Cell Biology

DNA Replication

DNA (Deoxyribonucleic Acid) •  Organism’s genetic blueprint

•  Resides in the nucleus of almost all body cells •  Does NOT leave the nucleus •  Can replicate itself! •  Instructs protein synthesis

Structure of DNA •  Macromolecule •  Composed of repeating

units called NUCLEOTIDES •  Polynucleotides

•  Sugar-phosphate backbone

What’s in a Nucleotide? •  A single deoxyribose sugar

•  A 5-Carbon monosaccharide (pentose sugar)

•  Phosphate group •  1 of 4 nitrogenous-containing bases

•  Adenine (A) •  Thymine (T) •  Guanine (G) •  Cytosine (C)

The Bases •  Recall: 4 different bases in

DNA •  A, T, C, G

•  Complimentary Base Pairing •  AT and GC sit next to each

other as a “rung” in the DNA double helix

Hint: •  Can you tell the difference between Purines

and Pyrimidines?

Note: •  The numbers refer to the carbons in the

sugar part of the nucleotide

Functions of DNA •  Stores genetic code in nuclei of cells •  The nucleus of a cell is ~6µm •  Nucleus contains ~1.8 m of DNA and in orde to

fit into nucleus, the DNA must CONDENSE into chromosomes!

•  Replicates itself to make an EXACT copy of itself when the cell divides

•  provides a code/template for the particular sequencing of AA that bond together to form a protein

RNA (Ribonucleic Acid) Three Types:

1.  Messenger RNA (mRNA) 2.  Ribosomal RNA (rRNA) 3.  Transfer RNA (tRNA)

Like DNA, RNA is a polynucleotide but is somewhat

different: •  Single stranded (no double helix) •  Contains RIBOSE sugar •  Leaves the nucleus to perform function •  Base pairing: Adenine - URACIL (replaces Thymine)

Deoxyribose vs Ribose Sugars

•  Note the absence of the hydroxyl (-OH) group on the 2’

carbon in the deoxyribose sugar in DNA

Functions of RNA

1.  Messenger RNA (mRNA) •  Produced from DNA by TRANSCRIPTION •  Contains bases complimentary to DNA

(remember: U replaces T in RNA) •  Carries DNA’s message out of nucleus to

ribosomes in cytoplam Q: What is the function of ribsomes?

Functions of RNA

2.  Ribosomal RNA (rRNA) •  Produced in nucleus and transported to

cytoplasm •  Combine with proteins to form ribosomes

3.  Transfer RNA (tRNA) •  Delivers individual AA’s to ribosomes during

protein synthesis •  Translates mRNA into AA sequence

DNA Replication - Why? n  DNA Replication MUST occur in Eukaryotic cells

before Mitosis or Meiosis occurs. §  A cell will continue to grow its entire life and it must divide

otherwise it becomes too large in VOLUME. There will not be enough SA to allow for adequate exchange of gases, nutrients, wastes.

Note: You may wish to review mitosis and meiosis to review what phases of the cell cycle replication occurs.

DNA Replication - simplified!

1.  DNA unzips 2.  RNA primase binds 3.  Complementary base pairing (H-bonds) 4.  Forms 2 sets of DNA double helices (semi-

conservative relication) - Replication is complete

1. DNA Unzips

n  DNA helicase, an enzyme, targets sections of Parent DNA (areas with many A-T bonds)

n  Helicase breaks H-bonds n  DNA unzips and unwinds to expose 2

complimentary (template) strands n  Initiation point where splitting occurs = “origin

of replication” –  forms “replication fork”

2. RNA Primase Binds

n  RNA primase binds initiation point of the 3’-5’ parent chain

n  RNA primase attracts DNA nucleotides which bind to DNA nucleotides

3. Complementary Base Pairing

n  Strands with exposed bases are no longer paired n  Free floating nucleotides in nucleus pair with

exposed bases §  A-T ; G-C (complementary base pairing)

n  DNA polymerase joins nucleotides together forming a new sugar-phosphate backbone

n  replication can only proceed from a 3’ to the

5’direction of the original molecule or the 5’ to 3’ direction of the new molecule

The other strand is running in the wrong direction ∴ the new nucleotides are linked into small segments called "Okazaki' fragments and inserted in the correct direction - joined by an enzyme called LIGASE.

4. Termination

n  After nucleotides attach to unzipped DNA strand, DNA ligase seals remaining breaks in backbone structure.

n  DNA polymerase proofreads new strand.

n  If errors occur, nucleases remove wrong nucleotides

n  DNA polymerase corrects mismatched base pairs

–  If not corrected, a MUTATION can result

Replication COMPLETE! n  Two DNA double helices have been created

n  Each helix contains one strand from the original helix and one new = SEMICONSERVATIVE