DNA Replication

SBI4U - Howes

DNA replication takes place during the __ phase of ___________

S, interphase

Meselson & Stahl hypothesis

DNA replication is semiconservative

describe Meselson & Stahl’s experiment

1. grew E.coli bacteria on a medium w. a heavy nitrogen isotope (¹⁵N)

2. since nitrogen is used to make the bases, after ~17 gens, almost all bacteria’s DNA contained ¹⁵N bases

3. some bacteria were then placed onto a medium w. a light nitrogen isotope (¹⁴N) and allowed to replicate

   
   

if DNA was conservative, what would the results be?

• the 1st gen would be 50% light, 50% heavy

• the 2nd gen would be 75% light, 25% heavy (the “untouched” OG dna)

since DNA is semiconservative, what were the results?

• the 1st gen was 100% intermediate weight

• the 2nd gen was 50% light, 50% intermediate

describe what it means to for DNA to be semiconservative

each new DNA molecule consists of one original (parental) strand and one newly synthesized strand

⇨ instead of conservative: the new DNA molecule is 100% new stuff

3 steps of DNA replication

1. DNA is prepped (unzipping)

2. New DNA is synthesized

3. The finishing touches

Helicase

an enzyme that unwinds and separates DNA strands by breaking H-bonds btn bases

when helicase separates DNA strands, this creates:

a replication bubble

__ to __ replication bubbles occur _____________ at many spots along the DNA strand; they _____ and then ______

1-3, simultaneously, grow, merge

Single-Stranded Binding Proteins (SSBP)

bind to exposed bases to prevent hydrogen bonds from reforming/bases from annealing

Topoisomerase

relieves tension: cuts both strands of DNA, allowing them to swivel, and then resealing

DNA is always synthesized in the __ to __ direction

5’ ➙ 3’

RNA Primase

makes an RNA primer, which it attaches to the parent strand

importance of RNA primers

indicates to DNA polymerase III where to start synthesizing

DNA Polymerase III

makes complementary DNA using deoxyribonucleoside triphosphates

DNA polymerase III can only attach a new __________ to the ___ end

nucleotide, 3’ end

Deoxyribonucleoside Triphosphates

composed of a deoxyribose sugar, nitrogenous base, and 3 phosphates

☆ the other 2 phosphates are used to provide energy!

Leading vs Lagging Strand

Leading: continuous, built towards replication fork

Lagging: discontinuous, built away, from replication fork in Okazaki Fragments

identify the lagging and leading strands

Top: LAGGING strand

Bottom: LEADING strand

DNA Polymerase I

cuts out the RNA primers and replaces them with the appropriate DNA nucleotides

DNA Ligase

forms phophodiester bonds that join the okazaki fragments together

After the finishing touches of DNA polymerase I and DNA ligase, the two new ________ _____ will then ____ into a double helix

daughter strands, twist

how is proofreading done during replication?

DNA polymerase I and III proofread the bases, and if a mistake is found it’s cut out and replaced witht the correct nucleotide