DNA

Reading Assignment:

•Read pages 287-290 and prepare to discuss.

•If you complete that before we are ready to discuss, please read through the rest of the section, page 294.

Questions:• What organelle is known as the “control

center” of the cell?▫Nucleus

• What structures are found in the nucleus?▫Chromosomes (Chromatin, DNA)

• What are located on chromosomes?▫Genes

• What are chromosomes composed of?▫DNA (wound around proteins)

• How do genes and chromosomes control the activity of a cell?▫By producing proteins that regulate the cellular

functions or become part of the cell structure.

Vocabulary

•Transformation

•Bacteriophage

•Nucleotide

•Base pairing

What was Griffith trying to do?

•Trying to learn whether or not a toxin produced by the bacteria was the cause of pneumonia.

•What are some questions he could ask?

Look at figure 12-2, page 288• What was Griffith trying to learn when he set up

this experiment? ▫How bacteria caused pneumonia

• How did Griffith show that the disease-causing bacteria were killed by the heat?▫He tried to grow them in a petri dish. If the

bacteria didn’t grow, then he knew he had killed it.• What result was Griffith expecting when he

injected the mixture of live harmless bacteria and the heat-killed disease-causing bacteria?▫The mouse to live.

Immunizations•How many of you remember getting

immunizations? •How do immunizations work?

▫Immunizations are toxoids or inactivated toxins from the toxins that are produced by the bacteria that make us sick.

•Why do you think it’s important to learn how bacteria cause disease?▫To find a cure for the disease, or a means to

prevent it.

•This was what Griffith was doing!

•He was showing how a toxin that is produced by the bacteria causes pneumonia.

•Then Avery expanded upon his work… He wanted to know what was most important for this transformation to occur.▫He isolated items and destroyed them, till he

found the one item, that when destroyed …. Transformation no longer occurred.

▫What was that item? DNA

Hershey-Chase Experiment

•A Bacteriophage is a virus that infects and kills a bacteria.

Hershey-Chase Experiment

How did they get the parts of the Bacteriophage radioactive?

•They soaked the bacteriophages in the radioactive isotopes – Because DNA has Phosphorus already, it incorporated the radioactive Phosphorus.

•They did the same with another set of bacteriophages with ulfur, and because the protein coat already contains sulfur, it incorporated the radioactive sulfur.

DNA History

•Griffith – ▫Experiment: Using mice, he observed

transformation – Live harmless bacteria transformed into disease causing bacteria.

▫Live, Disease Causing Bacteria▫Live, Harmless Bacteria▫Dead (Heat Killed) Disease Causing

Bacteria▫Dead (Heat Killed) Disease Causing

Bacteria mixed with Live Harmless Bacteria

Griffith Experiment (Pneumonia)

More DNA History

•Avery– •Experiment: Concluded that the genetic

material in bacteria was DNA not proteins▫Repeated Griffith’s experiments with

enzymes•Destroy proteins•Destroy lipids•Destroy carbohydrates•Destroy RNA•Destroy DNA

•Hershey - Chase– ▫Experiment: Used Bacteriophages to show

that DNA is what stores and transmits the genetic information from one generation to the next.

▫P32 (Phosphorus 32) Radioactive marker attached to DNA inside of

Bacteriophage▫S35 (Sulfur 35)

Radioactive marker attached to protein coat of Bacteriophage

The Discovery of DNA

•Rosalind Franklin – made x-ray of DNA •J. Watson & F. Crick – made the 1st model

of the structure of DNA! (using R. Franklin’s x-ray)

Watson & Crick

DNA

Watson & Crick – created the double helix model for

DNA.

Structure of DNA•DNA is a long molecule made up of units

called nucleotides.•Each nucleotide is made up of three parts:

▫a 5-carbon sugar called deoxyribose, ▫a phosphate group, and ▫a nitrogenous base (Nitrogen Containing).

•The backbone of DNA: sugar and phosphate groups

•The nitrogenous base stick out from the sides and can be joined together in any order, meaning that any sequence of bases is possible.

Nitrogenous Bases

•There are four kinds of nitrogenous bases.•They are divided into two classes: purines

and pyrmidines▫Purines – Adenine and Guanine▫Pyrimidines – Cytosine and Thymine

Structure of DNA

Shape = Double HelixMade up of nucleotides:

deoxyribose, phosphate group, and a nitrogenous base (A, T, C, or G)

Chargaff’s Rules

•Chargaff discovered how the nitrogenous bases bond together.

•He discovered that Adenine always bonds with Thymine and that Cytosine always bonds with Guanine.

Double Single Ring Ring

The NucleotidesPhosphate – sugar

“backbone”

The sugars also attach to the bases.

Bases attach to each other by a weak hydrogen bond.

The 2 strands are “complementary” to each other. (base pair rule!!!)

What is the base pair rule?Adenine (purine)

connects to Thymine

(pyrimidine)A-T

Guanine(purine)connects to

Cytosine(pyrimidine)

G-C

Prokaryotes & DNA

• In prokaryotes, DNA molecules are located in the cytoplasm of the cell.

•Most prokaryotic DNA is a single circular molecule that contains nearly all the cell’s genetic information.

Eukaryotes & DNA

•Many eukaryotes have 1000 times as much DNA as prokaryotes.

•DNA is located in the nucleus in the form of chromosomes.

•Chromosomes are DNA wound tightly around proteins called histones.

DNA Length

•E. Coli have about 4,639,221 base pairs. It is about 1.6mm in length. This sounds small until you realize the bacteria is only 1.6µm in diameter.

•Thus DNA must be wrapped tightly to fit into cells. Imagine fitting 900 yards (300m) of rope into a backpack.

DNA replication……

•Occurs during S phase of Interphase during the cell cycle.

•Makes an exact copy of all DNA.

DNA replication……

•Is “semi-conservative” – 1 strand is “old” & the other strand is “new”.

DNA Replication• The DNA molecule separates

into two strands

• Then produces two new complimentary strands following the rules of base pairing (Chargaff Rules).

• Each strand of double helix of DNA serves as a template, or model, for the new strand.

How It Occurs• DNA replication is carried

out by a series of enzymes.

• The enzymes unzip the DNA molecule creating two strands that serve as templates.

• Complimentary bases are added to the strands▫Example: ATTCGAG

. TAAGCTC.

What is needed for replication?

•Replication requires enzymes:

1) Helicase – unwinds . DNA

2) DNA polymerase . – puts new nucleotides . in place & proofreads

Replication Review

•Each new DNA molecule has one new strand and one strand from the original molecule.▫What is this called?▫Semi-conservative

•The enzyme DNA polymerase, the principal enzyme, “proofreads” the new DNA strands, helping to maximize the odds that each molecule is a perfect copy of the original.▫This is what I call: “The Police”

The process of DNA replication1) DNA unwinds &

unzips – uses Helicase

2) Replication fork forms (area where 2 strands separate)

* there are several replication forks at the same time.

3) DNA polymerase adds new nucleotides to each DNA strand, following the base pair rule!