Chapter 12

Identifying the Substance of Genes

Chapter Mystery

• Page 337

• UV Light– Why is UV light so dangerous?– How can these particular wavelengths of light

damage our cells to the point of causing cell death and cancer?

Hypothesis……….

Section 12.1Identifying the Substance of Genes• Objectives:

– What clues did bacterial transformation yield about the gene?

– What role did bacterial viruses play in identifying genetic material?

– What is the role of DNA in heredity

• Define:– Transformations– bacteriophage

I. Bacterial Transformations

• If the molecule that carries genetic information could be identified, it might be possible to understand how genes actually control the inherited characteristics of living things

• Fredrick Griffith was trying to figure out how bacteria make people sick (pneumonia)– He isolated 2 very similar types of bacteria

from the mice = 2 different varieties (strains) of the same bacterial species

• Both grew very well in culture plates• Only 1 caused pneumonia• Diseased-causing bacteria (S-strain) grew into

smooth colonies & harmless bacteria (R-strain) produced colonies w/ rough edges

• Differences in appearance made the 2 strains easy to tell apart

A. Griffith’s Experiments

• Griffith injected mice w/ disease-causing bacteria mice developed pneumonia mice died

• Injected mice with harmless bacteria mice stayed healthy

1. Heated S-strain bacteria killed bacteria then injected into mice mice survived = (suggested cause of pneumonia was not a toxin from disease-causing bacteria)

2. Mixed heat-killed S-strain w/ live, harmless bacteria from R-strain injected into mice (by themselves, neither type should have made mice sick) injected mice developed pneumonia many died- Examined lungs and found them to be full of harmful

bacteria!!!

Draw and Label a diagram of Griffith’s Experiments

B. Transformation

• Heat-killed bacteria passed disease-causing ability to harmless bacteria– Griffith’s hypothesis: when mixed, some chemical factor

transferred from heat-killed cells of S strain into live cells of R strain

• Transformation – process by which one type of bacteria (harmless) had been changed permanently into another (disease-causing form)– Ability to cause disease was inherited by the offspring of the

transformed bacteria – Griffith concluded: transforming factor had to be a gene

C. The Molecular Cause of Transformation

• 1944 – Avery repeated Griffith’s experiments to determine which molecule in the heat-killed bacteria was most important for transformation

• If they could find this particular molecule, it might reveal the chemical nature of the gene

• Extracted various molecules from heat-killed bacteria treated mixture w/ enzymes that destroyed proteins, lipids, carbs, nucleic acid RNA transformation still occurred (results)– Since all those molecules were destroyed, they could not

be responsible for the transformation (conclusion)• Repeated experiment used enzymes that broke

down DNA transformation did not occur (results) DNA was the transferring factor (conclusion)

• By observing bacterial transformation, Avery and other scientists discovered that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next

Avery’s Experiment

II. Bacterial Viruses• Scientists are skeptical• Takes several experiments to convince them of

something as important as chemical nature of gene

• Most important = 1952 = Alfred Hershey and Martha Chase– Studied viruses – tiny, nonliving particles that can

infect living cells

A. Bacteriophages

• Bacteriophage – kind of virus that infects bacteria– Bacteriophage enters bacteria

attaches to surface of cell injects genetic information into cell

– Viral genes act to produce as many bacteriophages (destroys bacterium)

– When cell splits open hundreds of new viruses burst out

Draw a diagram of how bacteriophage infects a bacteria

cell

B. The Hershey-Chase Experiment• Studied bacteriophage composed of DNA core and

protein coat• Wanted to determine which part of virus entered

bacterial cell to support or disprove Avery’s finding that genes were made of DNA– Used radioactive markers to tell which molecules actually

entered bacteria carrying genetic information of virus– Mixed marked viruses w/ bacterial cells waited few minutes

for viruses to inject genetic material separated viruses from bacteria tested bacteria for radioactivity

– Results: Nearly all radioactivity in bacteria indicated DNA– Concluded: genetic material of bacteriophage was DNA; not

protein• Hershey and Chase’s experiment w/ bacteriophages

confirmed Avery’s results, convincing many scientists that DNA was the genetic material found in genes – not just in viruses and bacteria, but in all living cells.

Hershey-Chase Experiment

III. The Role of DNA

• Scientists wondered how DNA, or any molecule, could do critical things that genes were known to do

• The DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell

A. Storing Information

• Foremost job of DNA– Genes that make a flower purple

must carry info needed to produce purple pigment

– Genes for blood type and eye color must have info needed for job

• Genes control patterns of development – Instructions that cause a single cell to

develop into an oak tree, sea urchin, or dog must be written into DNA of each of these organisms

B. Copying Information

• Before a cell divides must make complete copy of every one of its genes– Mechanism for this process

could not be proposed until structure of molecule was known

• Discussed in next section…

C. Transmitting Information• Genes are transmitted from one

generation to the next (per Mendel)• DNA molecule must be carefully

sorted and passed along during cell division– Especially important during formation of

sex cells in meiosis

• Chromosomes of eukaryotic cells contain genes made of DNA– Loss of any DNA during meiosis might

mean loss of valuable genetic information from one generation to the next

Section 12.2The Structure of DNA

• Objectives:– What are the chemical components of DNA?– What clues helped scientists solve the

structure of DNA?– What does the double-helix model tell us

about DNA?

• Define:– Base pairing

I. The Components of DNA

• Deoxyribonucleic Acid

• DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds

A. Nucleic Acids and Nucleotides

• Nucleic acids – long, slightly acidic molecules originally identified in cell nuclei

• Macromolecule made up of smaller subunits linked together to form long chains

– Nucleotides – building blocks of nucleic acids

• Made up of 3 basic components:1. 5-carbon sugar (deoxyribose)

2. Phosphate group

3. Nitrogenous base

B. Nitrogenous Bases and Covalent Bonds

• Nitrogenous bases – bases that contain nitrogen

– 4 types:1. Adenine (A)2. Guanine (G)3. Cytosine (C)4. Thymine (T)

– Nucleotides joined by covalent bonds formed b/w sugar of one nucleotide and phosphate group of next

– Nitrogenous bases stick out sideways from nucleotide chain

– Nucleotides can be joined together in any order any sequence of bases is possible

• Could carry coded genetic information– Bases have chemical that makes them

especially good at absorbing UV (ultraviolet) light

Mystery Clue

• Page 344…

• The energy from UV light can excite electrons in the absorbing substance to the point where the electrons cause chemical changes.

• What chemical changes might occur in the nitrogenous bases of DNA?

II. Solving the Structure of DNA

• Know DNA is made from long chains of nucleotides

• Must find out the way in which those chains are arranged in 3 dimensions

A. Chargaff’s Rule

• Percentages of adenine and thymine bases are almost equal in any sample of DNA– Same thing is true for guanine and cytosine

• Chargaff’s Rule: A=T and G=C– Samples from all organisms obey this rule

B. Franklin’s X-Rays• Rosalind Franklin (1950s) – used X-ray

diffraction to get info about DNA structure

1. Purified large amount of DNA2. Stretched DNA fibers in thin glass tube (so

most of strands were parallel)3. Aimed powerful X-ray beam at concentrated

DNA4. Recorded scattering pattern of X-rays on film5. Repeated until she obtained clear patterns

• Clues from X-ray:– X-shaped pattern strands in DNA are

twisted around each other like coils of a spring

• Shape = helix– Angle of X there are 2 strands in structure– Other clues nitrogenous bases near

center of DNA molecule

C. The work of Watson and Crick

• James Watson & Francis Crick• Built 3 dimensional models of

DNA made of cardboard and wire

• The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA

III. The Double-Helix Model

• Double helix looks like a twisted ladder– 2 strands twist around each other

like a spiral staircase

• The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together

• Also tells us how DNA can function as a carrier of genetic information

A. Antiparallel Strands

• 2 strands of DNA run in opposite directions = “antiparallel”– Enables nitrogenous

bases on both strands to come into contact at the center of molecule

– Allows each strand to carry a sequence of nucleotides

• Arranged almost like letters in a four-lettered alphabet

Mystery Clue

• Page 347…

• Our skin cells are exposed to UV light whenever they are in direct sunlight.

• How might this exposure affect base pairing in the DNA of our skin cells?

B. Hydrogen Bonding

• Relatively weak chemical forces• Could form b/w certain nitrogenous

bases• Provides just enough force to hold

2 strands together• Does it make sense that a

molecule as important as DNA should be held together by such weak bonds?– Yes!!– If 2 strands of helix were held together

by strong bonds it might be impossible to separate them

– Separation is critical to DNA’s functions

C. Base Pairing

• Watson and Crick’s model showed that hydrogen bonds could create a nearly perfect fit b/w nitrogenous bases along the center of molecule

• Bonds only form b/w certain base pairs– A with T– G with C

• Base pairing – nearly perfect fit b/w A-T and G-C– Explains Chargaff’s rule

• For every adenine exactly one thymine

• For each cytosine exactly one guanine

Section 12.3DNA Replication

• Objectives:– What role does DNA polymerase play in

copying DNA?– How does DNA replication differ in prokaryotic

cells and eukaryotic cells?

• Define:– Replication– DNA polymerase– Telomere

I. Carrying the Code• Base pairing in double helix explains

how DNA can be copied/replicated– Each base on one strand pairs w/ 1 (and

only 1) base on the opposite strand– Each strand has all the information

needed to reconstruct the other half by mechanism of base pairing

• Each strand can be used to make the other strand strands are complementary

A. The Replication Process• Before a cell divides it duplicates its

DNA – Process is called replication

– Occurs during late interphase

– Ensures that each resulting cell has same set of DNA molecules

• DNA separates into 2 strands• Produces 2 new complementary

strands– Follows rules of base pairing

– Each strand serves as template/model for new strand

• 2 strands of double helix unzip– Allow 2 replication forks to form

• As each new strand forms, new bases are added following rules of base pairing

• If base pair is A then T is added to new strand• G always pairs with C

– Example: strand w/ base sequence: TACGTT produces a complementary base sequence: ATGCAA

• Results: 2 DNA molecules identical to each other and to the original molecules

• Each new molecule has one new strand and one old strand

B. The Role of Enzymes• Enzymes – unzip molecule of DNA by

breaking hydrogen bonds b/w base pairs & unwinding the 2 strands

• Each strand serves as template for attachment of complementary bases

• DNA polymerase – principal enzyme involved in DNA replication

• DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA– Produces sugar-phosphate bonds that join

nucleotides together– “Proofreads” each new DNA strand, so that

each molecule is a near-perfect copy of original

Mystery Clue

• Page 351…

• How might UV-induced chemical changes in bases affect the process of DNA replication?

C. Telomeres• Telomeres – DNA at tips of chromosomes• Difficult to replicate• Cells use special enzyme telomerase

– Adds short, repeated DNA sequences to telomeres

• In rapidly dividing cells (stem cells) telomerase helps to prevent genes from being damaged or lost during replication

• Often switched off in adult cells• May be activated in cancer cells

enabling cells to grow and proliferate rapidly

II. Replication in Living Cells• DNA replication occurs during S phase

– Carefully regulated so it is completed before cell enters mitosis or meiosis

• Prokaryotes = single, circular DNA molecule in cytoplasm – contains nearly all genetic information

• Eukaryotes = 1000 times more DNA in nucleus, packaged into chromosomes– Chromosomes consist of DNA tightly

packed together w/ proteins = chromatin– DNA + histones = beadlike nucleosomes

• Histones = proteins chromatin is coiled around

A. Prokaryotic DNA Replication• DNA replication does not start until

regulatory proteins bind to a single starting point on the chromosome– Then proteins trigger beginning of S phase– Then DNA replication begins

• Replication in most prokaryotic cells starts from a single point and proceeds in 2 directions until entire chromosome is copied– 2 chromosomes are attached to different

points inside the cell membrane and separated when cell splits

B. Eukaryotic DNA Replication• Chromosomes = much bigger• Replication may begin at dozens

or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied– Although proteins check DNA for chemical

damage or base pair mismatches prior to replication system is not foolproof

– Damaged regions of DNA are sometimes replicated result in changes of DNA sequences that may alter certain genes and produce serious consequences

• 2 copies of DNA produced by replication in each chromosome remain closely associated until cell enters prophase of mitosis– Then chromosomes

condense 2 chromatids in each chromosome become clearly visible

– Chromatids separate from each other in anaphase of mitosis

– Produce 2 new cells each w/ complete set of genes coded in DNA

Solve the Chapter Mystery• Page 357…

1. Use your understanding of the structure of DNA to predict what sorts of problems excessive UV light might produce in the DNA molecule. How might these changes affect the functions of DNA?

2. All cells have systems of enzymes that repair UV-induced damage to their DNA. Some cellular systems block DNA replication if there are base pairing problems in the double helix. Why are these systems important? How might they work?

3. Analyze the effects that UV light might have on skin cells. What is UV light so dangerous? Why is the skin particularly vulnerable to it?

4. Among humans who inherit genetic defects in their DNA-repair systems, the incidence of skin cancer is as much as 1000 times greater than average. Based on this information, what can you infer about the effect of UV light on DNA?