Directionality of RNA and DNA

Overview

• Directionality:
  • DNA and RNA strands have a defined direction.
  • 5’ end at the top, 3’ end at the bottom.
  • Nucleotides can only attach to the 3’ end of a previous nucleotide.

DNA and RNA Structure

• Antiparallel Strands in DNA:
  • For base pairing between purines and pyrimidines.
  • Strands must run in opposite directions.
• Sugar-Phosphate Backbone:
  • Directionality from 5’ to 3’ prime direction.
  • Each strand has a 5’ end with a phosphate group and a 3’ end with a hydroxyl group.

Replication and Transcription

• Replication:
  • DNA nucleotides added to the 3’ end.
  • Both DNA strands act as templates.
• Transcription:
  • RNA nucleotides added to the 3’ end of mRNA.
  • Only one DNA strand used as a template.

Translation

• Process:
  • RNA carries sequence for polypeptide synthesis.
  • Ribosome moves along mRNA towards 3’ end.
  • Translation occurs from 5’ to 3’ direction.

Structure of a Nucleosome

Basic Unit

• Appearance:
  • Looks like beads on a string under a microscope.
  • Each bead is a nucleosome.
• Composition:
  • Composed of histones that package DNA to form chromosomes.

Function

• Supercoiling:
  • DNA coils around histones forming nucleosomes.
  • Makes DNA denser and occupies less space in the nucleus.
  • Occurs in eukaryotic cells during prophase of mitosis/meiosis.

Purpose of Nucleosome Packing

1. Compaction:
   • Supercoils DNA into a smaller volume for efficient storage.
2. Protection and Mobility:
   • Protects DNA from damage.
   • Allows chromosome mobility during cell division.
3. Gene Expression Control:
   • Controls gene expression and DNA replication.
   • Genes must be uncoiled to be transcribed.

Detailed Structure

• Histone Core:
  • Composed of 8 histone proteins, known as an octamer.
  • Positively charged, binding with negatively charged DNA.
• Linker DNA:
  • Connects nucleosomes.
  • Includes an additional H1 histone protein.

Investigation Tasks

1. Rotation of Molecule:
   • Identify two copies of each histone protein.
2. Observation:
   • Note 150bp of DNA wrapped around octamer core.
3. N-terminal Tail:
   • Projects from histone core.
4. Amino Acids:
   • Positively charged contributing to DNA association.

Resources

• Protein Data Bank:
  • Suggested to visit for further visualization and download images of nucleosomes.