DNA Replication and Telomeres (9/4-9/6)

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Semi-Conservative

One strand serves as the template

Bi-Directional (Hint: 2)

DNA synthesis initiates at an origin of replication and proceeds outwards in opposite directions

A replication bubble with two replication forks

Semi-Discontinuous

The entire DNA replication process that has both continuous and discontinuous processes

Replication Bubble

An open region of DNA where the double helix has been unwound, allowing DNA replication to occur

DNA Helicase (Hint: 3)

Binds at the replication origin

Moves away from each other

Unwind parental DNA strands forming a replication bubble with two replication forks

DNA Gyrase (Topioisomerase)

Removes supercoiling stress

Single-Stranded Binding (SSB) Proteins (Hint: 2)

Protect DNA

Stops from reforming double-stranded DNA

Primase

Synthesizes a complementary RNA primer at the origin on both parental strands

DNA Polymerase III (Hint: 2)

Catalyzes the addition of complementary nucleotides to the 3’-OH of the newly made strand

Synthesizes in the 5’→ 3’ direction ONLY

RNA Primer

Provides 3’-OH for DNA polymerase III

Leading Strand

DNA polymerase III adds complementary DNA nucleotides to the RNA primer

Continuous Strand

Leading strand

Lagging Strand

Once enough of the parental DNA is unwound, primase lays down another complementary RNA primer

Discontinuous Strand

Lagging strand

Okazaki Fragments (Hint: 2)

 DNA polymerase III extends until it hits the previous primer

Small sections of DNA that are formed during discontinuous synthesis of the lagging strand during DNA replication

Proofreading (Hint: 2)

DNA polymerase III can move “backwards” and excise an incorrect base and add the correct base

3’ to 5’ proofreading activity

Exonuclease

Remove a base from the end

DNA Polymerase I

Removes the RNA primer and fills in the gap with DNA nucleotides

DNA Ligase (Hint: 2)

Covalently links the ends together resulting in a seamless strand of DNA

Works with DNA polymerase I to link Okazaki fragments together

Replication Fork

Move away from the origin in a bidirectional fashion

End Replication Problem (Hint: 7)

On the lagging strand

Lagging strand lays down a primer

Removed at 5’ end

Can’t be filled in because there is no 3’-OH to attach DNA nucleotides to

Single stranded DNA can break and be lost

Results in shortened chromosome

Over many divisions chromosomes shortening can lead to loss of gene information near the chromosome ends

Telomeres (Hint: 5)

The ends of the chromosomes

Composed of hundreds of repeats

Protects the ends of chromosomes from degradation

Prevents ends from fusing

Overcomes the end replication problem

Telomerase

An enzyme that uses an RNA template to extend the 3’ end of chromosomes

Telomere Repeat

A G-rich repetitive sequence telomerase synthesizes so a primer can be synthesized and the end can be replicated

When does DNA replication take place in the cell cycle?

S phase

What are the 3 hallmarks of DNA replication?

Semi-Conservative

Bi-Directional

Semi-Discontinuous

Replicator of DNA Replication

The origin of replication

Initiator of DNA replication

Protein(s) that bind the origin and initiate replication

Origin of Replication

Site where DNA replication starts

Be able to fill in a blank DNA replication bubble with RNA primers, leading/lagging strands (continuous/discontinuous strands), helicase, replication forks and replication origin.

Proofreading: Why does the direction of proofreading mean that polymerization only occurs in the 5’-> 3’ direction?

Reaction could not proceed because there would be no high energy bond to cleave

What is the sequence of the human telomere repeat?

TTAGGG

Where (what types of cells) is telomerase usually expressed?

Expressed in actively dividing cells- single celled organisms, germ cells, early embryos, stem cells, bone marrow, intestinal epithelium, hair follicles

What are 2 of the conditions that are associated with shortened telomeres?

Aging

Werner’s Syndrome: Premature aging syndrome associated with shortened telomeres

Why do cancer cells express telomerase?

Necessary to keep dividing