Molecular Biology of the Gene

Chapter 10Honors 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

Watson & Crick

• Awarded the Nobel Peace Prize for discovering the structure of the DNA molecule

• Used Rosalind Franklin’s picture of an x-ray diffraction to determine the structure of the DNA molecule

• Her partner was Maurice Wilkins

James Watson & Francis Crick

Chargaff’s Rule

• Purines pair with pyrimidines

• Adenine and Guanine are purines

• Cytosine and Thymine are pyrimidines

• The purine bases have two carbon rings

• The pyrimidine bases have one carbon ring

DNA & RNA Structure

Types of RNA

• Messenger RNA (mRNA)-carries genetic information from DNA (copies info from DNA) out of the nucleus into the cytoplasm; 3 letter code is called a codon

• Transfer RNA (tRNA)-picks up an amino acid and brings it to the ribosome; 3 letter code is called an anticodon

• Ribosomal RNA (rRNA)-helps makes ribosomes(site of protein synthesis)

NucleotideBuilding Blocks of

Nucleic Acids• A Nucleotide is

composed of a phosphate group, a 5 carbon sugar and a nitrogenous base

• The sides or backbone of the DNA & RNA molecule consists of alternating phosphates and sugars

• The rungs or steps of the ladder contain the bases

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

DNA 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

Transcription

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

Translation

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?

Replication, Transcription, Translation

Mutations

• Any change that occurs in the genetic sequence (nitrogenous bases of DNA0

• May occur spontaneously• May result from overexposure from

X-rays, gamma rays or ultra-violet rays

• May result from pollution in the environment or hormones, preservatives in food

Types of Mutations• Chromosomal Mutations

• Affect the chromosome• Deletion-some of a gene is

removed • Duplication-part of a

chromosome is repeated• Translocation- part of one

chromosome is broken off & attached to another chromosome

• Inversion-when a fragment of the chromosome reattaches in a reversed direction

• Gene Mutations

• Point mutations-occur at a single point in the DNA sequence; one nucleotide and can change an amino acid sequence

• Frameshift Mutation– insertion/substitution of

a nucleotide that shifts the entire amino acid sequence and creates a new protein

Mutations