Chapter 10. Biologists knew that genes were located on chromosomes 2 components of chromosomes: ◦ Protein & DNA Proteins were in the lead ◦ Appeared.

Molecular Biology of the Gene

Chapter 10

Biologists knew that genes were located on chromosomes

2 components of chromosomes:◦ Protein & DNA

Proteins were in the lead◦ Appeared more structurally complex &

functionally specific

Is DNA the Hereditary Material?

Griffith (1928)◦ 2 Strains of Bacteria

R strain - harmless S strain – disease-causing

◦ Procedure of Experiment Heat-killed S strain Mixed with R strain Injected mice with mixture & some of the harmless

strain transformed to pathogenic



What materials were used & what was the set-up before the trials of the Hershey-Chase Experiment?◦ Materials: chem. containing radioactive isotopes,

radioactivity detector, blender, centrifuge◦ Procedure: radioactive isotopes to label DNA &

protein in T2 Grew T2 w/ E. coli in a solution containing radioactive

sulfur (protein contains sulfur, but DNA does not) - yellow New phages were made, radioactive sulfur atoms were

incorporated only into their proteins Grew separate batch in a solution containing radioactive

phosphorus (phosphorus is only in DNA) - green


DNA Structure

DNA Structure

RNA Structure

Double Helix

“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

Semiconservative model – each of the two daughter molecules will have one old strand and one new strand

DNA Replication

DNA Replication

DNA Replication

DNA Replication

http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html

Replication Process




Flow of Genetic Info

Discovery: Archibald Garrod (1909)◦ Genes dictate phenotypes through enzymes◦ Inherited disease reflects a person’s inability to

make a particular enzyme◦ Remember:

Each step in a metabolic pathway is catalyzed by a specific enzyme


Tatum & Beadle (1940s)◦ One gene-one enzyme hypothesis:

Function of a gene is to dictate the production of a specific enzyme

◦ Modifications: All types of proteins (not just enzymes)

Ex: keratin & insulin One gene-one protein

Many proteins are made from two or more polypeptide chains Ex: hemoglobin – two kinds of polypeptides, encoded by

2 different genes One gene-one polypeptide hypothesis


Cells governed by a molecular chain of command◦ DNA RNA Protein

Chemical language of DNA/RNA◦ Polymers of nucleotide monomers

A, T(U), C, G Written as a linear sequence of bases Specific sequence of bases = a gene

Consists of hundreds/thousands of bases in a sequence



DNA/RNA nucleic acid language is translated into Protein polypeptide language

Polypeptide language◦ Polymers made up of monomers of amino acids◦ Written in a linear sequence◦ Sequence of RNA dictates sequence of amino

acids◦ RNA is the messenger carrying genetic info from

DNA


DNA & RNA consist of 4 bases◦ In translation, these 4 must specify 20 amino

acids◦ Triplett code: Genetic instructions for the amino

acid sequence of a polypeptide chain are written in DNA & RNA as a series of 3-base words, called codons

◦ 3-base codons in DNA are transcribed into 3-base codons in RNA, and then the RNA codons are translated into amino acids that form the polypeptide


Set of rules giving the correspondence between codons in RNA and amino acids in proteins◦ 61 of 64 codons for amino acids◦ AUG – Methionine or start of polypeptide

sequence◦ 3 other codons do not designate amino acids, but

they stop the translation Redundancy but no ambiguity Nearly universal, shared by organisms from

bacteria to plants and animals

Genetic Code

Genetic Code

Genetic Code

Transfer of genetic info from DNA to RNA Occurs in nucleus (Eukaryotes) RNA is transcribed from DNA template

◦ RNA nucleotides follow same base-pairing rules that govern DNA replication

◦ U pairs with A, instead of T with A◦ RNA polymerase – link RNA nucleotides◦ Promoter – nucleotide sequence that says “start

transcribing” Binding site for RNA polymerase

Transcription

Conveys genetic info from DNA to translation machinery

Transcribed from DNA – Translated into polypeptides

Modified before leaving nucleus◦ Add nucleotides

Cap (G nucleotide) & tail (50-250 A’s)◦ RNA splicing

Remove introns and join exons Produce mRNA molecule that is continuous

mRNA

mRNA

Translates codons into amino acids Amino acids can not recognize mRNA

codons Up to the tRNA to match amino acids to the

appropriate codons 2 Functions must be met:

◦ Picking up the appropriate amino acids◦ Recognizing appropriate mRNA codons

tRNA

Structure◦ Single strand of RNA – 80 nucleotides◦ Twists & folds itself◦ Anticodon: single-stranded loop at 1 end that

contains a special triplet of bases Complementary to codon triplet

◦ Opposite end of anticodon contains the amino acid

◦ tRNA molecules a slightly different for each amino acid it specifies

Specific enzyme that joins amino acids to tRNA

tRNA

tRNA

Make up the large and small subunits of a ribosome

Ribosome is the site where translation occurs◦ tRNA & mRNA bind here

Prokaryotic & Eukaryotic Differences◦ Medically significant

Antibiotic drugs can inactivate prokaryotic ribosomes while leaving eukaryotic ribosomes unaffected

rRNA

rRNA

Initiation, Elongation, & Termination

Initiation◦ mRNA, tRNA w/ 1st amino acid, 2 subunits of ribosome◦ Role: establish exactly where translation begins – get

correct sequence of amino acids◦ 2 Steps:

1. mRNA molecule binds to small subunit, and tRNA (UAC) binds to start codon (AUG) – carries methionine

2. Large subunit binds to small; initiator tRNA fits into large subunit – P site – holds growing peptide; A site ready for next tRNA

Steps of Translation

Elongation1. Codon recognition: anticodon pairs w/ codon at

site A2. Peptide bond formation: polypeptide separates

from tRNA on P site & attaches by a peptide bond to the amino acid carried by the tRNA on the A site

3. Translocation: P site tRNA leaves, the ribosome translocates the tRNA in the A site to the P site

- Codon & anticodon remain bonded- Brings next codon & anticodon to the A site


Termination◦ When stop codon reaches the A site (UAA, UAG,

UGA) Don’t code for an amino acid Stop translation

◦ Polypeptide is released from last tRNA & exits ribosome

◦ Ribosome subunits separate



Chapter 10. Biologists knew that genes were located on chromosomes 2 components of chromosomes: ◦ Protein & DNA Proteins were in the lead ◦ Appeared.

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