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Molecular Basis of Inheritance

Q1. Describe the structure of double helix model of DNA.


  1. DNA is made up of two polynucleotide chains, where the backbone is made up of sugar and phosphate groups.

  2. The nitrogenous bases project towards the centre.

  3. The two Chains have anti-parallel polarity. It means, if one chain has the polarity 5'→3', the other has 3'→5'.

  4. Both Strands of DNA are helically coiled in a right-handed fashion.

  5. The nitrogen basses (adenine, guanine, cytosine, thymine) are paired through hydrogen bond forming base pairs (bp).

  6. Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine with three hydrogen bonds. As a result, always a purine comes opposite to a pyrimidine.

  7. The pitch of the Helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately 0.34 nm.

Q2. Mention and describe the salient features of genetic code.


  1. The codons are triplet: Out of 64 codons, 61 code for 20 amino acids and 3 codons (UAA, UGA, UAG) do not code for any amino acid, hence function as stop or terminating codons.

  2. Genetic codes are unambiguous and specific: One codon code for only one particular amino acid.

  3. Genetic codes are degenerate: Some amino acids are code by more than one codon.

  4. Genetic codes are without punctuations and comma less: The codon is read on mRNA in a continuous fashion.

  5. Genetic code is nearly universal: i.e., a particular codon codes for the same amino acid in all organisms except in mitochondria and few protozoa.

  6. AUG is a dual function codon, its codes for methionine (met) and it is also acts as initiator codon.

Q3. List out the goals of human genome project.


  1. Identify all the approximately 20,000 to 25,000 genes in human DNA.

  2. Determine the sequences of the 3 billion chemical base pairs that make up human DNA.

  3. Store this information in database.

  4. Improve tools for data analysis.

  5. Transfer related Technologies and information to other sectors, such as industries.

  6. Address the ethical, legal and social issues [ELSI] that may arise from the project.

Q4. Mention any five salient features of human genome project.


  1. The Human Genome contains 3164.7 million bases pairs.

  2. The average gene consists of 3000 bases; the largest known human gene being dystrophin at 2.4 million bases.

  3. The total number of genes is estimated to be 30,000 and 99.9% nucleotide bases are exactly the same in all people.

  4. The functions are unknown for over 50% of the discovered genes.

  5. Less than 2% of the genome codes for proteins.

  6. The Human genome contains large repeated sequences.

  7. Chromosome 1 has most genes (2968) and the Y has the fewest genes (231).

  8. Scientists have identified about 1.4 million locations where single base DNA sequence differences called single nucleotide polymorphism or SNPs occur in humans.

Q5. Explain the process of translation in eukaryotes.


  1. Charging of tRNA (aminoacylation of tRNA) - activation of amino acids in the presence of ATP and their linking to specific tRNA.

  2. Initiation - Binding of the ribosome (small sub-unit) to mRNA at the initiator codon (AUG).Binding of the initiator tRNA carrying the amino acid (methionine) to the initiator codon to initiate protein synthesis.