DNA Structure and Function

Chapter 13

Early and Puzzling Clues

1800s: Miescher found DNA (deoxyribonucleic acid) by examining pus cells

Early 1900s: Griffith transferred hereditary material from dead cells to live cells• Mice injected with live R cells lived• Mice injected with live S cells died• Mike injected with killed S cells lived• Mice injected with killed S cells and live R cells

died; live S cells were found in their blood

Avery and McCarty Find the Transforming Principle

1940: Avery and McCarty separated deadly S cells (from Griffith’s experiments) contained protein and nucleic acid components

When proteins, and RNA were destroyed, the remaining substance, DNA, still transformed R cells to S cells (transforming principle)

Confirmation of DNA’s Function

1950s: Hershey and Chase experimented with bacteriophages (viruses that infect bacteria)• Protein parts of viruses, labeled with 35S

(component of protein), stayed outside the bacterial but remained in the virus

• DNA of viruses, labeled with 32P (component of DNA), entered the bacteria

Conclusion: DNA is the material that stores hereditary information

The Hershey-Chase Experiments

13.2 The Discovery of DNA’s Structure

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

DNA’s Building Blocks

Nucleotide (three components)• A nucleic acid monomer consisting of a five-

carbon sugar, three phosphate groups, and one of four nitrogen-containing bases

DNA consists of four nucleotide building blocks• Two pyrimidines: thymine and cytosine (double

ring)• Two purines: adenine and guanine (single ring)

Chargaff’s Rules

The amounts of thymine and adenine in DNA are the same, and the amounts of cytosine and guanine are the same: A = T and G = C (If A is 31% then T is 31%)

The proportion of adenine and guanine differs among species (Ex. Humans/Adenine =31%, Fruit fly = 27%, and E.coli = 24%)

Watson and Crick’s DNA Model

A DNA molecule consists of two nucleotide chains (strands), running in opposite directions (anti parallel) and coiled into a double helix

Base pairs form on the inside of the helix, held together by hydrogen bonds (A-T and G-C)

Patterns of Base Pairing

Bases in DNA strands can pair in only one way• A always pairs with T; G always pairs with C

The sequence of bases is the genetic code• Variation in base sequences gives life diversity

13.3 DNA Replication and Repair

A cell copies its DNA before mitosis or meiosis I

DNA repair mechanisms and proofreading correct most replication errors (DNA polymerase)

Semiconservative DNA Replication

Each strand of a DNA double helix is a template for synthesis of a complementary strand of DNA

One template builds DNA continuously (leading strand) ; the other builds DNA discontinuously (lagging strand), in segments

Each new DNA molecule consist of one old (parent strand) strand and one new strand = semiconservative

Enzymes of DNA Replication

DNA helicase• Breaks hydrogen bonds between DNA strands

DNA polymerase• Joins free nucleotides or bases into a new strand

of DNA

DNA ligase• Joins DNA segments on discontinuous strand

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).

Semiconservative Replication of DNA

Fig. 13-8b, p. 209

The parent DNA double helix unwinds in this direction.

Only one new DNA strand

is assembled continuously.

5’

The other new DNA strand is

assembled in many pieces.

3’

3’

Gaps are sealed by DNA ligase.

5’ 3’3’ 5’

B Because DNA synthesis proceeds only in the 5’ to 3’ direction, only one of the two new DNA strands can be assembled in a single piece.

The other new DNA strand forms in short segments, which are called Okazaki fragments after the two scientists who discovered them. DNA ligase joins the fragments into a continuous strand of DNA.

Checking for Mistakes

DNA repair mechanisms • DNA polymerases (repair enzymes) proofread

DNA sequences during DNA replication and repair any damaged DNA

When proofreading and repair mechanisms fail, an error becomes a mutation – a permanent change in the DNA sequence (cancer or genetic disorders in offspring)

Checking for Mistakes

Mistakes can occur during base pairing process DNA is damaged when exposed to radiation or

toxic chemicals Mistakes can occur with chromosomes

(structure or the number)

Cloning

Clones• Exact copies of a molecule, cell, or individual• Occur in nature by asexual reproduction or

embryo splitting (identical twins)

Reproductive cloning technologies produce an exact copy (clone) of an individual. The offspring has one parent’s trait.

Reproductive Cloning Technologies

Somatic cell nuclear transfer (SCNT)• Egg cytoplasm reprograms differentiated (adult)

DNA to act like undifferentiated (egg) DNA• The hybrid cell develops into an embryo that is

genetically identical to the donor individual• Nucleus of an unfertilized egg is removed and

replaced with a donor nucleus. Figure 13.9

Therapeutic Cloning

Therapeutic cloning uses SCNT to produce human embryos for research purposes

Researchers harvest undifferentiated (stem) cells from the cloned human embryos

Video: Goodbye, dolly