PACKING OF DNA INTO CHROMOSOMESPRINCIPLES OF GENETICS
PRESENTED BY:- K.AMAR PRASAD RAM/15-35 M.Sc(Ag) GPBR
Relative position of chromosome in cell
Chemical composition of Eukaryotic chromosome Chemically chromosomes are nucleoprotein in nature means are composed of RNA, DNA and protein.Generally chromosomes contains 30-40% DNA, 50-65% protein and 0.5-10% RNADNA- The amount of DNA present in somatic cell is constant. DNA content of gametic cell is half of that of somatic cell. DNA of chromosome is of two types i) Unique DNA ii) Repetitive DNA
i) Unique DNA- unique DNA consists of those DNA sequence which are present in a single copy per genome and are unique in nature
Unique DNA is also known as non repetitive DNA. Codes for protein which requires in large quantity for cell. eg- storage protein ii) Repetitive DNA- Repetitive DNA consists of DNA nucleotides or base sequences, which are few to several hundred base pairs (bp) long and are present to several to a million copies per genome. Human genome contains 30% repetitive DNA. Repetitive DNA is further divided into Highly repetitive DNA and Moderately repetitive DNA2) RNA- Purified chromatin contain 10-15% RNA. RNA associated with chromosome is messenger RNA, transfer RNA and ribosomal RNA.
3) Protein- Protein associated with chromosome is classified into two broad groups Histone or basic protein Non histone protein Non histone proteins are acidic in nature and histone proteins are basic in nature because of basic amino acids.Histone protein- histones constitutes about 80% of the total chromosomal protein. They are present in an almost 1:1 ratio with DNA. Five fractions of histones are present like 1H1, 2H2a, 2H2b, 2H3 and 2H4 ii) Non histone protein- non histone proteins make up to 20% of the total protein mass. Content of non histone protein is different from species to species. Non histone protein includes many important enzymes like DNA and RNA polymerase.
Eukaryotic species contain one or more sets of chromosomesEach set is composed of several different linear chromosomesThe total amount of DNA in eukaryotic species is typically greater than that in bacterial cells
Chromosomes in eukaryotes are located in the nucleusTo fit in there, they must be highly compactedThis is accomplished by the binding of many proteinsThe DNA-protein complex is termed chromatin
A eukaryotic chromosome contains a long, linear DNA molecule
Three types of DNA sequences are required for chromosomal replication and segregationOrigins of replicationCentromeresTelomeres
Organization of Eukaryotic Chromosomes
A TYPICAL CHROMATID
DNA to chromosomes ????????????
The compaction of linear DNA in eukaryotic chromosomes involves interactions between DNA and various proteins Proteins bound to DNA are subject to change during the life of the cellThese changes affect the degree of chromatin compaction
Eukaryotic Chromatin CompactionNUCLEOSOME SOLENOID MODEL
The repeating structural unit within eukaryotic chromatin is the nucleosomeIt is composed of double-stranded DNA wrapped around an octamer of histone proteinsAn octamer is composed two copies each of four different histones146 bp of DNA make 1.65 negative superhelical turns around the octamerOverall structure of connected nucleosomes resembles beads on a stringThis structure shortens the DNA length about seven-fold.
Vary in length between 20 to 100 bp, depending on species and cell typeDiameter of the nucleosome
Histone proteins are basic They contain many positively-charged amino acids Lysine and arginineThese bind with the phosphates along the DNA backbone There are five types of histonesH2A, H2B, H3 and H4 are the core histones, Two of each make up the octamer H1 is the linker histone Binds to linker DNA, Also binds to nucleosomes But not as tightly as are the core histones
Play a role in the organization and compaction of the chromosome
Nucleosomes associate with each other to form a more compact zig-zag structure fiber of 30 nm. This was reveled by F.Thoma.Histone H1 plays a role in this compactionAt moderate salt concentrations, H1 is removedThe result is the classic beads-on-a-string morphologyAt low salt concentrations, H1 remains bound Beads associate together into a more compact morphology
Nucleosomes Join to Form a 30 nm Fiber
The 30 nm fiber shortens the total length of DNA another seven-fold
Its structure of 30 nm fiber has proven difficult to determineThe DNA conformation may be substantially altered when extracted from living cellsTwo models have been proposedSolenoid modelThree-dimensional zigzag model
Regular, spiral configuration containing six nucleosomes per turn
Irregular configuration where nucleosomes have little face-to-face contact
So far the DNA have been shortened the about 50-fold A third level of compaction involves interaction between the 30 nm fiber and the nuclear matrixThe nuclear matrix is composed of two partsNuclear laminaInternal matrix proteins10 nm fiber and associated proteinsFurther Compaction of the Chromosome
SCHEMATIC FIGURE SHOWS THE ARRANGEMENT OF THE MATRIX WITHIN THE CELL
The third mechanism of DNA compaction involves the formation of radial loop domains
Matrix-attachment regionsScaffold-attachment regions (SARs)orMARs are anchored to the nuclear matrix, thus creating radial loops25,000 to 200,000 bp
The attachment of radial loops to the nuclear matrix is important in two ways1. It plays a role in gene regulation 2. It serves to organize the chromosomes within the nucleusEach chromosome in the nucleus is located in a discrete and nonoverlapping chromosome territory
Further Compaction of the Chromosome