DNA

Replication of the genetic code

DNA = recipe book

• Instructions for ALL proteins are encoded by DNA

• DNA resides in the nucleus• To pass on instructions for life, need

to replicate DNA prior to reproduction

How do we know DNA is the genetic code of life?

• Some late 19th century observations of dividing cells gave us some clues

Observations

• Late 1800’s; Walther Flemming sees “threads” moving & changing during cell division

• Threads appear paired prior to cell division

• Paired threads separate just prior to division

• Named the “thread separating” process Mitosis

Observations on thread #• Thread # differs between species

– Roundworms = 4; Peas = 14; Humans = 46

• Thread # identical between individuals within a species– All roundworms = 4

• Same between cells within an individual• Threads were named chromosomes

which consist of both DNA & protein

Is it DNA or proteins that are important?

• Chromosomes consist of both, so how did scientists identify which one holds instructions for reproduction of cells?

DNA’s “discovery”• 1952: Hershey & Chase

find that bacteriophage virus infects and “reprograms” bacteria to make more virus– Consists only of external

protein coat and internal DNA

– Inserts its DNA into bacteria, protein coat remains outside

• A perfect model!

Label DNA & protein separately

• Radioactive Sulfur incorporates into proteins only. Why?

• Heavy bacterial cells settle, while lighter phage particles remain in solution. (where’s the radioactivity?)

Label DNA & protein separately

• Radioactive Phosphorous incorporated into DNA. Why?

• Heavy bacterial cells settle, while lighter phage particles remain in solution. (where’s the radioactivity?)

DNA encodes instructions for replication!

DNA’s structure

What’s it look like? Does its structure suggest how

replication is accomplished?

Monomers of DNA = Nucleotides

• Repeated phosphate, sugar, base motif of ALL nucleotides

• Phosphate-sugar backbone• Base = only difference between nucleotides

Nitrogenous Bases

• Purines: G, A; 2 nitrogenous rings• Pyrimidines: C, T; 1 nitrogenous ring

Who discovered the structure?

• J. Watson & F. Crick deduced double-stranded, helical structure from Rosalind Franklin’s X-ray crystallographic image of a DNA molecule.

Chemical structure

Conclusions• Molecule is of uniform width• Amounts of A & T are

identical; same for C & G• H-bonds hold bases of

neighboring strands together– suggests precise

complimentarity between nucleotides

– Adenine always pairs with Thymine; Cytosine always pairs with Guanine

Extensions

• Sequence possibilities are limitless (variation in sequence could account for the diversity of life.)

• Those “threads” (chromosomes) we saw separating with dividing cells must be DNA molecules

Structure also suggests mechanism of replication

• Pull strands apart; now each strand serves as template for a new strand

• Semiconservative model: ½ parent molecule is conserved in each daughter molecule

Replication• Begins @ multiple

replication centers

• Helicase unwinds and separates DNA strands (bubble)

• DNA polymerase adds bases opposite the template (parent strand)

Structure determines direction

• Strands are anti-parallel

• Each has a 5’ and a 3’ end– Refers to Carbon atom in

sugar ring (i.d. purposes)

• DNA polymerase can only add nucleotides to the 3’ end of a strand

Consequences of polarity

• One strand is continuously replicated

• The other is replicated in fragments (Okazaki fragments)

• DNA ligase joins these fragments to complete the new molecule

• Other polymerases proofread & edit