Maurice Wilkins and Rosalind Franklin used the technique of X-ray diffraction to determine the structure of DNA. At the same time James D. Watson and Francis crick built the scale model of DNA. All the data thus obtained strongly suggested that 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). 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 G and C pair. The two strands are wound around each other so that there are 10 base pairs in each turn of about 34 angstrom units (one angstrom = one 100-millionth of a centimeter) fig. 2.15).
Fig. 2.15:Model of DNA. Double helical structure of DNA proposed by Watson & crick (b). A hypothetical sequence of nucleotides (on the left side) shows hydrogen bonding between the complementary bases. Note a double bond between A and T, and triple bond between C and G (a).
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 (Table 2.3).
Table 2.3 amount of DNA/nucleus in different types of cells of a chicken (bird) and a carp (fish).
|
Type of cell |
Amount of DNA/nucleus (in pictogram) |
|
|
Chicken |
Carp |
|
| Red blood cells
Lever cells Kidney cells Sperm cells |
2.3 2.4 2.4 1.3 |
3.3 3.3 3.3 1.6 |
|
All the information for the structure and functioning of a cell is stored in DNA. For example in the chromosome of the bacterium<E.coli, each of the paired strand of DNA contains about 5 million bases arranged in a particular linear order, the information in those bases is divided into units of several hundred bases each. Each unit is a gene, a unit of biological inheritance. The genome consists of 4,639,221 base pairs, which code for at least 4288 proteins. |
|
Haemophilusinfluenza is the first microbe to have the genome completely sequenced and this was published on July 28,1995 |
RAN (Ribonucleic Acid)
Like DNA, RNA is a polymer of ribonucleotides. The RNA molecules occur as single strand, which may be folded back on itself, to give double helical characteristics. The nitrogenous bases form the usual complementary pairing viz. cytosine (C) with guanine (G) and uracil (U) with adenine (A). RNA is synthesized by DNA in a process known as transcription.
Types of RNA
Three main types of RNAs – messenger RNA (abbreviated as mRNA), transfer RNA (abbreviated as tRNA), and ribosomal RNA (abbreviated as rRNA) are recognized. All these three types of RNAs are synthesized from DNA in the nucleus and then are moved out in the cytoplasm to perform their specific functions.
Messenger RNA (mRNA): as the name indicates it takes the genetic message from the nucleus to the ribosomes in the cytoplasm to form particular 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. This type of RNA 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 3,000nucleotides. mRNA is about 3 to 4% of the total RNA in the cell.
Transfer RNA (tRNA): it comprises about 10 to 20% of the cellular RNA. Transfer RNA molecules are small, each with a chain length of 75 to 90 nucleotides. It transfers amino acid molecules to the site where peptide chains are being synthesized. There is one specific tRNA for each amino acid. So the cell will have at least 20 kinds of tRNA molecules. Transfer RNA picks up amino acids and transfers them to ribosomes, where they are linked to each other to form proteins.
Ribosomal RNA (rRNA): 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. It acts as 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.