The Molecular Basis of Inheritance - Brown Biology · DNA Replication • DNA Helicase- enzyme that...
Transcript of The Molecular Basis of Inheritance - Brown Biology · DNA Replication • DNA Helicase- enzyme that...
The Molecular Basis of Inheritance
Chapter 16
Griffith’s Experiment
• Transformation- a change in genotype and phenotype due to the assimilation of external DNA by a cell.
Hershey-Chase Experiments
• Bacteriophage (phage)- viruses that infect bacteria.
DNA
• DNA is a polymer of nucleotides – 5 Carbon Sugar: Deoxyribose – Phosphate Group – Nitrogen Base: Adenine, Thymine, Guanine,
Cytosine. • Chargaff’s Rules
– The base composition varies between species – Within a species, the number of A and T bases are
equal and the number of G and C bases are equal.
Rosalind Franklin
• X ray crystallography identified that DNA was a double helix structure.
DNA
• Covalent bonds link the units of each nucleotide.
• The two strands of DNA are held together by Hydrogen bonds between the base pairs.
• In Watson’s model of DNA, the sugar-phosphate backbones were antiparallel- with their subunits running in opposite directions.
DNA
Warm Up Exercise
• What type of bonds hold the DNA together? • Which bases are purines and which are
pyrimidines? • What happens in transformation?
DNA Replication
• Semiconservative Model- each DNA molecule is composed of one old strand and one new strand.
Messelson and Stahl
DNA Replication
• Origins of Replication- sites on the DNA where replication begins. – These areas have a specific sequence of
nucleotides. • Replication Fork- a Y-shaped region where the
parental strands of DNA is being unwound and new DNA is being synthesized.
DNA Replication
• DNA Helicase- enzyme that unzips the double helix at the replication fork, separating the two strands so replication can occur.
• Single Strand Binding Proteins- bind to unwound DNA to keep it from re-pairing.
• Topoisomerase- helps relieve strain on DNA by breaking, swiveling, and rejoining DNA strands.
• DNA Polymerase- helps synthesize new DNA by adding nucleotides to a preexisting chain. – DNA Pol III- adds DNA nucleotide to RNA primer and
continues adding nucleotides complementary to original DNA template strand.
DNA Replication
• Primer- short segment of RNA that indicates the start of a new DNA strand. (Synthesized by the enzyme primase).
DNA Replication
• Nucleotides added to a growing DNA strand come from a nucleoside triphosphate.
DNA Replication
• DNA polymerase can add nucleotides only to the 3’ end of a growing DNA strand. Thus the new strand is said to grow in the 5’ to 3’ direction.
DNA Replication
• Leading Strand- strand of new complementary DNA created as the replication fork progresses. – Only one primer required. – Synthesized in one long continuous segment.
• Lagging Strand- strand of new complementary DNA created in the direction away from the replication fork. – Primer required at the start of each fragment. – Synthesized discontinuously in a series of short
segments called Okazaki Fragments.
DNA Replication
• DNA Pol I- replaces RNA nucleotides of the primer with DNA nucleotides- moving from 5’ to 3’.
• DNA Ligase- seals Okazaki fragments and newly synthesized DNA into one continuous DNA strand. (joins sugar phosphate backbone)
Warm Up Exercise
• Briefly state the function of the following enzymes in your own words: – Ligase – Polymerase I – Polymerase III – Helicase – Topoisomerase – Primase
Proofreading and Repairing DNA
• During replication, DNA Polymerases proofread each nucleotide against its template as it is added to the growing strand.
• Mismatch Repair- enzymes remove and replace incorrectly paired nucleotides that have resulted from replication errors. (This is to catch errors that have been overlooked by DNA polymerase)
Proofreading and Repairing DNA
• Nucleotide Excision Repair- damaged segment of DNA is cut out, and gap is filled by DNA Pol I and DNA Ligase. – Nuclease- DNA cutting
enzyme. Removes damaged DNA.
Replicating the ends of DNA Molecules
• Telomeres- special nucleotide sequences found at the ends of a DNA molecule. – Do not contain
genes.
Replicating the ends of DNA Molecules
• Telomerase- catalyze the lengthening of telomeres in eukaryotic germ cells.
Chromatin Packing in a Chromosome
• Chromatin- complex of DNA and proteins found in the cell. – Exists as a
diffuse mass prior to cell division, but condenses to form chromosomes during division.
Chromatin Packing in a Chromosome
• Histones- proteins that help package the chromatin into chromosomes.
• Nucleosome- histones joined together in a beadlike structure.