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DNADNAChapter 16
Griffith - 1928
Streptococcus pneumoniae - bacteria that causes pneumonia in mammals R strain – harmless S strain – pathogenic
mixed heat-killed S strain with live R strain bacteria and injected this into a mouse the mice died
Transformation
change in genotype and phenotype due to the assimilation of a foreign substance (now known to be DNA) by a cell.
Oswald Avery 1944Oswald Avery 1944
World knows a molecule carries the World knows a molecule carries the genetic information.genetic information.
Doesn’t know if the molecule is a: Doesn’t know if the molecule is a: protein, lipid, carbohydrate, RNA, or DNAprotein, lipid, carbohydrate, RNA, or DNA
Avery performs Griffith’s experiment Avery performs Griffith’s experiment again with a twist.again with a twist.
Avery and other scientists discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation of an organism to the next.
What is the transforming substance?
Track the infection of bacteria by viruses
Viruses consist of a DNA enclosed by a protective coat of protein
To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery
Bacteriophages (phage)
Virus that specifically attacks bacteria
Hershey and Chase
T2 phage, consisting almost entirely of DNA and protein, attacks Escherichia coli
label protein and DNA and then track which entered the E. coli cell during infection
How?
Grew one batch of T2 phage in the presence of radioactive sulfur marking the proteins but not DNA
Grew another batch in the presence of radioactive phosphorus marking the DNA but not proteins
Allowed each batch to infect separate E. coli cultures
Chargaff Developed a series
of rules based on a survey of DNA composition in organisms
DNA is a polymer of nucleotides consists of a
nitrogenous base, deoxyribose, and a phosphate group
The bases = adenine (A), thymine (T), guanine (G), or cytosine (C).
The four bases are found in ratios
Wilkins and Franklin
X-rays are diffracted as they passed through aligned fibers of purified DNA
Used to deduce the three-dimensional shape of molecules
Watson and CrickDouble Helix
Purine and Pyrimidine
nitrogenous bases are paired in specific combinations: adenine with thymine and guanine with cytosine
DNA Replication
Because each strand is complementary to each other, each can form a template when separated
DNA Replication
semiconservative replication
each of the daughter molecules will have one old strand and one newly made strand
DNA replicationDNA replication
http://www.johnkyrk.com/http://www.johnkyrk.com/DNAreplication.htmlDNAreplication.html
Human Cells
Copy its billion base pairs and divide into daughter cells in a few hours
One error per billion nucleotides
Origin of Replication
Single specific sequence of nucleotides that is recognized by replication enzymes enzymes separate the strands, forming
a replication “bubble” Replication proceeds in both
directions until the entire molecule is copied
Anti-Parallel
Each DNA strand has a
3’ end with a free hydroxyl group attached to deoxyribose and a
5’ end with a free phosphate group attached to deoxyribose.
Bubbles and Forks
DNA Polymerase
DNA polymerases can only add nucleotides to the free 3’ end of a growing DNA strand
leading strand - used by polymerases as a template for a continuous complimentary strand
lagging strand -copied away from the fork in short segments (Okazaki fragments)
DNA polymerase and Primers
Polymerase cannot initiate synthesis of a polynucleotide can only add nucleotides to the end of
an existing chain To start a new chain requires a
primer a short segment of RNA
Replication ForkReplication Fork- Topoisomerases
- Helicases
- Single-strand binding proteins
- Primases (RNA primers)
- DNA Polymerases
- Ligases
Animation: Leading StrandAnimation: Leading StrandAnimation: Lagging StrandAnimation: Lagging Strand
DNA polymerase later replaces the primer with deoxyribonucleotides complimentary to the template
Animation: DNA Replication ReviewAnimation: DNA Replication Review
Mismatched nucleotides
Reactive chemicals, radioactive emissions, X-rays, and ultraviolet light can change nucleotides in ways that can affect encoded genetic information
Each cell continually monitors and repairs its genetic material over 130 repair enzymes
identified in humans
In mismatch repair, special enzymes fix incorrectly paired nucleotides
In nucleotide excision repair, a nuclease cuts out a segment of a damaged strand
telomeres = The ends of eukaryotic chromosomal DNA molecules – long repetitive sequences (no genes)
Telomerase
uses a short molecule of RNA as a template to extend the 3’ end of the telomere
Fig. 16-12Origin of replication Parental (template) strand
Daughter (new) strand
Replication fork
Replication bubble
Two daughter DNA molecules
(a) Origins of replication in E. coli
Origin of replication Double-stranded DNA molecule
Parental (template) strandDaughter (new) strand
Bubble Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
0.5 µm
0.25 µm
Double-strandedDNA molecule
Difference between Difference between bacterial bacterial chromosomes and chromosomes and eukaryotic eukaryotic chromososeschromososes
Chromatin Chromatin is a complex of DNA and protein, is a complex of DNA and protein, and is found in the nucleus of eukaryotic cellsand is found in the nucleus of eukaryotic cells
Histones Histones are proteins that are responsible for are proteins that are responsible for the first level of DNA packing in chromatinthe first level of DNA packing in chromatin
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Animation: DNA PackingAnimation: DNA Packing
Fig. 16-21aFig. 16-21a
DNA double helix (2 nm in diameter)
Nucleosome(10 nm in diameter)
Histones Histone tailH1
DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)
Fig. 16-21bFig. 16-21b
30-nm fiber
Chromatid (700 nm)
Loops Scaffold
300-nm fiber
Replicated chromosome (1,400 nm)
30-nm fiber Looped domains (300-nm fiber)
Metaphase chromosome
Chromatin is organized into fibersChromatin is organized into fibers 10-nm fiber10-nm fiber
DNA winds around histones to form DNA winds around histones to form nucleosome nucleosome “beads”“beads”
Nucleosomes are strung together like Nucleosomes are strung together like beads on a string by linker DNA beads on a string by linker DNA
30-nm fiber30-nm fiber Interactions between nucleosomes cause Interactions between nucleosomes cause
the thin fiber to coil or fold into this the thin fiber to coil or fold into this thicker fiberthicker fiber
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
300-nm fiber300-nm fiber The 30-nm fiber forms The 30-nm fiber forms looped domainslooped domains
that attach to proteinsthat attach to proteins Metaphase chromosomeMetaphase chromosome
The looped domains coil furtherThe looped domains coil further The width of a chromatid is 700 nmThe width of a chromatid is 700 nm
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Most chromatin is loosely packed in the Most chromatin is loosely packed in the nucleus during interphase and condenses nucleus during interphase and condenses prior to mitosisprior to mitosis
Loosely packed chromatin is called Loosely packed chromatin is called euchromatineuchromatin
During interphase a few regions of During interphase a few regions of chromatin (centromeres and telomeres) chromatin (centromeres and telomeres) are highly condensed into are highly condensed into heterochromatinheterochromatin
Dense packing of the heterochromatin Dense packing of the heterochromatin makes it difficult for the cell to express makes it difficult for the cell to express genetic information coded in these genetic information coded in these regionsregions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Histones can undergo chemical Histones can undergo chemical modifications that result in changes in modifications that result in changes in chromatin organizationchromatin organization For example, phosphorylation of a specific For example, phosphorylation of a specific
amino acid on a histone tail affects amino acid on a histone tail affects chromosomal behavior during meiosischromosomal behavior during meiosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings