QUESTION:
Write a detailed note on Nucleic acid?
NUCLEIC ACIDS:
Definition:
Nucleic acids were first isolated in 1870 by an Austrian physician Friedrich Miescher from the nuclei of pus cells. Due to their isolation from nuclei and their acidic nature, they were named nucleic acids.
Types of Nucleic Acids:
Nucleic acids are of two types:
1.Deoxyribonucleic acid or DNA
2. Ribonucleic acid or RNA
1. Deoxyribonucleic acid or DNA:
DNA occurs in chromosomes, in the nuclei of the cells and in much lesser amounts in mitochondria and chloroplasts.
2. Ribonucleic acid or RNA:
RNA is present in the nucleolus, ribosomes, in the cytosol and in smaller amounts in other parts of the cell.
Chemical Nature of Nucleic Acids:
Nucleic acids are complex substances. They are polymers of units called nucleotides. DNA is made up of deoxyribonucleotides, while RNA is composed of ribonucleotides.
Structure of Nucleotide: Each nucleotide is made of three subunits
1. 5-carbon monosaccharide (a pentose sugar)
2. Nitrogen containing base
3. Phosphoric acid.
Pentose Sugar: Pentose sugar in ribonucleotide is ribose, while in deoxyribonucleotide it is deoxyribose.
Nitrogenous Base: Nitrogenous bases are of two types:
1. Pyrimidines (single ringed): Pyrimidines are cytosine (abbreviated as C), thymine (abbreviated as T), and uracil (abbreviated as U).
2. Purines (double ringed): Purines are adenine (abbreviated as A) and guanine (abbreviated as G).
Phosphoric acid: Phosphoric acid (H,PO,) has the ability to develop ester linkage with OH group of pentose sugar.
Arrangement of molecules: In a typical nucleotide the nitrogenous base is attached to the position 1 of pentose sugar, while phosphoric acid is attached to carbon at position 3 of pentose sugar in front and to carbon at position 5 of pentose sugar behind it. Since phosphate forms a double ester inkage with pentose sugar, the linkage is known as phosphodiester linkage. The compound formed by combination of a base and a pentose sugar is called nucleoside. A nucleoside and a phosphoric acid combine to form a nucleotide.
Each nucleotide of RNA contains ribose sugar, whereas sugar nucleotide of DNA is deoxyribose (one oxygen removed from OH group at carbon number 2).
ATP is also an important nucleotide use as an energy currency by the cell.
DNA (DEOXYRIBONUCLEIC ACID):
DNA is the heredity material. It controls the properties and potential activities of cell.
DNA Nucleotides (Components of DNA): DNA is made of four kinds of nucleotides:
1. Adenosine monophosphate (AMP, also known as adenylic acid)
2.Guanosine monophosphate (GMP, also known as guanylic acid)
3. Cytidine monophosphate (CMP, also known as cytidylic acid)
4 Thymidine monophosphate (TMP, also known as thymidylic acid).
These nucleotides are united with one another through phosphodiester linkages in a specific sequence to form long chains known as polynucleotide chains. Two nucleotides join together to form dinucleotide whereas three join together to form trinucleotide. Nicotinamide adenine dinucleotide, abbreviated as NAD, is an example of dinucleotide. It is an important coenzyme in several oxidation-. reduction reactions in the cell.
Discovery of DNA: During 1950s, Linus Pauling worked out that DNA is a fibrous substance and the fibre is coiled into a helix. In 1951 Erwin Chargaff provided data about the ratios of different bases present in this molecule. This data suggested that adenine and thymine are equal in ratio and so are guanine and cytosine.
Maurice Wilkins and Rosalind Franklin used the technique of X-ray difraction to determine the structure of DNA. At the same time James D. Watson and Francis Crick built the scale model of DNA.
Fig . show amount of bases in DNA in (percentage)
STRUCTURE OF DNA:
Double helix: Al the data thus obtained strongly suggested thaf DNA is made of two polynucleotide chains or strands. The two strands are coiled round each, other in the form of a double helix. Coiling of two strands is opposite i.e. they are coiled antiparallel to each other. The two chains are held together by weak bonds (hydrogen bonds).
Attachment of nucleotides: Adenine (A) is always opposite to thymine (T), and guanine (G) and cytosine (C) are opposite to each other. There are two hydrogen bonds between A and T pair, and three hydrogen bonds between Gand C pair.
Dimensions of DNA: The two strands are wound around cach other so that there are 10 base pairs in each turn of about 34 Angstrom units (one Angstrom = one 100-millionth of a centimeter).
Amount of DNA: The amount of DNA is fixed for a particular species, as it depends upon the number of chromosomes. The amount of DNA in germ cells (sperms and ova) is one half to that of somatic cells. Amount of DNA/nucleus in different types of cells of a chicken (bird) and a carp (fish).
WORK ON DNA:
DNA as a hereditary material: DNA contains the genetic information, which is passed on from one generation to another.
Experimental evidence that DNA is hereditary material: The evidence that DNA is the genetic material came first from studies on a bacterium known as Pneumococcus.
Work of T. Avery, McLeod and McCarty: Avery, McLeod and McCarty provided experimental evidence to prove that virulence of Pneumococcus, which is due to its smooth outer surface, can be transmitted to bacteria, which have secondarily lost their virulence by mutation.
Work of Hershey and Martha Chase: Hershey and Martha Chase studied life history of bacterial viruses. Bacterial viruses consist of DNA, which is enclosed in a protein coat. At the time of infection, the viruses inject their DNA into a bacterium, while protein coat is left out in the process. The new viruses are made inside the host bacterium, under the instructions of viral DNA, which was injected into the bacterium. Obviously DNA is the genetic material.
RNA (RIBONUCLEIC ACID): RNA is a polymer of ribonucleotides.
Structure of RNA: The RNA molecules occur as single strand, which may be folded back on itself, to give double helical characteristics.
Attachment of nucleotides: The nitrogenous bases form the usual complementary pairing viz. cytosine (C) with guanine (G) and uracil (U) with adenine (A).
Synthesis of RNA: RNA is synthesized by DNA in the nucleus and then is mov ed out in the cytoplasm to perform its specific functions. The process of RNA synthesis is called transcription.
Types of RNA: There are three main types of RNAS.
Messenger RNA (abbreviated as mRNA)
Transfer RNA (abbreviated as RNA)
Ribosomal RNA (abbreviated as rRNA)
Messenger RNA (MRNA):
Amount: MRNA is about 3 to 4% of the total RNA in the cell.
Structure: MRNA consists of a single strand of variable length. Its length depends upon the size of the gene as well as the protein for which it is taking the message. For example, for a protein molecule of 1,000 amino acids, mRNA will have the length of 3,000 nucleotides.
Function: It takes the message from the nucleus to the ribosomes in the cytoplasm to form particular proteins.
Transfer RNA (IRNA):
Amount: It comprises about 10 to 20% of the cellular RNA.
Structure: Transfer RNA molecules are small, each with a chain length of 75 to 90 nucleotides.
Function: Transfer RNA picks up amino acids and transfers them to ribosomes, where they are linked to each other to form proteins. Messenger RNA carries the genetic information from DNA to ribosomes, where amino acids are arranged according to the information in mRNA to form specific protein molecule.
Ribosomal RNA (rRNA):
Amount: It is the major portion of RNA in the cell, and may be up to 80% of the total RNA. It is strongly associated with the ribosomal protein where 40 to 50% of it is present.
Function: It acts as a machinery for the synthesis of proteins. On the surface of the ribosome the mRNA and tRNA molecules interact to translate the information from genes into a specific protein .
Question:
What are conjugated molecules? Give some examples.
Conjugated Molecule:
Answer:
Definition:
Two different molecules, belonging to different categories, usually combine together to form conjugated molecules. These conjugated proteins are not only of structural, but also are of functional significance. They play an important role in regulation of gene expression.
Glycoproteins and Glycolipids: Carbohydrates may combine with proteins to form glycoproteins or with lipids to form glycolipids.
Functions:
1. Most of the cellular secretions are glycoproteins in nature.
2. Both glycoproteins and glycolipids are integral structural components of plasma membranes.
Lipoproteins: Combination of lipids and proteins form lipoproteins.
Function: They are basic structural framework of all types of membranes in the cells.
Nucleoproteins Nucleic acids have special affinity for basic proteins. They are combined together to form nucleoproteins.
Functions: The nucleohistones are present in chromosomes.
0 Comments