DNA—the thread of lifeBiol 100 – K.Marr

1. Topics this lecture

– DNA structure and Replication

– Chapter 10 in Essential Biology by Campbell et al

2. Lab 7. Modeling DNA Structure, DNA Replication and Protein Synthesis Read the introduction carefully

– This week: Part 1 (through page 9)—modeling DNA Structure and Replication

– Next week: Part 2—modeling transcription and translation

Outline for Next few Lectures

Chapter 10—Molecular Biology of the Gene

– Cystic Fibrosis Case Study

– Central Dogma of Biology

– Structure of DNA

– Replication of DNA

– Protein Synthesis

• Transcription = Reading of DNA to make RNA

• Translation = Reading of mRNA by ribosomes to make protein

Chapter 12—DNA Technology

– Application of Gene Therapy and biotechnology to cystic fibrosis

Cystic fibrosis—a genetic disease

1. View GATTACA trailer

2. View Cystic Fibrosis Case Study Movie

– available @ lecture page of class website

3. Optional Reading

– Cystic Fibrosis Foundation Website: http://www.cff.org

– Welsh, M.J. and A.E. Smith. Cystic fibrosis. Scientific American, December 1995

Salty sweat due toaltered salt secretionin sweat ducts

Lung

Pancreas

Testis

Infertilty inmales dueto cloggedsex ducts

Problems withdigestion dueto cloggedduct frompancreas

Mucus-cloggedAirways; Severe Respiratory infections

Symptoms of CF

Cell lining ductsof the body

Cystic Fibrosis: autosomal recessiveMost common lethal genetic disease

– 1 in 2000 children is born with CF in U.S.

– Untreated children die by age 5

– Ave. life expectancy: ~27 yrs

– Special diet + daily dose of antibiotic prevent infection

Carriers of CF gene:

– Hispanics: 1 in 46

– African Americans: 1 in 63

– Asian Americans: 1 in 150

– Caucasian of European descent: 1 in 25

• CF allele protects against the plague and many viruses

Cystic Fibrosis A single faulty protein is connected to the symptoms In 1989 the gene was mapped to chromosome #7

Our Goal

• To determine the connection between the symptoms associated with cystic fibrosis and DNA

• Let’s revisit the central dogma of biology…

Central Dogma of Biology

• Genes interacting with their environment determine the phenotype of an Organism

DNA

NucleusTranscription

RNA

Translation

Protein

Cytoplasm

Central Dogma of Biology• DNA specifies the

synthesis of proteins in two stages

1. Transcription____________________________________

2. Translation______________________________________

• Let’s learn a bit about the structure of DNA

DNA is like a rope ladder twisted into a spiral

Twist

DNA Structure

• Consists of 2 strands joined together by weak hydrogen bonds

• Rungs of the ladder are hydrogen bonded N-bases

• View Animation of DNA & RNA Structure

• Nucleotide: base + sugar + phosphate

Note carbon numbers on sugar

Nucleotides join to form a sugar-phosphate backbone

DNA and RNA: Polymers of Nucleotides

• Nitrogen Bases found in DNA & RNA

Uracil replaces thymine in RNA

Phosphate group Nitrogenous base

Sugar

Nucleotide

PolynucleotideSugar-phosphate backbone

Nitrogenous base(A,G,C, or T)

Thymine (T)

Phosphategroup

Sugar(deoxyribose)

DNA nucleotide

Representation of one Strand of DNA

3’ end

5’ end

DNA Structure

• Complementary pairing of bases

A to T

and

G to C• Note 5’ and 3’

ends of each strand

It’s in the genes...

1. What’s a gene?

• Sequence of DNA bases on a chromosome that determines the amino acid sequence of a protein

2. The kind of proteins an organism makes helps to determine it’s phenotype

3. Genes are...

• copied during __ S-phase of the cell cycle (DNA replication)_____

• passed to offspring __during fertilization and when cells divide__

4. Genes can change or mutate—how?

• View animation of DNA Replication

available at the lecture section of class website

Nucleus

One DNA molecule before replication

ParentalstrandsSeparated by Helicase

Free nucleotidesdiffuse in and pair upwith bases on theSeparated strands—A with T, G with G

DNA polymerase joinstogether the newly alignednucleotides

Parent strands conserved

New strands formed

DNA polymerase

DNA Replication

(simplified)

After Replication: 2 Identical DNA molecules(sister chromatids)

5’

5’

5’

5’3’

3’

3’

3’

3’

3’ 5’

5’

5’

5’

3’

3’

a.) Primer binding b.) Synthesis begins

Newly madestrands

Primer(5’ end)

DNApolymeraseand otherenzymes

Two strands ofthe parentalDNA molecule

DNA

Replication: DNA polymerase can only add nucleotides to the 3’ end of another nucleotide

(slide 1 of 2)

3’ end

Replicationfork moves

Replicationfork

5’ end

c.) Replication continues d.) Replication complete

Two identical daughterDNA molecules

Replicationfork moves

DNA Replication (slide 1 of 2)

This strand is synthesizedcontinuouslytoward thefork

This strand is made bydiscontinuous synthesis:Fragments aresynthesized and thenjoined together

DNA replication Self-test Questions

1. When during the cell cycle does it occur?

2. What do we start with?

3. What do we end with?

4. Where in a cell does DNA replication occur?

5. What’s needed for DNA replication to occur?

6. What is the sequence of events?

7. Why is DNA replication said to be semi-conservative?

8. What does proofreading mean in terms of DNA replication?

9. What does the proofreading during DNA replication?

Thymine(T)

AZT Part of a Thymine nucleotide

How do nucleotide analogues work to stop DNA replication in cancer cells and HIV?The drug AZT, below, is effective at preventing the spread of HIV. How?

DNA can be damaged by ultraviolet light

1. Enzymes (e.g. DNA polymerase) can repair the damage.

– Is the damage always repaired? Consequences?

2. Why does burnt skin peel? What gene is involved?

3. Role of Apoptosis?

4. Role of p53 gene?

– Advantages of peeling?

– Disadvantages?