4/11/13  

1  

Notes #3 – Replication of DNA Unit 9: DNA to Protein

TALK

Semiconservative Replication

•  When DNA is copied during the S phase (interphase) of the cell cycle it occurs by semiconservative replication.

•  This is a method by which the parental strands of DNA separate, serve as templates, and produce DNA molecules that have one strand of parental DNA and one strand of new DNA.

Semiconservative Replication

•  This replication process can be broken down into 3 steps: 1.  Unwinding

2.  Base pairing

3.  Joining

1. Unwinding

•  DNA helicase, an enzyme, is responsible for unwinding and unzipping the double helix.

•  As the double helix is unzipped, the hydrogen bonds between nitrogenous bases are broken, leaving single strands of DNA.

•  Proteins, called single-stranded binding proteins associate with the DNA too keep it from re-zipping.

•  Another enzyme, RNA primase, adds shorts segments of RNA, called an RNA primer, to each strand.

1. Unwinding

1. Helicase

2. Single-stranded Binding Proteins

3. RNA Primase

4. RNA primer

4/11/13  

2  

2. Base Pairing

•  The enzyme, DNA polymerase III catalyzes the addition of appropriate nucleotides to the new DNA strand.

•  The nucleotides are added to the 3’ end of the new DNA strand, making sure that A-T and G-C.

3’

3’ 5’

RNA Primer

DNA Polymerase III

2. Base Pairing

•  The two new strands are made slightly differently: •  Leading strand: It is elongated as the DNA unwinds;

built continuously by the addition of nucleotides to the 3’ end.

•  Lagging strand: elongates away from the replication fork; synthesized discontinuously in small segments called Okazaki fragments •  Still synthesized by DNA polymerase III reading in the 3’

to 5’ direction.

2. Base Pairing

5’ 3’

5’

5’ 3’

3’

1. Helicase

2. Single-stranded Binding Proteins

3. RNA Primase

4. DNA Polymerase III

4. DNA Polymerase III

Okazaki Fragment

LEADING STRAND

LAGGING STRAND

3. Joining

•  Once a good bit of the new DNA strand has been synthesized, the RNA primer is removed by DNA polymerase I and replaced with DNA.

•  However, this leaves a small gap in the new DNA strand, which is filled soon after by DNA Ligase.

3. Joining

5’ 3’

5’

5’ 3’

3’

1. Helicase

2. Single-stranded Binding Proteins

3. RNA Primase

4. DNA Polymerase III

4. DNA Polymerase III

Okazaki Fragment

5. DNA Polymerase I

6. DNA Ligase

LEADING STRAND

LAGGING STRAND

DNA Replication in Eukaryotes

•  Eukaryotic DNA unwinds in multiple areas.

•  As a result, multiple areas of replication occur along a large eukaryotic chromosome at the same time.

•  Multiple replication origins look like bubbles in the DNA strand.