Chapter Eight: From DNA to Proteins

Section One: Identifying DNA as the Genetic Material Discovery of DNA Oswald Avery said DNA must be the genetic material of the cell Alfred Hershey and Marsha Chase provided definitive proof through studies of bacteriophages Bacteriophage: a virus that takes over a bacterium’s genetic machinery and directs it to make more virus

Section Two: Structure of DNA DNA Structure Made of four nucleotides Nucleotides: small units that make up DNA Each nucleotide has 3 parts Phosphate group A ring shaped sugar called deoxyribose Nitrogen containing base

Section Two: Structure of DNA Nucleotides There are four types of nucleotides that only differ in what base they possess. Pyrimidines Cytosine Thymine Purines Adenine Guanine

Section Two: Structure of DNA DNA Shape Watson and Crick said DNA was shaped like a double helix Double helix: two strands of DNA wind around each other like a twisted ladder Rosalind Franklin used x-rays to visualize DNA

Section Two: Structure of DNA DNA Structure DNA nucleotides are joined together in the center of the helix by base pairing Base Pairing Rules Thymine pairs with adenine Guanine pairs with cytosine Sugars and phosphates form the back bone

Section Three: DNA Replication The process by which DNA is copied during the cell cycle Carried out by proteins DNA polymerases: a protein that bonds nucleotides together during DNA replication

Section Three: DNA Replication DNA Replication Steps Enzymes unzip the DNA double helix at the origin of replication Free floating nucleotides pair, one by one, with the bases on the template strand, which forms a new complementary strand

Section Three: DNA Replication

Section Three: DNA Replication DNA Replication Stages 3. Two identical DNA molecules are created. Each molecule has one strand from the original molecule and one new strand. This is why DNA replication is semiconservative.

Section Three: DNA Replication Fast and Accurate Many origins of replication allow many replications to happen at once DNA has a built in “proofreading” function so it can correct any copying errors

Section Four: Transcription Central Dogma of DNA DNA replication, then transcription, then translation Information flows from DNA to RNA to proteins RNA: a single stranded chain of nucleotides that are is used to make proteins

Section Four: Transcription The process of copying a sequence of DNA to produce a complementary strand of RNA A gene is translated into RNA RNA polymerases: enzymes bond together nucleotides in a chain to make a new strand of RNA

Section Four: Transcription Transcription Steps 1. RNA polymerase recognizes the transcription start site of the gene. A transcription complex assembles on the DNA strand and begins to unwind it.

Section Four: Transcription Transcription Steps 2.RNA polymerase make a complementary strand of RNA using the unwound DNA. Uracil instead of thymine binds with adenine. The DNA helix zips back together when the copying is done.

Section Four: Transcription Transcription Steps 3. Once the entire gene has been copied into RNA, the RNA strand detaches from the DNA.

Section Four: Transcription Involves 3 major types of RNA Messenger RNA (mRNA): an intermediate message that is translated to form protein Ribosomal RNA (rRNA): forms part of the ribosome, a cell’s protein factory Transfer RNA (tRNA): brings amino acids form the cytoplasm to a ribosome to help make a growing protein

Section Four: Transcription Transcription vs. DNA Replication Similar Both involve unwinding DNA Both involve complementary base pairing Both are highly regulated by the cell Different Transcription produces RNA Replication produces DNA Replication makes sure each cell has one copy of DNA Replication occurs only once during the cycle, transcription occurs hundreds or thousands of times

Section Five: Translation The process that converts an mRNA message into a polypeptide One or more polypeptides make up proteins The mRNA message is written using one of four nucleotides of RNA (cytosine, guanine, adenine, and uracil) The polypeptides or proteins are made of amino acids There are 20 amino acids

Section Five: Translation Amino acids are recognized by there codons Codon: a sequence of 3 nucleotides that code for an amino acid Many amino acids are coded for by more than one codon

Section Five: Translation

Section Five: Translation There are also stop and start codons that signal the end and the beginning of translation Stop Codons: signal the end of the amino acid chain UAA, UGA, UAG Start Codons: signals the start of translation and the amino acid methionine AUG

Section Five: Translation For the mRNA code to be translated correctly, the codons must be read the correct way The order in which they are read is called a reading frame 3 amino acids= 1 codon There are no spaces between codons

Section Five: Translation Translation Steps Occurs in the ribosome tRNA has an anticodon that matches the codon that allows it to pick up the correct amino acids Anticodon: a set of 3 nucleotides that is complementary to an mRNA codon For example: If the codon was CCC, the anticodon would be GGG

Section Five: Translation Translation Steps 1. The exposed codon of the mRNA in the ribosome attracts a tRNA with the correct anticodon and amino acid.

Section Five: Translation Translation Steps 2. The ribosome helps create a peptide bond between the two amino acids (the starting amino acid and the new one just added 3. The ribosome breaks the bond between the tRNA and the amino acid

Section Five: Translation Translation Steps 4. The ribosome pulls the mRNA strand down one codon so that the tRNA is moved into the exit site and leaves the ribosome. 5. The first site is empty again so that a new amino acid can be added.

Section Six: Gene Expression and Regulation Regulation of Gene Expression in Prokaryotes Promoter: a DNA segment that allows a gene to start transcription Operon: a region of DNA that includes the promoter, an operator, and one or more structural genes that code for a specific protein Usually found only in prokaryotes or worms

Section Six: Gene Expression and Regulation Regulation of Gene Expression in Eukaryotes Transcription factors and regulatory DNA sequences help start transcription RNA processing, the part that occurs after transcription, includes cutting out introns and putting together exons Introns: nucleotide segments that occur between exons and do not code for proteins Exons: nucleotide segments that code for part of a protein

Section Six: Gene Expression and Regulation

Section Seven: Mutations A change in an organism’s DNA sequence Mutations can be good or bad Types of gene mutations Point mutation: a mutation in which one nucleotide is substituted for another Frameshift mutation: the insertion or deletion of a nucleotide to the DNA that changes the reading frame

Section Seven: Mutations

Section Seven: Mutations Chromosomal Mutations When chromosomes do not correctly align with each other during crossing over, the segments that result may be of a different size. This could cause one chromosome to not have a particular gene and one chromosome to have two copies of one gene

Section Seven: Mutations Chromosomal Mutations Translocation: a mutation in which a piece of one chromosome moves to a non-homologous chromosome

Section Seven: Mutations Mutations and Phenotype Mutations may or may not affect phenotype Chromosomal mutations have a large effect on genes and could cause a gene to no longer work or create a new hybrid gene with a new function Gene mutations may effect enzyme functions, protein folding (which could inactivate a protein), or code for a premature stop codon

Section Seven: Mutations Mutations and Offspring Mutations in sex cells can affect offspring They are the underlying source of genetic variation Some can cause offspring to develop improperly or die before birth

Section Seven: Mutations What causes mutations? Replication errors Mutagens: agents in the environment that can change DNA Ex: UV rays, some industrial chemicals, cigarette smoke