Presentation on DNA The primary genetic material

Presented by-

Vinay k. Dubey

M. Pharm. 1st year

(p’cology)

DNA- an introduction• It stands for Deoxyribose Nucleic Acid.• The genetic material of all living organisms, both

prokaryotes and eukaryotes , is DNA. A number of other viruses have RNA as a genetic material.

• Research in genetics was revolutionized in 1972, when Paul Berg constructed the 1st recombinant DNA in vitro and , in 1973, when Herbert Boyer and Stanley Cohen cloned a recombinant molecule for the 1st time.

• The genetic material is organized into structures called chromosomes.

• The full DNA sequence of an organism or the full DNA or RNA sequence of virus is called its GENOME.

Genetics

It is the study about genetic material in living things.

The three major divisions of genetics are-• Transmission genetics, examines the principles of heredity;• Molecular genetics, molecular genetics deals with the gene

and the cellular processes by which genetic information is transferred and expressed;

• Population genetics concerns the genetic composition of groups of organisms and how that composition changes over time and space.

Overview of genome

Pre history of genetics

The ancient Greeks gave careful consideration to human reproduction and heredity. The Greek physician Alcmaeon conducted dissections of animals and proposed that the brain was not only the principle site of perception, but also the origin of semen. This proposal sparked a long philosophical debate about where semen was produced and its role in heredity.

The debate culminated in the concept of pangenesis, which proposed that specific particles, later called gemmules, carry information from various parts of the body to the reproductive organs, from where they are passed to the embryo at the moment of conception.

Comparision of theories

The Rise of Modern Genetics

• Dutch spectacle makers began to put together simple microscopes in the late 1500s, enabling Robert Hooke (1653–1703) to discover cells in 1665, that gave rise to the idea of preformationism. According to preformationism, inside the egg or sperm existed a tiny miniature adult, a homunculus, which simply enlarged during development.

• Preformationism meant that all traits would be inherited from only one parent—from the father if the homunculus was in the sperm or from the mother if it was in the egg.

Developments in cytology in the 1800s had a strong influence on genetics. Robert Brown described the cell nucleus in 1833. Building on the work of others, Matthis Jacob Schleiden & Theodor Schwann proposed the concept of the cell theory in 1839.

According to this theory, all life is composed of cells, cells arise only from preexisting cells, and the cell is the fundamental unit of structure and function in living organisms. Biologists began to examine cells to see how traits were transmitted in the course of cell division.

Charles Darwin, put the theory of evolution through natural selection and published his ideas in On the Origin of Species in 1856. Darwin recognized that heredity was fundamental to evolution, and he conducted extensive genetic crosses with pigeons and other organisms.

Walter Flemming observed the division of chromosomes in 1879 and published a superb description of mitosis.

August Weismann in 1890 finally laid to rest the notion of the inheritance of acquired characteristics. He cut off the tails of mice for 22 consecutive generations and showed that the tail length in descendants remained stubbornly long.

Weismann proposed the germ-plasm theory, which holds that the cells in the reproductive organs carry a complete set of genetic information that is passed to the gametes.

Twentieth-Century Genetics• The year 1900 was a watershed in the history of genetics. • The Father of Genetics, Gregor J. Mendel conducted

breeding experiments with pea plants from 1856 to 1863 and presented his results publicly at meetings of the Brno Natural Science Society in 1865.

• At the time, no one seems to have noticed that Mendel had discovered the basic principles of inheritance.

• The significance of Mendel’s discovery was unappreciated until 1900, when three botanists—Hugo de Vries, Erich von Tschermak, and Carl Correns—began independently conducting similar experiments with plants and arrived at conclusions similar to those of Mendel.

Cause of heredity..??

• The term gene was a word that Mendel never knew. It was not coined until 1909, when the Danish geneticist Wilhelm Johannsen first used it. a gene as an inherited factor that determines a characteristic.

• Mendel called them factor, Which control the phenotype of the plant.

• Each phenotype results from a genotype developing within a specific environment. The genotype, not the phenotype, is inherited.

• So the gene is responsible for the inheritance of specific character from one generation to next.

Mendel’s experiment

• Gene- A genetic factor (sequence of DNA) that helps to determine a characteristic.

• Allele- One of two or more alternate forms of a gene.• Locus -Specific place on a chromosome occupied by an

allele.• Genotype -Set of alleles that an individual possesses.• Heterozygote- two different alleles at a locus.• Homozygote - two of the same alleles at a locus.• Phenotype or trait- The appearance of a character.• Character or characteristic - An attribute or feature.

Mendel’s laws

The principle of segregation (Mendel’s first law) states that each individual diploid organism possesses two alleles for any particular characteristic.

These two alleles segregate (separate) when gametes are formed, and one allele goes into each gamete. Furthermore,

the two alleles segregate into gametes in equal proportions.

The concept of dominance (Mendel’s second law) states that, when two different alleles are present in a genotype, only the trait of the dominant allele is observed in the phenotype & other allele is called recessive.

The theory of independent assortment states that the daughter cells have same charecteristic as parents.

Chromosome theory of heredity• The theory states that genes are located on chromosomes

was developed in the early 1900s by Walter Sutton,

The chromosome theory of inheritance shows that The two alleles of a genotype segregate during anaphase I of meiosis, when homologous chromosomes separate. The alleles may also segregate during anaphase II of meiosis if crossing over has taken place.

Discovery of DNA• In late 1920, series of experiment were done for

identification of DNA as the genetic material-

• Griffith’s Transformation Experiment.• Avery’s Transformation Experiment. • Hershey-Chase Bacteriophage Experiment.

Griffith’s Transformation Experiment

• In 1928, Frederick Griffith a British medical officer, was working with Streptococcus pneumoniae. He used two strains of bacterium-

S strain- produced smooth shiny colony, capsulated –virulent.

R strain- rough colonies- non-virulent.

Conclusion-

He believed that the unknown agent responsible for the change in genetic material was a protein. He called the agent the transforming principle.

Avery’s Transformation Experiment

• In the 1930’s & 1940’s, American biologist Ostwald T. Avery, with his colleagues Colin M. Macleod & Maclyn McCarty, followed Griffith’s experiments, to identify the transforming principle by studying the transformation of R-type bacteria to S-type bacteria.

Conclusion-• Result shows that DNA was the genetic material, but still

criticized.

Avery’s Transformation Experiment

The Hershey-Chase Bacteriophage Experiments

RNA as genetic material

• In 1956, A. Giere & G. Schramm showed that when tobacco plants inoculated with the purified RNA of TMV.

• No lesions were produced when the RNA had been degraded by treatment with ribonuclease & then injected into the plant.

• In 1957, Heinz Fraenkel Conrat & B. Singer confirmed Giere’s conclusion.

• They isolate the RNA & protein content of two distinct TMV strains & reconstituted the RNA of one type with the protein of the other type & vice versa.

TMV experiment

Structure of genetic materials

• A series of experiments proved that the genetic material consist of two types of nucleic acid : DNA & RNA

• DNA is the genetic material for all living organisms & some viruses while RNA is of remaining viruses.

• DNA & RNA are polymers (made of monomers, linked together).

• That monomers are called as nucleotides.

nucleotidesA series of nucleotide sequence makes DNA & RNA.

Each nucleotides consist of three distinct part-

• A pentose sugar (five carbon ring)• A nitrogenous base, and• A phosphate group.

A pentose sugar-

A nitrogenous base-• These are basic compounds having nitrogen in their ring.

Its having following two types-

• Purine- 9 membered, double-ringed structure.

Adenine (A) and Guanine (G)

• Pyrimidine- 6 membered, single-ringed structure.

Thymine (T),Cytosine (C) and Uracil (U).

Structures nitrogenous base

Structure of nucleotide

Structure of polynucleotide

The DNA Double Helix model

• By the chemical treatment Ervin Chargaff had hydrolyzed the DNA of no. of organism and quantified the purines & pyrimidine released.

• Chargaff’s rule – Amount of adenine was equal to that of thymine and amount of guanine is equal to cytosine these equivalencies is known as Chargaff’s rule.

• Rosalind Franklin Working with Maurice Wilkins , studied isolated fibres of DNA by X-ray diffraction technique.

• She concluded that DNA is a helical structure with two distinct regularities of .34 and 3.4nm along the axis of the molecule.

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In 1953, James Watson and Francis Crick determined the structure of DNA with the help of Rosalind Franklin and Maurice Wilkins as well as Erwin Chargaff.

Watson and Cricks Model of DNA

• DNA consists of two helical chains wound around the same axis in a right-handed fashion aligned in an anti-parallel fashion.

• There are 10.5 base pairs, or 36 Å, per turn of the helix.

• Alternating deoxyribose and phosphate groups on the backbone form the outside of the helix.

• The planar purine and pyrimidine bases of both strands are stacked inside the helix.

Relationship of nucleotide

Different DNA structures

Structure of bonds in DNA

A-DNA B-DNA Z-DNA

Chromosomes

• The DNA in a cell is organized into physical structures called chromosomes.

• The full DNA sequence of an organism’s haploid set of chromosomes is its genome.

• In prokaryote, the genome is usually but not always, a single circular chromosome. But,

• In eukaryote it is typically distributed among a number of chromosomes in the cell nucleus.

• So it is important to understand organization of DNA molecules in chromosomes.

• The total amount of DNA in the haploid genome of a species is k/a its C value. e.g. homo sapiens have 3,400,000,000 bp of C value.

• A human cell has more than 700 times as much DNA as does E. coli.

• Without the compacting of the 6 109 base pairs of DNA in the diploid genome, the DNA of the chromosome of a single human cell would be more than 2 meters long (about 6.5 feet ) if the molecules were placed end to end.

• Several packing in chromatin enable DNA to short enough to fit into a nucleus of few micrometers in diameters.

Chromatin• It is a stainable material in a cell nucleus includes DNA &

Protein. Its fundamental structure is identical in all eukaryotes.

• Mainly two types of protein are associated with DNA in chromatin-

Histone- small basic protien contain large amount of arginine & lysine (with a net positive charge) so binds with DNA.

Nonhistone- all the protein associated with DNA except histones. e.g. proteins involves in replication, repair, transcription & recombination.

• They are acidic proteins (protein with a net negative charge) & are likely to bind with positively charged histones in chromatin.

Nucleosomes• Five main types of histones are associated with eukaryotic

DNA are H1, H2A, H2B, H3, and H4.• The amino acid sequence of H2A, H2B, H3, and H4

histones are highly conserved (very similar).• These four types involves in compacting DNA & formation

of nucleosomes.• The least compact form seen is the 10 nm chromatin fiber,

which has a characteristic “beads

on a string morphology”• These beads are nucleosomes of

diameter about 10nm. Consist of

a core of 8 histones.

Compaction of DNA

• Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

• 10 nm chromatin is produced in the first level of packaging.

• Linker DNA

• H1 • Histone octomer

nucleosomes

Condensation of chromatin• On a nucleosome 147 bp segment of DNA is wound about

1.65 times which compact the DNA by a factor of about 6 times.

• Now a single molecule of H1 binds both to the linker DNA at one end of the nucleosome & to the meddle of the DNA segment wrapped around core histones.

• It gives more regular appearance & nucleosome compact into a structure about 30 nm chromatin fiber.

• It shows by the solenoid model- has the nucleosome spiraling helically, with about six nucleosomes per complete turn.

30-nm chromatin fiber solenoid model

Chromosome organization

• Chromatin packing beyond 30 nm chromatin filament is less well understood.

• Current model shows 30-90 kb loops of DNA attached to a proteinaceous “scaffold” with a charecteristic X shape of the paired sister chromatid.

• Loop of chromatin consist of 180-300 nucleosomes organized into 300 nm fibers.

• An average human chromosome has approx. 2,000 looped domains.

• Current information indicates that each loop is held together at its base by nonhistone proteins that are part of the chromosome scaffold, but details of this structure are not known.

Chromosome organization

Chromosome organization

References-• iGenetics A Molecular Approach, by Peter J. Russell,

2nd Edition.• Genetics A Conceptual Approach by Benjamin Pierce.• MGA 8 – Griffiths• Internet source

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