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.