Chapter 12

Genes are Made of DNA
 Griffin’s experiment (1928)
 Avery’s experiment (1944)
 Hershey and Chase experiment (1952)

Griffith’s experiment -
1928

 Griffith showed that although a deadly strain of bacteria could be made harmless by heating it, some factor in that strain is still able to change other harmless bacteria into deadly ones. He called this the "transforming factor."

Avery’s experiment - 1944
 Transforming factor - Protein or DNA?
Avery and colleagues treated a mixture of heat- treated deadly strain and harmless strain of bacteria with:
 Protein-destroying enzyme
 DNA-destroying enzyme

Hershey and Chase experiment -1952

 The basic unit of the DNA molecule is called:
NUCLEOTIDE

A NUCLEOTIDE in DNA has three parts:
 A ring-shaped sugar called deoxyribose
 A phosphate group
 A nitrogenous base (single or double ring of carbon and nitrogen atoms)

Nucleotide monomers join together by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a sugarphosphate backbone.

Nitrogenous Bases

The bases pair up ( A-T
& G-C ) forming the
DOUBLE HELIX first described by Watson and Crick

Watson, Crick and
Franklin

Various ways to model DNA structure
• http://www.umass.edu/molvis/tutorials/dna/dnapairs.htm
manipulate DNA

Why does DNA need to be replicated?
Growth – new cells - reproduction
How does this process happens?

REPLICATION IN 3 STEPS
 DNA replication results in two new strands, each containing one new strand
( yellow ) and one original strand ( blue )

 Weak bonds
 Hydrogen bonds
 Comes apart easily
 Comes together easily

Overview of DNA replication
 DNA separates
 Complementary nucleotides are linked along separated strands

initiate
 Initiator protein guides unzipper protein
(helicase) to correct position on DNA

untwister
 Untwister
(topoisomerase) unwinds the
DNA double helix in advance of the unzipper

unzip
• Unzipper separates
DNA strands, breaking weak bonds between the nucleotides

assemble
• Builders
(polymerases) assemble new
DNA strand by joining nucleotides to their matching complements on the exposed strands

straightners
 Straighteners
(single-strand
DNA binding proteins) keep single strand of
DNA from tangling

Phosphate provides energy
• Phosphate bond energy from the new nucleotides is used to make the new bonds

Leading vs. Lagging strand
 Leading (top) strand is built continuously as the builder follows behind the unzipper, but the Lagging
(lower) strand builds in the opposite direction

Lagging strand
 Lagging
(lower) builder makes a loop with the
DNA strand and builds in opposite direction

Lagging strand
 Built in small sections
 Sections linked by enzyme ligase

Repairs of DNA
 Erasers (Repair
Nuclease): find poorly matched or damaged nucleotides and cut them out

Repairs of DNA
 Builders
(Polymerase): fill gaps using other DNA strand as a guide

Repairs of DNA
 Stitchers
(Ligase): uses
ATP to restore continuity of backbone of repaired strand

Big picture of DNA replication

Replication review

REPLICATION IN 3 STEPS
 DNA replication results in two DNA molecules, each with one new strand
( yellow ) and one old strand
( blue )

DNA replication animations
 http://www.youtube.com/watch?v=4jtmO
ZaIvS0
 http://www.youtube.com/watch?v=gW3qZ
F9cLIA
 http://www.youtube.com/watch?v=oNW_ ykH3AvA
 http://www.youtube.com/watch?v=hC_8y
8fNkCw

TED talk – DNA replication –
Chromosomes- malaria
 http://www.youtube.com/watch?v=dMPX u6GF18M