Packaging of DNA Helix: The DNA structure was proposed by Watson and Crick. According to them, the DNA is a double-helical structure with two polynucleotide strands running anti-parallel to each other. This double helix is negatively charged due to the presence of phosphate groups in the DNA backbone. The cell makes histone proteins that bind the DNA to counterbalance the negative charge. These histone proteins are involved in Packaging of DNA Helix.

Packaging of DNA Helix

• Roger Kornberg in 1974 reported that chromosomes are made up of DNA and proteins.
• Later, Beadle and Tatum reported that chromatin fibers look like beads on the string, where beads are repeated units of proteins.
• The proteins associated with DNA are true type – basic proteins (histone and protamine) and acidic non-histone chromosomal proteins.
• The negatively charged DNA molecule wraps around the positively charged histone proteins to form a structure called nucleosomes and nucleosomes is made up of four types of histone proteins H2A, H2B, H3 and H4 occurring in pairs.
• 200 BP of DNA helix is wrap around nucleosome by 1 1/3 turns plugged by H1 histone protein.
• Repeating units of nucleosome from chromatin (thyroid like) in nucleus.
• When chromatin is packed it from solenoid structure of 30nm in diameter and father supercoiling forms chromatin films (chromatin) which condense and coil during metaphase to form chromosomes.
• In prokaryotic cells (which do not have a defined nucleus) such as E coli DNA (being negatively charged) is held with some proteins (that have positive charge) in a region called a nucleoid. The DNA is nucleoid is organized in large loops held by proteins.

Core of Histone Molecules

Histone proteins play essential structural and functional roles in the transition between active and inactive chromatin states. Although histones have a high degree of conservation due to constraints to maintain the overall structure of the nucleosomal octameric core, variants have evolved to assume diverse roles in gene regulation and epigenetic silencing. Histone variants, post-translational modifications and interactions with chromatin remodeling complexes influence DMA replication, transcription, repair and recombination. The authors review recent findings on the structure of chromatin that confirm previous interparticle interactions observed in crystal structures.

Search of Deoxyribonucleic Acid (DNA)

• Frederick Griffith in 1928 performed a series of experiments using Streptococcus pneumoniae bacteria and mice.
• When Streptococcus pneumoniae are grown in cultural plate some produce smooth shiny colonies (S) while others produce rough colonies (R).
• Mice infected with S-strain die from pneumoniae whereas mice infected with R-strain does not.
• Griffith then killed the bacteria by heating them.
• Now when S-strain (heat-killed) were injected in mice they lived whereas when R lived and S killed were injected the mice died.
• He concluded that R-strain bacteria had somehow been transformed by the heat killed S-strain bacteria, i.e. some transforming material must be transferred from S-strain to R-strain that enabled the R-strain to synthesis smooth polysaccharides coat and become urilest.

Follow on Facebook

By Team Learning Mantras