Biology 3 Marks Question

→ Some of the salient observations drawn from human genome project are as follows:
(i) The human genome contains 3164.7 million bp.
(ii) The average gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million bases.
(iii) The total number of genes is estimated at 30,000 - much lower than previous estimates of 80,000 to 1,40,000 genes. Almost all (99.9 per cent) nucleotide bases are exactly the same in all people.
(iv) The functions are unknown for over 50 per cent of the discovered genes.
(v) Less than 2 per cent of the genome codes for proteins.
(vi) Repeated sequences make up very large portion of the human genome.
(vii) Repetitive sequences are stretches of DNA sequences that are repeated many times, sometimes hundred to thousand times. They are thought to have no direct coding functions, but they shed light on chromosome structure, dynamics and evolution.
(viii) Chromosome 1 has most genes (2968), and the Y has the fewest (231).
(ix) Scientists have identified about 1.4 million locations where single-base DNA differences (SNPs - single nucleotide polymorphism, pronounced as 'snips') occur in humans. This information promises to revolutionise the processes of finding chromosomal locations for disease-associated sequences and tracing human history.

→ In eukaryotes, there are two additional complexities
(i) There are at least three RNA polymerases in the nucleus (in addition to the RNA polymerase found in the organelles). There is a clear cut division of labour. The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S), whereas the RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs). The RNApolymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA).
(ii) The second complexity is that the primary transcripts contain both the exons and the introns and are non-functional.
→ Post-Transcriptional changes
→ Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order.
hnRNA undergoes additional processing called

→ It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation.
→ The significance of such complexities is now beginning to be understood. The split-gene arrangements represent probably an ancient feature of the genome.
→ The presence of introns is reminiscent of antiquity, and the process of splicing represents the dominance of RNA-world.
→ In recent times, the understanding of RNA and RNA-dependent processes in the living system have assumed more importance.

→ The salient features of genetic code are as follows:
(i) The codon is a triplet. 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons.
(ii) Some amino acids are coded by more than one codon, hence the code is degenerate.
(iii) The codon is read in mRNA in a contiguous fashion. There are no punctuations.
(iv) The code is nearly universal:for example, from bacteria to human UUU would code for Phenylalanine (phe). Some exceptions to this rule have been found in mitochondrial codons, and in some protozoans.
(v) AUG has dual functions. It codes for Methionine (met), and it also act as initiator codon.
(vi) UAA, UAG, UGA are stop terminator codons.

→ Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44), the genetic material was thought to be a protein.
→ They worked to determine the biochemical nature of 'transforming principle' in Griffith's experiment.
→ They purified biochemicals (proteins, DNA, RNA, etc.) from the heat-killed S cells to see which ones could transform live R cells into S cells.
→ They discovered that DNA alone from S bacteria caused R bacteria to become transformed.
→ They also discovered that protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not affect transformation, so the transforming substance was not a protein or RNA.
→ Digestion with DNase did inhibit transformation, suggesting that the DNA caused the transformation.
→ They concluded that DNA is the hereditary material, but not all biologists were convinced.

→ As polymorphism in DNA sequence is the basis of genetic mapping of human genome as well as of DNA fingerprinting, it is essential that we understand what DNA polymorphism means in simple terms.
→ Polymorphism (variation at genetic level) arised due to mutations.
→ New mutations may arise in an individual either in somatic cells or in the germ cells (cells that generate gametes in sexually reproducing organisms).
→ If a germ cell mutation does not seriously impair individual's ability to have offspring who can transmit the mutation, it can spread to the other members of population (through sexual reproduction).
→ Allelic sequence variation has traditionally been described as a DNA polymorphism if more than one variant (allele) at a locus occurs in human population with a frequency greater than 0.01.
→ In simple terms, if an inheritable mutation is observed in a population at high frequency, it is referred to as DNA polymorphism.
→ The probability of such variation to be observed in non-coding DNA sequence would be higher as mutations in these sequences may not have any immediate effect/ impact in an individual's reproductive ability.
→ These mutations keep on accumulating generation after generation, and form one of the basis of variability / polymorphism.
→ There is a variety of different types of polymorphisms ranging from single nucleotide change to very large scale changes.

→ From the very beginning of the proposition of code, it was clear to Francis Crick that there has to be a mechanism to read the code and also to link it to the amino acids, because amino acids have no structural specialities to read the code uniquely.
→ He postulated the presence of an adapter molecule that would on one hand read the code and on other hand would bind to specific amino acids.
→ The tRNA, then called SRNA (soluble RNA), was known before the genetic code was postualated.
→ However, its role as an adapter molecule was assigned much later.

→ tRNA has an anticodon loop that has bases complementary to the code, and it also has an amino acid acceptor end to which it binds to amino acids.
→ tRNAs are specific for each amino acid
→ For initiation, there is another specific tRNA that is referred to as initiator tRNA. There are no tRNAs for stop codons.
→ The secondary structure of tRNA has been depicted that looks like a clover-leaf.
→ In actual structure, the tRNA is a compact molecule which looks like inverted L.

→ The genetic code can be defined as the set of certain rules using which the living cells translate the information encoded within genetic material (DNA or mRNA sequences). The ribosomes are responsible to accomplish the process of translation.
→ During replication and transcription a nucleic acid was copied to form another nucleic acid. Hence, these processes are easy to conceptualise on the basis of complementarity.
→ The process of translation requires transfer of genetic information from a polymer of nucleotides to synthesise a polymer of amino acids.
→ Neither does any complementarity exist between nucleotides and amino acids, nor could any be drawn theoretically.
→ There existed ample evidences, though, to support the notion that change in nucleic acids (genetic material) were responsible for change in amino acids in proteins.
→ This led to the proposition of a genetic code that could direct the sequence of amino acids during synthesis of proteins.
→ If determining the biochemical nature of genetic material and the structure of DNA was very exciting, the proposition and deciphering of genetic code were most challenging.
→ In a very true sense, it required involvement of scientists from several disciplines physicists, organic chemists, biochemists and geneticists.
→ It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases.
→ He suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides.
→ This was a very bold proposition, because a permutation combination of 43 (4 x 4 x 4) would generate 64 codons; generating many more codons than required.
→ Providing proof that the codon was a triplet, was a more daunting task. The chemical method developed by Har Gobind Khorana was instrumental in synthesising RNA molecules with defined combinations of bases (homopolymers and copolymers)
→ Marshall Nirenberg's cell-free system for protein synthesis finally helped the code to be deciphered.
→ Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful in polymerising RNA with defined sequences in a template independent manner (enzymatic synthesis of RNA).