Question

Make a concept map to depict your understanding of the structure of DNA and the packing of DNA into chromatin and chromosomes.

(do not cross lines and create from higher to lower order)

Answer 1

1. The DNA is complex molecule which is tightly packed inside the small space of the nucleus. Just to give an idea, if you were to stretch out all of the DNA in your body end-to-end, it would be approximately 1.2 × 10^10 miles, which is about 70 trips to the Sun and back.
2. The double helix of DNA is highly negatively charged due to all the negatively charged phosphates in the backbone. All that negative charge must be counterbalanced by a positive charge, and the cell makes proteins called histones that bind DNA and aid in DNA's packaging. Histones are positively charged proteins that wrap up DNA through interactions between their positive charges and the negative charges of DNA. Double-stranded DNA loops around 8 histones twice, forming the nucleosome, which is the building block of chromatin packaging.
3. DNA can be further packaged by forming coils of nucleosomes, called chromatin fibers. These fibers are condensed into chromosomes during mitosis, or the process of cell division. However, packaging of chromatininto chromosomes that we are most familiar with occurs only during a few stages of mitosis. Most of the time, DNA is loosely packaged.
4. Histone Proteins are the packing briefcases for DNA: Histones are positively charged proteins that facilitate the packing of DNA into condensed chromatin fibers. Histones have many arginine and lysine amino acids that easily bind to the negatively charged DNA. DNA is highly negatively charged because of the phosphate group of each nucleotide is negatively charged. Histones are divided into two groups: Core histones are H2A, H2B, H3, and H4, where two H3/H4 dimers (H3 and H4 hooked together) and two H2A/H2B dimers (these two hooked together) form the octamer (all eight of these guys together). Linker histone H1 basically locks the DNA in place onto the nucleosome and can be removed for transcription while linker histone H5 is a variant of H1 predominantly used in birds.