Packaging of DNA Helix
DNA packaging is the folding of long DNA molecules with histone proteins into nucleosomes and higher-order chromatin so that DNA fits inside the nucleus and remains functionally organized.
In one line: Eu means easy to express; heterochromatin is heavily packed.
Concept Practice Coming SoonUnderstand it
- Eukaryotic DNA is extremely long compared with the size of the nucleus, so it cannot remain as a loose double helix.
- The negatively charged phosphate backbone of DNA is attracted to positively charged histone proteins that are rich in basic amino acids.
- About two turns of DNA wind around a histone octamer to form a nucleosome core, and short stretches of linker DNA connect one nucleosome to the next, giving the beads-on-string appearance.
- These nucleosomes coil and fold into thicker chromatin fibres, which can become highly condensed during cell division.
- Packaging is also functional: loosely packed euchromatin is more accessible to RNA polymerase and is generally transcriptionally active, while densely packed heterochromatin is less accessible and usually transcriptionally inactive.
- Thus, chromatin organisation helps both in fitting DNA into the nucleus and in regulating gene expression.
Example
Rule to remember
Key sequence: DNA double helix wraps around histone octamer, forms nucleosome, folds into chromatin fibre, and condenses further into chromosome during cell division.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
If asked why a long DNA molecule does not remain loose inside the nucleus, explain histone binding, nucleosome formation, and chromatin folding instead of saying it is simply coiled.
Method you can reuse
- 1First state the size problem: long DNA must fit into a small nucleus without losing functional control.
- 2Next mention charge attraction between negatively charged DNA and positively charged histones.
- 3Then write that DNA wraps around a histone octamer to form a nucleosome, with linker DNA connecting adjacent nucleosomes.
- 4After that, describe folding of nucleosomes into chromatin fibre and further condensation into chromosomes during cell division.
- 5Finally, connect packing level to gene activity: euchromatin is loosely packed and more transcriptionally active, while heterochromatin is densely packed and generally less active.
- 6Do not skip linker DNA or reverse the activity of euchromatin and heterochromatin, because these are common mark-losing points.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why can negatively charged DNA wrap around histone proteins?
Think first, then show answerHide answer
Common wrong answer
Writing that heterochromatin is more active in transcription than euchromatin.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
Answer plan
- 1
Enough for 1 mark
DNA packaging is the folding of long DNA molecules with histone proteins into nucleosomes and higher-order chromatin so that DNA fits inside the nucleus and remains functionally organized.
- 2
Add for 2 marks
Example: A human cell contains nearly two metres of DNA, yet it fits inside a microscopic nucleus because DNA is wrapped around histones and folded as chromatin.
- 3
Add for 3 marks
Eukaryotic DNA is extremely long compared with the size of the nucleus, so it cannot remain as a loose double helix.
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