Exam 3: #2 DNA Replication

DNA Replication

Process by which the genetic material is copied

• Original double stranded DNA strands are used as templates for synthesis of new strands with identical sequences

• Very quick, very accurate

Structure of DNA Replication

• A pairs with T

• G pairs with C

Process of DNA Replication

1. 2 Complementary DNA strands (Parental strands) come apart

• Each serves as a template strand for the synthesis of new complementary DNA strands

2. Two newly made DNA strands = daughter strands

3. Two original DNA strands = parental strands

DNA Synthesis 

DNA synthesis occurs from 5’ to 3’

• daughter strand grows from 5’ to 3’ 

Leading Strand

.

Lagging Strand 

.

Exam question (maybe): Where is the end on circular DNA?

Circles have no ends

Replication fork

Models of DNA Replication

1. Conservative Model

2. Semiconservative Model

3. Dispersive Model

Conservative Model

Both parental strands stay together after DNA replication

Semiconservative Model

Double stranded DNA contains one parental and one daughter strand after replication

Dispersive Model

Parental and daughter DNA segments are interspersed in both strands after replication

Mathew Meselson and Franklin Stahl Investigate

They distinguish between daughter and parental strands using light and heavy nitrogen

• Hypothesis: DNA replication is semiconservative

• Result: They found that DNA is semiconservative

Attendance Question: Hypothetically, what would the Meselson and Stahl experiment CsCl centrifugation results look like after one generation of bacteria growth if the mode of DNA replication is conservative?

There will be one heavy and one light band

Bacterial Chromosome Replication

• Two replication forks

Initiation of Replication

oriC: Origin of replication in E. coli

• Three types of DNA sequences in oriC:

1. DNA boxes: sites for the binding of DnaA protein

2. AT-rich regions: sites where the DNA strands separate

3. GATC methylation sites: sites that help regulate DNA replication

Events that Occur at oriC

1. DnaA proteins: bind to DnaA boxes and to each other

• Additional proteins bind and cause the DNA to bend

• Strands separate at AT-rich region

2. DnaB/Helicase: binds to the origin

• Further separates the DNA strands

• Travels along DNA in the 5’ to 3’ direction

• Uses ATP energy

*Replication occurs in both directions - bidirectional replication

DNA Helicase

DNA Helicase: separates the 2 DNA strands by breaking hydrogens bonds between them