Uploaded for students,2023-2024
DNA and Chromosome
20.1.1 define chromosomes with examples of some
organisms with different number of chromosomes
(penicillium, corn, sugarcane, mosquito, honey bee, mouse
and human being);
● a chromosome is a threadlike structure of nucleic acids
and protein found in the nucleus of most living cells,
carrying genetic information in the form of genes.
● it appear inside the nucleus at the time of cell
division.
● Chromosomes are the bearer/carries of hereditary
character in the form of genes.
● The number of chromosomes remain constant for a
particular species and it remains constant from
generation to generation.
● Their number however varies from species to species.
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Pencillium has only one pair of chromosomes.
some ferns have more than 500 pairs.
mosquito has 6 chromosomes
honeybee 32 chromosomes
corn 20 chromosomes
sugarcane 80 chromosomes
frog 26 chromosomes
mouse has 40 chromosomes
Human cells have 46 chromosomes, consisting of 23 pairs.
● The purines in DNA are adenine and guanine,
they are same in RNA.
● The pyrimidines in DNA are cytosine and
thymine; in RNA, they are cytosine and uracil.
● Purines are larger than pyrimidines because
they have a two-ring structure while pyrimidines
only have a single ring.
20.1.2 differentiate among types of chromosomes, i.e.
a. autosomes and sex chromosomes
b. homologous and non-homologous chromosomes,
c. telocentric, acrocentric, metacentric and submetacentric
chromosomes;
● autosomes:
● The pair of chromosomes that regulate the somatic
characters of the body are known as autosomes.
● Autosomes control the inheritance of all an organism's
characteristics except the sex-linked ones.
● sex chromosomes:
● A sex chromosome is a type of chromosome that
participates in sex determination.
● Females have two X chromosomes in their cells, while
males have both X and a Y chromosomes in their cells.
● Egg cells all contain an X chromosome, while sperm
cells contain an X or Y chromosome.
● homologous chromosomes:
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homologous chromosomes are similar paired
chromosomes.
They essentially have the same sets of genes , same loci
(gene position), same centromere location, and same
chromosomal length. Although they may have the same
sets of genes and same loci, but they may differ in alleles.
● homologous chromosomes are a set of one maternal and
one paternal chromosome that pair up with each other
inside a cell during fertilization.
● non-homologous:
● Non-homologous chromosomes are chromosomes that do
not belong to the same pair.
● Generally, the shape of the chromosome, that is, the length
of the arms and the position of the centromere, is different
in non-homologous chromosomes.
● Therefore, non-homologous chromosomes do not pair
during meiosis.
C.TYPES OF CHROMOSOMES:According to the position of
centromere, there are four different types of chromosomes.These
chromosomes- acquire different shapes at the time of anaphase
during cell division.
1. METACENTRIC: A chromosome with a centrally placed centromere
that divides the chromosome into two arms having approximately
equal length.chromosome look like V-shaped structure.
2. SUB METACENTRIC : chromosome whose centromere is located
near the middle. As a result, the chromosome arms are unequal in
length and may also form an L-shape or J-shaped structure.
3.TELOCENTRIC:Location
of centromere is at the end of
chromosome/at the terminal end of chromosome .chromosome
look like rod- shaped in structure.
4.ACROCENTRIC:The centromere is sub terminal in position.
.chromosome look like rod- shaped. with one arm very small and the
20.1.3 describe levels of eukaryotic chromosomal
organisation;
● Chromosomes are composed of DNA and protein. Most are
about 40% DNA and 60% protein. A significant amount of
RNA is also associated with chromosomes.
● The DNA of a chromosome is very long double stranded
fiber that stands unbroken through the entire length of
chromosome.
● NUCLEOSOME:The coiling of DNA is like string of beads.
DNA duplex cells around a core of eight histone protein
forming a complex structure called nucleosome.
● Mostly the proteins have negative charge but histone have
positive charge.
● The DNA have negatively charge so it is strongly attracted
to histone and wrap tightly around the histone core.
● The core thus acts as magnetic forms that promotes and
guide the coiling of DNA.
● Nucleoprotein (histone) gives DNA fiber as beaded
structure.
● The nucleosome is repeated after every 200 nucleotides of
DNA.
*SPACER DNA:
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The DNA present from one nucleosome (bead) to another is
called Spacer DNA.
*SUPER COIL:
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Further coiling of DNA occurs when the string of
nucleosome wraps into higher order coils called Super Coil.
● Genes :
● are present on chromosomes and they are very
small units .
● Genes are made up of DNA. Some genes act as
instructions to make molecules called proteins. However,
many genes do not code for proteins.
OR
● Every cell contains DNA. Each DNA contains specific
segments called genes. That control specific cellular
functions, either by synthesizing enzymes, proteins or by
regulating the action of other genes.
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chromosomes and genes are composed of DNA
and Protein.
20.1.4 Describe chromosome karyotype:
● Karyotyping is the process by which photographs of
chromosomes are taken in order to determine the
chromosome complement of an individual, including the
number of chromosomes and any abnormalities(number
/structures of chromosomes.).
● A karyotype is a laboratory technique that produces an
image of an individual's chromosomes.
(394) A Human Karyotype Preparation Animation - YouTube
(394) Steps of Karyotyping - YouTube
20.1.5 differentiate between heterochromatin and euchromatin;
When in prophase stage, chromosome is treated with
acetocarmine,two distinct regions of chromosome are visible
according to the amount of protein DNA and RNA that is
heterochromatin and euchromatin.
EUCHROMATIN
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These regions have light color .
They form large part of chromosome.
These are active regions and involved in transcription
(formation of RNA in Protein Synthesis).
● euchromatin is present in an open configuration and its
genes can be expressed.
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this region do not show contraction except in cell
division.
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HETEROCHROMATIN:
These regions have dark color /Stains deeply.
it is more coiled and compact/They contain double amount
of DNA.
They are present between small folds/These regions show
contraction.
They are inactive and do not take part in the formation of
RNA.
it is never expresses itself.
20.2.1 trace chromosomal theory of inheritance from Karl
Correns 1900 to Thomas Hunt Morgan 1910;
● Karl Correns rediscovered Mendel's laws in 1900’
● A central role for chromosomes in heredity was first
suggested in 1900 by the German geneticist Karl Correns,
Soon after, observations that similar chromosomes paired
with one another during meiosis led directly to the
chromosomal theory of inheritance.
● One was that reproduction involves the initial union of only
two cells, egg and sperm. If Mendel’s model was correct,
then these two gametes must make equal hereditary
contributions.
● Sperm, however, contain little cytoplasm, suggesting that
the hereditary material must reside within the nuclei of the
gametes.
● Furthermore, while diploid individuals have two copies of
each pair of homologous chromosomes, gametes have only
one.
● This observation was consistent(set of ideas)with Mendel’s
model, in which diploid individuals have two copies of each
heritable gene and gametes have one.
● Finally, chromosomes segregate during meiosis, and each
pair of homologous orients on the metaphase plate
independently of every other pair.
***OBJECTIONS:
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There is however one problem with this theory. If Mendelian
characters are determined by genes located on the
chromosomes, and if the independent assortment of
Mendelian traits reflects the independent assortment of
chromosomes in meiosis, why does the number of
characters that assort independently in a given kind of
organism often greatly exceed the number of chromosome
pairs the organism possesses? .
● This seemed a fatal objection to Sutton’s theory.
This has led many early researchers to have serious
reservations about Sutton’s theory.
20.2.2 infer (conclude from evidence and reasoning)chromosomal theory of
inheritance by Hunt Morgan 1910;
***In 1910 Thomas Hunt Morgan, studying the fruit fly/Drosophila
melanogaster, detected a mutant male fly, one that differed strikingly from
normal flies of the same species its eyes were white instead of red.
● Morgan crossed mutant male to a normal female. All F1 progeny had red
eyes.
● He then crossed red eyed flies from F1 generation with each other.
● Of the 4252 F2 progeny Morgan examined, 782 (18%) had white eyes.
Although the ratio of red eyes to white eyes in the F2 progeny was greater
than 3:1, the results of the cross nevertheless provided clear evidence that
eye colour segregates.
● However, there was something about the outcome that was strange and
totally unpredicted by Mendel’s theory - all of the white eyed F2 flies were
male!
● How could this result be explained? Perhaps it was
impossible for a white eyed female fly to exist; such
individuals might not be viable for some unknown reason.
● To test this idea, Morgan test crossed the female F1
progeny with the original white eyed male. He obtained both
white-eyed and red-eyed males and females in a 1:1:1:1
ratio, just as Mendelian theory predicted.
● Hence a female could have white eyes. Why, then were
there no white eyed females among the progeny of the
original cross?
● The solution to this puzzle involved sex. The gene causing
the white eye trait in Drosophila resides only on the X
chromosome. It is absent from the Y chromosome.
● A trait determined by a gene on the X chromosome is said
to be sex linked. Knowing that the white eye trait is
recessive to the red eye trait.
● the Morgan’s result was a natural consequence of the
Mendelian assortment of chromosomes.
● Morgan’s experiment was one of the most important in the
history of genetics because it presented the first clear
evidence that the genes determining Mendelian traits do
indeed reside on the chromosomes, as Sutton had
proposed.
● The chromosome theory of inheritance therefore, propounds
that genes are located on chromosomes.
● The segregation of the white-eye trait has one-to-one
correspondence with the segregation of the X chromosome,
in other words.
● Mendelian traits such as eye colour in Drosophila assort
independently because chromosomes do.
20.3.1 explain deoxyribonucleic acid (DNA) as a heredity
material with reference to the experiments conducted by
Frederick Griffith, Colin Macleod and Maclyn McCarty and
Alfred Hershey and Martha Chase;(TEACH IN LAB)
1.Frederick Griffith(SINDH TEXT BOOK)
● The first evidence of hereditary nature of DNA was provided
by a British microbiologist Frederick Griffith.
● Griffith attempted to develop a vaccine against a
bacterium streptococcus, pneumonia which causes the
lungs disease pneumonia.
● Griffith isolated two strains of bacterium which he
designated as , ‘S- type” ( smooth surface and virulent )
and “R” ( rough surface and non virulent).
1.Laboratory mice were injected with live “R” cells no
illness affected on mice and they were alive.
2.Mice were injected with live “S” cells the mice
contracted pneumonia and died.
3. “S” cells were killed with high temperature and mice
injected with these heat killed cells did not develop
Pneumonia and they were alive.
4. Live “R” cells were mixed with heat killed “S” cells and
injected, communication came down with Pneumonia and
mice died.
1.Frederick Griffith(PUNJAB TEXT BOOK):
● When he infected mice with a virulent strain of
Streptococcus pneumoniae bacteria , the mice died of blood
poisoning.The coat was apparently necessary for virulence.
● However, when he infected similar mice with a mutant
strain of S. pneumoniae that lacked the virulent strains
polysaccharide coat, the mice showed no ill effects.
● The normal pathogenic form of this bacterium is referred to
as the S form because it forms smooth colonies on a culture
dish.
● The mutant forms, which lacks an enzyme needed to
manufacture the polysaccharide coat, is called the R form
because it forms rough colonies.
● To determine whether the polysaccharide coat itself had a
toxic effect. Griffith injected dead bacteria of the virulent S
strain into the mice; the mice remained perfectly healthy.
● As a control, he injected mice with a mixture containing
dead S bacteria of the virulent strain and live coatless R
bacteria, each of which by itself did not harm the mice .
● Unexpectedly, the mice developed the disease symptoms
and many of them died. The blood of the dead mice was
found to contain high levels of live, virulent streptococcus
type S bacteria, which had surface proteins characteristic of
the live (previously R) strain.
● Somehow, the information specifying the polysaccharide
coat had passed from the dead, virulent S bacteria to the
live, coatless R bacteria in the mixture, permanently
transforming the coatless R bacteria into the virulent S
variety.
● Transformation is the transfer of. genetic material from one
cell to another and can alter the genetic make up of the
recipient cell.
● Griffith concluded that R strain bacteria had been
transformed by S strain bacteria. The R strain inherited
some 'transforming principle' from the heat-killed S strain
bacteria which made them virulent. And he assumed this
transforming principle as genetic material.
● The agent responsible for transforming Streptococcus went
undiscovered until 1944.
2.Oswald Avery along with Colin Macleod and Maclyn
McCarty:
Oswald Avery along with Colin Macleod and Maclyn McCarty
characterized “Transforming principle”.
● They first prepared mixture of dead S Streptococcus and live R
Streptococcus that Griffith had used.
● Then they removed as much of the protein as they could from
their preparation, eventually achieving 99.98% purity.
● Despite removal of nearly all the protein, the transforming
activity was not reduced.
● Moreover, the properties of transforming principle resembled
those of DNA.
● The protein digesting enzymes or RNA digesting enzymes did
not affect the principle’s activity,
● but the DNA digesting enzyme DNase destroyed all the
transforming activity.
● Avery’s conclusion was provided in 1952 by Alfred Hershey
and Martha Chase who experimented with bacteriophages T2.
3.Alfred Hershey and Martha Chase;
● in 1952 Alfred Hershey and Martha Chase experimented
with bacteriophages T2.
● In some experiments they labelled viruses with radio isotope
32P, which was incorporated into the newly synthesized
DNA of grooving phage.
● In other experiments, the viruses were grown on a medium
containing 35S, an isotope of sulphur which is incorporated
into the amino acids of newly synthesized protein coats.
● After the labelled viruses were permitted to infect bacteria,
the bacterial cells were agitated violently to remove the
protein coats of the infecting viruses from the surfaces of the
bacteria.
● This procedure removed nearly all of the 35S label from the
bacteria.
● However the 32P label had transferred to the interior of the
bacteria and was found in viruses subsequently released
from the infected bacteria.
● Hence, the hereditary information injected into the bacteria
that specified the new generation of viruses was DNA and
not protein.
● Conclusion: DNA was the genetic material. After a phage
particle attaches to a bacterium, its DNA enters through a
tiny hole while its protein coat remains outside.
20.3.2 describe the model of DNA as proposed by Watson
and Crick;
Watson and Crick suggested ladder type organization of DNA.
● Each molecule of DNA is made up of two polynucleotide chains
which are twisted around each other and form a double helix.
● The uprights of ladder are made up of sugar and phosphate parts
of nucleotide
● the rungs are made up of paired nitrogenous bases.
● The pairs are always as follows.Adenine always pairs with
thymine and cytosine always pairs with guanine.
● Two nucleotide chains are held together by weak hydrogen bond.
● There are two hydrogen bonds between A=T and three hydrogen
bonds between C=G.
● both polynucleotide strands remain separated by
20A°(Angstroms) distance.
● The coiling of double helix is right handed and complete turn
occurs after 34A°(Angstroms).
● since each nucleotide occupies 34A°(Angstroms) distance, along
the length of polynucleotide strand there are 10 mono-nucleotide
which occurs in each complete turn.
● The coiling of double helix is right handed and complete turn
occurs after 34A°(Angstroms).
• In their model, base pairs always consist of purines, which
are large, pointing toward pyrimidines which are small,
keeping the diameter of the molecule a constant 2 nm.
● Because hydrogen bonds exist between the bases in a base
pair, the double helix is stabilized as a duplex DNA molecule
composed of two antiparallel strands, one chain running 3’
to 5’ and the other 5’ to 3’.
● The base pairs are planar (Fat) and stack 0.34nm apart as a
result of hyperphobic interactions contributing to the overall
stability of the molecule .
● In the double helix, adenine forms two hydrogen bonds with
thymine, while guanine forms three hydrogen bonds with
cytosine.
● . Adenine will not form proper hydrogen bonds with
cytosine.and guanine will not form hydrogen bonds with
thymine.
● Consequently adenine and thymine will always occur in the
same proportion in any DNA molecule, as well guanine and
cytosine, because of this base pairing.
Nucleotide
Joining of nucleotide vertically
***DNA REPLICATION PROCESS:
1.Replication fork :
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A DNA molecule consists of two strands that wind around
each other like a twisted ladder.
In order to unwind DNA, interactions between base pairs
must be broken. This is performed by an enzyme known as
DNA helicase.
DNA helicase disrupts the hydrogen bonding between base
pairs to separate the strands into a Y shape known as the
replication fork.
Replication fork is bi-directional.
These two strands of fork serve as the template strand on which
new DNA strand grows/builds.
These two strands are known as leading strand and lagging
strand.
2.DNA polymerase role:
● DNA is read by DNA polymerase .
● DNA polymerase matches complementary nucleotides to the
template strand.
● DNA is read by DNA polymerase in the 3′ to 5′ direction,
meaning the new strand is synthesized in the 5' to 3' direction.
(Means DNA polymerase III is that it can add nucleotides only
to the 3’ end of a DNA strand.)
● · As replication always proceeds 5’ —» 3’ direction on a
growing DNA strand.
4.Leading
strand:
Leading strand, which elongates toward the replication
fork, is built up simply by adding nucleotides
continuously to its growing 3’ end.
5.lagging strand :
● The lagging strand is the strand of new DNA whose
direction of synthesis is opposite to the direction of the
growing replication fork. Because of its orientation,
replication of the lagging strand is more complicated as
compared to that of the leading strand. As a consequence,
the DNA polymerase on this strand is seen to "lag behind
(move more slow )" the other strand so DNA polymerase
cannot initiate synthesis on its own.
20.4.1 illustrate semi-conservative replication of DNA;
● The weak hydrogen bond that hold together the double
helix of DNA is broken up by an enzyme DNA helicase .
● starting from the ends like a zip, one by one, each purine
is separate from its pyrimidin partner.
● Each separation leaves an unmatched purine and
pyrimidine bases.
● Then free nucleotides which are present in cellular pool
could pair with exposed bases on both unwound strands.
● Each parent strand remains in fact and a new companion
strand is assembled on each one.
● During the replication, each parent strand is twisted into a
double helix with its new partner strand.
● Through these steps, a new upright, (of sugar and
phosphate) would be supplied for each ladder.
● Thus each strand will have replaced the nucleotide
partners it has lost with ones of exactly the same kind.
● In semi conservative replication, the two strands of the
duplex separate out each acting as a model or mold,
along which new nucleotides are arranged thus giving
rise to two new duplexes.
● The conservative model stated that the parental double
helix would remain intact and generate DNA copies
consisting of entirely new molecules.
semi-conservative replication of DNA
***The Meselson - Stahl Experiment:
● The three hypothesis of DNA replication were evaluated by
Mathew Meselson and Franklin Stahl of the California
Institute of Technology in 1958.
● They grew bacteria in a medium containing heavy isotope of
nitrogen, 15N, which became incorporated into the bases of
the bacterial DNA. After several generations, the DNA of
these bacteria was denser .
● Meselson and Stahl then transferred the bacteria from the
N15 medium to the N14 medium and collected the DNA at
various intervals.
● They dissolved the DNA in cesium chloride and then spun it
at a very high speed in an ultra-centrifuge. DNA strands of
diferent densities got separated.
The enormous centrifugal forces generated by the
ultracentrifuge caused the cesium ions to migrate toward the
bottom of the centrifuge tube, creating a gradient of CsCl,
and thus of density.
● Each DNA floats or sinks in the gradient until it reaches the
position where its density exactly matches the density of
cesium there.
● Because N15 strands are denser than N14 strands, they
migrate farther down the tubes to a denser region of the
cesium chloride gradient.
● The DNA collected immediately after the transfer, was all
dense.
● However, after the bacteria completed their first round of
DNA replication in the N 14 medium, the density of their
DNA had decreased to a value intermediate between N14DNA and N15 - DNA.
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● After the second round of replication, two density classes of
DNA were observed one intermediate and one equal to that
of N14 - DNA .
● Meselson and Stahl Result:
● Meselson and Stahl interpreted their results as follows:
1.after the first round of replication, each daughter DNA
duplex was a hybrid possessing one of the heavy strands of
parent molecule and one light strand.
● 2.When this hybrid duplex replicated, it contributed one
heavy strand to form another hybrid duplex and one light
strand to form a light duplex .
● Thus, this experiment clearly confirmed the prediction of the
Watson-Crick model that DNA replicates in a semiconservative manner.
DNA REPLICATION IS SEMICONSERVATIVE PROCESS
20.5.1 describe gene and genetic code;
Gene :
● Genes are made up of DNA.
● A gene is the basic physical and functional unit of
heredity.
● Genes control specific cellular functions, either by
synthesizing enzymes, proteins or by regulating the action
of other genes./ Some genes act as instructions to make
molecules called proteins. However, many genes do not
code for proteins.
Genetic code;
● Genetic code/codon is a combination of 3 nucleotides,
which specify a particular amino acid.
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A codon is a sequence of three DNA nucleotides that
corresponds (communicate)with a specific amino acid or
stop signal during protein synthesis. Each codon
corresponds (communicate) to a single amino acid (or stop
signal), and the full set of codons is called the genetic
code.
● So genetic code is triplet code and the reading occurs
continuously without punctuation between the three
nucleotide units. .
● Out of 64 codons, three codons UAA, UAG and UGA do
not code for any amino acid and hence are known as
nonsense codons/stop codons.These codons are usually
present at the end of the gene .
● Every gene starts with initiation codon /start codon AUG,
which encodes the amino acid methionine ,it is consider as
initiation codon /start codons .
● There are three nucleotides in a codon/Genetic code ,
because a two nucleotide codon would not yield enough
combinations to code for the 20 different amino acids that
commonly occur in proteins.
● If we take four DNA nucleotides, only 16 different
pairs of nucleotides could be formed
● but if the same nucleotide can be arranged in 64
different combination of three, more than enough to
code for 20 amino acids.
The genetic code is universal. It is the same in almost all the
organisms. For example AGA speciies arginine in bacteria, in
humans and all other organisms whose genetic code has been
studied. Because of the universality of codon, the genes can be
transferred from one organism to another and be successfully
transcribed and translated in their new host. The study of genetic
code of mitochondrial DNA however, showed that genetic code is
not that universal. For example UGA codon is normally a stop
codon but, in mitochondria it reads as tryptophan. Likewise AUA
was read as methionine instead of isoleucine and AG A and AGG
for termination of protein synthesis is instead of arginine. Thus it
appearsthatgenetic code is not quite universal.
20.5.2 describe one gene-one enzyme hypothesis;
genes produce their effects by specifying the structure of
enzymes and that each gene encodes the structure of one
enzyme . this relationship is called one - gene / one - enzyme
hypothesis.
● Archibald Garrod and William Bateson concluded in 1902
that certain diseases among their patients were more
prevalent (widespread/powerful)in particular families. By
examining several generations of these families, Garrod
found that some of the diseases behaved as if they were
the product of simple recessive alleles.Garrod investigated
several disorders in detail.
1.alkaptonuria (disorder);
● In normal individuals, urine contained homogentisic acid
which is broken down into simpler substances.
● In alkaptonuria the patients produced urine that contained
homogentisic acid. This substance oxidized rapidly when
exposed to air, turning the urine black.
● As Garrod concluded that patients suffering from
alkaptonuria lacked the enzyme necessary to catalyze
(accelerate )homogentisic acid breakdown.
● He speculated ( theory)that many other inherited diseases
might also reflect enzyme deficiencies.
● From Garrod’s finding, it could be inferred(conclude) that the
information encoded within the DNA of chromosomes acts
to specify particular enzymes.
● This point was not actually established, however, until 1941,
when a series of experiments by Stanford University
geneticists George Beadle and Edward Tatum provided
definitive evidence on this point.
● Beadle and Tatum deliberately set out (begin a journey)to
create Mendelian mutations in chromosomes and then studied
the effect of these mutations on the organisms.
● Beadle and Tatum exposed Neurospora (fungus) spores to Xrays,( expecting that DNA in some of these spores would
experience damage in the regions encoding the ability to make
compounds needed for normal growth.DNA changes of this kind
are called mutations and the organisms that have undergone
such changes are called mutants).
● Initially, they allowed the progeny(descendant) of the irradiated
spores(expose to radiation) to grow on a defined medium
containing all of the nutrients necessary for growth, so that any
growth deficient mutants resulting from the irradiation would be
kept alive.
● To determine whether any of the progeny of the irradiated spores
(expose to radiation)had mutations ,causing metabolic
deficiencies, Beadle and Tatum placed subcultures(a cultural
group within a larger culture) of individual fungal cells on a
“minimal” medium that contained only sugar, ammonia, salts, a
few vitamins and water.
● Cells that had lost the ability to make other compounds(enzyme)
necessary for growth would not survive on such a medium. Using
this approach, Beadle and Tatum succeeded in identifying and
isolating many growth deficient mutants.
● Next the researchers added various chemicals to the
minimal medium in an attempt to find one that would enable
a given mutant strain to grow.
● This procedure allowed them to pinpoint(identify) the nature
of the biochemical deficiency that strain had.
● The addition of arginine, for example, permitted several
mutant strains, dubbed(nickname) arg mutants, to grow.
When their chromosomal positions were located, the arg
mutations were found to cluster in three areas.
● we know Enzymes are responsible for catalyzing the
synthesis of all the parts of an organism. They are also
responsible for the assembly of nucleic acids, proteins,
carbohydrates and lipids. Therefore, by encoding(convert into
a coded form) the structure of enzymes and other proteins,
DNA specifies the structure of the organism itself.
● For each enzyme in the arginine biosynthetic pathway,
Beadle and Tatum were able to isolate a mutant strain(cell
resulting from genetic mutation) with a defective form of that
enzyme, and the mutation was always located at one of a
few specific chromosomal sites.
● Most importantly, they found there was a different site for
each enzyme. Thus, each of the mutant they examined had
a defect in a single enzyme, caused by a mutation at a
single site on one chromosome.
● Beadle and Tatum concluded that genes produce their
effects by specifying the structure of enzymes and that
each gene encodes the structure of one enzyme . They
called this relationship one - gene / one - enzyme
hypothesis.
● Because many enzymes contain multiple protein or
polypeptide subunits, each encoded by a separate gene,
the hypothesis is today more commonly referred to as “one
gene / one- polypeptide”.
20.5.3 explain mechanism of protein synthesis by means of
DNA and RNA;
GENE EXPRESSION (PROTEIN SYNTHESIS):The genetic
information resides in DNA flows down into RNA, which is then
converted into protein
The nature of protein is based on the number and
sequences of amino acids which are found in cell the
process of gene expression occurs in following steps.
1.Transcription
2.Translation
1.Transcription:
● All organisms use the same basic mechanism of reading
and expressing genes, which is often referred to as central
dogma.
● The first step of central dogma is the transfer of information
from DNA to RNA, which occurs when an mRNA copy of
the gene is produced. The process is called transcription.
● Transcription is initiated when the enzyme RNA polymerase
binds to a particular binding’ site called a promoter located
upstream ( toward the beginning of a stream )of the gene.
● The enzyme then moves along the strand into the gene and
mRNA is synthesized.
● At stop signal on the other end of gene, the enzyme
disengages itself from the DNA and releases the newly
assembled RNA chains.
● This chain is a complementary transcript of the gene from
which it was copied.
● The second step of the central dogma is the transfer of
information from RNA to proteins, which occurs when the
information contained in the mRNA is used to direct the
synthesis of polypeptides by ribosomes.
● This process is called translation, because the nucleotide
sequence of the mRNA is translated into an amino acid
sequence in the polypeptide.
● The two steps of central dogma taken together are also
means of gene expression.
● central dogma.(gene expression)
1.first step of central dogma
2.second step of the central dogma
● steps of transcription:
● Transcription starts at the RNA polymerase binding site
called promoter on the DNA template strand.
● The binding of RNA polymerase to the promoter is the first
step in gene transcription.
● In prokaryotes within promoter there are two binding sites
TTGACA also called -35 sequence and TAT A AT sequence
also called -10 sequence, which have affinity for the RNA
polymerase.
● In eukaryotes promoter sites are at -75 and -25 sites,
respectively.
● One of the subunits of RNA polymerase sigma factor, is
responsible for correct initiation of transcription process.
● Once the transcription has started the sigma factor is
released and the remaining part of the enzyme (core
enzymes) moves over template strand and completes the
transcription of the gene.
● The DNA strands open up at the place where enzyme is
attached to the template strand of DNA forming transcription
bubble.
● The transcription bubble moves down the DNA, leaving the
growing strand/mRNA protruding from the bubble.
● The stop sequences at the end of the gene terminate the
synthesis of mRNA.
● The simplest stop signal is a series of GC base pairs
followed by a series of AT base pairs.
● The RNA formed in this region forms a GC hairpin followed
by four or more U ribonucleotides. The hairpin causes RNA
polymerase to stop synthesis.
● In bacteria the newly synthesized mRNA is directly released
into the cytoplasm, when it is converted into polypeptide
chain.
● In eukaryotes however, it has to travel long distance from
inside the nucleus to ribosomes outside in the cytoplasm.
● The eukaryotic mRNA is therefore modified in several ways
to aid this journey. A cap and a tail is added so that the
molecule may remain stable during long journey to ribosome.
● The cap is in the form of 7 methyl GTP, which is linked 5’ to
5’ with the first nucleotide, whereas tail is in the form of polyA tail linked to 3’ end of the RNA.
● These caps and tails save the mRNA from variety of
nucleases and phosphatases.
Protein Synthesis| Transcription| Translation| - YouTube
from=140
●
In prokaryotes promoter sites:
Note:TRANSCRIPTION(IN EUKARYOTIC CELL);
In eukaryotic cell after the formation of pre-mRNA another
process takes place called RNA splicing .This RNA splicing
process form mature mRNA from pre-mRNA, after this
mature mRNA moves from nucleus to cytoplasm form
translation.
***RNA splicing process:
●
●
RNA splicing is a process in molecular biology where a
newly-made precursor messenger RNA (pre-mRNA)
transcript is transformed into a mature messenger RNA
(mRNA).
During transcription, the entire gene (DNA) is copied
into a pre-mRNA, which includes exons and introns.
During the process of RNA splicing, introns (noncoding regions of RNA, which do not code for any
proteins) are removed and exons (coding regions which
codes for proteins)joined to form a contiguous(together
in sequence) coding sequence.
***RNA splicing process:
● Splicing is usually needed to create an mRNA molecule
that can be translated into protein.
● Splicing occurs in the nucleus before the RNA migrates
to the cytoplasm.
*Intron:
●
An intron is apportion of a pre-mRNA that does not
code for amino acids/ enzymes or structural proteins.
● They come in between the exons.
●
If the introns are not removed, the RNA would be
translated into a nonfunctional protein.
●
Introns remain in the nucleus after being spliced out from
the mRNA transcript during RNA processing.
●
Most of these lariats (discarded byproducts of RNA
splicing) are destroyed within minutes in the cell nucleus.
●
Introns are found in eukaryotic organisms.
*Exons:
●
Exons are the nucleotide sequence in mRNA, which codes
for proteins.
OR
●
An exon is a coding region of a gene that contains the
information required to encode a protein.
OR
●
Exons are coding sections of an mRNA transcript, or the
DNA encoding it that are translated into protein.
●
Exons leave the nucleus to reach the cytoplasm after the
mature mRNAs are synthesized.
*Spliceosome/ spliceosome excision:
●
The spliceosome removes introns from a transcribed premRNA.
●
● Spliceosomes are protein-and-RNA complex found in
eukaryotic nucleus.
They assemble on RNA polymerase II transcripts from
which they excise (remove) introns and splice (joint)
together exons.
●
2.Translation:
● Translation is a stage of polypeptide formation on
ribosome by the messenger RNA, which directs
ribosome that what type of protein should be formed.
● In prokaryotes, translation begins when the initial portion of
an mRNA molecule binds to rRNA molecule on a
ribosome.
● The mRNA lies on the ribosome in such a way that only
one of its codons is exposed at the polypeptide site at any
time.
● A tRNA molecule possessing the complementary three
nucleotide sequence or anticodon, binds to the exposed
codon on the mRNA.
Protein Synthesis Animation Video - YouTube
●
As the ribosome moves along the messenger RNA,
successive codons on the mRNA are exposed and the
series of tRNA molecules bind one after another to the
exposed codons.
● Each of these tRNA molecules carries an attached amino
acid, which is added to the end of the growing polypeptide
chain.
● Particular tRNA molecules become attached to specific
amino acids through the action of activating enzymes called
aminoacyl-tRNA synthetase, one of which exists for each of
the 20 common amino acids.
● In prokaryotes, polypeptide synthesis begins with the
formation of initiation complex .
● First a tRNA molecule carrying a chemically modified
methionine (called N-formyl methionine) binds to the small
ribosomal subunit. Proteins called initiation factor.
● On ribosome in translation processes three site are
established.
● P site (peptidyl site), A site (for aminoacyl site),E site (for
exit site)
1.P site (peptidyl site) :where peptide bonds will form.
2. A site (for aminoacyl site):, where amino acid bearing
tRNAs will bind /enter
3. E site (for exit site) :where empty tRNAs will exit the
ribosome .
● This is called initiation complex.
● After the initiation complex has formed, the large ribosomal
subunit binds tRNA molecule with the appropriate anticodon
appears, proteins called elongation factors assist in binding
it to the exposed mRNA codon at the A site.
● The two amino acids which now he adjacent to each other
undergo a chemical reaction, catalyzed by the large
ribosomal subunit, which releases the initial methionine from
its tRNA and attaches it instead by a peptide bond to the
second amino acid .
● The ribosome now moves (translocates) three more
nucleotides along the mRNA molecule in the 5’ —> 3’
direction, guided by other elongation factors.
● This movement translocates the initial tRNA to the E site
and ejects it from the ribosome, repositions the growing
polypeptide chain (at this point containing two amino acids)
to the P site, and exposes the next codon on the mRNA at
the A site.
● When a tRNA molecule recognizing that codon appears, it
binds to the codon at the A site, placing its amino acid
adjacent to the growing chain. The chain then transfers to
the new amino acid, and the entire process is repeated.
● Elongation continues in this fashion until a chainterminating non sense codon is exposed .
● Nonsense codons do not bind to tRNA, but they are
recognized by release factors, proteins that release the
newly made polypeptide from the ribosomes.
Translation(protein synthesis)
20.6.1 describe types of mutation;
MUTATION:
● Mutation is a change in the nucleotide sequence of a
gene(DNA) or a chromosome.
OR
● A mutation is a change in a DNA sequence. Mutations can
result from replication mistakes (made during cell division),
exposure to ionizing radiation, exposure to chemicals
(mutagens), or infection by viruses.
● Germline mutations occur in the eggs and sperm and can
be passed onto offspring, while somatic mutations occur in
body cells and are not passed on.
● Mutations can broadly be classified as
(i) chromosomal aberration
(ii) point mutation/gene mutation.
1.chromosomal aberration :
● Chromosomal aberrations are concern with the visible
changes in the structure of chromosome .
● Chromosomal aberrations are mega changes .
● During meiosis when chromosomes become interwind (twist
together), there is plenty of opportunity for various kinds of
structural aberration in chromosome .
● chromosomal aberrations lead to syndromes like Down’s
syndrome, Klinefelter’s syndrome etc.
Chromosomal aberrations are of 4 major types:
(a) Deletion (b) duplication (c) inversion and (d) translocation
●
●
(A) Deletion or Deficiency:
there is a loss of segment of chromosome.
● Deletions are detected at the time of homologous pairing.
● The deletion in one chromosome of a pair may be harmful
for the individual but may be lethal if absent in both
chromosomes of a pair
B) Duplication:
● a segment of chromosome is repeated twice, i.e.,
duplicated.
OR
● A portion of the chromosome has been duplicated, resulting
in extra genetic material.
● Duplication produces abnormalities of form and function
(c) inversion:
● A section of the chromosome becomes changed by rotation
at 180° is called inversion. The order of the genes in it are
reversed.
● In this a portion of the chromosome has broken off, turned
upside down, and reattached, therefore the genetic material
is inverted.
● Inversion arises by the formation of loops on a chromosome.
Breaks may occur at the point of intersection of the loops .
● inversion reduce crossing over.
●
(D) Translocation:
● Transfer of a section of one chromosome to non homologous
chromosome is known as translocation.
● It also includes exchange of segments between non
homologous parts of a pair of chromosomes, e.g., ‘X’ or ‘Y’
chromosomes. The segment is neither lost or added it is just
exchanged.
● translocation may give rise to variation within species.
Reciprocal translocations occur when chromosomal
segments are exchanged between two non- homologous
chromosomes and is the most typical type of
translocation. Non-reciprocal translocations are a oneway transfer of a chromosomal segment to another
chromosome. Translocations have two genetic
consequences.
2.Point mutations /Gene mutation:
● Point mutations describe a change in single nucleotide of
DNA, such as that nucleotide is switched for another
nucleotide, or that nucleotide is deleted, or a single nucleotide
is inserted into the DNA that causes that DNA to be different
from the normal .
OR
● A point mutation is mutation where a single nucleotide base
is changed, inserted or deleted from a DNA or RNA sequence
of an organism's genome.
OR
● Point mutations are mutational changes which affect the
message itself, producing alterations in the sequence of DNA
nucleotide .
● If alterations involve only one or a few base pairs in the coding
sequence they are called point mutations.
● Point mutations affect the message itself./it changes genetic
message of a cell.
●
While some point mutations occur due to spontaneous
pairing errors that occur during DNA replication,
● others result from damage to the DNA caused by
mutagens, usually radiations or chemicals.
● modem industrial societies often release many chemical
mutagens into the environment.
● Sickle cell anemia and phenylketonuria are well known
examples of point mutation.
***sickle cell anemia :
***sickle cell anemia :
● it is an inherited red blood cell disorder
●
Sickle-cell is a point mutation( in the β-globin chain of
hemoglobin),in which the amino acid glutamic acid is
replaced with amino acid valine.
● In sickle cell anemia a point mutation leads to the
change of amino acid glutamic acid into valine .
●
This consequently alters the structure of the
hemoglobin molecule, reducing its ability to carry
oxygen.
● Normal red blood cells are round/disc shaped . which
gives them flexibility to travel through blood vessels
even through the smallest blood vessels.
● In people with sickle cell anemia, hemoglobin(a
substance in red blood cells)becomes defective and
causes the red blood cells to change its shape like
crescent /sickle shape.
● crescent /sickle shape, blocks blood from reaching
different parts of the body. This can cause pain and tissue
damage.
NOTE:
● Beta-globin is a component (subunit) of a larger protein
called hemoglobin, which is located inside red blood cells.
● GAG codon coding for Glutamic acid changes to GUG
codon for Valine.
● Sickle cell anemia is caused by a mutation in the gene that
tells your body to make the iron-rich compound that makes
blood red and enables red blood cells to carry oxygen from
your lungs throughout your body (hemoglobin).
NOTE:
● Hemoglobin is a protein in red blood cells that carries oxygen
to all parts of the body from the lungs.
● Hemoglobin contains iron, which allows it to pick up oxygen from the
the lungs/air we breathe and deliver it everywhere in the body.
● It's the hemoglobin that gives red blood cells their color, too.
● All red blood cells contain a red pigment/ protein
known as hemoglobin.
● Oxygen binds to hemoglobin, and is transported
around the body in that way.
● In tiny blood vessels in the lung, the red blood cells
pick up oxygen from inhaled (breathed in) air and
carry it through the bloodstream to all parts of the
body
● In 1953, an English biochemist Frederick Sanger,
described the complete sequence of amino acids of
insulin.
● Sanger’s achievement was significant, as it was
demonstrated for the first time that proteins consisted
of deinable sequences of amino acids. Soon it was
revealed that all enzymes and other proteins are
strings of amino acids arranged in a certain definite
order.
● Following Sanger’s pioneering work, Vernon Ingram in
1956 discovered the molecular basis of sickle cell
anemia, a protein defect inherited as a Mendelian
disorder.
● By analyzing the structure of normal and sickle cell
haemoglobin, Ingram, working at Cambridge University,
showed that sickle cell anemia is caused by a change
from glutamic acid to valine at a single position in the
protein .
● The alleles of the gene encoding hemoglobin differ only in
their specification of this one amino acid in the
hemoglobin amino acid chain.
The characteristics of sickle cell anemia and most other
hereditary traits are defined by changes in protein structure
brought about by an alteration in the sequence of amino
acids that make up the protein. This sequence in turn is
dictated by the order of nucleotides in a particular region of
chromosome.
For example, the critical change leading to sickle cell disease
is a mutation that replaces a single thymine with an adenine
at the position that codes for glutamic acid converting the
position to valine. The sequence of nucleotides that
determines the amino acid sequence of a protein is called a
gene.
NOTE:
● GAG codon coding for Glutamic acid changes to GUG codon for
Valine.
● In sickle cell anemia a point mutation leads to the change
of amino acid glutamic acid into valine at position 6 from N
terminal end in hemoglobin P chain. This consequently
alters the tertiary structure of the hemoglobin molecule,
reducing its ability to carry oxygen.
sickle cell disease is a mutation that replaces a single
thymine with an adenine at the position that codes for
glutamic acid converting the position to valine.
Phenylketonuria (PKU) :
● Phenylketonuria (PKU) is an inherited disorder that
increases the levels of a substance/amino acid called
phenylalanine in the blood.
● PKU is caused by a defect in the gene that helps to create
an enzyme(phenylalanine hydroxylase) which is needed to
break down phenylalanine.
● so phenylalanine is not degraded because of defective
enzyme phenylalanine hydroxylase.
● Phenylalanine consequently accumulates in the cells
leading to mental retardation, as the brain fails to develop in
infancy(early childhood). This disorder is because of the
point mutation.
differentiate between chromosomal aberration and gene
mutation;
● chromosomal aberration refers to a change in a
chromosome number or structure while gene mutation is
an alteration of the sequence of the gene which can cause
changes in genetic code.
● A chromosomal aberration always refers to a change in a
large segment of a chromosome, containing more than one
gene region while Point mutations describe a change in
single nucleotide of DNA.
● Chromosomal aberrations are mega changes while Point
mutations is a micro/small changes which affect the
message itself.
● chromosomal aberrations lead to syndromes like Down’s
syndrome, Klinefelter’s syndrome etc.Sickle cell anemia and
phenylketonuria are well known examples of point mutation.
*20.2.1 :trace chromosomal theory of inheritance from Karl
Correns 1900 to Thomas Hunt Morgan 1910;
***The chromosomal theory of inheritance:
***The chromosomal theory of inheritance:
OBJECTION:
*20.2.2: infer chromosomal theory of inheritance by
Hunt Morgan 1910;
*The fruit ly, Drosophila melanogaster :
● has eight chromosomes in the form of four homologous
pairs.
● T.H. Morgan (1911) noticed a peculiar difference in the
chromosomes of male and female Drosophila.
● Drosophila melanogaster cells are diploid, with three
pairs of autosomes and one pair of sex chromosomes.
● The chromosomes of the three homologous pairs were
similar in both of the sexes, but the fourth pair was very
different.
● The female had two similar rod shaped X-chromosomes
in the fourth pair, while the male had one rod shaped Xchromosome but the other a morphologically different, Jshaped Y chromosome in the fourth heteromorphic pair.
● X and Y chromosomes are called sexchromosomes
because these have genes for determination of sex.
● Chromosomes of the other three pairs are autosomes.
● All chromosomes other than sex-chromosomes are
called autosomes.
● Autosomes do not carry any sex determining gene
1- step:
2- step:
3- step:
4 step:
●CONCLUSION:
● The solution to this puzzle involved sex. The gene causing
the white eye trait in Drosophila resides only on the X
chromosome. It is absent from the Y chromosome.
●Morgan’s experiment was one of the most important in the
history of genetics because it presented the first clear
evidence that the genes determining Mendelian traits do
indeed reside on the chromosomes, as Sutton had proposed.
●The chromosome theory of inheritance therefore, propounds
that genes are located on chromosomes.
●The segregation of the white-eye trait has one-to-one
correspondence with the segregation of the X chromosome, in
other words.
● Mendelian traits such as eye colour in Drosophila assort
independently because chromosomes do.
***IMPORTANT FROM PTB:
● Genes are located at specific loci on chromosomes.
Independent assortment of genes depends upon
independent assortment of their chromosomes.
● All the genes present on a homologous pair of
chromosomes are linked to each other in the form of a
linkage group. These cannot assort independently.
● Those traits assort independently whose alleles are
riding non homologous chromosomes.
● Pea has seven homologous pairs of chromosomes.
Mendel knew nothing about chromosomes. The traits he
studied were conined to only four chromosomes.
***IMPORTANT FROM PTB:
● Mendel reported independent assortment of those traits
whose genes were either on different homologous
chromosomes, or were so far away from each other on
the same chromosome that they appeared to assort
independently due to crossing over.
20.6.4 discuss gene mutation and its causes, i.e. a.
ionisation radiation b. ultraviolet radiation c.
chemical mutagens;
1.Ionisation radiation/Ionizing radiation:
● High energy radiation such as X rays, gamma rays.is highly
mutagenic.
● Most of cell’s atoms reside in water molecules and not in
DNA.
● When this radiation reaches a cell it is absorb by the atoms,
that it encounters(experience/unexpectedly be faced with),
imparting (provide )energy to the electrons of their outer
shells and causing these electrons to be ejected from the
atoms.
● The ejected electron leaves behind ionized atom.
● These unpaired electrons called a free radical.
● The greater majority of free radical created by ionizing
radiation produced by water molecules and not DNA.
● Most of the damage that the DNA suffers is thus indirect. It
occurs because free radical is highly reactive
chemically,reacting violently with the other molecules of
the cell, including DNA.
● This free radical breaks both phosphodiester bond of a
DNA helix.
● The cell usual mutational enzymes cannot fix the damage.
● To repair a double strand break, the two fragments created
by the break must be immobilized(restrict the movements to
allow healing) end to end so that phosphodiester bond can be
reformed.
● Bacteria have no mechanism to achieve this alignment
and double strand break are lethal to their descendants.
● Eukaryotes mostly possess multiple copies of their
chromosomes are able to position the two fragments
created by double strand break end to end by using the
synaptonemal complex. That is assembled in meiosis to
pair the fragmented duplex with another copy of the
chromosome.
● In fact, it is thought that meiosis may have initially
evolved as a mechanism to repair double strand break in
DNA
● (The synaptonemal complex forms during the early stages
of meiotic prophase I, when it mediates the pairing of
homologous chromosomes.)
●
●
In chemistry, a radical is an atom, molecule, or ion that has an
unpaired valence electron.
A free radical can be defined as any molecular species capable
of independent existence that contains an unpaired electron in an
atomic orbital.
● Free radicals can build up in cells and cause damage to other
molecules, such as DNA, lipids, and proteins.
● A notable example of a free radical is the hydroxyl radical (HO•) (a
molecule that is one hydrogen atom short of a water molecule) .
● +ve charge means loss of electrons.-ve charge means gain(extra) of
electrons.
2.ULTRAVIOLET RADIATION:
● Ultraviolet radiation, the component of sunlight that lead
to suntan /sunburn is much lower in energy than X rays.
● When molecules absorb UV radiation, the radiation does
not impart enough energy to cause the molecules to eject
electrons.
● Consequently, free radicals are not formed.
● The only molecule that are capable of absorbing UV
radiation, infact are certain organic ring compounds.
●
Ultraviolet (UV) light kills cells by damaging their DNA. The
light initiates a reaction between two molecules of thymine, one
of the bases that make up DNA
3.CHEMICAL MUTAGENS:
● Many mutations result from direct chemical modification
of DNA bases. The chemical that act on DNA fall into
three classes.
I. Chemicals that look like DNA nucleotides, pair
incorrectly when they are incorporated into DNA.
II.The chemicals that removes amino group from adenine or
cytosine, causing them to mispair.
III.Chemical that adds hydrocarbon groups to nucleotide
bases also causing them to mispair.