Chapter 12: DNA & RNA

Essential Questions:

EQ: Why do you need DNA & RNA?EQ: How does the cell make protein?EQ: How do chromosomes carry traits?

Georgia Performance Standards: • Compare and contrast the structure and function of DNA and RNA. • Explain how DNA stores and transmits cellular information.

Hook:

• Link cell organelles (nucleus-chp. 7) and macromolecules (nucleic acids- chp. 2) to DNA & RNA (chp. 12).

• Probing Questions– Where in the cell is DNA located for eukaryotes and

prokaryotes?

– DNA is what type of macromolecule? (choose one)• Carbohydrate• Protein• Lipid• Nucleic Acid

Warm-up Review:Section 12-1

Go to Section:

• Macromolecules:– Carbohydrates (sugars)

– Lipids

– Proteins

–Nucleic Acids (DNA & RNA)

Nucleic Acids• Structure:

– Contain hydrogen, oxygen, nitrogen, carbon, and phosphorus.

– Monomers of nucleotides

– 5-carbon sugar– Phosphate group– Base

• Function:– Nucleic acids store and

transmit hereditary, or genetic, information.

• Types:– Ribonucleic acid (RNA)

– Deoxyribonucleic acid (DNA).

A Nucleotide (Monomers of Nucleic Acids):

DNA Nucleotides

• DNA is made up of a series of monomers called nucleotides.

• Each nucleotide has three parts: a deoxyribose molecule, a phosphate group, and a nitrogenous base.

• There are four different bases in DNA: adenine, guanine, cytosine, and thymine.

The backbone of a DNA chain

• Formed by alternating sugar and phosphate groups of each nucleotide.

• The nitrogenous bases stick out sideways from the chain and are linked by hydrogen bonds.

• The nucleotides can be joined together in any order, meaning that any sequence of bases is possible.

Chargaff’s Rules

• Erwin Chargaff showed that the percentages of guanine and cytosine in DNA and adenine and thymine are almost equal.

•A=T (2 hydrogen bonds hold them together)•G C (3 hydrogen bonds hold them together)

What Shape Does DNA Have?

• X-Ray Evidence – In the early 1950s, a British

scientist named Rosalind Franklin began to study DNA.

– She used a technique called X-ray diffraction to get information about the structure of the DNA molecule.

– The angle of the X suggests that there are two strands in the structure.

– Other clues suggest that the nitrogenous bases are near the center of the molecule.

What Shape Does DNA Have?

• The Double Helix Shape of DNA– Francis Crick and James Watson were trying to understand the

structure of DNA by building three-dimensional models of the molecule.

– Then, early in 1953, Watson was shown a copy of Franklin’s remarkable X-ray pattern.

– Within weeks, Watson and Crick had figured out the structure of DNA.

– Watson and Crick’s model of DNA was a double helix, in which two strands were wound around each other.

What is a Double Helix?

• A double helix looks like a twisted ladder or a spiral staircase.

• The double helix accounted for many of the features in Franklin’s X-ray pattern but did not explain what forces held the two strands together.

• Watson and Crick found the answer. They discovered that hydrogen bonds could form between certain nitrogenous bases and provide just enough force to hold the two strands together.

• The principle of base pairing (A-T and G-C), explained Chargaff’s rules.

• DNA is a double helix in which two strands are wound around each other.

• Each strand is made up of a chain of nucleotides.

• The two strands are held together by hydrogen bonds between adenine and thymine and between guanine and cytosine.

Group Activity:

• Use plastic models to build a DNA structure.

• Nitrogenous Bases:– A = orange– G = Yellow– T= Green– C = Blue

• Black Pieces = Sugar (Deoxyribose)

• White tube = phosphate group

• Toothpicks = hydrogen bonds

What Carries Genetic Material (DNA or Protein)?

• Many Experiments were carried out to verify that genetic material is carried by DNA and not protein.

1. Griffith and Transformation: DNA can be transformed (Video Clip)

2. Avery and other scientists discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation of an organism to the next.

3. Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

Avery Repeats Griffith’s work.

• Why? to determine which molecule in the heat-killed bacteria was most important for transformation.

• If transformation required just one particular molecule, that might well be the molecule of the gene.

• Avery and his colleagues made an extract, or juice, from the heat-killed bacteria.

• He treated the extract with enzymes that destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA. – Transformation still occurred. – Obviously these molecules

were not responsible for the transformation.

– If they had been, transformation would not have occurred, because the molecules would have been destroyed by the enzymes.

• He repeated the experiment, this time using enzymes that would break down DNA. – Transformation did not occur. – Therefore, DNA was the

transforming factor.

Checkpoint Questions:

1. Describe Watson and Crick’s model of the DNA molecule. What was Rosalind Franklin’s contribution to the discovery of the shape of DNA?

3. What are the four kinds of bases found in DNA? Which ones are purines and which are pyrimadines?

4. How did Watson and Crick’s model explain why there are equal amounts of thymine and adenine in DNA? Hint: Chargaff's Rule

4. List the conclusions Griffith, Avery, Hershey, and Chase drew from their experiments.

5. Why did Hershey and Chase grow viruses in cultures that contained both radioactive phosphorus and radioactive sulfur? What might have happened if they had used only one radioactive substance?

12–2 Chromosomes and DNA Replication 

• Prokaryotic cells – No nuclei and many of the organelles found in

eukaryotes.

– DNA in the cytoplasm.

– Most prokaryotes have a single circular DNA molecule called a plasmid.

– The plasmid is considered the chromosome

Chromosome

E. coli bacterium

Bases on the chromosome

Prokaryotic Chromosome Structure

Go to Section:

DNA and Chromosomes• Eukaryotic cells

– DNA is in the nucleus

– Chromosome form (cell division)

– # of chromosomes varies from one species to the next.• Humans have 46• Fruit flies have 8

– Chromosomes contain both DNA and protein, which is tightly packed together to form chromatin.

• Protein is called histone

– The DNA and histone collectively are called nucleosomes• fold long lengths of DNA into the tiny nucleus.

– The nucleosomes are tightly packed to form supercoils

Chromosome Structure of Eukaryotes

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

Section 12-2

Go to Section:

12-2 Warm-up:

• A Perfect Copy

• When a cell divides, each daughter cell receives a complete set of chromosomes.

• This means that each new cell has a complete set of the DNA code.

• Before a cell can divide, the DNA must be copied so that there are two sets ready to be distributed to the new cells.

12-2 Warm-up:1. On a sheet of paper, draw a curving or zig-zagging line

that divides the paper into two halves. Vary the bends in the line as you draw it. Without tracing, copy the line on a second sheet of paper.

2. Hold the papers side by side, and compare the lines. Do they look the same?

3. Now, stack the papers, one on top of the other, and hold the papers up to the light. Are the lines the same?

4. How could you use the original paper to draw exact copies of the line without tracing it?

5. Why is it important that the copies of DNA that are given to new daughter cells be exact copies of the original?

DNA Replication

• The double helical shape of DNA allows for strands to be copied.– Strands are considered to be complimentary

• DNA must be replicated (copied) before a cell divides. – The DNA molecule separates into 2 strands– 2 new complimentary strands are produced– Each strand serves as a template for the new strand.

 DNA ReplicationSection 12-2

Go to Section:

Growth

Growth

Replication fork

DNA polymerase

New strand

Original strand DNA

polymerase

Nitrogenous bases

Replication fork

Original strand

New strand

DNA Replication

• Prokaryotes– DNA replication begins

at a single point in the chromosomes and proceeds in two directions until the entire chromosome is replicated.

– Takes about 20 minutes

• Eukaryotes– DNA replication occurs

in hundreds of places and proceeds in both directions until each chromosomes is completely copied

– The sites where replication and separation occur are call replication forks.

How Does Replication Occur?

• Many enzymes are used– Some unzip the strands, some hold the

strands open, some glue the strands together.

– DNA Polymerase • enzyme that adds the complimentary bases to

each strand.• Proofreads each strand

– See video

Checkpoint Questions:1. Explain how DNA is replicated. 2. Where and in what form is eukaryotic DNA found?

3. How are the long DNA molecules found in eukaryotes packed into short chromosomes?

4. How are histones related to nucleosomes?

5. What is the role of DNA polymerase in DNA replication?

6.  How is the structure of chromosomes in eukaryotes different from the structure of chromosomes in prokaryotes?

Homework

• Compare and contrast the process of DNA replication in prokaryotes and eukaryotes.– Compare the location, steps, and end

products in each kind of cell.

• You may use a Venn diagram, a chart or table, a drawing, or write an essay.