Chapter 10 – DNA: The Chemical Nature of the Gene

Early DNA studies Johann Friedrich Meischer – late 1800s –Studied pus (contains white blood cells) –Isolated nuclear material Slightly acidic, high phosphorous content Consisted of DNA and protein –Called in “nuclein” – later renamed nucleic acid By late 1800s –Chromatin thought to be genetic material, but protein or DNA?

Early DNA studies Tetranucleotide theory –DNA made up of 4 different nucleotides in equal amounts Nucleotide – pentose sugar, phosphate group, nitrogenous base –Under this assumption, DNA doesn’t have the variety needed for genetic material Protein composed of 20 different amino acids; complex structures Erwin Chargaff 1940s –Base composition of DNA among different species had great variety, but consistent within a single species –Adenine amount roughly equals thymine amount; guanine amount roughly equals cytosine amount

Fred Griffith 1928 Worked with different strains of the bacteria Streptococcus pneumoniae Transformation – bacteria acquired genetic information from dead strain which permanently changed bacteria

Oswald Avery published 1944 Based on Griffith’s findings What was transforming principle – protein, RNA, or DNA? Conclusion: when DNA is degraded, no transformation occurs; DNA genetic material

Alfred Hershey and Martha Chase 1952 DNA or protein genetic material? Conclusion: phage injects DNA, not protein, into bacteria; DNA genetic material

Maurice Wilkins and Rosalind Franklin early 1950s Worked independently on X ray crystallography Diffraction pattern gives information on molecular structure

James Watson and Francis Crick Published paper detailing DNA structure in 1953 –Based on published data and unreleased information 1962 won Nobel prize along with Maurice Wilkins

Heinz Fraenkel Conrat and Bea Singer 1956 RNA can serve as genetic material in viruses Created hybrid virsuses; progeny particles were of RNA type

Nucleotide structure Pentose (5 carbon) sugar –1′ to 5′ “′” refers to carbon in sugar (not base) –RNA – ribose -OH at 2′ carbon Less stable –DNA – deoxyribose -H at 2′ carbon Phosphate group –Phosphorous and 4 oxygen –Negatively charged –Attached to 5′ carbon

Nucleotide structure Nitrogenous base –Covalently bonded to 1′ carbon –Purine Double-ringed; six- and five-sided rings Adenine Guanine –Pyrimidine Single-ringed; six-sided ring Cytosine Thymine (DNA only) Uracil (RNA only)

Nucleotide structure Nucleoside –Base + sugar Nucleotide –Nucleoside + phosphate

Polynucleotide strands Nucleotides covalently bonded – phosphodiester bonds –Phosphate group of one nucleotide bound to 3′C of previous sugar Backbone consists of alternating phosphates and sugars –Always has one 5′ end (phosphate) and one 3′ end (sugar –OH)

DNA double helix 2 antiparallel strands with bases in interior Bases held together by hydrogen bonds –2 between A and T; 3 between G and C Complementary base pairing; complementary strands

Helices B-DNA –Watson and Crick model –Shape when plenty of water is present –Right hand/clockwise turn; approx 10 bases per turn A-DNA –Form when less water is present; no proof of existence under physiological conditions –Shorter and wider than B form –Right hand/clockwise turn; approx 11 bases per turn Z-DNA –Left hand/counterclockwise turn –Approx 12 bases per turn –Found in portions with specific base pair sequences (alternating G and C) –Possible role in transcription regulation?

Genetic implications Watson and Crick indicated structure revealed mode of replication –H bonds break and each strand serves as a template for new strand due to complementary base pairing Central dogma –Replication DNA from DNA –Transcription RNA from DNA –Translation Polypeptide/protein from mRNA

Special structures Sequences with a single strand of nucleotides may be complementary and pair – forming double- stranded regions Hairpin –Region of complementary bases form base; loop formed by unpaired bases in the middle Stem –No loop of hairpin

Special structures Cruciform –Double-stranded –Hairpins form on both strands due to palindrome sequences Complex structures can form within a single strand

DNA methylation Addition of methyl groups to certain bases Bacteria is frequently methylated –Restriction endonucleases cleave unmethylated sequences Amount of methylation varies among organisms –Yeast – 0% –Animals – 5% –Plants – approx 50% Methylation in eukaryotic cells is associated with gene expression –Methylated sequences are low/no transcription

Bends in DNA Series of 4 or more A-T base pairs cause DNA to bend –Affects ability of proteins to bind to DNA’ affects transcription SRY gene –Produces SRY protein Binds to certain DNA sequences; bends DNA –Facilitates binding of transcription proteins; activates genes for male traits