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

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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

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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

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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

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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

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