NEET Exam  >  NEET Notes  >  Biology Class 12  >  Structure & Packaging of DNA Helix

Structure & Packaging of DNA Helix | Biology Class 12 - NEET PDF Download

Structure of DNA Double Helix

  1. DNA is a long polymer of deoxyribonucleotides. It is made up of two polynucleotide chains, where the backbone is constituted by sugar-phosphate and the bases project inside.
  2. The two chains have anti-parallel polarity, i.e. 5′ > 3′ for one, 3′ > 5′ for another.
    Fig: Double Helix Structure of DNA
    Fig: Double Helix Structure of DNA
  3. The bases in two strands are paired through hydrogen bond (H—bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with thymine from the opposite strand and vice-versa. Guanine bonds with cytosine by three H—bonds. Due to this, purine always comes opposite to pyrimidine. This forms a uniform distance between the two strands.
  4. The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there is roughly 10 bp in each turn. Due to this, the distance between a base pair in a helix is about 0.34 nm.
  5. The plane of one base pair stacks over the other in a double helix. This confers stability to the helical structure in addition to H—bonds.

Question for Structure & Packaging of DNA Helix
Try yourself:What is the structure of DNA double helix?
View Solution

The length of a DNA double helix is about 2.2 meters (6.6 x 10bp x 0.34 x 10-9 m/bp) 

Therefore, it needs special packaging in a cell.

Packaging of DNA Helix

DNA packaging is the process of tightly packing up the DNA molecule to fit into the nucleus of a cell.

Have you ever wondered how DNA is present in a nucleus smaller than it?

The DNA is an organic, complex, molecular structure, found in both prokaryotic and eukaryotic cells and also in many viruses. It is a hereditary material that is found in the nucleus of the cell and is mainly involved in carrying genetic information.

The DNA structure has the following characteristics:

  • The strands of the DNA are helically wounded, every single strand forms a right-handed coil.
  • The pitch of each helix is 3.32 nm and about 10 nucleotides make up one turn.
  • The distance between two succeeding base pairs is 0.34 nm
  • The total length of a DNA is the distance between two succeeding base pairs and the product of a total number of base pairs.
  • A typical DNA has an extent around 2.2 meters, which is much longer than a nucleus.
  • Prokaryotic cells can be distinguished from eukaryotic cells by the presence of a well-defined nucleus. However, their negatively charged DNA is arranged in a region called the nucleoid. They appear as a loop wrapped around a protein molecule having a positive charge.
  • All eukaryotes have a well-defined nucleus that contains DNA. DNA is a negatively charged polymer, packed compactly within the chromatin engirdling the histone proteins, a ball of positively charged proteins.
  • The octamer of histone proteins is wrapped with a DNA helix, giving rise to a structure called a nucleosome. The nucleosomes are further coiled which results in the formation of chromatin fibres. Chromatin fibres are stained thread-like structures whereas nucleosomes are beads present over it. These chromatin fibres condense to form chromosomes during mitosis.
    Structure & Packaging of DNA Helix | Biology Class 12 - NEET

Question for Structure & Packaging of DNA Helix
Try yourself:
Why does DNA need special packaging in a cell?
View Solution

Histones

Histones are the proteins promoting the DNA packaging into chromatin fibres. Histone proteins are positively charged possessing several arginine and lysine amino acids binding to the negatively charged DNA. 

Structure & Packaging of DNA Helix | Biology Class 12 - NEET

There are two types of Histones:

  1. Core Histones
  2. Linker Histones
  • H2A, H2B, H3 and H4 are the core histones. Two H3, H4 dimers and two H2A, H2B dimers form an octamer.
  • Linker histones lock the DNA in place onto the nucleosome and can be removed for transcription.
  • Histones can be modified to change the amount of packaging a DNA does. The addition of the methyl group increases the hydrophobicity of histones. This results in tight DNA packaging.
  • Acetylation and phosphorylation make the DNA more negatively charged and loosens the DNA packaging.
  • Enzymes that add methyl groups to histones are called histone methyltransferases. The enzymes that add acetyl groups to the histones are called histone acetyltransferases while the ones that remove the histones are called histone deacetylases.
  • The packaging of chromatin at higher level requires additional set of proteins which are collectively called Non-Histone Chromosomal (NHC) proteins.
  • In a nucleus, some regions of chromatin are loosely packed (stains light) called euchromatin (transcriptionally active chromatin). In some regions, chromatin is densely packed (stains dark) called heterochromatin (inactive chromatin).
The document Structure & Packaging of DNA Helix | Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
All you need of NEET at this link: NEET
158 videos|393 docs|239 tests

FAQs on Structure & Packaging of DNA Helix - Biology Class 12 - NEET

1. How is the structure of DNA double helix formed?
Ans. The structure of DNA double helix is formed by two strands of DNA that are twisted around each other in a spiral shape, with the bases of the nucleotides on each strand pairing up in a specific way (A with T, and G with C).
2. What is the role of histones in the packaging of DNA?
Ans. Histones are proteins that help in the packaging of DNA by forming complexes with the DNA strands. These complexes, known as nucleosomes, help to condense the DNA and regulate gene expression.
3. How does the packaging of DNA helix impact gene expression?
Ans. The packaging of DNA helix, through the formation of nucleosomes and higher order chromatin structures, can regulate gene expression by controlling access to the DNA strands. This packaging can either facilitate or inhibit the binding of transcription factors and other regulatory proteins to the DNA.
4. What are the different levels of DNA packaging beyond nucleosomes?
Ans. Beyond nucleosomes, DNA can be further packaged into higher order structures such as solenoids, loops, and domains. These structures help to compact the DNA even more and play a role in gene regulation and chromosome organization.
5. How does the packaging of DNA helix impact DNA replication and repair processes?
Ans. The packaging of DNA helix can impact DNA replication and repair processes by influencing the accessibility of the DNA strands to the enzymes and proteins involved in these processes. Compacted DNA may be more difficult to replicate or repair compared to loosely packaged DNA.
158 videos|393 docs|239 tests
Download as PDF
Explore Courses for NEET exam

How to Prepare for NEET

Read our guide to prepare for NEET which is created by Toppers & the best Teachers
Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Download the FREE EduRev App
Track your progress, build streaks, highlight & save important lessons and more!
Related Searches

MCQs

,

mock tests for examination

,

video lectures

,

Sample Paper

,

Objective type Questions

,

past year papers

,

study material

,

Viva Questions

,

shortcuts and tricks

,

Free

,

Structure & Packaging of DNA Helix | Biology Class 12 - NEET

,

practice quizzes

,

Summary

,

Extra Questions

,

Structure & Packaging of DNA Helix | Biology Class 12 - NEET

,

ppt

,

pdf

,

Structure & Packaging of DNA Helix | Biology Class 12 - NEET

,

Exam

,

Semester Notes

,

Previous Year Questions with Solutions

,

Important questions

;