Chromosomes and DNA | HSC Part-II – Biology Notes

CHAPTER – 6

CHROMOSOMES

Definition

The thread like structure present inside the nucleus bearer of hereditary character in the form of genes is called Chromosomes.

Discovery
Chromosomes were discovered by a German Biologist Walther Flaming in 1882. He placed the larva of salamander in a chemical Perkins Aniline, then he observed that the chromosomes become darker than other organelles of a cell.

Number of Chromosomes

The number of chromosomes varies from species to species.

For Example

§  Penicillin => 2 Chromosomes

§  Mosquito => 6 Chromosomes

§  Drosophila => 8 Chromosomes

§  Pea => 14 Chromosomes

§  Frog => 26 Chromosomes

§  Human => 46 Chromosomes

§  Sugar Cane => 80 Chromosomes

§  Fern => 100 Chromosomes

Structure of Chromosome

Chromosomes can only be seen when the cells are divide. A chromosome is made up of

1. Chromatid

2. Centiomere

1. Chromatids

Each chromosome consists of two very fine thread like structure called Chromatids.

Chromonema
Each chromatids of a chromosomes consists of one or more fine thread called Chromonema.

Chromomere
Chromonema contain deeper staining regions along their length, giving the threads like appearance of strings of beads. These regions are called Chromomeres.

Types of Chromotids

There are two types of chromatids.

i. Sister Chromatids

ii. Non-Sister Chromatids

i. Sister Chromatids

The two chromatids of same chromosome are called Sister Chromatids.

ii. Non-Sister Chromatids

The chromatids of different chromosomes are called Non-Sister Chromatids.

2. Centromere

The region or point in which the chromatids are linker together is called Centromere.

Kinetochore

Centeomere is small spherical zone on the chromosome, within centromere a disc shape protein structure called Kinetochore is present to which the spindle fibres attached during cell division.

Composition of Chromosome

Chromosome are complex structure made up of Deoxyribonuleo protein. This protein composed of

1. DNA

2. Protein

3. RNA

1. DNA

Chromosome have 30-40% DNA. DNA is made up of billion of units called Nucleotide and the nucleotide made up of three components.

§  Phosphate group

§  Deoxyribsoe sugar

§  Nitrogenous base

There are two groups of nitrogenous bases.

i. Purines

ii. Pyrimidines

i. Purines

Nitrogenous base that have double ringed structure called Purines.

Example

§  Adenine (A)

§  Guanine (G)

ii. Pyrimidines

Nitrogenous structure that have single ringed structure called Pyrimidines.

Example

§  Cytosine (C)

§  Thymine (T)

2. Protein

Chromosome have 50-65% protein. The most abundant chromosomal protein is called Histones. It is a basic protein.

3. RNA

Chromosome have very less amount of RNA about 1-10%

Ultra Structure of Chromosome

Eukaryotic chromosomes are composed of chromatin, a complex of DNA and protein and less amount of RNA.

Deoxyribonucleic Acid (DNA)

The DNA of a chromosome is one very long double stranded fiber, duplex which extend unbroken through the entire length of chromosome.
A typical human chromosome contain about 140 million nucleotide in its DNA.

Size
If a strand of DNA from a single chromosome were laid out in a straight line, it would more than 7 feet long (2 meter). It is too much long to fit into a cell. In the cell, however, the DNA is coiled, thus fitting into a much smaller space.

Nucleosome
DNA resemble a string of beads. Even 200 nucleotides, the DNA duplex is coiled around a core eight histone proteins forming a complex known as Nucleosome.

Protein

Histones are positively charged proteins due to an abundance of the basic aminoacids, arginine and lysine. They are thus strongly attached to the negatively charged phosphate groups of the DNA.

Further coiling occurs when the string of nucleosomes wraps up into higher order coils called Super Coils.

Heterochromatin
Highly condensed portions of the chromatin are called Heterochromatin.

Euchromatin
The reminder of the chromosome called Euchromatin is condensed only during the cell division.

Karyotype

The particular array of chromosome which an individual possess is called the Karyotype in which chromosomes are paired by matching bonding and arranged by size and shape.

Applications

§  Karyotypes show marked differences among species and sometime even among individuals of the some species.

§  It help in determining many issue as the culprits of difference offences and settle the disputed paternity.

§  It help in detecting genetic abnormalities, arising from extra or lost chromosome.

Types of Chromosome on the Bases of Centromere

There are four types of chromosome according to the position of centromere are as follows.

i. Telocentric Chromosome

ii. Acrocentric Chromosome

iii. Sub-Metacentric Chromosome

iv. Metacentric Chromosome

i. Telocentric Chromosome

When the centromere is placed at the extreme end of the chromsome, the chromosome is called Telocentric Chromosome.

ii. Acrocentric Chromosome

When the centromere is placed near the end of the chromosome, the chromosome is called Acrocentric Chromosome.

iii. Sub-Metacentric Chromosome

Chromosome with unequal arms (chromatids) that resembling the “j” shape are called Sub-meta Centric chromosome.

iv. Meta Centric Chromosome

Chromosome with equal arms and centromere in center is called a Metacentric Chromosome.
There are three more types of chromosomes

i. Homologous Chromosome

ii. Autosomes

iii. Sex Chromosomes

i. Homologous Chromosomes

Those chromosomes which are morphologically similar with same set of genes in which one chromosome comes from male and another from female is called as Homologous Chromosome.

ii. Autosomes

The somatic paired chromosome which contain the genes of various characters except reproductive organs are called Autosomes.
These are same in male and female both.

iii. Sex Chromosome

Those chromosomes in a cell by which the sex of organism can be determined are called Sex-Chromosome.
There are different types of distribution of Sex-Chromosome.

i. XY type distribution

ii. ZW type distribution

i. XY Type Distribution

Females have a pair of similar sex chromosomes called X but males have a mismatch set and X and another chromosome called a Y, so the male XY and the female is XX.

ii. ZW Type Distribution

In some fishes, moths and birds, the male have similar chromosome called Z and the female has different so the male is ZZ and the female is ZW.

Chromosomes as Carrier of Genes

Genes are small bodies found in the chromosomes.

Chromosomes are considered as the carrier of genes.

§  The chromosomes can be separately identified visually but the genes are very small units. And so far have not been seen even with the best microscope.

§  The chromosomes and gene behave as hereditary units but the genes can not be considered outside the chromosome.

§  At the time meiosis, the separation of homologous chromosomes takes place which result in the segregation of gene pairs.

§  In the genotype of every individual one member of each pair of genes is contributed by one parent and the other by the other parent.

CHROMOSOMAL THEORY OF HEREDITARY

Introduction
The chromosomal theory of inheritance was first formulated by the American Biologist “Walter Sutton” in 1902.

Postulates
The main postulates of this theory are as under

i. Hereditary Material

ii. Segregation of Chromosomes

iii. Number of Chromosome

iv. Independent Assortment

i. Hereditary Material

Reproduction involves the initial union of only two cells, egg and sperm If Mendel’s model is correct then these two gametes must make equal hereditary contributions. Sperm, however contains little cytoplasm, therefore the hereditary material must reside within the nuclei of the gametes.

ii. Segregation of Chromosomes

Chromosomes segregated during meiosis in a manner similar to that exhibited by the elements of Mendel’s model.

iii. Number of Chromosome

Gametes have one copy of each pair of homologous chromosomes, diploid individuals have two copies.

iv. Independent Assortment

During meiosis each pair of homologous chromosomes orients on the metaphase plate independent of any other pair.

FREDRICK GRIFFITH’S EXPERIMENT

Introduction
Fred Griffith in 1928 provided the evidence of hereditary material in bacteria.

Experimental Material

He was working on strains of streptococcus pneumoniae, which occurs in two distinct different forms.

R-Type
Rough surfaced, non-capsulated bacteria, without the capability of producing pneumonia.

Example
Smooth surfaced, capsulated bacteria with the capability of producing pneumonia i.e. virulent.

Steps of Experiment

1. He observed that, when he injected R-type bacteria in the mice, there was no ill effect.

2. When he injected the S-type, they proved to be fatal.

3. He also observed, when he injected both the bacteria separately after killing them by heating under high temperature, the mice did not develop the disease.

4. He also observed that when the injected the living R-type with heat killed S-type there was high mortality among the mice.

Conclusion
Fred Griffith concluded that the R-type bacteria gained genetic property of S-type inactive bacteria when they kept together, so R-type bacteria converted into virulent S-type by the activity of DNA. Hence by this experiment, it can be proved that DNA is a genetic material.

Avery, Macleod and McCarty’s Experiment

Introduction
In 1944, after a decade of research, Oswald Avery, Maclyn McCarty and Colin Macleod discovered that the transforming agent had to be DNA.

Experiment
They performed various experiments and found out that the only substance, which carried the transforming capability was DNA because if the enzyme deoxyriba-nuclease was added to the bacteria, the transforming capability was lot.

HERSHEY AND CHASE’S EXPERIMENT

Introduction
1n 1952, Hershey and chase performed experiment to proof that DNA is a hereditary material.

Experiment at Material

Hershey and chase labeled protein coat and DNA of Bacteriophage separately. Protein coat was labeled with radioactive sulphur and DNA with radioactive phosphorus. These two viruses used to attack bacterial cells.

Step of Experiment

1. Hershey and chase observed that if cultures of bacteriophage are labeled with radioactive phosphorus or with sulphur.

2. Bacteriophage is ruptured, the DNA is released and treated with deoxyribsonucleus, the DNA breaks up into fragments in the solution.

3. The empty protein coats of the ruptured membrane appear as coats all the phosphorus and sulphur were made to inject bacteria and multiply by the help of special technique, all the sulphur labeled protein were removed.

4. The new phage formed contained only phosphorus indicating the presence of DNA molecule.

Conclusion
The conclusion appears similar to the transforming principle in bacteria, showing that DNA is the genetic material in phage, transmitted from one generation to the next.

Watson and Crick’s Model of DNA

Introduction
James Watson and Francis Crick, in 1953 proposed structure of the DNA molecule.

Structure of DNA

Watson and Crick suggested a ladder like organization of DNA.

i. Double Helix

ii. Backbone of DNA

iii. Pairing of Bases

i. Double Helix

Each molecule of DNA is made up of two polynucleotide chains which twisted around each other and form a double helix.

ii. Backbone of DNA

The uprights of the ladder are made up of sugar and phosphate parts of nucleotide and the rungs are made up of a paired nitrogenous bases.

iii. Pairing of Bases

The pairs are always as follows

§  Adenine always pairs with thymine and cytosine with Guanine.

§  The two polynucleotide chains are complimentary to each other and held together by hydrogen bonds.

Hydrogen Bonding

There are two hydrogen bonds between Adenine and Thymine (A=T) and three between Cytosine and Guanine.

Distance

§  Both polynucleotide strands remain separated by 20 A⁰ distance.

§  The coiling of double helix is right handed and complete turn occurs after 34 A⁰. In each turn 10 nucleotide pairs are present, therefore the distance between two pairs is about 3.4 A⁰.

Genes – The Unit of Hereditary Information

Introduction
Archibald Garrod discovered in 1902, that certain diseases were more prevalent among some families and were inherited as a recessive Mendelian trait.

Alkaptonuria
Alkaptonuria is a disease in which the urine contained a substace called “Alkapton” now known as “Homogentisic acid” which was immediately oxidizes to black when exposed to the air.

Causes

§  He suggested that this disease occurred due to absence of an enzyme, which could break the “Alkapton” down to other products so it would not build up in the urine.

§  He proposed that the condition was “inborn error of metabolism” which is occurring due to changes in the hereditary information, which must have occurred in one of the ancestors of a affected families.

Conclusion
He concluded that the inherited disorders might reflect enzyme deficiencies.

GENOME

Definition

The total genomic constitution of an individual is known as Genome.

Example
In a bacterial cell, a single chromosome along with plasmid is genome of bacteria, while in a human being all twenty two pairs of autosome along with a pair of sex-chromosomes constitute genome.

REPLICATION OF DNA

Definition

The mechanism in which DNA prepares its copies is called DNA replication.

OR

When the formation of new DNA molecule takes place in the cells without any change, it is known as Replication of DNA.

SEMI CONSERVATIVE REPLICATION

Definition

The type of replication in which new daughter double helical duplex contain one strand old and another newly synthesized is called Semi Conservative Replication.

THE MESELSON STAHL EXPERIMENT

Introduction
Mathew meselson and Frank Stahl performed experiments to test the semi conservative method of DNA replication.

Experiment

§  They grew bacteria in a medium containing Nitrogen-15 (N15), a heavy isotope of the nitrogen.

§  The DNA after several generations became denser than normal because the entire bacterial DNA now contained Nitrogen-15 (N15).

§  They then transferred the bacteria into a new medium containing lighter isotope Nitrogen-14 (N14) and analyzed the cultures for changes in the DNA.

§  At first DNA, which the bacteria synthesized, was all heavy.

§  After one round the density of the DNA fell exactly to the value one half between the all heavy isotope DNA and all light isotope DNA.

Result
This showed that after one round of replication, each of the daughter DNA duplex contained one strand of heavy isotope, after two rounds half contained none of the heavy isotope strand to form light duplex and half contained one of the heavy strand isotope.

It was now confirmed that the semi conservative method of the replication of DNA replication was true.

ONE GENE ONE ENZYME HYPOTHESIS

Introduction
George Beadle and Coworker Edward L.Tatum proved that the information coded within DNA of a chromosome is used to specify particular enzymes.

Method of Study

§  Beadle and Tatum created Mandelian in the chromosomes of the fungus call Neurospora by the use of the x-rays.

§  They studied the effect of the mutations caused by them and suggested “one gene one enzyme hypothesis”

Choice of Material

§  They choose the bread mold, neuropora crassa as an experimental organism. It had a short life cycle and was easily grown on a defined medium, containing known substances, such as glucose and NaCl.

§  The nutrition of Neurospora could be studied by its ability to metabolize sugars and other chemicals the scientist could add or delete from the mixture of the medium.

Production of Mutations

§  The induced mutations in Neurospora spores by using x-rays.

§  The mutated spores were placed on complete growth media enriched with all necessary metabolites, so keeping the strains alive because the strains were deficient in producing certain compounds necessary for fungus growth due to damaged DNA by earlier irradiation, hence called Mutants.

Identification of Mutant Strains

§  To test the mutations, they grew the mutated strains on the animal media containing sugar, ammonia, salt, a few vitamins and water.

§  A strain that had lost the ability to make a necessary metabolite, failed to grow on such media.

§  Using this approach they succeeded in identifying and isolating the different mutants.

Identification of Specific Mutations

§  To determine the specific nature of each mutation, they added various chemicals to minimal media, to make the strains grow.

§  Using this technique, they were able to pinpoint the biochemical problem and thus the genetic deficiency of the mutants.

§  Many of the mutants were unable to synthesize a single amino acid or a specifc vitamin.

§  If a spore lacked the ability to synthesize a particular amino acid, such as Arginine, it only grew if the Arginine was added in the growth medium Such mutants were called as arg mutants.

§  Chromosomes mapping studies on the organism facilitated their work and they mapped three areas clusters of mutant Arginine genes.

§  For each enzyme in the arginine biosynthetic pathway, they were able to isolate a mutant strain with a defective form of that enzyme and mutation always proved to be located at one of a few specific chromosomal sites, different for each enzyme.

Conclusion
They concluded that genes produced effects by specifying the structure of enzymes and that each gene encodes the structure of a single enzyme. This was called “One gene one enzyme hypothesis”.

RNA

Definition
The single stranded helical polynucleotide contain ribose sugar and uracil instead of thymine is called RNA.

Location
RNA is found in the nucleus (in nucleolus 10%) as well as in the cytoplasm (90%).

Types of RNA

There are three types of RNA.

1. Ribosomal RNA (rRNA)

2. Transfer RNA (tRNA)

3. Messenger RNA (mRNA)

1. Ribosomal RNA (rRNA)

The class of RNA found in ribosomes is called ribosomal RNA.

Function
During polypeptide synthesis it provides the site on the ribosome where the polypeptide is assembled.

2. Transfer RNA (tRNA)

A second class of RNA is called transfer RNA is much smaller. Human cell contains more than 40 different kinds of tRNA molecules.

Function
During polypeptide synthesis tRNA molecules transport the amino acid into the ribosome for the synthesis of polypeptide chain.

3. Messenger RNA (mRNA)

It is long strand of RNA that passes from the nucleus to the Cytoplasm.

Function
During polypeptide synthesis, mRNA molecules brings information from the chromosomes to the ribosomes to direct the assembly of amino acids into a polypeptide.

GENE EXPRESSION

Definition

All functions in the body of an organism are controlled by genes. A function expressed or performed by a gene is called gene expression.

Process of Gene Expression

The process of gene expression occurs in two phases

1. Transcription

2. Translation

1. TRANSCRIPTION

Definition

The process in which an RNA copy of the DNA sequence encoding the gene is produced with the help of an enzyme RNA polymerase is called Transcription.

Step of Transcription

§  Transcription is initiated when a special enzyme called RNA polymerase binds to a particular sequence of nucleotide on one of the RNA strands. This strands is known as template strands or Antisense strands while the other strand is called coding or sense strand.

§  RNA polymerase proceeds to assemble to assemble a single strand of RNA with a nucleotide sequence complementary to that of the DNA pairing Adenine to Uracil and Guanine to Cytosine and vice versa.

§  Only one strand of DNA is transcribed and when the RNA polymerase reaches a specific stop sequence at the far end of the gene, it disengages itself from the DNA and release the newly assembled RNA chain.

§  This RNA chain is called the primary RNA transcript (copy) of the DNA nucleotide sequence of the gene or simply mRNA.

2. TRANSLATION

Definition
The process of formation of the polypeptide chains using the messenger RNA is called Translation.

Steps of Translation

1. Binding of mRNA

2. tRNA Binds Amino Acids

3. Reading Or Decoding of mRNA

4. Polypeptide Chain Synthesis

1. Binding of mRNA

The process of translation begins with the binding of one end of the mRNA with a rRNA on a ribosome.

2. tRNA Binds Amino Acids

A tRNA molecule possessing the complementary three nucleotide sequence or anticodon, binds to the exposed codon on the mRNA, because this tRNA molecule bind with a particular amino acid and put amino acids at correct place on the elongated polypeptide chain.

3. Reading Or Decoding of mRNA

The ribosome then starts to move along the mRNA molecules in increment of three nucleotides, adding a specific amino acid at each step through tRNA.

4. Polypeptide Chain Synthesis

It continues until it reaches the stop sequence, after which it stops the process. It then disengages itself from the mRNA and releases the newly assembled polypeptide.

DECODING

Definition

Messenger RNA (m-RNA) contains gentic code in three nitrogen bases and t-RNA contains anticodon triplet and it transfers amino acids to the ribosome, if anticodon triplet is attached the codon triplet of m-RNA. This process is called Decoding.

MUTATION

Definition

Any change in the amount, structure and content of genetic material is called Mutations.

Mutations can appear in both sex chromosomes as well as in autosomes.

Types of Mutations

There are two main types of mutations.

1. Chromosomal Mutation

2. Gene Mutation

1. Chromosomal Mutation

The change in amount arrangement and the nature of genetic material on a chromosome is called Chromosomal mutations. It is also called Chromosomal aberration.

§  This mutation is visible under the microscope.

Types of Chromosomal Aberration

There are following types of this mutation.

i. Deletion

ii. Duplication

iii. Inversioniv. Translocation

i. DELETION

Definition

When a small portion of a chromosome is missing the situation is called Deletion.

Effects of Deletion

Pseudo-Dominance
Deletion may cause Pseudo dominance in heterozygous condition.

Lethal Effect

If deletion takes place in both homologous chromosomes then it has the lethal effect on the organism.

ii. DUPLICATION

Definition

The repetition of a segment on a chromosome is called Duplication.

Effects of Duplication

Due to the duplication different physiological and morphological functions are disturbed.

iii. INVERSION

Definition
When the arrangement of genes on a chromosome is changed then the mutation is called Inversion.

Effect of Inversion

Inversion reduced crossing over.

iv. TRANSLOCATION

Definition
The transfer of a chromosomal segment to a non-homologous chromosomes is called Translocation.

Effect of Translocation

Translocation may give rise to varieties within species.

2. GENE MUTATION

Definition

When small changes occur in the molecular structure of DNA, these are called Gene-Mutations.

§  This mutations can not be detected by the microscope.

§  These changes can produce drastic changes in the expression of the genetic messages.

Types of Gene Mutations

There are following types

i. Point Mutation

ii. Transposition

i. POINT MUTATION

Definition
The change of the sequence of one or a few nucleotides is called Point Mutation.

ii. TRANSPOSITION

Definition
Individual genes may move from one place to another place on their own chromosome which is called Transposition.

Effects
This chromosomal rearrangement often brings alternation in the expression of the genes or that of neighboring genes.

DNA Damage (Causes of Mutation)

There are three major important causes of DNA damage, they are

1. Ionizing Radiation

2. Ultra Violet Radiation

3. Chemical Mutagens

1. Ionizing Radiation

§  High energy radiations such as X-rays and Gamma rays are highly mutagenic Nuclear radiation is also of this sort.

§  These radiations release unpaired electrons which are called free radical.

§  These free radicals are highly reactive chemically, reacting violently with the other molecules of the cell including DNA.

2. Ultra Violet Radiation

§  Ultra violet radiation is the component of sunlight.

§  When molecules absorb UV radiation little damage is produce in these molecules.

§  Mostly certain organic ring compounds are affected by UV-radiation.

3. Chemical Mutagens

§  The chemicals which are capable of damaging DNA are called Mutagens.

§  There are three main types of mutagens.

§  Chemicals resembling DNA nucleotides but pair incorrectly when they are incorporated into DNA.

§  Chemicals that remove the amino group form Adenine or cytosine, causing them to pair wrongly.

§  Chemicals that add hydrocarbon group to nucleotide bases also causing them to pair wrongly.

Hereditary Diseases Due to DNA Damage

The two main hereditary disease due to DNA damage are

1. Sickle Cell Anaemia

2. Phenylketonuria

1. Sickle Cell Anaemia

An inherited autosomal recessive condition that causes abnormal haemoglobin in blood cells, leading to defective oxygen carrying, infections or organ damage is called Sickle Cell Anaemia.

Cause
The disorder occurs due to presence of abnormal haemoglobin (Glutamic acid is replaced by valine in β-chain at 5th position). It is produced due to mutation in gene. By the activity of changed gene abnormal haemoglobin is formed and sickle cell disease is produce.

Symptoms

§  The abnormal haemoglobin has loco binding capacity with oxygen.

§  The R.B.C containing the defective haemoglobin deform to a sickle shape during relative oxygen scarcity, haemoglobin molecules become insoluble and combine with one another forming stiff, rod like structures. Because of stiffness and irregular shape, these R.B.Cs may form clots blocking the small vessels.

2. Phenylketonuria

An inborn error of metabolism caused by the lack of an enzyme which breaks down amino acid phenylalanine, resulting in conversion of phenylalanine to other chemicals is called Phenylketonuria.

Causes
Phenylketonuria is a ressisive disorder caused by a mutant allele of the gene encoding the enzyme that normally breaks down phenylalanine. Only individual homozygous for the mutant allele development the disorder. It is because of the point mutation.

Symptoms

§  The abnormal derivatives of phenylalanine are harmless because they interfere with the development of brain cells causing irreversible brain damage to the infants and can lead to severe, progressive mental retardation.

§  Affective individuals rarely live for more than 30 years.

Treatment

It can be treated, if early detected by restricting the diet and avoiding phenylalanine.

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