Organism

Organs

Tissues

Cells(DNA)

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DNA is the molecular repository of genetic information. The ability to store and transmit genetic information from one generation to the next is a fundamental condition for life.

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What is the genetic material?

Chromosomes are made of protein and DNA. So, one or other or combination of both should be the genetic material.

Fundamental property required of the genetic material is that it must be able to exist in an infinite variety of forms:- Each cell contains a large number of different genes- Each gene controls different heritable trait- Each gene should have different structure

These speculations led biologists conclude that protein (not DNA) was the genetic material. DNA was believed to be small invariant molecule, whereas proteins were known to be macromolecules made of long polymers of amino acids.

Genetic Material

By late 1930s DNA was a long polymer and, like protein, could exist in an almost infinite number of variable forms

Identification of the chemical nature of the transforming principle:- Transforming principle can change the bacterium Streptococcus pneumoniae, from one form to another (avirulent to virulent form).- Frederick Griffith 1928 bacterial transformation

Cell membrane

Cell wall

Slime layer or capsule

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S = Smooth; with slime layer or capsule; virulentR = Rough; without capsule; avirulent

Griffith’s Experiment (bacterial transformation)

IS

mouse

death

IIR

mouse

alive

IS

Δ 600C for 3h

mouse

alive

Heat killed IS +

living IIR

mouse

death

- Virulent S bacteria can be obtained from the mouse postmortem- Some property of the dead IS bacteria can transform the live IIR into IS- The transforming principle is the genetic material

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The transforming principle is DNA

Oswald Avery, Colin Macleod and Maclyn McCarty 1944

Filterate from heat-killed S cells

Trypsin, chemotrypsin

(degrade protein)

SIII enzyme(degrade

polysaccharide)

Ribonuclease(degrade RNA)

Deoxyribonuclease(degrade DNA)

transformation transformation transformation No transformation

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Components of DNA

Nucleotides (the monomer in DNA)- the sugar component (pentose; 2’-

deoxyribose)- the nitrogenous base (double-ring purines

A & G; single-ring pyrimidines T & C)- the phosphoric acid component (three

phosphate groups can be attached)

5 carbons in the pentose sugar

C1’ base is attachedC2’ ribose or deoxyriboseC3’ point of attachment for more nucleotidesC4’ completes ring via oxygen which bridges to the C1C5’ hangs away from the ring; point of attachment for its –PO4

O

H

HHO

H

OH

HH

HOCH2

1’

2’3’

4’

5’

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Structural elements of the nucleotides

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Polynucleotides:

- Nucleotides are joined together by phosphodiester bonds

- Polynucleotides have distinct ends (5’-P terminus and 3’-OH terminus)

- Polynucleotides can be any length and have any sequence

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The double helix

J. Watson and F. Crick (Nobel Prize in 1962) 1953 discovered double helix using model building based on all the available information.

E. Chargaff 1945-50 demonstrated that the base ratios in DNA are constant.

Result: A=T and G=C; total purines (A+G) is equal to total pyrimidines (T+C); but A+T ≠ G+C; GC content varies from species to species

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1. DNA is double-stranded

2. Sugar-phosphate backbone is on the outside, and the bases are stacked on the inside of the helix

3. Bases of the two strands interact by hydrogen bonding (A-T and G-C)

4. Ten base pairs per turn of the helix; pitch of the helix is 34A0 i.e. spacing between adjacent base pairs is 3.4A0 ; the helix is 20A0 in diameter.

5. The two strands are antiparallel

6. The double helix has two different grooves; major and minor

7. The double helix is right-handed

Structure of DNA

April 20, 2023

DNA movie

Circumfrance of Earth = 4 X 104 Km

Sun – Earth distance = 1.5 X 108 Km

Total length of your DNA = 2 X 1011 KmOne human being contains ≈ 1014 cellsOne human cell contains 2 meters of DNA

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Supercoiling

Relaxed and supercoiled plasmid DNAs

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Bacterial DNA is a compact nucleoid

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Nucleosome is the structural unit of chromatin

Chromatin spilling out of lysed nuclei

Digestion of chromatin with micrococcal nuclease. Nucleosomes are 10A0 beads.

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Beads on a String—10 nm Fiber

10 nm filament; nucleosomes

histones (= 1g per g DNA) DNA

proteinpurification

H1

H3H2A

H2BH4

•Basic (arg, lys);•+ charges bind to - phosphates on DNA

deoxyribonuclease I (DNase I) digestion

nucleosomes

•Conclude: histones in a nucleosome protect 200 nt from DNase I attack.

Separate DNA from protein

H1

2H32H2A

2H2B2H4

the histoneoctamer

proteins

200 nt fragments

DNA

A Nucleosome has 200 bp DNA and core histones

200

400

600

800

bases

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Organization of histone octamer

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Path of nucleosomes in the chromatin fiber

10 nm fiber 30 nm fiberApril 20, 2023 20Chromosome movie

Folding of 30 nm fiber

Loops of DNA attached to a nuclear scaffold.

Numerous loops of DNA attached to a scaffold like structure.

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Net result: Each DNA molecule has been packaged into a chromosome that is 10,000-fold shorter than its extended length

Packaging of chromatin fiberTwo chromatids(10 coils each)

One coil(30 rosettes)

One rosette(6 loops)

One loop(75000 bp)

30 nm fiber

10 nm fiber

DNA

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THANK YOU

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