·
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
·
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:
Distinguish between unique/single copy and highly repetitive DNA (which is used for fingerprinting)
The pictures are simplified without any unnecessary information.
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