M.S.Thanusan

Nucleic acids are capable in directing their own replication from their monomers. In most organisms, DNA is found as the genetic material. However , some viruses such as infl uenza virus, consist RNA as their genetic material. Accurate replication of DNA, its transmission from one generation to the other and its ability to store and express hereditary information facilitates DNA for acting as vital genetic material in organisms.

DNA Double Helix Model: James Watson and Francis Crick, based on the X-ray crystallography of a DNA molecule obtained by Rosalind Franklin, proposed the double helix model to describe how the six molecules; de-oxy ribose sugar, phosphate group and four diff erent nitrogenous bases, are organized into DNA and explain its properties. According to this model, DNA is a twisted ladder (spiral staircase) in which the rails are composed of alternating phosphate and sugar molecules forming the back bone. The steps of the ladder are paired nitrogenous bases. A purine is paired with a pyrimidine as per the base pairing rule with either two hydrogen bonds (A=T) or three (G ≡ C). T.H. Morgan and his group, through their experiments concluded that chrosomes are made up of DNA and proteins and genes as regions of chromosomes. Architecture of Chromosomes Architecture of chromosomes is the way the DNA molecules are arranged in the nucleoid or nuclear region of the cytoplasm of the prokaryotic cells or in the nucleus of eukaryotic cells.

Nucleus

Nucleolus Chromatin

Eukaryote

Nucleoid (Folded Chromosome)

Prokaryote Figure 7.1: DNA is packaged into a nucleus in eukaryotes and a nucleoid in prokaryotes. 1G.C.E.(A/L) Biology

DNA of both prokaryotes and eukaryotes are called chromosomes. However, true chromosomes

are present only in eukaryotes.

Prokaryotic (bacterial) chromosome is a single double stranded circular DNA molecule associated

with a few protein molecules. Eukaryotes have several chromosomes, each consisting of a single

double stranded linear DNA associated with histones and other protein molecules.

When the size of all chromosomes of an organism are considered, it is an enormous amount

of DNA. These facts illustrate that, a cell has a huge problem in having its DNA either in the

nucleoid of prokaryotes or the nucleus of eukaryotes. The containment of the genome/DNA in the

nucleoid or in the nucleus is called DNA packaging.

Packaging of DNA in prokaryotes is facilitated by the proteins associated with the DNA molecule.

The protein molecules cause the DNA to coil (or fold or loop) and supercoil, so the DNA molecule

is compacted into the nucleoid. The DNA molecule is initially coiled into loops and these loops

then independently supercoil into domains identifi able in electron micrographs. The loops of

compacted mass of DNA are bound to a ‘core’ consisting of RNA and protein. The ‘core’ also

attaches the chromosome to the membrane. The supercoiled DNA can be relaxed by introducing

single strand nicks. Since the chromosomes are attached to the membrane and held together by

the ‘core’ acting as a barrier for rotation, the domains can relax and supercoil independently.

This is important in the transcription of specifi c genes. Removal of RNA will lead to loss of

independence of the loops.

RNA-Protein core

350μ

30μ

2μ

a) Circular, unfolded

Chromosome

b) Folded chromosome

40 to 50 loops

c) Supercoiled,

chromosome

Figure 7.2:Folding and supercoiling compact the prokaryotic chromosomes

In addition to the chromosomal DNA, certain prokaryotes have extra chromosomal genetic

elements called plasmids. They are also circular DNA and are coiled and supercoiled.

2Resource Book G.C.E.(A/L) Biology

Eukaryotic chromosomes are associated with a large number of proteins called histones that helps to organize the DNA inside the nucleus. This DNA-protein complex is known as chromatin. The chromatin may be lightly packed as in euchromatin or tightly packed as in heterochromatin. Euchromatin is rich in genes and is probably active in transcription. Heterochromatin consists nucleotide sequences which are mostly inactive. They may contribute in gene regulation, epigenetic inheritance and protection from chromosomal integrity.

In the fi rst level, the double helix winds around a complex of eight histone molecules. These are called nucleosomes and they look like beads of a necklace. The adjoining beads of nucleosomes are linked together by a stretch of DNA: linker DNA. (Figure 7.3)

H2A H2B DNA (a) 550 Linker

Nucleosome "bead" (8 histone molecules + 146 basez pairs of DNA)

(b)

Figure 7.3: a) First level of packing: Forming beads of nucleosomes joined together by linker DNA, b) Electron micrograph showing nucleosome (beads) and linkers (strings)

In the second level, the nucleosomes twist and pack in a spiral fashion to form a chromatin fi bre of roughly 30 nm in diameter:30 nm fi bres from 10 nm fi bres. (Figure 7.4)

Nucleosome core particle

30nm DNA 10 nm fi ber 10 nm 30 nm fi ber Figure 7.4: Formation of a 30 nm fi ber (Cylindrically coiled) 3 A 10nm Linker DNA 1100 A