Molecular Biology What Is DNA and How Does It Work?
DNA Structure Must Be Compatible with Its Four Roles • DNA makes copies of itself. • Occurs during S phase of the cell cycle before mitosis or meiosis. • DNA encloses information. • Information that gives rise to discernible traits in organisms.
DNA Structure Must Be Compatible with Its Four Roles • DNA controls cells and tells them what to do. • Determines function of the cell. • DNA changes by mutation. • Structure must be able to change.
Building Blocks of DNA • Nucleotides • Three components: • Five-carbon sugar • Phosphate group • Nitrogen-containing base
Building Blocks of DNA • Four nitrogenous bases in DNA • Adenine • Thymine • Guanine • Cytosine
Structure of DNA • Maurice Wilkins and Rosalind Franklin • Attempted to determine structure of DNA. • Discovered DNA was a helix.
Chargaff’s Ratios • 1950 • Erwin Chargaff • Observed that the four nitrogenous bases conformed to a rule: • Amount of Adenine = Amount of Thymine • Amount of Cytosine = Amount of Guanine • Served as a clue to help Watson and Crick determine DNA structure!
Watson and Crick • Early 1950s • They were young scientists at Cavendish Laboratory in Cambridge, England. • Using Chargaff’s ratios and Franklin’s data, Watson and Crick determine DNA structure is a double helix
DNA Double Helix • Consists of two strands of nucleotides. • Nucleotides bonded together with covalent bonds. • Adenine hydrogen bonds with Thymine. • Cytosine hydrogen bonds with Guanine. • Structure was compatible with four roles of DNA
How Does DNA Copy Itself? • DNA replication • Precedes cell division. • Process: • DNA strands separate • New complementary base pairs are added forming a new strand • Result: two double helices. • Each containing one old strand of DNA and one new strand of DNA
Meselson and Stahl • Proved the mechanism of DNA replication. • Called semiconservative mechanism. • Grew bacteria in medium containing various radioactive nitrogen isotopes. • Separated DNA by density using a dense, viscous sugar solution.
How is the information in DNA expressed? • Genome • Information to make proteins stored in all of the DNA of a single set of chromosomes. • Gene: blueprint for the synthesis of a protein. • Proteins • Polymers made of amino acids connected end-to-end • Similar to beads on a string.
How is the information in DNA expressed? • Chromosomes containing DNA contained in nucleus. • DNA codes for the construction of proteins using an intermediary molecule: • Ribonucleic acid or RNA. • Decoding information in DNA requires two processes: • Transcription. • Translation.
DNA vs. RNA • RNA: • Contains the sugar ribose. • Contains adenine, uracil, cytosine and guanine. • Single helix • DNA: • Contains deoxyribose. • Contains adenine, thymine, cytosine and guanine. • double helix.
DNA vs. RNA • RNA: • Smaller, mobile. • Degrades easily. • Travels form nucleus to cytoplasm. • DNA: • Larger, immobile. • Lasts the life of cell. • Resides in nucleus.
Types of RNA • Messenger RNA • Carries genetic information from DNA in nucleus to cytoplasm. • Information is used to synthesize a protein. • Codon: three nucleotide sequence that codes for one amino acid.
Types of RNA • Transfer RNA • Functions as the “interpreter” • Transfer amino acids to the sites where the information in the mRNA is being used to make a protein • Anticodon: three nucleotide sequence that is complementary to a particular codon in mRNA
Types of RNA • Ribosomal RNA • Combine with proteins to form ribosomes • Ribosomes • Site of translation • Large subunit • Small subunit
Protein Synthesis • Two processes: • Transcription • Occurs in the nucleus • Produces RNA • Translation • Occurs in the cytoplasm • Produces proteins
Translation • To line up the appropriate amino acids in the proper order requires: • mRNA • tRNA • Ribosomes
Translation • Codon (mRNA) must be complementary to the anticodon (tRNA). • Translation continues until ribosome encounters a stop codon.
Genetic Code • Three nucleotides in mRNA (codon) code for one amino acid. • Some sequences serve as starting points. • AUG codes for the amino acid methionine which also indicates to start translation. • Some sequences do not have complementary tRNA. • Indicate to the ribosome to stop translation.
What Makes Cells Different From Each Other? • Due to the information in the DNA, a cell could manufacture 50,000 different proteins, but it doesn’t. • The proteins a cell produces influences its function. • Example: red blood cells and hemoglobin
Gene Expression • Some genes are always transcribed and translated. • Others can be turned on or off by environmental signals • Gene expression is highly regulated.
Gene Expression in Prokaryotes • Jacob and Monod • Studied digestion of lactose in bacteria. • Discovered the lac operon. • Prokaryotes regulate gene expression at the level of transcription
Gene Expression in Eukaryotes • Regulated at the level of transcription. • Transcription requires transcription factors. • They recognize and bind to DNA sequences called regulatory sequences • Transcription factors can increase or decrease the rate of transcription • Longevity of RNA molecule also influences gene expression.
How Does DNA Change Over Time? • Mutations: a permanent change in the genetic material of a cell or organism. • Can be inherited. • Can involve whole chromosomes or changes in DNA sequences.
Whole Chromosome Mutations • Polyploid: organism or cell containing three or more sets of chromosomes. • Occurs due to a cell division error. • Frequently seen in plants, rare in animals. • Can have advantageous results.
Whole Chromosome Mutations • Nondisjunction: instances when paired chromosomes fail to separate during mitosis or meiosis • Can result in an aneuploid: individual whose chromosome number is greater or less than normal
Whole Chromosome Mutations • Down’s Syndrome • Due to nondisjunction with chromosome 21. • Characterized by mental retardation, distinctive facial features.
Whole Chromosome Mutations • Transposons: • Variety of DNA sequences that can randomly insert themselves by transposition in various non-homologous regions on chromosomes and other DNA. • Can generate new gene combinations • Can also induce genetic errors
Mutations Involving Single DNA Nucleotides • Point Mutations: • Change in a single nucleotide base pair. • Example: sickle cell anemia.
Mutations Involving Single DNA Nucleotides • Frame-shift mutation: • A change in the reading frame resulting from an insertion or deletion of nucleotides in the DNA sequence for a protein. • Extremely harmful. Normal: JOE ATE THE HOT DOG After deletion: JEA THE OTD OG