Chromosomal Substances - Bacterial Genetics

Chromosomal Substances

Structure of DNA

The DNA is the key basic component of gene, which carries the genetic information that is transcribed onto ribonucleic acid and then translated as the particular polypeptide. The basic structure of DNA molecule was first described by Watson and Crick for which they were honored with the Nobel Prize in Medicine. The DNA molecule is composed of two strands of complementary nucleotides wound together in the form of a double helix. The double helix has a diameter of 2 nm. Each full turn of the double helix contains 10 nucleotide pairs and is 3.4 nm in length.

◗ DNA strand

Each DNA strand has a backbone of deoxyribose (sugar) and phosphate group residues arranged alternately. It has a sugar– phosphate backbone substituted with purine and pyrimidine bases. It contains four nitrogenous bases, two purines (adenine and guanine), and two pyrimidines (thymine and cytosine).

The two complementary strands are held together by hydro-gen bonds between the nitrogenous bases on the opposite strands. The hydrogen bonding follows a specific binding manner in which hydrogen bonds are formed only between guanine and cytosine and between adenine and thymine. Guanine and cytosine form a complementary base pair and adenine and thymine form another base pair.

A molecule of DNA therefore contains as many units of adenine as thymine and of guanine as cytosine. For example, when the arrangement of bases along one strand is AGCTAG, the arrangement on the other strand will be TCGATC. The ratio of adenine and thymine to guanine and cytosine is constant for each species, but varies widely from one bacterial species to another.

During replication, the DNA molecule replicates, first by unwinding at one end to form a fork and then by separation of strands at the other end. Each strand then acts as a template for the synthesis of a complementary strand with which it then forms a double helix.

◗ Gene

It is a segment of DNA that carries codons specifying for a particular polypeptide. A DNA molecule consists of a large num-ber of genes, each of which contains hundreds of thousands of nucleotides. The DNA of a bacterial chromosome is usually arranged in a circular form and when straightened, it measures around 1000 μ. The length of DNA is usually expressed as kilo-bases (1 kbp = 1000 base pairs, or bp). Bacterial DNA measures usually 4000 kbp and the human genome about 3 million kbp.

Structure of RNA

Basically, the structure of RNA is similar to that of DNA except for two major differences:

a)        In DNA, the sugar is D-2-deoxyribose; in RNA, the sugar is D-ribose.

b)       The RNA contains the nitrogenous base uracil instead of thymine that is present in DNA.

On the basis of structure and function, the RNA can be differentiated into three types:

c)        Messenger RNA (mRNA),

d)       Ribosomal RNA (rRNA), and

e)        Transfer RNA (tRNA).

The RNA molecules range in size from the small tRNAs (which contain fewer than 100 bases) to mRNAs (which may carry genetic messages extending to several thousand bases). Bacterial ribo-somes contain three kinds of rRNA with respective sizes of 120, 1540, and 2900 bases and a number of proteins. Corresponding rRNA molecules in eukaryotic ribosomes are somewhat larger.

A few RNA molecules have been shown to function as enzymes (ribozymes). For example, the 23S RNA in the 50S ribosomal subunit catalyzes the formation of the peptide bond during protein synthesis. Some small RNA molecules (sRNA) function as regulators either (a) by binding near the 5' end of an mRNA, preventing ribosomes from translating that message, or (b) by base pairing directly with a strand of DNA near the promoter, preventing transcription.