pruitt ppt ch06
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Molecular BiologyMolecular BiologyWhat Is DNA and How Does It
Work?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
Transcription
Translation
• To line up the appropriate amino acids in the proper order requires:– mRNA– tRNA– Ribosomes
Translation
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.
Genetic Code
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