Question

What groups are found on the exterior of the DNA double helix? In the nucleus, DNA strands are wrapped around proteins called histones. Would you expect histones to be neutral, positively charged, or negatively charged? Based on your answer, which amino acids do you expect to be abundant in histones and why?

Short answer

DNA's exterior has negatively charged phosphate groups; histones are positively charged, rich in lysine and arginine.

Step-by-step solution

Identify Groups on the DNA Exterior

DNA is a double helix with a backbone composed of sugar-phosphate groups. These phosphate groups on the exterior of the DNA are negatively charged due to their structure.

Analyze the Role of Histones

Histones are proteins that DNA wraps around to form nucleosomes, which help in the packaging of DNA within the nucleus. To properly bind with the negatively charged DNA, histones need to have an opposite charge.

Determine the Charge of Histones

Since DNA's exterior is negatively charged, histones must be positively charged to facilitate attraction and binding. Proteins with a positive charge can interact with and neutralize the DNA's negative charge.

Identify Amino Acids in Histones

Histones have an abundance of positively charged amino acids. These would include lysine and arginine, both of which have side chains that carry positive charges at physiological pH. Their positive charge helps in the binding with negatively charged DNA.

Key concepts

DNA double helix

The DNA double helix is an elegant structure that lies at the core of genetic operation in living organisms. The hallmark feature of this structure is the spiral arrangement of two long strands that intertwine around one another. Each strand in the helix is composed of what is called a sugar-phosphate backbone. This backbone is on the exterior of the helix and gives the DNA its stability and shape.

Crucially, the phosphate groups in this backbone carry a negative charge. This property is vital when it comes to DNA interactions within the cell. The negative charge helps keep the DNA soluble in the cellular environment and is also significant when the DNA interacts with other molecules. Feel free to imagine the DNA double helix as a twisted ladder, where the sides of the ladder are the sugar-phosphate backbones, and the rungs are the nitrogenous base pairs. This mental image can help you appreciate the structural and functional beauty of DNA.

histones

Histones are specific proteins that play a crucial role in organizing DNA in the nucleus of eukaryotic cells. Because the DNA double helix is so long, it needs to be efficiently packed so it can fit inside the cell nucleus. Histones come into action here, helping to package the DNA into what are called nucleosomes, forming a chromatin structure.

These nucleosomes resemble beads on a string when viewed under a microscope. The 'beads' being the histones, each wrapped with a segment of DNA. For histones to effectively interact with DNA, they possess a positive charge. This is due to the presence of certain positively charged amino acids that we'll talk about in a bit. This positive charge allows histones to bind closely with the negatively charged DNA, facilitating the coiling and packing of DNA.

amino acids

Amino acids are the building blocks of proteins and play a pivotal role in various cellular processes. In the case of histones, the amino acids lysine and arginine are particularly important. These amino acids are abundant in histones because they carry positive charges on their side chains at physiological pH.

The positive charge of lysine and arginine allows them to interact favorably with the negatively charged phosphate groups in the DNA backbone. This relationship is essential for the stable binding of histones to DNA, ultimately allowing the packaging and protection of genetic material in the nucleus. It highlights how specific characteristics of amino acids are crucial in biological interactions at the molecular level.

protein-DNA interaction

Protein-DNA interaction is fundamental to numerous biological processes, such as replication, transcription, and repair. The interaction between DNA and histone proteins serves as a classic example. As mentioned earlier, the negatively charged DNA is wrapped around positively charged histone proteins, forming a structural unit known as the nucleosome.

The successful bonding of DNA and histones is primarily driven by electrostatic attractions between opposite charges. Besides histones, these interactions can involve other proteins like transcription factors that can bind to specific DNA sequences, either promoting or inhibiting the transcription of particular genes. Understanding these interactions is key to appreciating how genes are regulated and expressed within a cell. This process is akin to complex dance choreography, where each step is integral in determining the final performance.