Bead-like structures composed of proteins and tightly wound DNA; these structures 'supercoil' the DNA inside the cell.

Bead-like structures composed of proteins and tightly wound DNA; these structures 'supercoil' the DNA inside the cell.

• A. Nucleosome
• B. Histone
• C. Chromosome
• D. Chromatin

Answer: (A)

DNA packaging in eukaryotic cells begins with nucleosomes, the basic units of chromatin. A nucleosome forms when about 147 base pairs of DNA wrap around a histone octamer (two copies each of H2A, H2B, H3, and H4). This wrapping creates a bead-like appearance along the DNA, with a short segment of linker DNA connecting one bead to the next. This arrangement tightens and constrains the DNA, promoting supercoiling and enabling efficient packaging of the genome in the nucleus while still allowing access for processes like transcription and replication. Histones are the protein components of these beads; chromatin refers to the entire DNA–protein complex; chromosomes are the highly condensed forms of chromatin seen during cell division. So the bead-like structures described are nucleosomes.

DNA packaging in eukaryotic cells begins with nucleosomes, the basic units of chromatin. A nucleosome forms when about 147 base pairs of DNA wrap around a histone octamer (two copies each of H2A, H2B, H3, and H4). This wrapping creates a bead-like appearance along the DNA, with a short segment of linker DNA connecting one bead to the next. This arrangement tightens and constrains the DNA, promoting supercoiling and enabling efficient packaging of the genome in the nucleus while still allowing access for processes like transcription and replication. Histones are the protein components of these beads; chromatin refers to the entire DNA–protein complex; chromosomes are the highly condensed forms of chromatin seen during cell division. So the bead-like structures described are nucleosomes.