SBI4U: DNA Replication
SBI4U: DNA Replication
Why do we need to replicate our DNA? ____________________________
When does DNA replication happen in a Cell? _______________________
Background:
Cell Division: ________________ + _________________
DNA is replicated in __________________ prior to mitosis
Each _______________________ must have an exact copy of the _____________ DNA
How does DNA Replication Occur? - The Meselson-Stahl Experiment
In the 1950s there were 3 possible models of DNA Replication:
1. Semi-Conservative:
• Two parental strands separate and each serves as
as a _____________ for a new ________________
• Newly synthesized DNA molecule has __________
_________________________________________
2. Conservative
• The two parental strands _____________________
and produce another daughter helix with ________
_________________________________________
• Newly synthesized DNA has ___________________
3. Dispersive
• DNA becomes fragmented so that new and old DNA _______________________ after replication.
• Newly-synthesized DNA molecule has __________________________________
Meselson-Stahl Experiment:
• Purpose: to find the ______________________________
• Experimental model: _________________ (bacteria)
Centrifuged the DNA within a density gradient: Separates components according to density
*A centrifuge is a device that spins a solution at high speeds, the spinning splits up the different components in a mixture based on density
Meselson-Stahl Experiment (continued)
Observed:
First generation – One intermediate band
Second generation – One light/one intermediate
Conclusion:
DNA replication is semi-conservative
DNA Replication-3 steps: An overview:
Stage 1: Initiation
◦ DNA strands are ____________________
◦ A small portion of RNA is __________ to the
exposed strands to “prime” them for replication
Stage 2: Elongation
◦ DNA polymerase III builds a __________
of DNA by incorporating _________________
Stage 3: Termination
Stage 1: Initiation
Stage 2: Elongation
Characteristics of DNA Replication
1. Bi-directional
2. Semi-discontinuous
1. Elongation is bi-directional
← Elongation proceeds in _________________, outwards from the _________________
← The junction where the strands are still joined is called the _____________________
← DNA synthesis occurs simultaneously using _________________________________
← A ____________________________ forms between two replication forks
[pic]
2. Elongation is semi-discontinuous
← DNA synthesis always occurs in the_________________________ (of the new strand!)
← The two template strands are antiparallel
← ____________________________________________________
Removal of the RNA primers, and joining of the Okazaki fragments:
|Enzyme |Role |
|DNA polymerase I |removes the ________________; |
| | |
| |replaces them with the proper |
| |deoxyribonucleosides |
|DNA ligase |joins the fragments together |
| |(___________________________) |
Stage 3: Termination
← Two _____________________ meet each other; or
← DNA Polymerase III reaches the ______________________
-----------------------
Separation of Strands:
← DNA strands are “______________________” by ______________________
◦ Hydrogen bonds between complementary bases are ____________
← Single-stranded binding proteins (SSBs) bind to exposed strands to _______________________________________________
← DNA gyrase relieves ___________________________________________________
Priming:
← DNA polymerase cannot start incorporating nucleotides on its own
◦ Needs ___________________________________________
← A short segment of ________________ (a “______________” – 10 to 60 nucleotides long) provides that 3’ end
← ____________________ synthesizes the primer and anneals it to the template strand.
← DNA polymerase can then add on _______________________
← New strand is synthesized in the _______________ (added on to the end with the -OH group)
← Catalyzed by _______________
__________________________
← Free bases are floating in the nucleoplasm as ____________ _________________________
← Energy required for DNA synthesis is provided by ______ ____________________________________________________
← Leading strand – Uses the 3’ to 5’ template strand as its guide
← Is built continuously, towards the replication fork
← Lagging strand – Uses the 5’ to 3’ template strand as its guide
◦ Is built __________________ in short ___________________
◦ RNA primase constantly adds new RNA primers along the template strand.
◦ The fragments are called __________ __________________
◦
Problem: Shortening of telomeres
← Telomeres: The ends of DNA. Contain repetitive sequences.
← Protects the chromosome from degradation.
← Loss of telomeric DNA occurs on the ____________________________________
← Approximately 50 replications before the telomeres become ____________________
← Telomere shortening linked to ___________
← Telomerase - enzyme that prevents ____________________________________
← Present in cells that need to divide constantly: white blood cells, ______________________
← May be present in ______________________
Proofreading:
← _____________________________________ are constantly proofreading the progeny strand as it is synthesized.
← Both have ______________________ activity
← can identify incorrectly added nucleotides, backtrack, and excise them (cut them out) before continuing synthesis.
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