Bio1 11/9 PCV: DNA Replication

What are the stages of the cell cycle?

interphase (G₁, S, G₂), M (mitosis or meiosis), and cytokinesis

What are the first steps in DNA replication (before DNA polymerase)?

Helicase attaches to DNA at the sequence of nucleotides called the origin of replication.

As the helicase unwinds the double-stranded DNA helix, single-strand binding proteins stabilize the single stranded regions of the DNA.

For the two parent DNA strands, an RNA polymerase called primase creates a short nucleotide sequence called an RNA primer that is complementary to the parent DNA.

Describe the steps in DNA replication for the leading strand.

Discuss direction.

1. DNA Polymerase III continuously adds deoxyribose nucleotides to the 3’ end of the primer on the leading strand, elongating the strand in the direction of the replication fork (from 5’ to 3’).

2. At the very end of replication, an enzyme replaces the RNA primer with DNA.

3. DNA ligase forms phosphodiester bonds to join the two adjacent DNA segments.

Describe the steps in DNA replication for the lagging strand.

Discuss direction.

1. Primase makes another RNA primer complementary to the parent DNA strand.

2. DNA Polymerase III discontinuously builds the strand—from that primer to the previous primer—as a series of short segments called Okazaki fragments. It is built in a direction away from the replication fork (from 5’ to 3’).

3. When the DNA Polymerase III reaches the first RNA primer, it is replaced by DNA Polymerase I, which replaces the RNA primer with DNA.

4. DNA ligase attaches and forms phosphodiester bonds between the Okazaki fragments.

5. As DNA is further unwound, new primers are created and DNA polymerase III begins creating another Okazaki fragment. The process repeats.

Describe telomeres and the end replication problem.

Telomeres, the ends of eukaryotic chromosomes, shorten with every cell division due to the limitation of DNA polymerase requiring a primer to add nucleotides to.

At the end of the lagging strand, where there isn’t room for a primer to be placed, one segment remains single stranded and eventually falls off.

Over multiple rounds of division, the telomeres shorten.

Explain one solution to the end replication problem.

Some cells have telomerase which can extend telomeres and prevent shortening.

• The RNA portion acts as a template for telomerase to extend the parent strand so that DNA polymerase can fill in the original missing part of the lagging strand.

What is telomerase made of?

Telomerase is an enzyme formed of both protein and RNA.

What are three types of cells that have telomerase?

bone marrow, gametes, and cancer cells