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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Genes Page 1

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Gene :


  • Gene is a unit of heredity. It is made up of DNA and controls the inheritance and expression of a character.




Griffith's Experiment:


  • Griffith worked on two strains of bacteria Diplococcus pneumoniae:
    1. Smooth strain (s ), which was capsulated and pathogenic.
    2. Rough strain (R), which was non-capsulated and non-pathogenic.
  • He performed a series of experiments and made the following observations:
    1. When he injected the living R-strain into mice, they did not suffer from pneumonia.
    2. Then he injected the living s-strain into mice and they died of pneumonia.
    3. He then injected the heat-killed s-strain into the mice. The mice did not suffer from pneumonia.
    4. In the last case, he injected the mice with a mixture of heat-killed s-strain and living R-strain. The mice suffered from pneumonia and died.
    5. After culturing the strain from the blood of dead mice, he found the colonies of
    6. This clearly proved that the change in character of R-strain had occurred due to transformation of genetic material from S to R strain.
    7. The exact cause of this transformation was experimentally proved by Avery, Maccleod and McCarty.
    8. They found that DNA fragments from heat-killed s-strains were transferred into R-strains, transforming R-strains into pathogenic s-strains.
    9. When the enzyme DNA are which destroys DNA was added to this culture, the transformation did not take place.
    10. Thus, it was proved that DNA is responsible for transmission of genetic information.
    11. Later on, it was confirmed by Zinder and Lederberg that the process of transferring genetic information from one bacterium to another also takes place through a virus.
    12. This process is called transduction and was shown using the virus T2 bacteriophage.




Packaging Of Hereditary Material, Structure Of Chromatin:


  1. Chromatin is seen in the nucleus during interphase as a loose network of threads.
  2. It consists of DNA and two types of proteins, i.e. histones and non-histones.
  3. Histones belong to five types, i.e. H1, H2A, H2B, H3 and H4.
  4. Under an electron microscope, the chromatin gives a beaded appearance
    (string and bead arrangement), with identical chromatin units.
  5. These units are called nucleosomes.
  6. Each nucleosome consists of a histone core and DNA.
  7. The histone core consists of an octomer, which is formed by two each of H2A, H2B,
    H3 and H4 histones.
  8. This core is wound around by two rounds of DNA strand.
  9. The structure of nucleosome is comparable to that of a solenoid.
  10. The unwound strand of DNA between the two successive nucleosomes is called as lintees DNA.




Chemical Components Of DNA


  1. DNA (Deoxyribose Nucleic Acid) is a complex biomolecule.
  2. It is made up of a chain of subunits called nucleotides, which are linked to form polynucleotides.
  3. Each nucleotide is composed of:
    1. A pentose sugar called deoxyribose
    2. A phosphoric acid
    3. One of the four nitrogen bases, which are attached to the pentose sugar.
  4. The nitrogen bases are of two types:
    1. Purines - adenine (A) and guanine (G)
    2. Pyrimidines - cytosine (C) and thymine (T)




Watson and Crick Model of DNA


  1. According to Watson and Crick, the DNA molecule is made up of two polynucleotide chains, which are twisted about one another in the form of a rope-like coiled structure called double helix.
  2. The two strands of DNA are anti-parallel and run in opposite directions.
  3. The two strands of DNA twist around its axis at 36 in such a manner that it shows a
    major groove, which is followed by a minor groove.
  4. The structure of DNA can be compared to a ladder where the sides of the ladder are composed of deoxyribose sugar and phosphoric acid and the rungs of the ladder are composed of nitrogen bases, which are held by weak hydrogen bonds.
  5. It should be noted that the rungs of DNA show complementary nitrogen base pairing. A purine, adenine (A) pairs with a pyrimidine thymine (T) and guanine (G) pairs with cytosine (C).
  6. A and T are joined by two weak hydrogen bonds, i.e. A = T, whereas G and C
    are joined by three weak hydrogen bonds, i.e. G
  7. In a DNA molecule the purines and pyrimidines are always equal in number.

Therefore, the ratio  

  is constant for a particular species.




Functions of DNA:


  1. DNA contains the genetic information in a coded form and transfers it from
    one cell to the other by the process of replication.
  2. It, thus, plays an important role in the transmission of hereditary characters
    from one generation to the other.
  3. It determines and regulates the synthesis of enzymes and proteins required
    by the cell.
  4. A change in the nucleotide sequence in a DNA molecule can result in mutations.




Circular DNA Molecules


  1. In prokaryotes such as bacteria and virus, the genetic material is a single circular chromosome, which consists of a double stranded DNA.
  2. This double stranded DNA is called the genome or nucleoid, which is attached
    to the cell membrane at least at one point.
  3. It is also referred to as the circular DNA or bacterial chromosome.
  4. It is not surrounded by a nuclear membrane or the nucleolus and lies freely in the cytoplasm.
  5. It contains 80 percent DNA and 20 percent proteins and RNA.
  6. In bacteria such as Escherichia coli, the DNA is a of highly twisted, loop-like
    structure called super-coiled DNA or looped domains.
  7. The looped domains show many super coils, which are independent with respect to other parts.
  8. Sometimes, the DNA appears without any super coiling and is said to be relaxed.




Replication of DNA in Eukaryotes:


  • The property by which a DNA molecule makes exact copies or replicas of itself is called self-duplication or replication.
  • It takes place during the interphase of cell division and results in two identical molecules from one molecule.
  • The process includes the following steps:

    1. The two strands of DNA uncoil by breaking the hydrogen bonds with
      the help of DNA unwinding protein (enzyme helicases) and form a
      replicating fork (Y-shaped structure).
    2. Each strand of DNA fork acts as a template.
    3. The replication process is initiated by a small RNA molecule called RNA primer, which gets attached to the template strand and attracts the complementary nucleotides from the surrounding nucleoplasm.
    4. The base pairing A = T and C G is established with the help of enzyme DNA polymerase, which moves along the DNA strand.
    5. This results in the formation of complementary strands between the two old strands of the DNA fork.
    6. These strands are exact replicas of the old strands.
    7. The arm of the fork on which the complementary DNA is synthesised in a continuous manner in the 5' - 3' direction is called the leading strand.
    8. On the other strand DNA synthesis take place in the form of small fragments called Okazaki fragments.
    9. This strand is called the lagging strand.
    10. Okazaki fragments are joined with each other with the help of enzyme
      DNA ligase.




Semi-conservation Method


  1. This method of replication is known as the semi-conservative method as out of the two newly synthesised DNA strands, one strand which is old or retained from the original molecule and the other strand is new.
  2. It was experimentally proved by Meselson and Stahl in 1958.
  3. They grew E. coli bacteria in a culture medium containing heavy isotope of Nitrogen (15N)
  4. They, then, transferred them to another eight culture medium containing (14N) and observed the replication.
  5. They saw that the replicated DNA was a hybrid, formed by one old strand containing 15N and one new strand containing 14N.


Circular DNA: The circular chromosome is called circular DNA or prokaryotic DNA.




Supercoiled DNA: The circular DNA of Escherichia coli has 50 or more highly twisted loop-like structures called supercoiled DNA or looped domain of DNA. Circular DNA molecule without coiling is called relaxed DNA.





Replication of Prokaryotic Chromosome


  1. The prokaryotic DNA is double stranded and circular.
  2. Its replication starts at a specific point called point of origin.
  3. The process is bi-directional, i.e. it proceeds in both the directions from the origin.
  4. The replication is brought about with the help of the enzyme DNA polymerase.
  5. A theta (q) shaped structure appears during replication and ultimately the two circular molecules get separated.





  1. Plasmids are small circular DNA molecules, which are extra-chromosomal and are often seen in the cytoplasm of most bacterial cells.
  2. They are also called Mini chromosomes.
  3. They show antibiotic resistance.
  4. They are widely used in genetic engineering.
  5. Episomes are specific plasmids, which may be present in the cytoplasm and
    can replicate independently or integrate into bacterial DNA.




Ribo Nucleic Acid (RNA)


  1. RNA is also a type of nucleic acid present in both nucleus and cytoplasm.
  2. Unlike DNA, it is short, mostly single stranded and made up of a few hundred nucleotides.
  3. The nitrogen bases present in RNA are similar to those of DNA, except that thymine (T) is replaced by uracil (U).
  4. It may be genetic or non-genetic.
  5. Non-genetic RNA has no genetic role, e.g. cellular RNA.
  6. Genetic RNA has an important genetic role and is sometimes double stranded.




Types of Non-genetic RNA and their Role in Protein Synthesis:


  • There are three types of non-genetic RNA:
    1. Messenger RNA (mRNA)
    2. Ribosomal RNA (rRNA)
    3. Transfer RNA (tRNA)




Messenger RNA:


  1. It is linear and formed in the nucleus on one of the strands of DNA by transcription.
  2. It is also called as template RNA.
  3. It carries information for protein synthesis from DNA to ribosomes.
  4. This information is always carried in triplet nitrogen bases as codons.
  5. Each codon codes for a specific amino acid.




Transfer RNA:


  1. It is single stranded but folded.
  2. Generally, it is trifoliate or clover leaf-like.
  3. Sometimes it is hair pin-like.
  4. It is also called soluble RNA.
  5. It is the shortest of all three types.
  6. The nitrogen base triplet present on tRNA is called anticodon. It is
    complementary to the codon of mRNA.
  7. tRNA transfers the specific amino acids from cytoplasm to the complementary codons on mRNA in the correct sequence. Thus, it helps in formation of amino acid chains.




Ribosomal RNA:


  1. It is also single stranded but double at some regions due to folding.
  2. It provides the necessary binding sites for mRNA on the ribosomes.
  3. It helps to orient mRNA so that the codons are read properly.
  4. It helps to release tRNA and mRNA after the transfer of activated amino acid to the peptide chain.




The Genetic Code:


  • The genetic code is the basic genetic information on DNA in code language made of 3 alphabets out of 4, i.e. A, G, C and T, which represent the nitrogen bases.

    Main Characteristics of Genetic Code :
    1. It is a triplet code.
    2. It is commaless and non-overlapping.
    3. The code is universal and used by practically all organisms.
    4. There are 64 codons which represent 20 amino acids. The genetic code, is therefore, said to be degenerate as there are two or more different codons for the same amino acid.




Wobble Hypothesis:


  1. The phenomenon of unusual pairing of codon anti-codon, in which the first two base pairs match with each other but the third one does not match is referred to as, Wobble Hypothesis.
  2. It was suggested by Crick.
  3. It explains the degeneracy of genetic code.




Protein Synthesis (In Eukaryotes)


The process of protein synthesis is completed in two main steps:

  1. Transcription:
    1. The process by which mRNA is synthesised from a single strand of DNA in the presence of enzyme RNA polymerase is called transcription.
    2. This process takes place in the nucleus and then the mRNA moves to the cytoplasm.
  2. Translation:
    • The process by which formation of a polypeptide chain takes place using the sequence of codons on mRNA is known as translation.
    • It involves the following steps:
      1. Activation of Amino Acids
        • The amino acids which are present in the cytoplasm are activated by ATP and enzymes amino acyl tRNA synthetases.
        • The activated amino acid gets attached to the C-C-A end (carrier end) of the specific tRNA to form amino acyl tRNA complex.
      2. Transfer of Amino Acids
        • Protein synthesis starts on the mRNA at the start codon AVG
        • The specific amino acid is transferred to ribosome by tRNA.
        • The first amino acid carried by tRNA is methionine.
      3. Formation and Termination of Peptide Chain:
        • The ribosomes move along with mRNA and at each step an amino
          acid is added to the chain with the help of codon-anti codon sequence.
        • The amino acids are joined by peptide bonds, with the help of enzymes peptide synthetase, which is present in ribosomes.
        • A polypeptide chain is thus formed.
        • The synthesis of a polypeptide chain is terminated at the site of one of the stop codons, i.e. UAA, UAG or UGA.
        • The tRNA, mRNA and ribosome separate and the polypeptide chain is released with the help of releasing factors.




Protein Synthesis in Prokaryotes:


The process of protein synthesis in prokaryotes is more or less similar to eukaryotes, however, there are some differences which are summarised below:

  1. The prokaryotic ribosomes and mRNA are much smaller compared to eukaryotes.
  2. As there is no well-defined nucleus in prokaryotes, the process of translation may begin at one end of mRNA molecule even as the process of transcription is going on at the other end.



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