Molecular Genetics

Chapter 12Honors Biology

Mrs. Stewart, Rm 806

Experiments Showed that DNA is the Genetic Material

• In 1928, Frederick Griffith reported studies on a species of bacterium. He studied two varieties of a bacterium, a pathogenic strand and a variant, that was harmless

• A transformation occurred which means that one strain of bacteria was transformed into another one

Griffith’s Experiment

Transformation

Oswald Avery, Colin MacLeod, Maclyn McCarty

Avery, McCarty, and Macleod• Repeated Griffith’s experiment and

used the heat-killed bacteria and made a juice or extract from it

• Added enzymes to the juice that would break down lipids, proteins, and carbohydrates, and RNA

• Transformation occurred in all of these except when tested using DNA

Avery, McCarty, and MacLeod’s Experiment

Bacteriophage• Is a virus that infects a bacterium• Stages of infection

– Attachment– Entry or injection– Replication– Assembly– Lysing of the cell

Lytic Cycle of Viral Infection

Alfred Hershey and Martha Chase

• Were interested in knowing which part of the virus infected the bacterium

• They labeled two batches of viruses with radioactive sulfur-35 and phosphorus-32

• The protein was labeled with S-35 and the DNA core with P-32

• The viruses’ DNA entered the bacteria, and the protein coat remained outside the bacteria

Hershey and Chase’s Experiment

Base Pairing Charts• DNA to DNA

A-TT-AC-GG-C

DNA to RNAA-UT-AC-GG-C

• RNA to RNAA-U U-AC-G G-C

DNA Replication• The process by which DNA duplicates itself• Unzipping occurs when the hydrogen

bonds between the base pairs are broken and the two strands unwind. Each of the separated strands serves as a template for the attachment of complementary bases

• DNA helicase unzips the original strand• DNA polymerase base pairs free

nucleotides to the original strand• DNA ligase ties the strand back together

Replication

Transcription• RNA Polymerase attaches to special

places on the DNA molecule, separates the two strands, and synthesizes a mRNA

• mRNA is complementary to one of the DNA strands

• The base pairing mechanism ensures that mRNA will be a complementary copy of the DNA strand that serves as its template

• RNA Polymerase unwinds and unzips DNA

• Complementary NTP’s (nucleoside triphosphates) add to template

DNA strand from 5’ to 3’• RNA Polymerase begins

transcribing the DNA at a specific point

• RNA strand is identical to the non-coded DNA (and complementary

to the template strand)

Transcription

Translation (Protein Synthesis)• The two subunits of the ribosome bind to a

molecule of mRNA• The initiator codon, AUG, binds to the first

anticodon of tRNA, signaling the beginning of a polypeptide chain

• Soon the anticodon of another tRNA binds to the next mRNA codon

• This tRNA carries the 2nd amino acid that will be placed into the chain of the polypeptide

Translation Continued• A peptide bond (covalent bond)

forms between two amino acids• This polypeptide chain continues to

grow until the ribosome reaches a stop codon of mRNA

• Once here, the new polypeptide and mRNA are released from the ribosome

So proteins are determined by the chain of amino acids that make them up

But how do our protein makers know which amino acids to add?