Sunday, September 1, 2013

DNA Replication


DNA Replication

·      Deoxyribonucleic Acid
o   DNAàRNA à Protein
o   Replicationà Transcription à Translation
·      DNA Structure
o   Nucleotides
o   Four different base pairs: adenine, thymine, guanine
o   Nucleotides linked up with phosphodiester  (strong covalent) bond
o   Purines (A and G)- have 2 rings of carbon and nitrogen
o   Pyrimidines (C and T)- have a single ring of carbon and nitrogen
o   Hydrogen bonds between bases; A=t G=-C
o   Double helix which has uniform diameter along its entire length
o   Both helices are right handed
o   5’ to 3’
o   two polynucleotide chains are antiparallel
o   6 billion base pairs per diploid cells




·      Nucleosomes
o   Double helix has major and minor grooves on its outer diameter
o   These chemical groups within DNA are bonded to proteins. DNA is bonded to proteins called histones
o   Nucleosomes consist of DNA wrapped around 8 histones and held together by another histone (H1 linker)
o   1.65 turns of the DNA in a histone; 146 DNA bases
o   basic unit of DNA packaging
o   supercoiling condenses the DNA molecule by a factor of x15,00
o   histones are responsible for the packaging of DNA at the different levels
o   H1 molecules interact with each other, causing the chromatin to forma spiral, with 6 to nucleosomes per turn of the spiral- the structure is called a solenoid or 30 nm chromatin fiber
o   Chromatin is tightly packed, and the DNA is genetically inactive

H1 linker-->

·      DNA Sequences in the human genome
o   Exons (1-2%)
o   Introns (24%)
o   (Highly repetitive sequences- HRs (45%)
o   structural DNA (20%)
o   inactive genes (2%)
o   other (7-8%)

·      Introns and Exons
o   Eukaryotic organism have non-coding regions within the gene called introns.
o   Exons are coding regions
o   Introns are transcribed but not translated.
o   The mature mRNA is ready for export from the nucleus to the cytoplasm where translation occurs.
o   These are copied when the gene is transcribed to produce pre-mRNA.
o   The intron-RNA is edited out, through splicing, to form mature mRNA.
o   SNRNPs (small nuclear ribonucleoprotein) combine with other proteins to form a complex called a spliceosomes.
o   Spliceosomes remove the introns
o   Exons are translated into proteins.

·      DNA Replication: moves in a 5’ to 3’ direction
o   Replication Leading Strand
1.     Helicase uncoils the DNA by breaking the hydrogen bonds of the complementary base pairs forming a Replication Fork.
2.     RNA primase adds a 1-2 RNA nucleotides to the template strand. This acts as a primer allowing the DNA polymerase III to bind.
3.     DNA Polymerase III adds nucleotides in the 5’ to 3’ direction and moves in the same direction as the replication fork.
4.     DNA polymerase I removes the RNA primer and replaces it with DNA.
o   Replication Lagging Strand
1.     Helicase uncoils the DNA by breaking the hydrogen bonds of the complementary base pairs forming a Replication Fork.
2.     RNA primase adds a short length of RNA attached by base pairing to the template strand. This acts as a primer allowing the DNA polymerase III to bind.
3.     DNA polymerase III starts the replication next to the primer and adds complementary nucleotides in the 5’ to 3’ direction. It moves away from the replication fork.
4.     DNA polymerase I removes the RNA primer and replaces it with DNA. A nick is left where two nucleotides are still unconnected.
5.     Okazaki fragments are the short lengths of DNA formed between the RNA primers on the Lagging Strand only.

6.     DNA ligase seals up the nicks by making another sugar phosphate bond.

Prokaryotes v. Eukaryotes
  •          Prokaryotes have one spot- ori is the starting point; ter in the stopping point
  •           Eukaryotes have multiple spots


Enzymes in DNA Replication
       Helicase- unwinds the DNA double helix and separates it into two template strands, by breaking the hydrogen bonds between the bases.
       DNA Polymerase III- adds nucleotides in the 5’ to 3’ direction on both strands and is a proofreading mechanism.
       DNA Polymerase I- removes the RNA primer and replaces it with DNA on the 5’ to 3’ original strand.
       RNA Primase- Adds a short length of RNA to the template strand on the 5’ to 3’ original strand.
       DNA Ligase- seals the nicks on the 5’ to 3’ original strand by making another sugar phosphate bond, joins Okazaki fragments.

Speed of Replication
  •            DNA replication can take a few hours and this limits the speed of cell division.
  •            Bacteria can replicate quickly because of the relatively small amount of DNA.
  •            Eukaryotic organism's accelerate DNA replication by having thousands of replication forks along the length of the DNA molecule.


Complementary Base Pairing
  •        Ensure that genes (base sequences) are faithfully passed from one generation to the next.


Semi-Conservative
  •         The mechanism of semi-conservative replication produces two descendent double helices that each contains one of the original polynucleotide chain
  •        Half old, half new

2 comments:

Unknown said...

Distinguish between unique/single copy and highly repetitive DNA (which is used for fingerprinting)

Unknown said...

The pictures are simplified without any unnecessary information.