Directionality of RNA and DNA
The DNA and RNA strands have a defined direction with the 5’ end at the top and 3’ end at the bottom.
Any additional RNA or DNA nucleotide in a strand can only attach to the 3’ end of a previous nucleotide. For pairing in DNA between purines and pyrimidines to occur, the two strands must run in antiparallel directions.
Directionality of RNA and DNA
5’ end
And the importance of directionality...
The linking of nucleotides between the 3rd carbon and the 5th carbon results in a directionality of the sugar- phosphate backbone – it is said to go from 5’ (“prime”) to 3’ direction.
Each strand has a 5’ end with a terminal phosphate group and a 3’ end with a terminal hydroxyl group.
Directionality of RNA and DNA
RNA shows the same directionality as DNA. The phosphate groups act as connectors between the sugar rings, by forming covalent phosphodiester bonds between the 3rd carbon atom of the one sugar ring and the 5th carbon atom of the second sugar ring in the growing strand of RNA.
In replication, DNA nucleotides can only be added on to the 3’ end of the growing polymer of nucleotides. The 5’ end is already bonded to a phosphate group. Both DNA strands acts as templates.
In transcription, the 5’ ends of free RNA nucleotides are only be added on to the 3’ end of the growing polymer of mRNA nucleotides. Only one of the two strands is used as a template.
A molecule of RNA carries the sequence information for making a polypeptide by linking amino acids together. The ribosome moves along the mRNA towards the 3’ end. Translation is in 5’ -> 3’ direction.
Structure of a nucleosome
When eukaryotic DNA is more closely examined under the microscope, it looks like beads on a string.
Each ”bead” represents a nucleosome. Nucleosomes are the basic units of eukaryotic DNA and are composed of packaging proteins called histones, and which hold the DNA together to form the frame of chromosomes.
Structure of a nucleosome
The DNA double strand coils around histone proteins to form nucleosomes. This process of (called supercoiling) makes DNA denser, so takes up less space in the nucleus. Nucleosome packing and supercoiling only takes place in eukaryotic cells and usually during prophase of mitosis/meiosis.
Structure of a nucleosome
Nucleosomes are useful, because they help to pack the long strands of DNA composed of billions of base-pairs into the nucleus of a size between 5 – 10μm.
Watch the videos and annotate your diagram to show how the histone proteins contribute to the packaging process.
Structure of a nucleosome
The core is composed of 8 histone proteins with a positively charge. This structure is called an octamer.
Each octamer consists of two copies of 4 different types of histones.
The “linker “ DNA connects one nucleosome to the next
Nucleosomes are linked by an additional histone protein (H1 histone)
Structure of a nucleosome What is the purpose of the nucleosome packing?
Supercoiling helps to protect the DNA from damage and allows chromosomes to be mobile during mitosis and meiosis.
It helps to control gene expression and
DNA replication – for a gene to be transcribed it must be uncoiled.
DNA into a smaller volume to fit in the cell, resulting in a greatly compacted structure that allows for more efficient storage
Nucleosomes help to supercoil and package the
Nucleosomes
http://www.rcsb.org/pdb/home/home.do
Rotate the molecule to see the two copies of each histone protein. They are identified by the tails that extend from the core. Each protein has such a tail.
Notetheca.150bpofDNAwrapped nearly twice around the octamer core
NotetheN-terminaltailthatprojects from the histone core for each protein.
Visualize the positively charged amino acids on the nucleosome core. Suggest how they place a role in the association of the protein core with the negatively charged DNA.
Visit the protein data bank and download the image of a nucleosome.