Chromosomes

DNA double helix

Chromosomes:

A chromosome is a nuclear large macromolecule of extensively coiled DNA double helix (which carries the genetic information). The word chromosome is derived from the Greek chroma (= color) and soma (= body) owing to the fact that it can be stain with different dyes.

Living cells can be divided into two main groups: prokaryotic and eukaryotic according to the organization of their cellular structures. Animals, plants, fungi, protozoan, and algae are eukaryotic cell types. Only bacteria have prokaryotic cell types which are generally much smaller and of more simple structure than eukaryotic.

In prokaryotic cells, chromosomes are in the form of small circular DNA molecules known as plasmids.

In eukaryotic cells, the DNA is linear, extremely long (about 2 meters) and is extensively coiled and packed into a chromosome of 2 to 10 micrometers in length. Human cells contain 46 chromosomes divided into 23 pairs.

The DNA double helix is bound to proteins called histones.  The histones have positively charged (basic) amino acids to bind the negatively charged (acidic) DNA.

DNA

Double stranded polymer (helix) of polynucleotide chains.

Bind together by hydrogen bonds between the bases of the nucleotides in complementary manner. A=T and GΞC.

The two strands run antiparallel (in opposite direction). Thus one of them is running in the 5` to the 3` direction, and the other runs in the direction of the 3` to the 5` direction.

Note that transcription (reading the genetic code) occurs in the direction of the 5` to the 3` direction.

RNA

It is transcribed form the DNA into the form of a single strand polymer of nucleotide.

The pyrimidine Uracil replaces the thymine in DNA.

The pentose sugar is ribose.

It is less stable than DNA.

There are four main types: messenger mRNA, transfer tRNA, and ribosomal rRNA