DOUBLE HELIX MODEL OF DNA
Watson
and Francis Crick (1953) proposed a double-helix model of DNA, based on the
X-ray diffraction data, produced by Maurice Wilkins and Rosalind Franklin.
Later
Ervin Chargaff observed that in a double stranded DNA, the ratios between
adenine and thymine and that between guanine and cytosine are constant (A=T
& G=C). DNA is made up of two polynucleotide chains. The bases in the two
strands are held together by hydrogen bonds forming base pairs; adenine pairs
with thymine through two hydrogen bonds and guanine with cytosine through three
hydrogen bonds.
The
two chains have an antiparallel polarity (one chain has a 5'’-3'’
polarity and the other has 3'’-5'’
polarity).
The
stacking of bases creates two types of grooves called major and minor grooves. The
two chains are coiled in a right handed fashion. The pitch of the helix is 3.4
nm (34A0) .There are about 10 base pairs in each turn with
0.34nm(3.4A0) between two base pairs.
PACKAGING OF DNA
The
long sized DNA are accommodated in small areas( about one micro millimeter in E
coli and 5 micron nucleus in human beings) only through packing or compaction.
DNA
is acidic due to presence of a large number of phosphate groups. Packaging
occurs by folding and attachment of DNA with basic proteins. Non histone in prokaryotes and
histones in eukaryotes.
In
prokaryotes with no well defined nucleus, the DNA(negatively charged) lies in
cytoplasm. It is super coiled with the help of RNas and non histone basic
proteins (that have positive charges) like polyamines. The compacted mass
of DNA is called nucleoid or prochromosome.
In
eukaryotes, there is a set of positively charged basic proteins called
histones. Histones are rich in the basic amino acid residues lysines and
arginines. Histones are of five types H1,H2A,H2B,H3, AND H4. To assist in
packaging of very long DNA molecules in a very small cell histones are
organized to form a unit of eight molecules called histone octamer (2 molecules
of each H2A,H2B,H3& H4).
The
negatively charged DNA is wrapped around positively charged histone octamer
into a structure called nucleosome. Histone H1 is a linker histone which binds
DNA of two adjacent necleosomes. A typical nucleosome consists of 200 bp of DNA
helix.
The
nucleosomes are the repeating units that form chromatin fibres. The chromatin
fibres condense at metaphase stage of cell division to form chromosomes. (Dna,
nucleosome, solenoid, chromatin fibre, chromatid and chromosome.)
The
packaging of chromatin at higher level requires additional set of proteins
called non-histone chromosomal (NHC) proteins. In a nucleus certain regions of
the chromatin are loosely packed and they stain lighter than the other regions,
these are called euchromatin. The other regions are tightly packed and they
stain darker and are called heterochromatin. Euchromatin is transcriptionally
more active than heterochromatin.
Ribonucleic acid
(RNA)
RNA is a non hereditary nucleic acid except in some viruses (retroviruses). It is a polymer of ribonucleotides and is made up of ribose sugar, phosphoric acid and nitrogenous bases( adenine, uracil, cytosine and Guanine). The chemical composition of RNA differ from DNA in two aspects:
The pentose sugar in RNA is ribose sugar while it is deoxyribose sugar in DNA.Thymine in DNA is replaced by uracil in RNA. All other nitrogenous bases are same in both.
Types of RNA
The RNA may be mainly two types - genetic RNA and non genetic RNA.
Genetic RNA is seen in most of the plant viruses and some animal viruses eg. In TMV, poliovirus , influenza virus, etc. RNA act as genetic material.
Types of non genetic RNA
On the basis of molecular size and function three main forms of non genetic RNA are mRNA, tRNA and rRNA. mRNA constitutes about 3.5 % of cellular RNA, tRNA is about 15 % and rRNA about 80%.
Messenger ribonucleic acid (mRNA)
It is also called informational RNA or template RNA. It is a molecule of RNA that is transcribed from a gene and then translated by ribosomes in order to manufacture protein. It is responsible for carrying the genetic information present in DNA, It act as a messenger of information contained in DNA.
In eukaryotes mRNA caries information for one polypeptide only. It
is monocistronic because it is transcribed from a single cistron ( gene).
In prokaryotes mRNA carries information for more than one
polypeptide chains. Such an mRNA
is said to be polycistronic because it is transcribed from many adjascent genes.
tRNA( transfer RNA)
It is also known as soluble RNA, acceptor RNA or adaptor RNA. It
is the smallest RNA. It transfers an
amino acid from cytoplasm to the site of protein synthesis. It has a cloverleaf shaped two dimensional structure with
four recognisable sites.
Amino acid binding site
tRNA molecule have single stranded CCA-OH sequence at the 3 prime end. This is called aminoacid attachment site.
DHU loop
DHU loop contains dihydrouridine. It is binding site for aminoacyl synthetase enzyme.
T€ C loop
It contains pseudouridine. The loop is the site for attaching to ribosomes.
Anticodon loop
It is made up of three nitrogen bases for recognising and attaching to the codon of mRNA.
Extra arm is a variable site or loop which lies between T€C loop and anticodon loop.
Ribosomal RNA
It is a component of the ribosomes, the protein synthetic factories in the cell. It is formed in nucleolus. It is the most stable type of
RNA. Depending upon sedimentation
coefficient Eukaryotic ribosomes contain for different rRNA molecules:
18S, 5.8S, 28S and 5S rRNA.
Prokaryotic ribosomes have three types of rRNA: 23S, 16S, and 5S.
Small nuclear RNA (snRNA)
It is the name used to refer to a number of small RNA molecules found in the nucleus. They are always found associated with specific proteins and the complexes are referred to as small nuclear ribo nucleoproteins or sometimes as snurps.
CHARACTERISTICS OF GENETIC MATERIAL
DNA versus RNA
RNA as a catalyst is also more reactive. Therefore DNA chemically is less reactive and structurally more stable when compared to RNA.
Both DNA and RNA are able to mutate. In fact RNA being unstable, mutate at the faster rate. Hence
DNA has the property to be genetic material.
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