Chapter 12 Molecular Genetics 12.1 DNA: The Genetic Material
Discovery of the Genetic Material • Chromosomes are about 50% nucleic acid and 50% protein, which is the genetic material? • Most scientists thought that protein was the genetic material because protein is more complex • Griffith performed the first major experiment that led to the discovery of DNA as the genetic material
Discovery of the Genetic Material • Significance of Griffith’s work (1928) • One strain of bacteria transformed into another strain • Did not identify what the transforming substance was
Discovery of the Genetic Material • 1944 Oswald Avery identified that DNA was the transforming substance in Griffith’s experiments • Most leading scientists did not believe him
1952 Hershey and Chase used radioactively labeled DNA and radioactively labeled protein and proved that DNA is the genetic material Discovery of the Genetic Material
DNA is made of subunits called nucleotides Three parts to a DNA nucleotide Sugar Phosphate Nitrogen Base DNA Structure NUCLEOTIDE
Four Different Nitrogen Bases Purine (two rings) Adenine Guanine Pyrimidine (one ring) Cytosine Thymine Uracil (not found in DNA) DNA Structure
Chargaff determined in 1950 that the amount of adenine equals the amount of thymine and the amount of guanine equals the amount of cytosine Chargaff’s Rule: A=T and C=G DNA Structure Chargaff’s Rule
Rosalind Franklin’s (1951) famous photo of X ray diffraction of DNA DNA Structure: X Ray Diffraction
IN 1953 Watson and Crick astounded the scientific community with their announcement of DNA’s structure DNA Structure: Double Helix
Watson and Crick, using Franklin’s photo, determined that DNA is a double helix with: Outside strands of alternating sugar and phosphate C bonds with G with three hydrogen bonds A bonds with T with two hydrogen bonds DNA Structure: Double Helix
DNA called a twisted ladder Sugar is deoxyribose in the upright rails of the “ladder” alternating with phosphate (spacers) Rungs of the “ladder” have a purine base H-bonded to a pyrimidine base DNA Structure: Double Helix
DNA strands are antiparellel (one strand right side up and other stand upside down) Stands named by their Carbon orientation, C-5 (5’) or C-3 (3’) DNA Structure: Double Helix
An average sized chromosome would be 5 cm long if the DNA were stretched out DNA is packaged to be condensed in the cell’s nuclues Chromosome Structure
Chapter 12 Molecular Genetics 12.2 Replication of DNA
DNA original strand untwists New base pairs bond to open existing strands following base paring rules (A=T, C=G) New strands twist; each new helix is half new half original Semiconservative Replication
Untwisting by DNA helicase Strands kept apart by single-stranded binding proteins Add “starter” RNA segment by RNA primase Add new nucleotides by DNA polymerase This is only the highlights; there are many other enzymes involved Enzymes Control DNA Replication
DNA Replication • Because DNA is antiparallel and new nucleotides can only be added to the 3’ end, each strand replicates slightly differently
DNA Replication • Leading strand replicates by continuous addition of nucleotides to the 3’ end • Lagging strand replicates by producing short DNA sections called Okazaki fragments • Enzyme ligase “glues” the fragments together
Eukaryotes have multiple areas of DNA replication along one chromosome Prokaryotes have one circular chromosome and have only one origin of replication Comparing DNA Replication in Eukaryotes and Prokaryotes
Chapter 12 Molecular Genetics 12.3 DNA, RNA, and Protein
How does the information in DNA, located in the nucleus, allow for the production proteins in the cytoplasm? RNA is another form of nucleic acid that relays the information. Central Dogma
RNA Single helix Ribose sugar Bases: adenine, guanine, cytosine, and uracil Several types of RNA DNA Double helix Deoxyribose sugar Bases: adenine, guanine, cytosine, and thymine One type of DNA RNA versus DNA
Types of RNA • Messenger RNA (mRNA): long strands (hundreds of nucleotides) that are formed complementary to DNA; leave the nucleus to carry information to the cytoplasm • Transfer RNA (tRNA): short (80-100 nucleotides) T-shaped RNA that transport amino acids • Ribosomal RNA (rRNA): along with protein make up the ribosomes
DNA to RNA to Protein • Two step process: transcription and translation • Transcription (rewrite): RNA is made from DNA; occurs in the nucleus • Translation (change language): protein is made from RNA code; occurs in the cytoplasm at the ribosome
A section DNA (ave. size 8000 nucleotides) in the nucleus untwists and unzips. RNA nucleotides, following base pairing rules, bond on the leading strand of DNA Like DNA replication controlled by many enzymes Transcription Occurs in the nucleus
RNA when it is transcribed must be processed GTP cap is added to 5’ end to protect and give attach signal to ribosome Introns (intervening sequences) are cut out Exons (expressed sequences) are put together Poly-A tail (30-200 A nucleotides) added to 3’ end to protect and “get out of nucleus” signal RNA Processing
Starts when mRNA, tRNA carrying amino acids, and small and large ribosomal subunits come together Concludes when a polypeptide chain in produced Translation: Making Protein
The Code • There are 20 amino acids, each is coded for by a sequence of 3 nucleotides called a codon. • Discovered during the 1960’s.
mRNA has the codon tRNA has the anticodon (complementary to the codon) Example: mRNA codon AUG would code for the amino acid methionine which is also the start codon Redundancy exists: more that one codon per amino acid (UAU and UAC codes for tyrosine) Ambiguity does not exist: UAU only codes for tyrosine not any other amino acid. The Code mRNA Genetic Code
Translation • All the players come together • First tRNA with anticodon UAC carrying methionine bonds with mRNA codon AUG at the P-site of the ribosome • Second tRNA with anticodon carrying another amino acid bonds with complementary mRNA codon at A-site of ribosome • Polypeptide bond forms between two amino acids • Ribosome moves down the mRNA so that the first tRNA is now in E-site of ribosome (and is released) • A-site is now empty to attach the third tRNA carrying the third amino acid • Steps 4-7 repeated until mRNA codon for stop is signaled, then polypeptide chain released
The Beadle and Tatum experiment showed that one gene codes for one enzyme. We now know that one gene codes for one polypeptide. One Gene-One Enzyme
Chapter 12 Molecular Genetics 12.4 Gene Regulation and Mutation
Prokaryote Gene Regulation • Ability of an organism to control which genes are transcribed in response to the environment • An operon is a section of DNA that contains the genes for the proteins needed for a specific metabolic pathway.
Try Operon What would an off Try operon look like?
Eukaryote Gene Regulation • Controlling transcription: transcription factors ensure that a gene is used at the right time and that proteins are made in the right amounts • Promoters: stabilize binding of RNA polymerase • Regulatory proteins: control rate of transcription • The complex structure of eukaryotic DNA also regulates transcription.
Hox genes are responsible for the general body pattern of most animals. Hox genes code for transcription factors that are active in zones of the embryo that are in the same order as the genes on the chromosome Eukaryote Gene Regulation
Eukaryote Gene Regulation • RNA interference can stop the mRNA from translating its message.
Mutations • Mistakes occur in copying the DNA during replication. • Mechanisms exist for correcting these mistakes • If the mistakes are permanent then a mutation occurs • If a mutation in the DNA occurs, then the protein that is made from this DNA instruction can be absent or nonfunctional.
Pieces of chromosomes get deleted, duplicated, inverted, inserted or translocated Visible on karyotype Chromosomal Mutations
Fragile X chromosome is due about 30 extra repeated CGG codons near the tip of the X chromosome Results in many mental and behavioral symptoms Chromosomal Mutations