Chapter 14: You should know: Mendel’s two laws of inheritance –The law of segregation –The law of independent assortment You should know how to do mono-hybrid and di-hybrid crosses and the associated vocabulary Understand types of interaction that are more complicated than those witnessed by Mendel –Co-dominance, incomplete dominance, multiple alleles, complex traits

Chapter 15: You should know: The relationship between Mendelian inheritance and chromosomes –As indicated by Morgan’s work Patterns of inheritance with linked genes –Sex-linked –Linked traits

The molecular basis of inheritance

Our plan: The importance of DNA The evolution of our knowledge about DNA DNA replication DNA repair

The importance of DNA DNA is the information storage molecule of life The continuity of life is based on DNA Since Watson and Crick published their model of DNA structure in 1953, it has become the most celebrated molecule of our time

The search for the genetic material Since Mendel’s work was published (1850’s) biologists have generated a lot of knowledge about DNA

The search for the genetic material Morgan’s contributions (~1910) Morgan’s group noticed that genes are located on chromosomes

The search for the genetic material Morgan’s contributions (~1910) Morgan’s group noticed that genes are located on chromosomes Strands of DNA coiled around proteins

The search for the genetic material Morgan’s contributions (~1910) Morgan’s group noticed that genes are located on chromosomes Strands of DNA coiled around proteins DNA and protein became the two candidates for storing genetic material –Proteins had more support initially due to their complexity –Nucleic acids seemed too uniform

The search for the genetic material Hershey and Chase (1952) Used a bacteriophage (virus that infects bacteria) to study DNA Bacterial cell Phage head Tail sheath DNA

The search for the genetic material Hershey and Chase (1952) Used a bacteriophage (virus that infects bacteria) to study DNA Bacterial cell Phage head Tail sheath DNA Viruses contain DNA in a protein coat They insert their DNA into host cells to replicate

The search for the genetic material Hershey and Chase (1952) Grew phages in media labeled with radioactive isotopes of phosphorous or sulfur P was incorporated into viral DNA S was incorporated into viral proteins Detected the radioactive DNA in the bacterial cell Concluded DNA is the genetic material

Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge Pellet Pellet (bacterial cells and contents) Radioactivity (phage protein) in liquid Radioactivity (phage DNA) in pellet The search for the genetic material Hershey and Chase (1952)

Chargaff (1950) The structure of nucleotides was known –Three phosphate groups, a 5-C sugar, and a nitrogenous base –Four types of nitrogenous bases Purines (A and G) Pyrimidines (C and T) The search for the genetic material

Chargaff (1950) The structure of nucleotides was known It was also known that DNA was polymer of nucleotides The search for the genetic material

Chargaff (1950) The structure of nucleotides was known It was also known that DNA was polymer of nucleotides Chargaff determined that there were patterns in the amounts of nitrogenous bases –Base composition differed between species (further evidence for its role as genetic material) –Amount of A=T –Amount of G=C The search for the genetic material

Chargaff (1950) The structure of nucleotides was known It was also known that DNA was polymer of nucleotides Chargaff determined that there were patterns in the amounts of nitrogenous bases –Base composition differed between species (further evidence for its role as genetic material) –Amount of A=T –Amount of G=C The search for the genetic material By this time most scientists agreed DNA was the genetic material. The new challenge was to determine the structure.

Determining the structure of DNA Franklin (1950’s) Used x-ray diffraction to generate images of DNA She concluded DNA was a very long and thin molecule Sugar and phosphate groups were on the outside There was a repeating pattern every 0.34 and 3.4 nanometers

Determining the structure of DNA Watson and Crick (1953) Made a model that conformed to Franklin’s images

Determining the structure of DNA Watson and Crick (1953) Made a model that conformed to Franklin’s images Concluded DNA was double helix with a sugar-phosphate backbone

Determining the structure of DNA Watson and Crick (1953) Made a model that conformed to Franklin’s images Concluded DNA was double helix with a sugar-phosphate backbone Nitrogenous bases paired in the middle (A to T and G to C) Maintains the uniform width of the helix Triple hydrogen bonds between G and C Double hydrogen bonds between A and T

Determining the structure of DNA Watson and Crick (1953) Made a model that conformed to Franklin’s images Concluded DNA was double helix with a sugar-phosphate backbone Nitrogenous bases paired in the middle (A to T and G to C) Bases were paired across the helix every 0.34 nanometers and the helix turned every 3.4 nanometers

Determining the structure of DNA Watson and Crick (1953) Made a model that conformed to Franklin’s images Concluded DNA was double helix with a sugar-phosphate backbone Nitrogenous bases paired in the middle (A to T and G to C) Bases were paired across the helix every 0.34 nanometers and the helix turned every 3.4 nanometers Anti-parallel molecule (two-way street)

Determining the structure of DNA Watson and Crick (1953) Made a model that conformed to Franklin’s images Concluded DNA was double helix with a sugar-phosphate backbone Nitrogenous bases paired in the middle (A to T and G to C) Bases were paired across the helix every 0.34 nanometers and the helix turned every 3.4 nanometers Anti-parallel molecule (two-way street) The linear sequence of bases can be varied (yielding genetic diversity)

DNA Replication When does DNA replicate? Why is this important?

DNA Replication “It has not escaped our notice that the specific pairing we have postulated immediately suggest a possible copying mechanism for genetic material” Watson and Crick (1953)

DNA Replication “It has not escaped our notice that the specific pairing we have postulated immediately suggest a possible copying mechanism for genetic material” Watson and Crick (1953) Each strands has the information to code for the other Complimentary strands serve as a template for the construction of new strands New nucleotides line up on the template and are linked together

DNA Replication Replication begins at origins of replication (AT- rich regions) One in proks Many in euks Proteins recognize the sequence and begin separating the strands Replication proceeds in both directions until entire DNA molecule is copied Origin of replication Parental (template) strand Daughter (new) strand Replication fork Replication bubble Two daughter DNA molecules (a) Origins of replication in E. coli Origin of replicationDouble-stranded DNA molecule Parental (template) strand Daughter (new) strand Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes 0.5 µm 0.25 µm Double- stranded DNA molecule

DNA Replication Replication begins at origins of replication (AT- rich regions) One in proks Many in euks Proteins recognize the sequence and begin separating the strands Replication proceeds in both directions until entire DNA molecule is copied Origin of replication Parental (template) strand Daughter (new) strand Replication fork Replication bubble Two daughter DNA molecules (a) Origins of replication in E. coli Origin of replicationDouble-stranded DNA molecule Parental (template) strand Daughter (new) strand Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes 0.5 µm 0.25 µm Double- stranded DNA molecule How does replication proceed?

DNA Replication Helicase unwinds the helix at the replication forks Topoisomerase helps relieve strain from untwisting the helix Topoisomerase Helicase Primase Single-strand binding proteins RNA primer

DNA Replication Helicase unwinds the helix at the replication forks Topoisomerase helps relieve strain from untwisting the helix Strands are stabilized by proteins Topoisomerase Helicase Primase Single-strand binding proteins RNA primer

DNA Replication Helicase unwinds the helix at the replication forks Topoisomerase helps relieve strain from untwisting the helix Strands are stabilized by proteins Two separated strands are available as a templates Topoisomerase Helicase Primase Single-strand binding proteins RNA primer

DNA Replication Helicase unwinds the helix at the replication forks Topoisomerase helps relieve strain from untwisting the helix Strands are stabilized by proteins Two separated strands are available as a templates Primase adds complimentary RNA nucleotides as part of a short primer Topoisomerase Helicase Primase Single-strand binding proteins RNA primer

DNA Replication Topoisomerase Helicase Primase Single-strand binding proteins RNA primer Helicase unwinds the helix at the replication forks Topoisomerase helps relieve strain from untwisting the helix Strands are stabilized by proteins Two separated strands are available as a templates Primase adds complimentary RNA nucleotides as part of a short primer Nucleotides are added to the 3’ end of the primer

DNA Replication New strands only grow in the 5’ to 3’ direction This creates a leading and lagging strand Leading strand: Elongating toward the fork One primer is required Leading strand Overview Origin of replication Lagging strand Leading strandLagging strand Primer Overall directions of replication Origin of replication RNA primer “Sliding clamp” DNA poll III Parental DNA

DNA Replication New strands only grow in the 5’ to 3’ direction Lagging strand: Elongating away from the fork Overview Origin of replication Leading strand Lagging strand Overall directions of replication Template strand RNA primer Okazaki fragment Overall direction of replication

DNA Replication New strands only grow in the 5’ to 3’ direction Lagging strand: Elongating away from the fork Requires numerous primers Overview Origin of replication Leading strand Lagging strand Overall directions of replication Template strand RNA primer Okazaki fragment Overall direction of replication

DNA Replication New strands only grow in the 5’ to 3’ direction Lagging strand: Elongating away from the fork Requires numerous primers DNA polymerase adds complimentary nucleotides until it reaches another primer Overview Origin of replication Leading strand Lagging strand Overall directions of replication Template strand RNA primer Okazaki fragment Overall direction of replication

DNA Replication New strands only grow in the 5’ to 3’ direction Lagging strand: Elongating away from the fork Requires numerous primers DNA polymerase adds complimentary nucleotides until it reaches another primer Synthesized in fragments Overview Origin of replication Leading strand Lagging strand Overall directions of replication Template strand RNA primer Okazaki fragment Overall direction of replication

DNA Replication New strands only grow in the 5’ to 3’ direction Lagging strand: Elongating away from the fork Requires numerous primers DNA polymerase adds complimentary nucleotides until it reaches another primer Synthesized in fragments Primers are replaced with DNA (by DNA polymerase) Fragments are joined by DNA ligase Overview Origin of replication Leading strand Lagging strand Overall directions of replication Template strand RNA primer Okazaki fragment Overall direction of replication

DNA proofreading and repair

It is important that replication is done correctly DNA repair mechanisms fix mismatched bases before the DNA is replicated again Errors become mutations –Mutations may lead to defects (a misspelled word that does not mean anything) –Mutations can be advantageous (the invention of a new word that has great meaning)

DNA proofreading and repair DNA polymerase proofreads as it adds bases Mismatched bases are cut my nucleases The resulting gap is filled by DNA polymerase and sealed by DNA ligase Nuclease DNA polymerase DNA ligase

You should understand: The importance of DNA The evolution of our knowledge about DNA DNA replication DNA repair