Chromosome structure

Chromosome structure

1. CHROMOSOME STRUCTURE

2. Chromosomes are organized structure of DNA and proteins found in cells. they are thread-like structures located inside the nucleus of animal and plant cells . Chromosomes are made up of proteins and a molecule of deoxyribonucleic acid (DNA). Chromosomes are passed on from parents to offspring. The term chromosome is derived from a Greek word ' chroma ' which means 'color' and 'soma' which means 'body'.  

3. Chromosomes play an important role that ensures DNA is copied and distributed accurately in the process of cell division . In most of the organisms chromosomes are arranged in pairs in the nucleus of the cell . We have 23 pairs of chromosomes

4. TYPES OF CHROMOSOME:

5. PROKARYOTIC CHROMOSOME STRUCTURE:

6. E.COLI CHROMOSOME : The bulk of DNA in E.Coli cell consists of a single circular DNA molecule of length 4.6 million base pairs.

7. DNA DOMAINS: When the DNA from E.Coli is isolated and observed,The DNA consists of 50-100 domains/ loops.These loops are about 50-100kb in size. The ends of loops are binded to a protein member core. SUPERCOILING OF GENOME: The E.Coli chromosome as a whole is negatively supercoiled .( Δ LK/LK °=- 0.06). Some domains may not be supercoiled because the DNA has become broken in one end. The attachment of the DNA to the protein member core may act as a barrier to rotation of DNA.

8. DNA BINDING PROTEINS: Proteins HU-a small basic,positively charged,dimeric protein which binds DNA non specifically. H-NS protein(formally protein H 1 )-A monomeric neutral protein which binds DNA non specifically(Also called histone like proteins-it has the effect of compacting the DNA,which is essential for packing of DNA into nucleoid .)

9. HU PROTEIN H-NS PROTEIN

10. EUKARYOTIC CHROMOSOME STRUCTURE:

11. DIFFERENCE BETWEEN PROKARYOTIC & EUKARYOTIC CHROMOSOMES:

12. CHROMATIN STRUCTURE: Chromatin is comprised of histones and DNA: 147 base pairs of DNA wraps around the 8 core histones to form the basic chromatin unit, the nucleosome .

13. HISTONE PROTEINS:

14. NUCLEOSOMES: The histone octomer (made of core histone ) forms a wedge shaped disk,around when the 146bp of double helix DNA is wrapped in 1.8 turns in a left handed direction. The left handed wrapping of the DNA around the nucleosome corresponds to negative supercoiling . Nucleosome is a basic unit of chromatin. Nucleosomes are separated by spacer DNA to which histone H 1 is attached.This continous string of nucleosomes represents beach on a string forms chromatin.

15. ROLE OF H 1 : One molecule of histone H 1 bind to the nucleosomes and act to stabilise the point at which the DNA enter and leaves the nucleosome core. In presence of H 1 a further 20bp of DNA is protected from nuclease digestion. A nucleosome core plus H 1 is known as chromatosome . The larger the size of H 1 compared with core histones is due to presence of an additional c-terminal tail,which stabilises the DNA between the nucleosome core.

16. LINKER DNA: The beads on a string structure,comprises globular particles( nucleosomes ) connected by thin strand of DNA These thin strands are known as linker DNA. The average length of linker DNA between core particles is 55bp.

17. HIGHER ORDER STRUCTURE: The 10nm fiber is further coiled to produce 30nm fiber which has a solenoid structure with 6 nucleosomes,in every turn. These 30nm fibers are further organised into non condensed loops (300nm) and then into condensed form of chromosome(700nm) metaphase chromosome(1400nm). Chromatins are believed to operate directly through the histone proteins which carry out packaging.

18. GENOME COMPLEXITY: Genome- The total DNA(genetic information) contained in an organism or a cell is regarded as the genome.Thus the genome is the store house of biological information. Genomics-The study of the structure and function of genome. It includes the chromosome in the nucleus and DNA in mitochondria and chloroplasts.

19. The genome of most eukaryotes are more complex and larger than prokaryotes. It contains Functional genes(coding sequence of DNA) DNA sequence that does not code for protein. Coding sequence are extrons and non coding sequence are introns .

20. FLOW OF GENETIC INFORMATION: The biological information from DNA to RNA and form proteins. The DNA is trascriped to an RNA molecule(mRNA) And then the message is translated into a protein sequence according to genetic code .