Nucleic acid, DNA and RNA

NUCLEIC ACIDS 

Nucleic acids are so called because they are slightly acidic and present mainly in the nucleus.

The two types of nucleic acid, DNA and RNA, both contain carbon, hydrogen, oxygen, nitrogen and phosphorus.

Nucleic acids are polymers of nucleotides.

Nucleotides are the building blocks of nucleic acids. A nucleotide consists of three units:

  • A sugar (ribose or deoxyribose),
  • A phosphate group,
  • A nitrogen-containing base.

In a typical nucleotide the nitrogenous base is attached to position 1 of pentose sugar, while phosphoric acid is attached to carbon at position 5 of pentose sugar.

DNA has nucleotides in which the sugar is deoxyribose, while RNA contains the sugar ribose.

DEOXYRIBONUCLEIC ACID (DNA)

A German chemist, Friedrich Miescher, discovered DNA in 1869. Miescher extracted a white substance from the nuclei of human cells and fish sperm. He called this substance nuclein because it was associated with the nucleus. Since nuclein was acidic, it was called as nucleic acid.

Most of the DNA in a eukaryotic cell is in the nucleus and in very less amount in Mitochondria and Chloroplasts.

The nucleotides in DNA contain the nitrogenous (nitrogen-containing) bases which are: adenine, guanine, cytosine or thymine.

Names of four nucleotides of DNA are:

  • d-adenosine monophosphate (d-AMP).
  • d-guanosine monophosphate (d-GMP).
  • d-cytidine monophosphate (d-CMP).
  • d-thymidine monophosphate (d-TMP).

When DNA replicates (copies itself), it makes new strands by adding nucleotides. These are available as free molecules in the cytoplasm. Generally, cells can synthesis their own nucleotides.

How DNA Carries the Genetic Code?

DNA has two remarkable characteristics:

  • It is a store of genetic information.
  • It can copy itself exactly, time after time.

How the Bases Pair?

Adenine and guanine are purines (large/double ring) while thymine and cytosine are pyrimidines (small/single ring).

Because of the shape of the two types of molecules, each purine always bonds with only one pyrimidine. So, in DNA, adenine always bonds with thymine, and cytosine with guanine. In RNA, cytosine bonds with guanine and adenine bonds with uracil:

DNA: A = T                 RNA: A = U

G  C                          G  C

The base pairs are held together by hydrogen bonds. There are two H-bonds between A and T (or U) and three between C and G.

A nucleotide has ester bond. A polynucleotide chain has phosphodiester bonds.

In phosphodiester linkage, one phosphate group is linked to the two sugars by means of a pair of ester (P-O-C) bonds.

According to Erwin Chargaff the quantity of A and T are almost equal. Similarly the quantity of G and C are almost equal. It means A with T and G with C in DNA double strand.

British Chemist Rosalind Franklin carried on an X-ray diffraction analysis of DNA in the Lab of Maurice Wilkins.

Maurice Wilkins (British biochemist) prepared DNA fibres.

The diffraction pattern suggested that the DNA molecule had a shape of a helix with a diameter of 2 nm and a complete helical turn every 3.4 nm.

James Watson and Francis crick proposed structure of the DNA molecule.

In DNA the sides are formed by alternating sugar-phosphate units, while the base pairs form the cross-bridges, like the rungs of a ladder. Each base pairing causes a twist in the helix and there is a complete 3600 turn every 10 base pairs.

DNA & Histones form Eukaryotic chromosomes.

A gene is a unit of biological inheritance.

The E. coli genome consists of 4,639,221 base pairs. It codes for about 4288 proteins.

The first microbe whose genome is completely sequenced is Haemophilus influenzae. It was published in July 28, 1995.

The arrangement of genes on the DNA and the arrangement of nucleotides on the genes is called Genome sequencing.

RIBONUCLEIC ACID (RNA)

RNA is synthesized by DNA in a process known as transcription.

RNA is present in the Nucleus/nucleolus, ribosomes, cytosol and in smaller amounts in other parts of the cell.

The RNA molecule has single strand. It may fold back on itself, to give double helix.

ATP is highly unstable nucleotide. It is used as energy currency by the cell.

All types of RNAs are synthesized from DNA in the nucleus and then are moved out in the cytoplasm to perform their specific functions.

Three of the bases in RNA — adenine, guanine and cytosine — are the same as those in DNA. The fourth is different: RNA contains uracil instead of thymine.

RNA molecules are much smaller than DNA molecules.

DNA can consist of over 300,000,000 nucleotides; RNA usually consists of a few hundred.

RNA is also less stable.

DNA molecules are the permanent store for genetic information and last for many years. In contrast, RNA molecules have a short-term function and are easily replaced.

There are three forms of ribonucleic acid (RNA) in the cell:

  1. Messenger RNA (mRNA) can be thought of as a mobile copy of gene. Small lengths of mRNA are assembled in the nucleus using a single gene within the DNA as a template. When a complete copy of the gene has been produced, the mRNA moves out of the nucleus to the ribosome, where the protein is synthesized according to the code taken from the DNA. For a protein molecule of 1,000 amino acids, the length of mRNA will be 3,000 nucleotides. The mRNA is about 3 to 4% of the total RNA in the cell.
  2. Transfer RNA is found in the cytoplasm and is a carrier molecule, bringing amino acids to the ribosomes for assembly into a new amino acid chain, according to the order specified on the mRNA code. There is one specific tRNA for each kind of amino acid. So there are at least 20 kinds of tRNAs in the cell. tRNAs are about 10 to 20% of the total RNA in the cell. Human cells contain about 45 different kinds of tRNA molecules.
  3. Ribosomal RNA makes up part of the ribosome. Ribosomal RNA is up to 80% of the total RNA in the cell. In ribosome, rRNA is 40—50%. During translation, rRNA provides the site where polypeptides are assembled.

CONJUGATED MOLECULES

Two different molecules, belonging to different groups, combine to form conjugated molecules.

When carbohydrates combine with proteins, glycoproteins are formed. Cellular secretions are mostly glycoporteins. When carbohydrates combine with lipids, glycolipids are formed.

Glycolipids are present in cell membranes along with glycoproteins.

Lipoprotein is formed by lipids and proteins. It forms the basic structure of all types of membranes in the cells.

Nucleic acids are combined with basic proteins to form nucleoproteins. The nucleoproteins/nucleohistones are present in chromosomes. These proteins play important role in regulation of gene expression.

IMPORTANT POINTS:

  1. Disaccharide: A sugar made up of two monosaccharides, such as glucose and fructose, joined together by a glycosidic bond.
  2. Glycosidic Bond: A covalent bond linking two monosaccharides via oxygen atom.
  3. Hemophilia: A genetic disease linked to the X chromosome in which the body lacks a protein necessary for normal blood clotting.
  4. Hydrolysis is a “splitting with water” process in which one larger molecule is split into two monomers by the addition of water molecules.
  5. Antibodies: Globular blood proteins that are produced by B lymphocytes and that bind specifically to foreign antigenic materials in the body, and destroy them.
  6. Macromolecules: An extremely large molecule, with a molecular weight of about 10,000 daltons or more that is an aggregation of smaller molecules and contributes to the diversity of organic structure.
  7. Immunoglobulin: A protein antibody molecule.
  8. Messenger RNA (mRNA): The type of RNA (ribonucleic acid) that encodes information from DNA and is translated into the corresponding protein structure (amino acid
  9. Monosaccharide: A sugar monomer that is the basic carbohydrate subunit of more
  10. Myosin: A mechanoenzyme protein that, in the form of thick filaments, interacts with actin to bring about the contraction of muscle cells.
  11. Nucleic acid: A polymar chain made up of nucleotide sub-units that are arranged in a specific linear sequence. The two types of nucleic acids are deoxyribonucleic acids (DNA) and ribonucleic acid (RNA).
  12. Nucleotide: The building block of nucleic acid, made up of a nitrogen containing base, a five carbon sugar and a phosphate group.
  13. Phospholipid: A lipid molecule in which the glycerol is linked to at least one other molecule containing a phosphate group. Phospholipids are fundamental components of the cell membranes.
  14. Pyrimidine: (pie-rim-ih-deen) a single ringed nitrogen containing base that is a structural component of nucleic acids. The bases cytosine, thymine, and uracil are pyrimidines.
  15. Steroid: Any member of the class of lipid compounds that is composed of four interconnected rings of carbon atoms linked with various functional groups. Some steroids act as vitamins, other as hormones.
  16. Polysaccharide: A long chain carbohydrate made up of large number of monosaccharides linked by Glycosidic bonds.

RELATED READINGS 1 ADDITIONAL SYLLABUS OF NUST 1 AGHA KAHN

  1. Water is 65 — 89% in different organisms (20% water in bone cells and 85% in brain cells of human).
  2. The number of calories required to raise the temperature of 1g of water from 15 to 16C is called as the specific heat capacity of water. Its value is 1.0 for water.
  3. Water works as temperature stabilizer for the organisms and protects living material from sudden thermal changes. This is the importance of specific heat capacity of water in biology.
  4. The amount of heat required to convert liquid into gas is called heat of vaporization. OR Heat of vaporization is the calories absorbed per gram vaporized. The specific heat of vaporization of water is 574 Kcal/kg.
  5. Heat of vaporization provides cooling effect to the plants when water is transpired. Similarly it provides cooling effect to animals when water is respired. Heat of vaporization plays an important role in the regulation of heat produced by oxidation. This is the importance of heat of vaporization in biology.

SOME IMPORTANT POINTS TO REMEMBER

Property / Character Value / No.
% of proteins out of total dry weight of body Over 50%
A turn of DNA 34 angstroms
Amino acids as constituents of proteins 25
Amino acids in hemoglobin 574
Amino acids in Insulin 51 (21 + 30)
Amino acids in most of the proteins 20
Amount of solar energy required to synthesize 10 g glucose 717.6 Kcal
Amount of water in bone cells 20%
Amount of water in brain cells 85%
Concentration of H+ / OH in pure water 10-7 mole/liter
Glucose in human blood 0.08%
Heat of vaporization of water 574 Kcal/kg
Melting point of butyric acid -8C
Melting point of palmitic acid 63.1C
Number of amino acids in each turn of alpha helix 3.6
Number of base pairs in each turn of DNA 10
Number of base pairs in genome of E. Coli 4,639,221 for 4288 proteins
Number of carbon atoms in alkanes of waxes 25-35
Number of carbon atoms in fatty acids 2-30
Number of carbon atoms in natural monosaccharides 3-7
Position of nitrogenous base on pentose sugar 1
Position of phosphate on pentose sugar in polynucleotide chain 3, 5
Proteins in human body More than 10,000
Range of water in different organisms 68-89%
Specific gravity of fats and oils 0.8
Specific heat capacity of water 1.0
Total amino acids 170

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