XII BIOLOGY
CH-4 Molecular Basis of Inheritance PART-I The Genetic Material is a DNA
The Discovery of DNA:
1)Modem understanding of
DNA has evolved from the discovery of nucleic acid to the development of the
double-helix model.
2)In 1869, Friedrich Miescher began working with white blood cells which are
the major component of pus from infections.
3)He collected a lot of
pus from bandages at the local hospital.
4)He used a salt solution
to wash the pus off the bandages.
5)When he added a
weak alkaline solution to the cells, the cells lysed and nuclei precipitated
out of thesolution.
6)From the cell nuclei, he
isolated a unique chemical substance to which he called nuclein.
7)Chemically, nuclein has
high phosphorus content.
8)It showed acidic
properties. Hence it was named as nucleic acid.
9) By the early 1900s, we
knew that Miescher's nuclein was a mix (mixture) of proteins and nucleic
acids.
10)There are two kinds of
nucleic acids. -
1) DNA (deoxyribonucleic acid)
2) RNA (ribonucleic acid). The Genetic Material is a DNA:
1)By the early 1900s,
geneticist knew that genes control the' heritance of traits, that genes are
located on chromosome and that chemically chromosomes are mainly composed of
DNA and proteins.
2)Initially, most
geneticists thought that protein are rge, complex molecules and store
information needed to govern cell metabolism.
3)Hence it was assumed
that proteins caused the variation observed within On the other hand, DNA thought to be small
,simple molecule whose composition varied little among species.
4)Over the time,
these ideas about DNA were shown to be
wrong.
5)In fact, DNA molecules are large and
vary tremendously within and among species.
6)Variations in the DNA
molecules are different than the variation in shape, electrical charge and
function shown by proteins so it is not surprising that most researchers
initially favored proteins as the genetic material.
7)Over a period of roughly
25 years (1928-1952), geneticists became convinced that DNA and not protein,
was the genetic material.
1)In 1928, a British e 'teal officer Frederick Griffith performed an
experiment on bacterium Streptococcus pneumoniae that ca ses pneumonia
in humans and other mammals.
2)Griffith used two
strains or two genetic varieties of Streptococcus to find a cure for
pneumonia, which was a common cause of death at that time.
3)The two strains used :
-
.
i. Virulent, smooth,
pathogenic and encapsulated S type.
ii. Non-virulent, rough, non-pathogenic
and non-capsulated R type.
4)Griffith conducted four
experiments on these bacteria.
5)First, when he injected bacteria of strain R to mice, the
mice survived because it did not develop pneumoma.
6)Second, when he injected bacteria of strain S to mice, the
mice developed pneumonia and died.
7)In the third experiment, he injected heat-killed strain S bacteria to
mice, once again the mice survived.
8)In fourth experiment, he mixed heat-killed S bacteria with
live bacteria of strain R and injected to mice.
9)The mice died and
Griffith recovered large numbers of live strain S bacteria from the blood of
dead mice.
10)In these four experiments, something had caused harmless strain R bacterium
to change into deadly S strain bacterium.
11)Griffith showed that
the change was genetic.
12)He suggested that genetic material from heat-killed strain S
bacterium had somehow changed the living strain R bacterium into strain S
bacterium.
13)Griffith concluded that
the R-strain bacterium must have taken up, to what he called a "transforming
principle" from the heat-killed S bacterium, which allowed R strain to
get transformed into smooth-coated bacterium and become virulent.
Avery, McCarty and MacLeod's experiment:
In 1944, after some 10 years of research and experimentation, U. S. microbiologists Oswald T.
Avery, Colin M. MacLeod and M lyn McCarty (all at Rockefeller University in New York) first evidenced to prove the DNA is enetic material (transforming principle), through the experiments.
They purified DNA, RNA proteins (enzymes) and other materials from cell free extract of S cells strain and mixed with heat killed S strain and R cells separately to confirm which one could transform living R cell into S cells.
Only DNA was alJ e to transform harmless strain R into deadly strain S.
Hershey - Chase
Experiment:
1)Hershey and Chase worked
with viruses that infect bacteria i.e. bacteriophages, which are composed of
DNA and protein.
2)They used radioactive
phosphorous 32p in the medium for some viruses and radioactive Sulphur 35S for
some others.
3)They grew some viruses
on a medium that contained radioactive phosphorus and some others on medium
that contained radioactive Sulphur.
1)Length of DNA double helix molecule, in a typical mammalian cell is approximately 2.2 meters. (This can be worked out simply by multiplying the total number of base pairs with distance between the consecutive base pairs).
2)Approximate size of a typical nucleus is 106 m.
3)It is very difficult to accommodate long DNA in small nucleus, therefore, must be condensed, coiled and super coiled to fit inside such small nucleus.
Packaging in Prokaryotes:
1)In prokaryotes like E. coli, cell size is almost 2-3 μ long.
2)They do not have well organized nucleus.
3)It is without nuclear membrane and nucleolus.
4)The nucleoid is small, circular, highly folded, naked ring of DNA which is 1100 μ long m perimeter, containing about 4.6 million base pairs.
5)The 1100 μ long (approximately 1.1 mm, if cut and stretched out) nucleoid is to be fitted or packaged into a cell which is hardly 2-3 μ long.
6)Hence the negatively charged DNA becomes circular, reducing the size to 350 um in diameter.
7)This is further reduced to 30 u m in diameter because of folding/ loopin .
8)40-50 domains (loops) are formed.
9)Formation of loops is assisted by RNA connectors.
10)Each domain is further coiled and supercoiled, there by reducing the size down to 2μ in diameter.
11)This coiling (packaging) is assisted by positively charged, (Histone like DNA binding proteins) proteins and enzymes like DNA gyrase and DNA topoisomerase I, for maintaining super coiled state.
Packaging in
Eukaryotes:
1)Eukaryotes show well
organized nucleus containing nuclear membrane, nucleolus and thread-like
material in the form of chromosomes.
2) In the chromosomes, DNA
is associated with histone and non-histone proteins as was reported by
R.Kornberg in 1974.
3)The organization of DNA
is much more complex in eukaryotes.
4)Depending upon the
abundance of amino acid residues with charged side chains, a protein acquires
its charge.
5)Histones are the
proteins which are rich in lysine and arginine residues.
6)Both these amino acid
residues are basic amino acids and carry positive charges with them.
7)histones are a set of
positively charged, basic proteins (histones + protamine).
8)These his tones organize
themselves to make a unit of 8 molecules known as histone octamer .
9)The negatively charged
helical DNA is wrapped around the positively structure known as nucleosome.
10) The nucleosome core is
made up of two molecules of each of four types of histone proteins viz. H2A,
H2B,H3B and H4.
11) H1 protein binds the
DNA thread where it enters and leaves the nucleosome.
12)One nucleosome
approximately contains 200
basepair long DNA helix wound
around it .
13)About 146 base pair
long segment of DNA f ains present in each nucleosome.
14)Nucleosomes are the
repeating units of chro" arm, which are thread-like, stained (coloured)
bodies present in nucleus.
15)These look like 'beads-on-string', when observed under an electron
microscope.
16) DNA helix of 200 bps
wraps around the histone octame 1¾ turns.
17)Six such nucleosomes
get coied an then form solenoid that looks like coiled telephone wire.
18)The chromatin is packed
to form a solenoid structure of
30 nm diameter (300A0) and
further supercoiling tends to form a looped structure called chromatin
fiber.
19)which further coils and
condense at metaphase stage to form the chromosomes.
20The packaging of em
&atin at higher levels, need additional set of proteins that are called
Non-Histone Chromosomal prote ns (NHC).
Non-Histone Chromosomal proteins (NHC):
These are additional sets of proteins that
contribute to the packaging of chromatin at a higher level.
Heterochromatin and Euchromatin:
1)Heterochromatin:
i)In eukaryotic cells, some segments of chromonema/ chromosome during interphase and early prophaseremain in a condensed state.
ii)These regions constitute heterochromatin. This term was proposed by Heitz.
iii)These regions are localized near centromere, telomeres and are also intercalated.
iv)It is genetically mostly inactive. It stains strongly and appears dark.
v) Heterochromatin is 2 to 3 times richer in DNA than in the euchromatin.
2)Euchromatin:
i)The regions of chromonema which are in non-condensed state, constitute euchromatin.
ii)Euchromatic regions stain light.
iii)Euchromatin is genetically very much active and fast replicating.
iv)Euchromatin is transcriptionally active, while heterochromatin is transcriptionally almost inactive.
1)What is the backbone of the DNA structure?
Deoxyribose sugar (pentose sugar) and phosphate group makes the backbone of DNA
2)Name the nitrogen bases of DNA.
The four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C).
3)What are Nucleoside and Nucleotide?
Nucleoside consists of a nitrogenous base covalently attached to a sugar (ribose or deoxyribose) but without the phosphate group.
A nucleotide , is one of the structural components, or building blocks, of DNA and RNA. It consists of a base (one of four chemicals: adenine, thymine, guanine, and cytosine) plus a molecule of sugar and one of phosphoric acid.
4)Is the double helix riqht or left handed?
DNA is a right-handed helix. Normal B-DNA, as first described by Watson and Crick, is a right-handed helix. GC-rich DNA can also exist in a form known as Z-DNA, which forms a left-handed helix
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