CHAPTER 13 DNA

THE ROLE OF DNA IN HEREDITY Stores information needed for traits and

cell processes Copying information needed for new

cells Transferring information from generation

to generation

GRIFFITH’S EXPERIMENTS

THE HERSHEY-CHASE EXPERIMENTS

13.2 THE DISCOVERY OF DNA’S STRUCTURE

Watson and Crick’s discovery of DNA’s structure (double helix) was based on almost fifty years of research by other scientists

COMPONENTS OF DNA Made of nucleotides (3 parts)

Sugar (deoxyribose)Phosphate groupBase (nitrogenous base)

A (adenine) T (thymine) C (cytosine) G (guanine)

SOLVING THE STRUCTURE OF DNA

Chargaff discovered that the percent of adenine and thymine in DNA were the same.

The percent of guanine and cytosine are also equal.

The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules.”

SOLVING THE STRUCTURE OF DNA

Rosalind Franklin (1950) – used X-ray diffraction (aimed X-rays at DNA and looked at the scatter pattern) to find clues about the structureShowed DNA has 2 strandsThe DNA strands are twisted around each

other like a spring (helix shaped)The bases are in the center Died at age 37 from cancer (x-ray

exposure?)

SOLVING THE STRUCTURE OF DNA Watson and Crick – built models of DNA

Discovered the double helix structure (2 strands twist around each other like staircases)

Explained Franklin’s and Chargaff’s earlier discoveries

Discovered that hydrogen bonds hold the DNA strands together Weak forces that enable the DNA to come apart

BASE PAIRING Adenine pairs with Thymine Guanine pairs with Cytosine

12.3 DNA REPLICATION

Before a cell divides, it duplicates its DNA in a copying process called replication

This process ensures that each resulting cell has the same complete set of DNA molecules

DNA REPLICATION How does the double helix structure of

DNA make replication (copying) possible?????? Each strand of the double helix has all the

information needed to reconstruct the other half by the mechanism of base pairing.

Because each strand can be used to make the other strand, the strands are said to be complementary

DNA REPLICATION The DNA molecule separates into two strands

and then produces two new complementary strands following the rules of base pairing.

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

DNA REPLICATION The two strands of the double helix separate,

or “unzip,” allowing two replication forks to form.

New bases are added following the rules of base pairing (A-T and C-G) to the newly forming strand.

RESULTS OF DNA REPLICATION Each DNA molecule has one original strand

and one new strand (semi-conservative). The result of replication is two DNA molecules

identical to each other and to the original molecule.

SEMICONSERVATIVE REPLICATION OF DNA

Stepped ArtFig. 13-6, p. 208

D DNA ligase seals any gaps that remain between bases of the “new” DNA, so a continuous strand forms. The base sequence of each half-old, half-new DNA molecule is identical to that of the parent DNA molecule.

C Each of the two parent strands serves as a template for assembly of a new DNA strand from free nucleotides, according to base-pairing rules (G to C, T to A). Thus, the two new DNA strands are complementary in sequence to the parental strands.

B As replication starts, the two strands of DNA are unwound. In cells, the unwinding occurs simul- taneously at many sites along the length of each double helix.

A A DNA molecule is double-stranded. The two strands of DNA stay zippered up together because they are complementary: their nucleotides match up according to base-pairing rules (G to C, T to A).

DNA REPLICATION AND ENZYMES

DNA replication is carried out by enzymes. DNA helicase

Breaks hydrogen bonds between DNA strands

DNA polymerase Joins free nucleotides into a new strand of

DNA DNA ligase

Joins DNA segments on discontinuous strand

DISCONTINUOUS SYNTHESIS OF DNA

VIDEOS OF DNA REPLICATION Computer animation of DNA Replication

2 minutes 4 minute DNA REPLICATION 2 minutes detailed DNA replication

Bozeman Biology

DNA WHITEBOARDING Draw, label and explain DNA replication Include the following terms

Nucleotide (deoxyribose, base, phosphate)A, T, C, GDNA polymeraseDNA ligaseDNA helicase3’end and 5’endContinuous replication and discontinuous

replication Old strand and new strandOkazaki fragmentsSemi-conservative replication

DNA WHITEBOARDING Draw, label and explain DNA replication Include the following terms

Nucleotide (deoxyribose, base, phosphate)A, T, C, GDNA polymeraseDNA ligaseDNA helicase3’end and 5’endContinuous replication and discontinuous

replication Old strand and new strandOkazaki fragmentsSemi-conservative replication

CHECKING FOR MISTAKES

DNA repair mechanisms DNA polymerases proofread DNA sequences

during DNA replication and repair damaged DNA

When proofreading and repair mechanisms fail, an error becomes a mutation – a permanent change in the DNA sequence

TELOMERES The tips of chromosomes are known as telomeres

Telomeres are hard to copy. DNA may be lost from telomeres each time a chromosome is replicated.

An enzyme (telomerase) adds short, repeated DNA sequences to telomeres, lengthening the chromosomes and making it less likely important gene sequences will be lost during replication.

WE WILL TALK CLONING LATER THIS YEAR – BUT HERE IS SOMATIC CELL NUCLEAR TRANSFER (SCNT)

CLONING

1997 Dolly News Story GMA Pet Cloning

NATIONAL GEOGRAPHIC Bringing Them Back To Life – Read the

article and answer the questions on the next slide.

REVIVING EXTINCT SPECIES

Read the entire article then answer the following in complete sentences. Synthesize information from the entire article to answer the questions completely. Incomplete answers will only receive partial credit.

1) Have we been able to “bring an extinct species back to life”? If so, what was it and what was the result?

2) Do scientists think we will be able to bring a dinosaur back to life? Why or why not?

3) Discuss the benefits of de-extinction. In other words, why would we want to bring extinct species back to life?

4) What are some challenges of bringing a mammoth back to life? What is the procedure researchers would use to bring it back to life?

5) Explain why some people do not think scientists should try to revive extinct species?

6) Do you think that research should be done to recreate extinct species? Support your answer using support from the article.