DNA: Structure and Replication

1
DNA Structure and Replication
2
Two Types of Nucleic Acids
Slide 2
Nucleic Acids carry the genetic instructions
for all life
Nucleic Acid Stands for Type of Sugar of strands Function
DNA Deoxyribonucleic acid deoxyribose 2 double helix Stores genetic material
RNA Ribonucleic acid ribose 1 single Transfers genetic material
3
Discovery of Nucleic Acid
Slide 3

• 1869 Friedrich Miescher discovered nuclein
• Noticed that nuclein had a lot of phosphate
• He found nuclein in every type of cell he
  studied.

Mieschers lab
4
Griffiths Experiment
Slide 4

• 1. 1928 Frederick Griffith
• a. Showed that bacteria could be
  transformed
• b. transformation process in which
  bacteria can take up the genes of
  another bacteria and express those genes

5
Averys Experiment
Slide 5
Avery continued Griffiths experiment!
Avery showed that DNA is the transforming agent!
6
Hershey and Chase Experiment
Slide 6

• 1. 1952 Hershey Chase used radioactive
  markers on viruses
• a. showed that virus only injects
  nucleic acid into bacteria
• b. bacteria take up nucleic acid
  and can express the new genes
• c. demonstrated that DNA is the
  genetic material of the cell

7
Components of DNA
Slide 7
DNA is made up small, repeating units (monomers)
called nucleotides.

• Nucleotide has 3 parts
• 1. A phosphate group
• 2. A sugar (deoxyribose)
• 3. Nitrogen base (4 different bases)
• Adenine A
• Thymine T
• Cytosine C
• Guanine G

Adenine
Guanine
Cytosine
Thymine
Phosphate group
Deoxyribose
Go to Section
8
Chargaff Discovered Ratios of Nitrogen Bases
Slide 8

1. 1940s Erwin Chargaff discovered that the of
   base A was almost equal to the of base T
   the percent of base C was nearly equal to the
   of base G didnt know why
2. Also discovered that composition of DNA ( of A,
   T, C, G) varies from species to species
3. Both discoveries gave support that DNA carries
   the genetic code!

Chargaff
9
Percentage of Bases in Four Organisms
Slide 9
Source of DNA A T G C
Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9
18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 2
9.4 19.9 19.8
Go to Section
10
Rosalind Franklin The Famous Picture Taker
Slide 10

1. 1952 Rosalind Franklin an expert in x-ray
   crystallography.
2. She took the x-ray picture of DNA -- shows DNA is
   a double helix.
3. 1958 Franklin died (37 years old)

11
James Watson Francis Crick Puzzle Solvers
Slide 11

• 1953 Discovered the structure of DNA
• Described DNA as a double helix (twisted ladder)
• a. Sides of ladder are made up of sugar
  phosphate groups
• b. Steps of ladder are made up of nitrogen base
  pairs (A-T C-G)
• -- Applied Chargaffs rules
• c. Base pairs (steps) are held together by weak
  hydrogen bonds
• d. Sequence (order) of nitrogen bases determines
  the genetic instructions / genetic code of
  organism.

James Watson
Francis Crick
12
Knowing the Structure Tells how DNA Replicates
Slide 12

1. DNA Replication makes 2 identical DNA strands
   by copying the original model
2. Each new strand contains one old (parent) strand
   one new (daughter) strand
3. DNA replication occurs during cell division
   inside the nucleus

Parent strand gray Daughter strand red
13
Steps in DNA Replication
Slide 13

• DNA unwinds (DNA Helicase)
• DNA polymerase breaks hydrogen bonds that hold
  nitrogen bases together
• DNA ligase hydrogen bond new nitrogen bases onto
  DNA parent strand
• Chargaffs rules applied A-T C-G
• Nucleotides added in 5 to 3 direction
• Sugar-phosphate groups bond to nitrogen bases to
  complete daughter strand
• Result 2 identical DNA strands each contains 1
  parent strand 1 daughter strand

14
Steps in DNA Replication
Slide 14
Daughter strand
Parent strand
Strands are antiparallel
15
When Does Replication Occur?
Slide 15

• 1. The cell replicates (makes a copy) its DNA
  right before cell division.
• 2. When two new cells are produced in cell
  division, each new cell needs its own copy of
  the DNA (identical copy!)
• 3. After DNA replication, the cell will divide in
  half (cell division) and give each new cell
  a copy of the DNA