understanding dna biology 12 ms. bowie. understanding dna dna stands for deoxyribonucleic acid dna...
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Understanding DNA
Biology 12
Ms. Bowie
Understanding DNA
• DNA stands for deoxyribonucleic acid
• DNA is the genetic material. – transfer from parent cell to daughter cell and – from parent organism to offspring.
• The 3 main components of DNA are:
Understanding DNA
• There are four nitrogenous bases in DNA.
– Adenine (A),
– Guanine (G)
– Thymine (T)
– Cytosine (C)
– Uracil (U)
The Nitrogenous Bases
which are double ringed which are double ringed purinespurines(Hint: “pure” = (Hint: “pure” = AAlways lways GGood)ood)
which are single ringed which are single ringed pyrimidinespyrimidines(Hint : “pyro” = (Hint : “pyro” = TTake ake CCare)are)
found in RNA instead of found in RNA instead of thyminethymine
• DNA is made of polymers of many nucleotides held together by phosphodiester bonds between the phosphate group and the adjacent sugars.
• Nitrogenous bases are held together by hydrogen bonds. Two between A & T and Three between C & G
Bonds in DNA
• Erwin Chargaff determined that: – Adenine always pairs with Thymine
– Cytosine always pairs with Guanine
Chargaff’s Rule
Numbering Carbons in the Ring
• Carbons are numbered clockwise, starting with the carbon atom to the immediate right of the oxygen atom.
• The first one would be called 1’, the next would be 2' (2 prime)...
• The nitrogenous base attaches to the sugar at the 1' location (via a glycosyl bond).
• The phosphate group attaches to the sugar at the 5' location (via an ester bond).
The Race for DNA
• Linus Pauling – building ball and stick models
using his expertise in chemistry.
• He proposed 1 model– it did not show how copying of
the DNA could occur.
• His son, Peter Pauling, went to work with another group in Cambridge, England.
The Race for DNA• Rosalind Franklin and Maurice Wilkins
– King's College in London – studies using the diffractions with X-ray
crystallography. – Work was based on serious scientific evidence,– They have great difficulty working TOGETHER.– Franklin uncovered the shape of DNA when she
found an x-ray diffraction in a cross pattern. – Wilkins shared this data with Watson who used it to
further his own work.
The Race for DNA• James Watson and Francis Crick
– considered lazy slackers
– neglected experimentation and evidence
– focused on the structure of DNA using 3D modelling
– when Wilkins showed the x-pattern of Franklin's x-ray diffraction, he immediately knew what the structure might look like.
– They came up with the modern structure of DNA that accurately answered the question as to how DNA can create perfect copies of itself.
The Race for DNA
• Watson, Crick and Wilkins won Noble Prizes for their Discovery of the structure of DNA.
• However, Franklin did not receive one as she died before the awards were given.
DNA Double Helix Structure
• DNA is made of 2 antiparallel strands of nucleotides.
• It is important to be able to identify the 3' and 5' ends of each strand.
DNA Double Helix Structure
• Thanks to Chargaff's Rule, we only state the 5' to 3' strand, since the complementary strand can be easily deduced.
5' - GCAATCTA - 3'3' - CGTTAGAT - 5'
Replicating DNA
• DNA must be copied in order for single cells to be replaced as they age or are damaged.
• It also happens so organisms can grow.This process is known as mitosis.
Replicating DNA
• Semiconservative: each DNA molecule is made of one parent strand and one newly synthesized strand.
• Conservative: each DNA molecule reforms so that both new strands stay together and both parent strands stay together.
• Dispersive: where bits and pieces of the parent and new strands are interspersed in both strands following the replication.
The Process of Replicating DNA• The two DNA strands separate
• Each strand serves as a template
• DNA replication begins at the origin of replication.
The Process of Replicating DNA
• DNA replication begins at the origin of replication.
The Process of Replicating DNA
• Replication proceeds (following the AT/CG rule) outward from the origin in opposite directions.
• This is a process called bidirectional replication.
• In simple bacteria, which have a small circular chromosome, there is a single origin of replication.
The Process of Replicating DNA
• The origin of replication forms a bubble that creates two DNA replication forks.
• Replication begins near the opening of each fork.
The Process of Replicating DNA
• The building of the new DNA always begins with a primer.
• New DNA is always built in the 5' to 3' direction.
The Process of Replicating DNA
• One strand will be the leading strand. The other will be the lagging strand.
• The leading strand moves in the same direction as the fork is moving.
• The leading strand is replicated as one long continuous molecule.
The Process of Replicating DNA
• The lagging strand is made in a series of small fragments which will eventually form a continuous strand.
• Synthesis of this strand goes in the direction away from the fork.
• These fragments are known as Okazaki fragments.
DNA Replication ProteinsStep 1: Formation and Movement of the Replication ForkStep 1: Formation and Movement of the Replication Fork
DNA Replication ProteinsStep 1: Formation and Movement of the Replication ForkStep 1: Formation and Movement of the Replication Fork
DNA Replication Proteins
» In prokaryotes, 3- 5 enzymes that serve to replicate and repair the DNA strand. These are DNA polymerase I, II, III, IV & V.
» In eukaryotes, there are 12+ DNA polymerases involved.
Step 2: Synthesis of the Leading and Lagging StrandsStep 2: Synthesis of the Leading and Lagging Strands
DNA Replication Proteins
» As DNA polymerase III slides along the DNA, free nucleotides with 3 phosphate groups, called deoxynucleoside triphosphates, hydrogen bond to the exposed bases in the template strand according to the AT/GC rule. DNA polymerase III breaks the bond between the 1 and 2nd phosphate groups releasing a pyrophosphate (2 phosphate groups, which are recycled.
»
Step 2: Synthesis of the Leading and Lagging StrandsStep 2: Synthesis of the Leading and Lagging Strands
DNA Replication Proteins
» DNA polymerase III can't polymerize a DNA strand unless a DNA or RNA strand is already attached to the template.
» An enzyme called DNA primase is needed if the template is bare. DNA primase makes a complimentary primer that is actually a short segment of RNA, usually about 10-12 nucleotides long. At a later stage in replication, the RNA primer is removed and replaced with DNA by DNA Polymerase I.
Step 2: Synthesis of the Leading and Lagging StrandsStep 2: Synthesis of the Leading and Lagging Strands
DNA Replication Proteins
» DNA polymerase can only synthesize new DNA in the 5' to 3' direction (of the new strand).
» The Okazaki fragments must be linked together (bonded) on the lagging strand.
» This bond is reformed by an enzyme known as DNA ligase.
Step 2: Synthesis of the Leading and Lagging StrandsStep 2: Synthesis of the Leading and Lagging Strands
DNA Replication Proteins
• There are many other proofreading enzymes known as exonucleases that correct and repair mistakes in the newly forming DNA strands.
• If they detect a mismatched pair, they will backtrack, cut it out and replace it with the correct pair.
• If the repairs are done immediately to avoid being copied in the next replication. Mistakes that make it through can result in mutations.
Step 3: Proofreading and Repair of newly formed DNA strandsStep 3: Proofreading and Repair of newly formed DNA strands
DNA Replication ProteinsStep 3: Proofreading and Repair of newly formed DNA strandsStep 3: Proofreading and Repair of newly formed DNA strands
• http://www.wiley.com/college/pratt/0471393878/instructor/animations/dna_replication/index.html
• http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter3/animation__dna_replication__quiz_1_.html
• http://www.johnkyrk.com/DNAreplication.html