Bio Unit 7.2: DNA Replication

DNA

DeoxyriboNucleicAcid

Nucleotide structure

phosphate, pentose sugar (deoxyribose in DNA), nitrogen base

Base pairs are connected with _______ bonds and the phosphate-sugar backbone is connected with _____ bonds

hydrogen (can be easily unzipped), covalent (provides integrity)

Purine + examples

2 ring nitrogen base (adenine and guanine)

Pyrimidine + examples

1 ring nitrogen base (cytosine and thymine)

The four nitrogen bases and what they pair with

Adenine - Thymine

Cytosine - Guanine

Why can each base only go with one other base? (2)

1) Number of rings

• Purine + purine = too wide

• Pyrimidine + pyrimidine = too narrow

• Purine + pyrimidine = perfect (2nm)

2) Number of hydrogen bonds

• Bases are held together with hydrogen bonds

• C-G has three hydrogen bonds

• A-T has two hydrogen bonds

4 major enzymes in DNA replication

• Helicase

• DNA polymerase

• Primase

• Ligase

Helicase

 unzips DNA by breaking the hydrogen bonds

DNA polymerase

builds the new strand of DNA and proofreads the strand at the end

Primase

adds primer (RNA) to the DNA strand for the polymerase to bind to and tells polymerase where to start

Ligase

glues fragments together

Steps of DNA replication (5)

1. Helicase unzips the enzyme

2. Primase adds primer

3. Polymerase builds on nucleotides

4. The DNA rezips and rewinds

5. Polymerase “proofreads” the DNA for mistakes

2 things that keep the DNA from winding back up while replicating

• SSB proteins bind to the DNA strands to keep them separated

• Topoisomerase keeps the DNA from supercoiling (overwinding of DNA)

Antiparallel

strands are parallel but they go in opposite directions

3’ and 5’- how does polymerase build?

• The carbons on the sugar are numbered clockwise after the oxygen

• The number at the top to the number at the bottom is the name for the direction

• Polymerase can only BUILD from 5’ → 3’ but MOVES from 3’ → 5’ along the old strand

Leading strand

 strand being built while moving from 3’ → 5’

Lagging strand

strand being built while moving from 5’ → 3’

Okazaki fragments

result from lagging strand having to keep jumping forward after building a chunk to get to the unraveling DNA

Semiconservative

 the two copies each contain one original strand

Place of origin

• where replication starts

• multiple origin points

• form bubbles that expand into each other

  • ligase glues them together

Replication speed (example and why)

• Human cells can replicate a full set of chromosomes in 8 hours

• there are multiple points of origin

• Strand is unzipped and more nucleotides are added immediately

Chase and Hershey

Couple who discovered DNA was genetic material using bacteriophage

Bacteriophage + experiment

• used radioactive isotopes to track the DNA and protein in bacteriophage

  • After the virus infected the bacteria, the protein was detected outside the cell and the DNA was detected inside the cell

• Virus lysed the cell (caused it to explode) by “hijacking it”

  • Injects its own DNA into the cell so the cell produces more virus

  • virus burst out of cell once they are formed

Rosalind Franklin

X-rayed DNA and provided evidence it is a double helix shape

Chargaff(‘s rule)

within an organism the ratio of A:T and C:G is 1:1

Watson and Crick

• put the puzzle pieces together

• discovered the structure of DNA

  • 2nm wide, double helix, purine + pyrimidine, antiparallel