THE STRUCTURE OF DNASection 13-1

WHICH IS MORE LIKELY TO BE GENETIC MATERIAL?Protein DNA

THE TALE OF DNA

1904: Thomas Hunt Morgan – white-eyed fly showed that chromosomes were the heart of inheritance

Big question: What are genes? (Like thought and memory today.)

FREDERICK GRIFFITH’S EXPERIMENTS

Two types of bacteria:

S-strain covered by a capsule and causes pneumonia

R-strain had no capsule and was not dangerous

Heat-killed S bacteria (capsule still present) were harmless

FREDERICK GRIFFITH’S RESULTS

FREDERICK GRIFFITH’S RESULTS

R bacteria (safe) mixed with heat-killed S bacteria (capsule still present) killed the mouse

Griffith’s discovery: TRANSFORMATION, the uptake of foreign genetic material

OSWALD AVERY’S EXPERIMENT

Repeated Griffith’s experiments, utilizing enzymes to destroy DNA, RNA, and protein, respectively

Missing protein? Transformation still occurs

Missing DNA? No transformation

HERSHEY-CHASE EXPERIMENT

How do proteins and DNA cross cell membranes?

STEP 1: Viral proteins and DNA “marked” with two different radioactive elements

HERSHEY-CHASE EXPERIMENT

STEP 2: Viruses “infect” bacteria

STEP 3: Radioactive elements can be detected by machines

STEP 4: Examine infected bacteria to find radioactive proteins or DNA

UNRAVELING DNA

Linus Pauling (Caltech) pioneered X-ray crystallography

James Watson and Francis Crick working on structure at Cambridge (England) using cardboard cutouts “like a puzzle”

Rosalind Franklin’s X-ray images taken by Maurice Wilkins and shown to Watson

DNA

DNA = DeoxyriboNucleic Acid

Sugar (“-ose”) in DNA is called deoxyribose

Each NUCLEOTIDE (piece of DNA) consists of (1) a sugar, (2) a phosphate, and (3) a nitrogenous base ?

NITROGENOUS BASES

4 different bases; 2 types

Purines (Adenine and Guanine) are 2-ringed

Pyrimidines (Thymine and Cytosine) are 1-ringed

ERWIN CHARGAFF’S OBSERVATIONS Amount of adenine always equals amount of

thymine (And C = G, too!)

WATSON AND CRICK: “THE DOUBLE HELIX”

DNA’S SHAPE

“Spiral staircase” or double helix

1953 publication

1962 Nobel Prize for Watson, Crick, and Wilkins

BASE PAIRS

Adenine – Thymine

Cytosine – Guanine

Two strands held together by weak hydrogen bonds

BASE PAIRS

Adenine – Thymine

Cytosine – Guanine

Two strands held together by weak hydrogen bonds

WHAT IS THE COMPLEMENTARY STRAND?

A C G C C A G T A

T G C G G T C A T

REPLICATION OF DNASection 13-2

REASON FOR REPLICATION

Each new cell contains a complete, exact copy of DNA

REPLICATION: Process by which DNA is copied

STEPS IN REPLICATION Helix unwinds,

creating Y-shaped “replication forks”

New strand created when complementary nucleotides are paired with each original strand

Eventually, two new identical helixes form

PROTEINS AND ENZYMES

Proteins: most important structural and functional molecules in the cell

Enzymes: proteins that act as catalysts (cause reactions)

Enzymes end in -ase

DNA HELICASE Wedges between two strands of double helix

Breaks weak hydrogen bonds

Unwinds DNA before replication

DNA POLYMERASE Moves along

strand, adding complementary nucleotides

Not able to continue if wrong base is paired (“proofreading”)

THE WORK OF DNA POLYMERASE

What DNA strand would be created to match this strand?

G A C G A T C G A

TRANSCRIPTION AND TRANSLATIONSection 13-3

WIKIPEDIA Transcription may refer to:

Transcription (linguistics), the conversion of spoken words into written language. Also the conversion of handwriting, or a photograph of text into pure text

Transcription (music), either notating an unnotated piece, common in ethnomusicology, or rewriting a piece, either simply recopying (as for clarity), or as an arrangement for another instrument

FUNCTION OF DNA

DNA codes for proteins , which perform most cell functions

Where are proteins made?

Where is DNA?

See a problem?

DNA CODES FOR PROTEINS

A molecule called RNA carries the message from DNA to the ribosome, where the protein is created

Very similar to DNA – few exceptions

RNA VS. DNA

Double-stranded

Deoxyribose sugar

Thymine (T) base

Single-stranded

Ribose sugar

Uracil (U) base

DNA RNA

DNA PROTEIN

RNA created from the message in a strand of DNA

Occurs in nucleus

Protein created from message in a strand of RNA

“Language” of N.A.s changed to “language” of amino acids

Occurs at ribosome

Transcription Translation

TRANSCRIPTION

TRANSCRIPTION

RNA polymerase bonds to DNA at a specific site called the promoter and unwinds DNA

Complimentary RNA bases added as RNA polymerase moves along the DNA

Result is a strand of mRNA, one of three types

RESULTS OF TRANSCRIPTION

What strand of mRNA would be made from the following strand of DNA?

DNA: T C C G A C C A G T C A

RESULTS OF TRANSCRIPTION

What strand of mRNA would be made from the following strand of DNA?

DNA: T C C G A C C A G T C A

RNA: A G G C U G G U C A G U

TRANSLATION

JOB OF MRNA

mRNA (messenger RNA) takes DNA “message” from nucleus to ribosome

Ribosomes read mRNA and create appropriate chain of amino acids (polypeptide or protein)

CODONS

Three-base sections of mRNA

“Code” for one of 20 amino acids

How many different combinations can be made?

HOW TRANSLATION OCCURS

Changes message from “language” of nucleic acids into “language” of proteins

Three types of RNA interact

TWO MORE TYPES OF RNA

rRNA – makes up a large portion of the ribosome structure

tRNA – compact molecule which “grabs” amino acids and matches them to mRNA

STEPS OF TRANSLATION

1. Each tRNA molecule has an anticodon, complementary to a codon, and carries a specific amino acid

2. Specific tRNA molecule binds to correct codon of mRNA

STEPS OF TRANSLATION

3. Ribosome moves down the mRNA strand

4. Second tRNA binds to next codon

5. Peptide bond forms between two amino acids

6. First tRNA leaves the molecule

STEPS OF TRANSLATION

7. Continues until a “Stop Codon” is reached

8. Chain of amino acids (called a polypeptide or protein) falls away from mRNA molecule and ribosome

EXAMPLE FROM TEXTBOOK

Keratin is on of the proteins in hair. The gene for keratin is transcribed and translated by certain skin cells. The sequence below is part of the mRNA molecule that is transcribed from the gene for keratin.

U C U C G U G A A U U U U C C

ANSWER THESE QUESTIONS.

U C U C G U G A A U U U U C C

1. Determine the sequence of DNA that was transcribed to create the mRNA strand shown above.

2. Determine the sequence of amino acids that will result from the translation of the segment of mRNA above.

2nd Base1s

t Bas

e

GENE EXPRESSION Complex relationships

between genes and their outcomes

Most traits are not controlled by one gene or one protein

Mistakes and variations occur regularly

Not all cells express all genes

REVIEW Answer the questions regarding the strand of

DNA below.

T A C G C G A G T A C C A C T

1. What is the complementary strand of DNA?

2. What RNA strand would be created through transcription? (Use original DNA strand.)

3. What polypeptide (chain of amino acids) would be created through translation?

MUTATIONSSection 14-1

MUTATIONS

Changes in structure/amount of DNA

Cause genetic variation (or worse)

Essential to evolution

Darwin is my favorite scientist

CAUSES Some mistakes in replication are not repaired

Increased rate due to radiation and some chemicals

EFFECTS

Changes in amino acids (proteins) result in specific diseases or in non-functional cell parts

Some mutations can be beneficial over time.

TYPES

Point Mutation: change in single base

Insertion

Deletion

Substitution

HOW DO MUTATIONS AFFECT PROTEINS?

A T G C C A T A G

What amino acids result from this DNA?

1. Change second A to T. (Silent mutation.)

2. Change second C to A. (Missense mutation.)

3. Add G before first C. (Frameshift mutation.)

OTHER TYPES Nonsense mutation

– amino acid code changed to a “stop” signal.

EFFECTS OF GENETIC CHANGE

Changes in egg/sperm cells are heritable (genetic disorders)

Mutations in genes that control cell growth and division cause tumors

New alleles (new traits) can be created – beneficial or harmful?

GENETIC DISORDERS Sickle cell anemia

Tay-Sachs disease

Cystic Fibrosis

Hemophilia A

Huntington’s disease

Downs Syndrome***

REGULATING GENE EXPRESSIONSection 14-2

GENE EXPRESSION

Which genes are “used” by a cell?

Controlled by complex regulatory systems which take into account environmental and other factors

Some expression can be turned on/off to use materials more efficiently

OPERONS

Gene-regulation system in which DNA controls the expression of related genes

Common in prokaryotes

Often controlled by environmental factors

LAC OPERON E. Coli cells live in your stomach and digest

lactose

Need 3 different enzymes (3 different genes) to do this

Controlled by system called “lac operon”

IN EUKARYOTES

Transcription factors affect the binding of RNA polymerase to the promoter

Examples: Activators and repressors bond to operators

REGULATION AFTER TRANSCRIPTION

Some sequences of RNA do not code for proteins

Coding sequences: exons

Non-coding sequences: introns

REGULATION AFTER TRANSCRIPTION

Certain proteins remove introns and splice (re-join) exons together

Exons leave nucleus to be translated in ribosome