DNA: the Molecule of Heredity

What is DNA?

• Deoxyribonucleic acid

• DNA determines an organism’s traits

• DNA achieves control by producing proteins– Remember: proteins give us structural

building material and allow function (enzymes)

• DNA is the information for life

Time Line – Early History of GeneticsKey Players

1840’s MendelEarly 1900’s T.H. Morgan1928 Griffith1944 Avery1952 Hersey and Chase1947 ChargaffEarly 1950’s Franklin and WilkinsEarly 1950’s Pauling1953 Watson and Crick

Place these names and dates on your timeline under “Scientists”

DNA is the genetic material

• Early 1900’s, the identification of the molecules of inheritance loomed as a major challenge to biologists

• T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material

Early History of Genetics

• The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928

• He worked with two strains of a bacterium, one pathogenic and one harmless. He did transformation experiments

• Basically he found that harmless bacteria became deadly when they took in DNA from dead pathogenic bacteria

Fig. 16-2

Living S cells (control)

Living R cells (control)

Heat-killed S cells (control)

Mixture of heat-killed S cells and living R cells

Mouse diesMouse dies Mouse healthy Mouse healthy

Living S cells

RESULTS

EXPERIMENT

Avery - 1944He separated the components of the bacteria and found only the DNA extract caused mice to die

Evidence That Viral DNA Can Program Cells

• More evidence for DNA as the genetic material came from studies of viruses that infect bacteria

• Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research

Fig. 16-4-3

EXPERIMENT

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA

Batch 1: radioactive sulfur (35S)

Batch 2: radioactive phosphorus (32P)

Empty protein shell

Phage DNA

Centrifuge

Centrifuge

Pellet

Pellet (bacterial cells and contents)

Radioactivity (phage protein) in liquid

Radioactivity (phage DNA) in pellet

Hersey and Chase

Next Steps… What is DNA made of?

• It was known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group

• In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next, however that the nitrogen based are found in predictable ratios:

A = T and C = G

Finding the Structure of DNA• After most biologists became convinced that DNA

was the genetic material, the challenge was to determine how its structure accounts for its role

• Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure

• Franklin produced a pictureof the DNA molecule using this technique

The Discovery of DNA

• Watson and Crick – 1953– Double Helix – long twisted zipper

Segment with James Watson

Structure of DNA

• DNA is a long molecule• Composed of nucleotides

– Simple sugar – deoxyribose– Phosphate group– Nitrogen base – Adenine

- Guanine

- Cytosine

- Thymine

DNA Structure Cont.

• Animation on how DNA is packaged into the nucleus

S – A ..…T – S| |P P| |

S – G ..…C – S| |P P| |

S – C ..…G – S| |P P

A pairs with T

C pairs with G

Weak Hydrogen Bond

DNA Instructions for life

• The sequence of nitrogen bases forms the genetic instructions for an organism

A-T-T-G-A-C

is different than

T-T-C-A-A-G

They code for different proteins and therefore structure and function of an organism

How can we use DNA?

• Nucleotide sequences can be used to determine evolutionary relationships– Organisms that are closely related have

similar DNA• Ex. Gorilla and Chimp – very similar

Gorilla and Rose Bush – very different

• It can be used to determine if two people are related

• DNA can be used to compare DNA from a crime scene to DNA from a suspect

Complementary Strands

• If one side of the DNA molecule consisted of the following nucleotide bases, what would the other side be?

• ATC CTG GAT TAT GAC CAT ATG

DNA Replication

DNA Replication

• You have learned that cells divide through the process of mitosis and meiosis

• In order to do this, each cell has to make a copy of its DNA

• DNA is copied through the process of DNA Replication

• What might happen DNA replication did not occur prior to cell division?

How DNA Replicates

• Remember: DNA is composed of two strands– A pairs with T– C pairs with G

• So if you know the order of bases on one side, you know the order on the other side (the complementary strand)

• During replication, each strand serves as a pattern

Fig. 16-9-1

A T

GC

T A

TA

G C

(a) Parent molecule

*What has to happen first in to make a copy of the DNA?

Fig. 16-9-2

A T

GC

T A

TA

G C

A T

GC

T A

TA

G C

(a) Parent molecule (b) Separation of strands

What type of molecule might help the two sides of the DNA molecule separate?

Fig. 16-9-3

A T

GC

T A

TA

G C

(a) Parent molecule

A T

GC

T A

TA

G C

(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand

(b) Separation of strands

A T

GC

T A

TA

G C

A T

GC

T A

TA

G C

What types of molecules might be used to add nucleotides and bind the sides together?

Enzymes involved in DNA Replication

• Helicase – unwinds the DNA strand to begin replication(it’s like unzipping a zipper)

• DNA Polymerase – adds nucleotides, one at a time to the open DNA strand (in humans up to 50 nucleotides per second)

• Ligase - joins the sugar-phosphate backbones of the newly formed strand.(it’s like gluing the sides together)

Steps of DNA Replication

• Step 1 – An enzyme breaks the H+ bonds between the nitrogen bases that holds the two strands together (un-zipping the molecule)

• Step 2 – Free floating nucleotides in the cell bond to the complementary bases on each of the original strands

• Step 3 – An enzyme secures the two strands together, forming two new chains

DNA Replication Cont.

• DNA replication results in the formation of two identical strands from the one original DNA molecule.

What do you think the word “semiconservative” means?

DNA Replication is Semiconservative

• Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand

• DNA replication animation

Animation 2

HHMI animation

A T

GC

T A

TA

G C

(a) Parent molecule

A T

GC

T A

TA

G C

(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand

(b) Separation of strands

A T

GC

T A

TA

G C

A T

GC

T A

TA

G C

Compare the two new strands of DNA. Are they the same or different? Why?

From DNA to Protein

From DNA to Protein

• The sequence of nucleotides in DNA contains information that produces proteins

• Proteins – Structures– Enzymes

• By controlling protein production, DNA controls cells

RNA

• Different from DNA in 3 ways– RNA – single strand– Sugar in RNA is ribose

(DNA = deoxyribose)– RNA has uricil (U)

instead of thymine

The cell works like a factory

• DNA provides “workers” with instructions for making proteins

• “workers: bring over the parts (amino acids) to the assembly line– Workers = RNA

3 Types of RNA

• Messenger RNA (mRNA) – brings the info from the nucleus to the factory floor

(cytoplasm)

• Ribosomal RNA (rRNA) – ribosomes are made of rRNA– Clamp onto mRNA and use its info to assemble

amino acids

• Transfer RNA (tRNA) – “supplier”– Transports amino acids to the ribosomes where they

are assembled into proteins

RNA Transcription

• Read steps in figure 11.6 (pg 296)

• Explain how it is different from DNA replication

• Animation of Transcription

• HHMI animation

The role of tRNA

• For proteins to be built, the 20 different amino acids dissolved in the cytoplasm must be brought to the ribosomes

• This is the role of tRNA

tRNA

• Composed of about 80 nucleotides

• Each tRNA only recognizes only one amino acid

• The amino acid bonds to the tRNA

• Located on the base of the tRNA molecule are three nitrogen bases, called an anticodon, that pair up with an mRNA codon during translation

tRNA

• Basically, the tRNA molecule transfers the information for making proteins to the correct codon on the mRNA.

• If the mRNA has the codon for that particular amino acid, the tRNA binds, if it does not, the tRNA doesn’t bind and the amino acid that the tRNA is carrying is not made.

Amino Acids to Proteins

• Proteins are made in the Ribosomes

• Proteins are made of Amino Acids

• As multiple tRNA molecules attach to the mRNA, an enzyme joins the two amino acids by forming a peptide bond.

Translation of DNA to Protein

• Translation Animation

• HHMI animation

The Genetic Code• A code is needed to

convert the language of mRNA into the language of proteins amino acids

• There are 20 different amino acid

• mRNA only has 4 bases (AUCG)

Ala: Alanine  Cys: Cysteine  Asp: Aspartic acid  Glu: Glutamic acid

Phe: Phenylalanine Gly: Glycine His: Histidine  Ile: Isoleucine 

Lys: Lysine Leu: Leucine  Met: Methionine Asn: Asparagine

Pro: Proline Gln: Glutamine Arg: Arginine Ser: Serine

Thr: Threonine Val: Valine Trp: Tryptophane Tyr: Tyrosisne

The Genetic Code Cont.• Scientist found that

a group of 3 nucleotides codes for 1 amino acid

• Each set of 3 nucleotides that code for an amino acid is called a codon

The Genetic Code Cont.

• Some codons don’t code for amino acids, they are instructions for assembling proteins– Stop codon = UAA – Start codon = AUG

Genetic Code Cont.

• All organisms use the same genetic code for assembling proteins– UAC = tyrosine in humans, birch trees, and

bacteria

Genetic Code Cont.

Try these:DNA base sequence

Process mRNA codon

Process tRNA anticodon

Amino Acid

AAT

GGG

ATA

AAA

GTT

Critical Thinking Questions

• How specific are the tRNA molecules?

• How does energy play a role in all this hustle and bustle?

• How does translation begin and end?

• What happens to the mRNA strands?

Compare and contrast Transcription and Translation

Transcription Translation

Where?

What is used as a template?

What is used to synthesize the new strand?

What is the new strand made of?

Compare and Contrast Replication and Protein Synthesis

Replication Protein Synthesis

Where?

When?

Purpose?

Starting point?

What enzyme is used to synthesize the new strand?

Associated proteins?

Nucleotides?

Finishing Processes?

Where does the finished “product” go?

Read the Help Wanted ad below. Based on your notes, tell me “who” is qualified to fill each position. Your choices are DNA, tRNA, and mRNA.

Help Wanted!• Positions Available in the genetics industry. Hundreds of entry-

level openings for tireless workers. No previous experience necessary. Must be able to transcribe code in a nuclear environment. The ability to work in close association with ribosomes is a must.

• Accuracy and Speed vital for this job in the field of translation. Applicants must demonstrate skills in transporting and positioning amino acids. Salary commensurate with experience.

• Executive Position available. Must be able to maintain genetic continuity through replication and control cellular activity by regulation of enzyme production. Limited number of openings. All benefits.

• Supervisor of production of proteins—all shifts. Must be able to follow exact directions from double-stranded template. Travel from nucleus to the cytoplasm is additional job benefit.

Central Dogma of Genetics

Genetic Changes

Mutations: Changes in DNA

• Mutation – any change in the DNA sequence that also changes the protein it codes for

• Mutations can happen in reproductive cells and in body cells (cancer)

Point Mutation

• A change in a single base pair in DNA• Look at this simple analogy

– THE DOG BIT THE CAT– THE DOG BIT THE CAR

Frameshift Mutation

• When a single base is added or deleted from a DNA strand

• It shifts the reading of the codons by one base

More about Mutations• Look at table 12.3 on pg 346. Gather some

information about the different types of mutations. Look at the examples of the diseases associated with each type of mutation

• Read pg 349.

-What are the results of mutations to body cells?

-What are the results of mutations to sex cells?-Why is a mutation in a sex cell considered potentially more harmful than one in a body cell?

Chromosomal Mutations

• Changes that occur at the level of the chromosomes

• Occurs when parts are broken off and lost during mitosis or meiosis

• Few chromosome mutations are passed on to the next generation because the zygote usually dies or is sterile

• Video Clip