
  • Wax on leaf and fruit surface prevent excess water loss Protects from parasites and insects
  • Spermaceti wax in Sperm whale helps in echolocation and keeping buoyancy
Related topics: 
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What are Glycerides? Neutral glycerides vs Phosphoglyceride

 Glycerides are Glycerol containing lipids

Glycerides can be classified into

1. Neutral glycerides and

2. Phosphoglyceride

This is a simplified 6 minute video on What are Glycerides? Nuetral glycerides vs Phosphoglycerides for better understanding

What are Glycerides

What are neutral glycerides?

1. Neutral glycerides are nonionic and non-polar. They are electrically neutral; therefore called as neutral glycerides.

Example: Triglycerides (fats)

Fats are the most abundant lipid, made up of two types of smaller molecules: glycerol and fatty acids

Glycerol is a 3-C alcohol with a hydroxyl group attached to each carbon

A fatty acid consists of a carboxyl group attached to a long carbon skeleton

In a fat, 3 fatty acids are joined to 1 glycerol by ester linkage, forming triacylglycerol, or triglyceride.

Fats are solid at room temperature whereas oils are liquid at room temperature.

fatty acid