GN 301 Module 3: DNA Replication

Meselson and Stahl Experiment

Describe the Meselson and Stahl results and conclusion:

Meselson and Stahl grew E coli. in a petri dish in N15 media and watched it grow for a few generations. Then, they transferred cells to N15 media let the E coli. replicate. This allowed them to rule out conservative and dispersive replication.

(15N is heavier than 14N)

Which 3 possibilities did Meselson and Sthal consider?

Conservative, Dispersive, and Semiconservative

What is meant by semiconservative replication?

In this model, the two strands of DNA unwind from each other, and each acts as a template for synthesis of a new, complementary strand. This results in two DNA molecules with one original strand and one new strand.

Replication

Nucleotides are joined to each other by the enzyme ==polymerase== which makes ==phosphodiester== bonds

==Continuous v Discontinuous Synthesis:==

Continuous DNA synthesis occurs from the 3' end to the 5' end of the parent strand. This is often referred to as the leading strand with new nucleotides being added to the 3' end. Discontinuous DNA synthesis occurs from the 5' end to the 3' end of the parent strand.

In which direction does replication occur?

==5’ to 3’==

Leading and Lagging Strands

Define Okazaki fragments and state where they are found.

==Okazaki fragments== are fragments of DNA that are formed during DNA replication of the lagging strand. Okazaki fragments were discovered by Reiji Okazaki in 1968. During DNA synthesis, DNA polymerase can only add new nucleotides in the 5' to 3' direction of the DNA strand.

Compare and identify Leading vs Lagging strands

The major difference between a lagging and leading strand is that the lagging strand replicates discontinuously forming short fragments, whereas the leading strand replicates continuously

What is meant by bidirectional replication?

Bidirectional replication involves replicating DNA in two directions at the same time which results in a leading strand and a lagging strand.

Why is an RNA primer used in DNA replication?

RNA primers are key to initiating DNA synthesis. They provide the initiation sequence necessary for DNA polymerase to synthesize new strands. The RNA primer is introduced at the template DNA site by primase enzyme on both the leading and lagging strands.

\n Telomeres

Describe the problem at the ends of chromosomes and the role of telomerase in solving the problem.

The ends of chromosomes, known as ==telomeres,== consist of repetitive DNA sequences that protect the chromosomes from damage. With each cell division, telomeres shorten, and when they become too short, the cell can no longer divide and dies.

==Telomerase== is an enzyme that can add repetitive DNA sequences to the ends of chromosomes, thereby extending the length of telomeres. This allows cells to continue dividing. Abnormal telomerase activity can contribute to the development of cancer, as it allows cells to continue dividing uncontrollably. On the other hand, activating telomerase in normal cells could have therapeutic potential for treating age-related diseases and regenerative medicine.