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DNA - Chapter 12

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DNA

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Searching for the “Secret of Life” 1. What is the fundamental unit of

all living things?The Cell

2. Which cellular structure controls the cell and carries the heredity information?

The nucleus3. What substance in the nucleus

carries the code of life?DNA

I. Griffith & TransformationA. Fredrick Griffith - trying to learn how

certain bacteria caused pneumonia.B. Isolated two different strains of

pneumonia bacteria from mice and grew them in his lab.

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C. Griffith set up four types of experiments:

1. Experiment 1: Mice were injected with pneumonia causing bacteria – developed pneumonia and died.

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2. Experiment 2: Mice were injected with harmless strain of bacteria – didn’t get sick.

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3. Experiment 3: Griffith heated the disease-causing bacteria. Then injected the heat-killed bacteria into the mice. The mice survived.

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4. Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria . The injected mixture gave the mice pneumonia & they died.

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D. Griffith concluded that the heat-killed bacteria passed their disease-causing ability to the harmless strain.

E. Griffith called this process transformation – The harmless strain had changed permanently into a disease-causing strain.

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Hershey and Chase Transformation Experiment (1952)

… What did it prove?

DNA of the virus and not the protein of the virus enters the bacteria and conducts the transformation of the bacteria into a viral factory

This experiment is predicated on the idea that DNA contains P and not S… and Protein contains S and not P

Bacterialcell

Phagehead

Tail

Tail fiber

DNA

100

nm

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Chromatin

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DNAI. DNA – Deoxyribose Nucleic Acid

A. Structure1. DNA is made of repeating

units called nucleotides.2. Nucleotides have 3 parts:

a. A nitrogenous baseb. A 5-carbon sugar

(deoxyribose)c. A phosphate group

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Fig 12-5 – nucleotide structure

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B. The Four Nitrogenous Bases1. The Purines – 2 carbon rings

Why are these called nitrogenous bases?

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B. Four Nitrogenous Bases (cont)2. The Pyrimidines – 1 carbon ring

How are the pyrimidines different from the purines?

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C. The Sugar

D.The Phosphate

Deoxyribose

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II. Double HelixA. The shape of DNA is described as

a double helix (a twisted ladder). What force holds the two strands

together?

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II. Double Helix (cont)B. A simple model of DNA

Draw this!!

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III. Principles of Base Pairing (Chargaff)A. The opposing bases always pair the

same: Adenine (A) pairs with Thymine (T) Guanine (G) pairs with Cytosine (C)

What would be the complimentary base sequence for the strand of

DNA below?A C G G T T A C A A T G AT G C C A A T G T T A C T

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IV. Chromosomes & DNA Replication

A. Chromosomes contain DNA and protein, tightly packed together to form chromatin.1. Chromatin consists of DNA

tightly coiled around proteins called histones.

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Eukaryotic Chromosome StructureChromosome

Supercoils

Nucleosome

DNA double helix

Histones

Coils

Fig. 12-10

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B.DNA Replication1.Before a cell divides, it

duplicates its DNA in a process called replication.

2.Replication ensures that each copied cell will have a complete set of DNA.

3.In replication the DNA separates into two strands. Each strand produces two new complementary strands by base pairing.

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Nitrogen Bases

Replication Fork

DNA Polymerase

Replication Fork

Original strandNew Strand

GrowthGrowth

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C. How Replication Occurs1. DNA replication is carried out

by enzymes that “unzip” a molecule of DNA.

2. Hydrogen bonds between base pairs are broken and the two strands of DNA unwind.

3. The principal enzyme involved in DNA replication is DNA polymerase.

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DNA Replication

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V. How does DNA control living things?A. DNA codes for the amino acid

sequence in proteinsB. Every three bases (called a codon)

codes for one amino acid.1. Examples:TTT codes for phenylalanineGAA codes for leucineTGG codes for tryptophan

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The first step in DNA replication is a. producing two new strands.

b. separating the strands.c. producing DNA polymerase.

d. correctly pairing bases.

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A DNA molecule separates, and the sequence GCGAATTCG occurs in one strand. What is the base sequence on the other strand?a. GCGAATTCGb. CGCTTAAGCc. TATCCGGATd. GATGGCCAG