chapter 10. biologists knew that genes were located on chromosomes 2 components of chromosomes:...
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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
Is DNA the Hereditary Material?
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
Is DNA the Hereditary Material?
“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
http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html
Replication Process
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
Flow of Genetic Info
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
Flow of Genetic Info
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
Flow of Genetic Info
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
Flow of Genetic Info
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
Flow of Genetic Info
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
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
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
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
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
Steps of Translation
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
Steps of Translation