6.4 Flashcards
(13 cards)
DNA Repplication is semiconservative
- New parent strands separate
- Complementary strands and made for each
- New molecule consists of one parent strand and one new strands
Meselson and Stahl
- Carried out and experiment that demonstraght that DNA replication is semiconservative
- Used heavy isotopes of Nitrogen 15, and normal Nitrogen 14.
- 1/2 rounds of replication
- Any new DNA with lighter 14 incorpreted into its structure demonstraghts the semiconservative structure of DNA
DNA Replication: The Process
Three Steps Involved:
- Parental strands of DNA are separated
- Complementary strands are assembled
- New strands are proofread and repaired
Step 1:
Strand separation
DNA Helicase binds to a specific nucleotide sequence (replication origin)
Unwinds DNA by breaking the Hydrogen bonds between the base pairs
Forms a y-shaped structure, Replication Fork
Two Problems in Strand Separation
Tension:
- Can lead to twists and tangles
-Topoisomerase relieves tension by cutting one or two strands near the replication fork (stands can untangle and rejoin)
Separated Strands Tends to Anneal
- This is because the strands are complimentary
- Single-strand binding proteins bind to the separated base pairs
- Keeps strands separated
Replication Bubble
- DNA helicase complex separetes DNA in both directions
-Many replication bubbles at a given time
- Eventually meet and merge
- Multiple replication origins is needed, otherwise replication would take too long
In Euk.: DNA replicates in 50bp/sec
Otherwise: Replicating the entire human genone would take 1 month
Building complimentary strands
- DNA Polymerase adds new nucleotides to build DNA
- New strands are made from the 5’ to 3’ direction
- Template strands are read from the 3’ to 5’ direction
How doe DNA Polymerase use Energy
- DNA polymerase need energy
- Via hydrolysis of nucleoside triphosphate, to break of 2 Pi
-The formation of phosphierster bond with hydroxyl group + remain phosphate +3-carbon on carbon strand
- Energy released drives DNA synthesis
Nucleoside: Base + sugar
Nucleotide: Sugar + Base + Phosphate
Continued with RNA Primerase
- When replication fork opens, RNA primase starts replication processes
- Builds complementary RNA segments (RNA primers)
- One from strands is origanted in the 3’ to 5’ direction, and the other from the 5’ to 3’ direction
Leading strand as RNA Primer are placed
-DNA polymerase III adds nucleotides
- DNA Polymerase keeps building towards the fork continuesly without needing any more RNA Primers
Lagging strand as RNA primers are in place
- On opposite strand
- DNA Polymerase III adds nucleotides away from the replication fork
- Called the lagging strand becuase it is not made in one coninual proccess
Okazaki Fragments: Smaller fragments made by multiple RNA primers
- Longer in prkaryotes than in Eukaryotes
Replication Process Continued
- DNA polymerase I removes
the RNA primers one at a
time and replaces them with
DNA nucleotides
DNA Ligase: catalyzes the formation of phosdiester bonds b/w the two fragments
Step 3: Dealing with errors
- DNA polymerase enzymes proofread and correct errors
- DNA polymerase III cannot move
forward if base-pairs are mismatched
Repair mechinisms: DNA Polymerase 1 and 2
DNA polymerase II is a slow
enzyme that repairs damage
* b /c incorrect bps don’t bond
properly, they distort DNA
shape
- Repair mechanisms find the
distortion and remove a
portion o f the strand around
the mismatch, and the resulting
gap is filled by a DNA
polymerase and sealed with
DNA ligase
