DNADNADNADNA

Chapter 25Chapter 25

Discovery of DNA FunctionA. Discovery of DNA

1. first isolated from cells in 18682. unknown function

B. Griffith Experiments (1928)

1. worked with a pneumonia-causing bacteria2. two strains

R strain: non-fatal S strain: fatal

3. experiment: mice injected with

a. R (rough) cells - livedb. S (smooth) cells - diedc. heat killed S cells - livedd. heat killed S cells + live R

cells - mice died !!

4. analysis of last group showed live S cells and no R cells

1. Transformation of R cells into S cells meant that some Genetic Material must have gone from into the R-cells from the S.

2. Source of Genes: protein or nucleic acids

Hershey-Chase Experiment

1. Was DNA or proteins the genetic material?

2. Bacteriophages - viruses used

a. protein coatb. nucleic acid interior (DNA core)

3. Radioactive labeling of protein & DNA

4. Results showed that DNA directed the formation of new viruses, not the protein coat

Discovery of DNA Structure

Components of DNA 1. DNA consists of a sugar, base and phosphate group

2. one of 4 possible bases:

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

Purines

Pyrimidines

DNA Structure – The Twisted Ladder

The Nucleotide:

1. Sugar

2. Phosphate Group

3. Nitrogenous Base

Ribose Sugar

Phosp

hate

Base

Experiments - 1949

1. the amount of A = amount of T

2. the amount of C = amount of G

3. ratios vary by species

X-Ray experiments

1. Used X-rays to examine structure of DNA

2. Observed thata. DNA is helicalb. DNA exists in long strandsc. DNA is highly repetitive

Watson-Crick analysis

1. Constructed a model of DNA structure 2. Phosphate groups from backbone3. Double-stranded molecule4. Base=pairing of A-T and G-C5. Helical shape

DNA Replication & Repair

A. Replication

1. replication is semi-conservative

2. major moleculesa. helicase - unwinds DNA strandsb. DNA polymerase - copies DNAc. DNA ligases - seal newly formed

DNA strands

B. DNA Repair

1. DNA polymerase “proofreads” the new strands

2. DNA ligases help to repair mismatched bases

DNA Organization

A. Chromosome structure1. DNA is organized into long molecules called chromosomes

2. chromosome are stabilized by proteins called histones

3. histones organize the DNA