Bio-molecules Nucleic acids Preparation and Properties
→ The polymeric molecules made up of carbon, nitrogen, hydrogen oxygen and phosphorous which determinesb heredity characters of living beings are called nucleic acid. The characteristic features are transmitted from one generation to the next, by the process of inheritance.
→ The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of biomolecules called nucleic acids. The combination of protein and nucleic acids are called nucleoproteins.
→ Nucleic acid are very important for life as the ‘genes’ having genetic code in chromosomes are made from them.
→ Nucleic acids are helpful in control of protein synthesis in various cells.
Molecular Organisation of Nucleic acids :
→ First of all, in 1869, Swiss scientist Friedrich Miescher isolated nucleic acids from nuclei of pus cells and was named as nuclein. After that in 1889, Richard Altman, gave them the name ‘Nucleic acids’.
→ Nucleic acids are colurless, complex and shapeless molecules. There are two types of nucleic acids. They are:
- Deoxyribonucleic acid (DNA) and
- Ribonucleic acid (RNA).
→ DNA is found mostly in nucleus and to some extent in cytoplasm, motochondria, and chloroplast, RNA is mostly found in cytoplasm and to some extent in nucleus. On hydrolysis, nucleic acids give nucleotides which on further hydrolysis gives nucleosides and phosphoric acid.
→ Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases). In DNA molecules, the sugar moiety is α-D-2-deoxyribose whereas in RNA molecule, it is β-D-ribose. Hydrolysis of nucleic acids can be shown as :
Thus, hydrolysis of nucleic acids gives three types of products:
- Phosphoric acid
- Organic Bases.
Phosphoric acid : It forms esters to — OH groups of the sugars to bind nucleotide segments together.
Sugar : Nucleic acid have two types of pentose sugars—ribose and deoxyribose. In deoxyribose, H—C—H— group. The sugar moiety in DNA is β-D-2-deoxyribose whereas in RNA molecule, it is β-D-ribose. Both the sugars molecules are in furanose form.
Organic Base : These are nitrogenous bases. Two types of organic bases are present in nucleic acids. These are purines and pyrimidine.
Purines : One purine forms a six membered ring which condenses with five membered ring. Adenine and guanine are purines. These are found in both DNA and RNA.
Pyrimidines : These are heterocyclic units. Uracil (u), Thymine (T) and cytosine (C) are main pyrimidines found in nucleic acids. DNA have thiamine, cytosine whereas RNA has uracil and cytosine. In RNA. adenine form bond with uracil and cytosine with guanine. (AU and CG).
Arrangement of Constituents ¡n Nucleic acid :
→ Nucleoside : A unit formed by the attachment of a base to 1’ position of sugar is known as nucleoside. In nucleoside, the sugar carbons are numbered as 1’, 2’, 3’, etc. In order to distinguish these from the bases. In a nucleoside C-1 of sugar, N-1 of pyrimidine base is attached with N9 of purine.
Example; Adenosin (Adenine and ribose-Adenosine) RNA and DNA contains four different types of nucleosides.
- Adenine + Ribose → Adenosine
- Guanine + Rjbose → Guanosine
- Cytosine + Ribose → Cytidine
- Uracil + Ribose → Uridine
- Thymine + deoxyribose → Deoxythymidine
→ Nucleotide : When nucleoside is linked to phosphoric acid at 5-position of sugar moiety, we get a nucleotide. Nucleotides are joined together by phosphodiester linkage between 5’ and 3’ carbon atoms of the pentose sugar. In ribose sugar, it can attach to C-2 sugar. Nucleotide can be represented by three capital letters.
Example: Adenosine + Phosphoric acid — Adenosine monophosphate (AMP).
→ Polynucleotide : Nucleotides joined at C-5 and C-3 of sugars by phosphodiester linkage and forms polynucleotides. These polynucleotides are nucleic acids. These can represented as:
→ The sequence in which sugar, phosphate and base are attached in called primary structure of nucleic acids. These can be represented as:
Difference between DNA and RNA
|RNA is single stranded except in some viruses.||DNA is double stranded except in few viruses.|
|RNA have ribose sugar.||DNA have deoxyribose sugar.|
|Bases present are adenine, guanine, cytosine and uracil.||Bases present are adenine, guanine, cytosine, and thymine.|
|Adenine pairs with uracil.||Adenine pairs with thymine.|
|Purine is not equal to pyrimidine.||Purine is equal to pyrimidine (Chargaffs rule)|
|Regions having complementary nucleotides, pairs and form hair pin loop like structure and helical.||Complementarynucleotides are present throughout the length of the DNA.|
|RNA is genetic material in some viruses.||DNA is the genetic material in all living organisms.|
|Length of RNA is short consisting of only few thousands nucleotides.||Length of DNA is quite large consisting ofmillions of nucleotides.|
|Three types of RNA are present in an organism : mRNA, rRNA, tRNA.||DNA occurs only in one form in an organism.|
|mRNA occurs in nucleolus, rRNA and tRNA occur in cytoplasm.||DNA occurs in nucleus, nucleolus and extrachromosmomal DNA in mitochondria and chioroplast.|
Molecular Structure of DNA :
→ DNA is a double stranded molecule. The primary structure of DNA is shown in Fig. 14.7. DNA is a large molecule whose weight is about thousands. Chargaff in 1950 done chemical analysis is DNA and concluded that (i) number of adenine is always equal to thymine and number of cytosine is equal to guanine. (ii) The ratio of bases in DNA is thfferent in different group but A always bond to T and C with G.
→ In 1953, Wilkins and his companions studied structure of DNA by X-ray crystallorgraphy. After J.D. Wilkins, Watson and F.H.C. Crick in 1953, give double helical structure of DNA by X-ray diffraction studies. He got noble prize for his work in 1962.
Double Helical Structure of DNA :
→ Double helical structure of DNA is given by Watson and Crick. According to them.
→ It is a double helix with two right handed helical polydeoxyribonucleotide strands twisted around the same central axis.
→ The two strands are antiparallel. The phosphodiester linkages of one of these strands runs in 5’ to 3’ direction while the other strand runs in 3’ to 5’ direction. The bases are stacked inside the helix in planes perpendicular to the helical axis.
→ These two strands are held together by hydrogen bonds. In addition to hydrogen bonds, other forces like, hydrophobic interactions between stacked bases are also responsible for stability and maintenance of double helix.
→ Adenine always pairs with thymine while guanine always pairs with cytosine. A-T pair has 2 hydrogen bonds while G-C pair has 3 hydrogen bond. Hence, G C is more stronger than A = T.
→ The content of adenine is equal to the content of thymine and the content of guanine is equal to the content of cytosine. This is Chargaffs rule.
→ which is proved by the complementary base pairing in DNA structure.
→ The genetic information is present only on one strand known as template strand. The double helix structure contains major and minor grooves in which proteins interact with DNA.
→ The diameter of double helix is 2 nm. The double helical structure repeats at intervals of 3.4 nm (one completer turn) which corresponds to 10 base pairs.
→ The 13-conformation DNA hiving the right handed helix is the most stable. On heating the two strands of DNA separate from each other (known as melting) and on cooling these again hybridize (annealing). The temperature at which the two strands separate completely in known as its melting temperature (Tm) which is specific for each specific sequence.
Biological Functions of Nucleic Acids :
There are two main biological functions of nucleic acids :
- Replication of DNA
- Synthesis of Protein.
Replication DNA :
→ Replication can be defined as “the process in which a single DNA molecule produces two identical daughter molecules of itself”. This replication is a semi-conservative process.
- Due to this property of DNA, hereditary characters are transferred to offsprings.
- Replication is a catalytic process.
→ The parent DNA has two complementary strands, which unwinds by breaking hydrogen bonds. The separate strand acts as templates for the synthesis of new strands. As a result two identical copies of DNA are produced.
→ These daughter molecules contain one parental strand and one new strand. Hence, this is known as semi -conservative replication.
→ DNA replication follows the base pairing rules by which A pairs with T and G pairs with C. Thus each daughter molecule is an exact replication of the present molecule.
→ The replication of DNA occurs in 5’ to 3’ direction. The DNA synthesis differs in both strands that is lagging strand and leading strand. In leading strand the synthesis of DNA is continuous where as in lagging strand it is discontinuous. These discontinuous pieces are called as Okazaki fragments. They are joined to from continuous strand by using DNA ligase.
Synthesis of Proteins
→ Protein synthesis is another important function of nucleic acids. The process of protein synthesis is a very complex process. As stated earlier, 20 main acids combine in different ways to synthesise protein. Living cells has above 200 enzymes and more than 70 RNA takes part in protein synthesis. Protein synthesis is done by different RNA molecules. DNA provides code for protein synthesis.
The synthesis of proteins can be explained as:
Protein synthesis takes place in two steps:
- Transcription : Transmission of heredity information to RNA.
- Translation : Transmission of information from RNA to protein, the synthesis of protein from DNA can be represented as:
→ Transcription : Transcription is the process by which DNA gives rise to RNA. It can also be defined as, the process of copying genetic information from one strand of the DNA into RNA is termed as transcription.
→ Three types of RNA takes part in protein synthesis (m-RNA, r-RNA and t-RNA) which are synthesised by DNA.
→ Transcription involves the binding of RNA-polymerase at the promoter site on DNA. As it moves along (through structural gene), the DNA unwinds and one of the two strands acts as template to synthesize a meaningful RNA and other strand act as non-coding. A complementary RNA strand is synthesized with A. U, C and G as bases. RNA synthesis is terminated when the RNA-polymerase falls off a terminator sequence on the DNA.
→ Translation : This process is very complex. The formation of protein from m-RNA is known as translation. More than loo macromolecules are used in this process such as m-RNA, t-RNA, ribosomes etc. It refers to the process of polymerisation of amino acids are defined by the sequence of bases in the m-RNA. The amino acids are joined by a bond which is known as a peptide bond.
→ Protein synthesis takes place in cytoplasm. After transcription, m-RNA moves to ribosome in cytoplasm from nucleus. r-RNA are components of ribosomes. The base sequence in RNA is studied in three groups. This group of three nucleiotide triplet is known as codon.
→ Each codon represents a particular amino acid. t-RNA has specific sequence of bases at the ends which are complementary to base sequence of m-RNA. At the other end of t-RNA a specific amino acid is attached.
→ In ribosomes, amino acids retransferred from t-RNA to m-RNA. Those amino acids whose base sequence of t-RNA which is complementary to base sequence in m-RNA are bonded by m-RNA thorough peptide linkage. It this way, polypeptide chain is formed and protein synthesis takes place.
→ In proteins, sequence of amino acids is determined by m-RNA and in m-RNA, base sequence is determined by DNA. This means that translation of amino acids takes place by DNA on ribosomes.
Genetic Code :
→ Genetic code refers to the relationship between the sequence of nucleotides (introgen bases) on m-RNA and the sequence of amino acids in proteins. Each code in known as codon with three nucleotides (triplet). It has been deciphered by Nirenberg, Khorana, Severo Ochoa and Crick.
→ As stated earlier, code for protein synthesis is determined by DNA. The sequence of nucleotides on DNA is known as gene. In living cells, each protein has a specific gene.
→ 20 amino acids combine to form all types of proteins. Thus, nucleotides should have proper units to code these 20 amino acids. To represent 20 amino acids, three letters are used because if 4 or 2 letters are used, total 16 mixtures are found which are not adequate to represent all amino acids. By taking three letters, total number of mixture formed is 64 which are sufficient to represent all 20 amino acids. There sequences codons.
Some of the features of genetic codes are:
- The codon is triplet. 61 codons code for 20 different amino acids and 3 condons do not for any amino acids, hence they function as stop condons (UAG, UGA and UAA).
- One codon codes for only one amino acid, hence, it is unambiguous and spcific.
- Some amino acids are doded by more than one codon, hence the code is degenerate.
- The codonis read in m-RNA in a contiguous fashion. There are no punctuations.
- The code is nearly universal. For example, from becteria to human. UUU would code for Phenylalanine (phe) amino acid.
- AUG has dual function. It codes for methionine (met), and it also act as initiator codon.