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

DNA is the genetic material

• Early in the 20th century, 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

• The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 who worked with two strains of a bacterium, one pathogenic and one harmless. He did transformation experiments, now defined as a change in genotype and phenotype due to assimilation of foreign DNA

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

Additional Evidence That DNA Is the Genetic Material

• 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

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

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

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

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

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.

• DNA replication animation

Animation 2

HHMI animation

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

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

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.

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

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