Section 2Molecular and Morphological Structure of Chromosomal DNA

Experiments proving that Chromosomes Posses the DNA:•Chemical analysis (Ris & Marasky)•Feulgen’s reaction•Ultraviolet absorption•Enzymatic digestion Lambproush chromosomes•Autoradiography

Section 2Molecular and Morphological Structure of Chromosomal DNA

1- Enzymatic Digestion of Lampbrush Chromosomes:

A Lampbrush chromosome is the largest chromosome known and is found in the amphibian oocytes (immature eggs). Lampbrush chromosomes occur during the diplotene stage of meiosis I. They are meiotic bivalents, each consisting of 2 sister chromatids. Lampbrush chromosomes are clearly visible even in the light microscope, where they are seen to be organized into a series of large chromatin loops emanating from a linear chromosomal axis.

Rnase +proteinase Matrix disappeared DnaseChromosome

lyses

Section 2Molecular and Morphological Structure of Chromosomal DNA

2- Feulgen’s Reactiona specific staining procedure for DNA: mild acid hydrolysis makes the aldehyde group of deoxyribose available to react with schiffs reagent to give a purple colour.

Warm and diluted Acid

DNA hydrolysis

Schiff’s reagent+

Nucleic Acid Source(CHO- Ribose)

magenta or purple color

develops

Section 2Molecular and Morphological Structure of Chromosomal DNA

3- Ultraviolet AbsorptionDNA can absorb UV waves in 2600 Å (260n.m) wave length due to the cyclic structure of the nitrogen bases.

High radiatio

n

Low

rad

iation

Chemical Analysis (Ris and Marasky)

90 %

The main compound

10 %

The secondary compound

45 % DNA

55 % Histone proteins

14 % RNA

2.6 % DNA

80 % Non Histone Proteins

Animal

( Cow)

Thyroid

glands

Autoradiography

Genome Organization• Eukaryotes Vs. Prokaryotes• Genome• Haploid• Diploid

http://www.youtube.com/watch?v=ke7PDxrKbj8

Eukaryotic Chromosome

http://www.youtube.com/watch?v=ke7PDxrKbj8

DNA Packaging Why package DNA?

Levels of Packing:

1-Nucleosome

2-30 nm fiber/Solenoid

3-Loops

4-Rossettes

5-Chromosome

• ChromosomesNucleosome (Level 1)A nucleosome core particle contains: – 146 base pairs of supercoiled DNA– Around a core of eight histone molecules

Histone coreTwo copies of H2A, H2B, H3 and H4– Octamer

H1 resides outside of core– Linker histone– Binds to linker DNA– Connecting one nucleosome core particle

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