Ch. 9 How Genes Work

Ch. 9-1 Understanding DNA How Scientists Identified the Genetic Material Scientists knew that chromosomes were involved in the transmission of traits Wanted to know which chemical substance in the chromosomes, DNA or protein, store and transmit the genetic traits

Griffith Studies Bacteria pg.191 Frederick Griffith (British, 1928) Trying to find out HOW bacteria make people sick Wanted to prepare a vaccine against pneumonia Background: Studied 2 forms of the bacteria Pneumococcus: Isolated from mice, both grow well in culture; only 1 kind caused pneumonia

2 types of Pneumococcus: Type S – surrounded by a smooth capsule virulent - disease causing Type R – no capsule with rough edges nonvirulent - does NOT cause disease vaccine – substance prepared from killed or weakened pathogens and introduced into body to produce immunity

Griffith’s Experiments Type S grows in culture Type R grows in culture Nothing

Conclusion: Some “factor” from dead Type S transformed (or changed) Type R into Type S Griffith discovered: Transformation - the transfer of genetic material from one organism to another, as when a bacterium takes up foreign DNA

Avery Identifies the Agent of Transformation (1944) Avery modifed Griffith’s experiment: Wanted to identify the transforming factor (the substance that made nonvirulent bacteria (Type R) become virulent bacteria (Type S) Was it DNA or protein?

Experiment 1: Extracted DNA from smooth (Type S) colonies of bacteria and added it to rough (Type R) colonies of bacteria Some of the bacteria that grew from this mixture formed smooth (Type S) colonies Transformation occurred

Experiment 2: Added protein-destroying enzymes to bacteria and transformation still occurred Added DNA-destroying enzymes to bacteria and transformation did NOT occur!!! Conclusion: Avery concluded that DNA was the genetic material

Hershey and Chase Confirm that DNA is the Genetic Material (1952) Background: Used bacteria-infecting viruses called bacteriophages Structure of viruses: DNA core surrounded by a protein coat

Viruses attach to the surface of bacteria, inject their hereditary information which directs the synthesis of new viruses What was injected, the DNA or protein? When new viruses are mature, they burst out of infected bacteria and attack new cells.

Experiment (pg. 192): Used viruses with radioactive isotope labels One group of viruses labeled with 35 S which labels protein (with sulfur) Another group of viruses labeled with 32 P which labels DNA (with phosphorus) Each kind of virus infected 2 separate cultures of bacteria After allowing enough time for the viruses to infect bacteria (by injecting their genetic material), they used a blender to separate the viruses and bacteria (centrifuge) and then measured for radioactivity.

Results: No radioactivity in bacteria infected with 35 S labeled viruses Viruses still radioactive Radioactivity in bacteria infected with 32 P labeled viruses Viruses NOT radioactive

Conclusion: Viral DNA enters bacteria and directs the synthesis of new viruses DNA is the genetic (hereditary) material

DNA (deoxyribonucleic acid) – nucleic acid that contains the sugar deoxyribose and acts as the hereditary material - Stores and transmits genetic information from one generation of an organism to the next AND codes for the production of a cell’s proteins

Nucleotide - structural unit of a nucleic acid consisting of 3 parts 3 parts of a nucleotide are: 5-C sugar (deoxyribose) phosphate group nitrogenous base (4 types) 4 types of N bases are:adenine (A) guanine (G) thymine (T) cytosine (C) How Scientists Determined the Structure of DNA Chemical components (building block of DNA & RNA):

Adenine Nucleotide

2 Types of nitrogenous bases: purines - one of the 2 larger nucleotide bases (double ring structure) ex. adenine and guanine pyrimidines - one of the 2 smaller nucleotide bases (single ring structure) ex. cytosine and thymine in DNA (uracil in RNA)

Chargaff’s Rules or base-pairing rules Analyzed amount of N bases in DNA from different organisms Found that: Amount of adenine (A) in a DNA molecule always equals the amount of thymine (T) Amount of guanine (G) always equals the amount of cytosine (C)

Rosalind Franklin And X-Ray Diffraction (early 1950’s) The X near the center shows that fibers that make up DNA are twisted to form a double helix Groups of molecules in DNA are spaced out at regular intervals Her pictures provided important clues about the structure of DNA

James Watson and Francis Crick Double Helix (1953)

They applied clues provided by Chargaff’s rules and Franklin’s X-ray diffraction studies a.DNA was shaped like a spiral staircase composed of 2 strands of nucleotides whose N bases face each other b. Double helix – spiral of 2 strands, as in DNA structure c. Two strands of nucleotides held together by weak H bonds between N bases Adenine forms 2 H bonds only with thymine Guanine forms 3 H bonds only with cytosine

d.Overall structure of DNA 2 side chains made of alternating 5-C sugar (deoxyribose) and phosphate groups Pair of N bases make up the “steps” and are linked by weak H bonds N bases are attached to the deoxyribose (5-C sugar) Francis Crick and James Watson were awarded the Nobel Prize in 1962

DNA molecular structure needed to explain HOW DNA could be copied exactly Within a few weeks Watson and Crick wrote another paper describing the copying mechanism

Pairing Between Bases Guanine Cytosine Adenine Thymine Base-pairing rules - rule that states that cytosine pairs with guanine and adenine pairs with thymine in DNA (adenine with uracil in RNA)

Complementary base pairs - Characteristic of nucleic acids in which the sequence of bases on one strand is paired to the sequence of bases on the other strand.

Replication - copying of DNA before cell division (during Interphase - S) DNA must be copied precisely every time a cell divides Watson - Crick model suggested HOW this could occur 1.The 2 stands of the double helix are complimentary to each other The sequence of bases of the one strand determines the sequence of bases on the other strand Ex. If one strand is: A T T G C A T the complementary strand is: T A A C G T A

2. When the 2 strands separate, each strand exposes information to build 2 identical strands Replication fork - area where the double helix (DNA) separates 3. DNA replication requires: several enzymes : One enzyme: DNA helicase - breaks H bonds between base pairs and untwists strands Another enzyme: DNA polymerase - attaches the correct complimentary nucleotides; catalyzes the formation of the DNA molecule

Replication usually preserves the sequence of bases in an organism’s DNA however, mutations can occur Mutations - abrupt change in the genotype (DNA) of an organism Mutagens - environmental agent that can alter the structure of DNA ex. Radiation, chemicals 4. Checking for errors - DNA polymerase has a “proof-reading” role, checking for errors

The Rate of Replication Prokaryotes have a circular chromosome; 2 replication forks that begin at the same point and work away from each other Eukaryotes have multiple chromosomes made of long strands of DNA Each chromosome has multiple replication forks (may have about 100) and can replicate in 8 hours (If there was 1 replication fork would take 33 days!)

DNA Replication