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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