SECTION 1: THE STRUCTURE OF DNA

Chapter 13 DNA, RNA and Proteins

DNA

DNA (deoxyribonucleic acid) contains the information of life

It holds the blueprints (instructions) for making essential proteins that are needed for life

Three major experiments led to the conclusion that DNA is the genetic material in cells. These experiments were performed by Griffith, Avery, Hershey, and Chase.

Griffith’s Experiment of Transformation

Griffith’s Experiment of Transformation

Two types of bacteria: S strain with capsule (causes pneumonia and victim dies) and R strain without capsule (does not cause pneumonia and victim lives) Mice injected with regular S strain die Mice injected with heat-killed S strain live Mice injected with R strain live Mice injected with combination of R strain and heat-killed

S strain die; R strain had “transformed” into S strainConclusion: genetic material could be

transferred between cells; this is transformation

Oswald Avery’s Experiment with Nucleic Acids

R strain bacteria that lacked protein or RNA could transform into S strain bacteria

R strain bacteria that lacked DNA could not transform into S strain bacteria

Conclusion: DNA is responsible for transformation in bacteria

Hershey and Chase Bacteriophage Experiment

Hershey-Chase Experiment

DNA Structure

DNA is a very long molecule that is capable of holding lots of information

It is a nucleic acid (as is RNA) and is composed of four nucleotides.

Each DNA nucleotide consists of these 3 parts (there are some differences for RNA): Sugar (deoxyribose, the name of the sugar in DNA

that gives it its name) Phosphate Nitrogen base (guanine, thymine, cytosine, or

adenine)

A nucleic acid is a series of nucleotides.

How do the nucleotides come together to make a nucleic acid? Condensation (water is removed to connect monomers)

There are two main categories of bases: purines and pyrimidines.

The purines consist of two rings. Guanine and adenine are purines.

The pyrimidines consist of one ring. Cytosine and thymine are pyrimidines (notice all three of these words have a “y” in them—use that to help you remember).

DNA consists of many nucleotides in the form of a double helix (spiral). The sugars and phosphates comprise the “backbone”, and the bases pair on the inside.

The base-pairing rules: (Chargaff) Adenine – Thymine (just remember the word “at”) Cytosine – Guanine

The base pairs on the inside of the DNA ladder are like the teeth of a zipper

The base pairs are held together by weak hydrogen bonds. Adenine forms two hydrogen bonds with thymine, while

cytosine forms three hydrogen bonds with guanine

Base-Pairing Rule

Characteristics

1. Nucleotides bond to each other to form one strand2. Two strands bond to each other (via complementary base

pairing) to form the double stranded DNA molecule3. Two strands twist to form the double helix (visualize a

ladder that is twisted)4. Sides of the ladder are formed by alternating sugar and

phosphate units5. Rungs of the ladder consist of bonded pairs of nitrogen

bases (they are joined by weak hydrogen bonds)6. Rungs of the ladder are always uniform in length because

one base is always a purine (2 ring) and the other is always a pyrimidine (1 ring); A-T, C-G

7. Right hand twist of the ladder

Complementary strands

Sugar-Phosphate Backbone

Bases in the middle

Weak hydrogen bonds between

bases

Discovery of the Structure of DNA

James Watson and Francis Crick—using research collected by Erwin Chargaff, Maurice Wilkins and Rosalind Franklin—proposed the structure of DNA in 1953.

Erwin Chargaff— the amount of adenine is always equal to the amount of thymine and the amount of cytosine is always equal to the amount of guanine

Rosalind Franklin and Maurice Wilkins—used X-ray diffraction that showed the DNA molecule resembled a coiled helix and was composed of two chains of nucleotides

Complementary strands: the arrangement of nitrogen bases along one strand is the exact complement of the bases on the other strand.

Ex: if one strand is ATTCGCCA, then the other strand is TAAGCGGT.

* Note that the sequence of DNA is not random—it serves as a code for the making of proteins.

ATTCGCCA TAAGCGGT

Two Primary Activities of DNA

1. Segments of DNA called genes store information for making proteins

2. It can copy itself exactly for new cells process of replication

Section 13.2: Replication of DNA

Purpose is so that every new cell receives a complete copy of the genetic code.

Replication occurs in the nucleus during the S phase of interphase.

When the two complementary strands of DNA are separated, each strand can serve as a pattern to make a new complementary strand.

Process: (pages 300-301)

1. An enzyme, DNA helicase, unwinds the two complementary strands of DNA and breaks the hydrogen bonds between nitrogen bases that hold the strands together; “Unzipping the DNA”

2. Nucleotide bases floating free in the nucleus react with unpaired bases that are now exposed on each strand

3. The complementary bases bond via hydrogen bonds by DNA polymerase

4. Two new DNA molecules are formed that are exact replicas of the original DNA

DNA is copied many sections at a time, so it goes quickly.

DNA is copied very accurately because of “proofreading” mechanism in DNA polymerase.

If a mistake is made, this enzyme can backtrack and replace the wrong nitrogen base.

Prokaryotic vs. Eukaryotic Replication

Because prokaryotic DNA is a single loop, replication begins at one place along the loop

Replication occurs in opposite directions until both replication forks meet

Eukaryotic DNA is linear and replication occurs at many sites along a chromosome This allows faster replication than

prokaryotic replication Replication “bubbles” are formed along the

chromosomeIf a scientist took all of the DNA in a

single human cell and laid the DNA in one line, the line would be 2 m long!

Replication of an entire human chromosome occurs in about eight hours