Biology Chapter 16 - WCJCfacultyweb.wcjc.edu/users/kevind/documents/BIOL_1406_files/Lecture... ·...

Biology – Kevin Dees

Chapter 16The Molecular Basis of

Inheritance


DNA

• Life’s instructions!!!!• Deoxyribonucleic Acid• Nucleic acid polymer from nucleotide

monomers

• Unique in that it can:– Self replicate– Carry information


History of DNA• James Watson

(American) and Francis Crick (English) have been granted the majority of the credit with the “discovery of DNA” in 1953..however, they merely put the puzzle pieces together.


• Fredrick Griffith – British medical officer studying the bacteria

which causes pneumonia in 1928 • Oswald Avery

– American bacteriologist, 1944

– DNA was the controlling agent in cells


• Edwin Chargaff– (1947) Analyzed DNA from a biochemical standpoint– Examined ratios of nitrogenous bases which were

known to be part of nucleotides• In human DNA

– A=30.3 %– T= 30.3 %– G=19.5%– C=19.9%

– Chargaff’s rules explained how nucleotides could carry information; still used to explain base-pairs

– But the structure of DNA remained a mystery


• Rosalind Franklin– X-ray diffraction of

DNA• Watson and Crick

used these images to finally put the pieces together

• Watson and Crick (along with Franklin’s colleague – Wilkins, Franklin died of cancer in 1958) won the Nobel prize in 1962


The double-helix

• DNA consists of nucleotides

• Each nucleotide monomer has:– Phosphate– Deoxyribose (pentose

sugar)– One of four

nitrogenous bases• Adenine• Thymine• Guanine• Cytosine

• Phosphate-sugar backbone


The double-helix

• Notice that the sugar-phosphate backbone also gives “direction” to the strands of DNA monomers

• 5’ end• 3’ end


• DNA forms the double helix with the nitrogenous bases forming hydrogen bonds– Base-pairs

• Adenine with Thymine• Guanine with Cytosine

• The sugar-phosphate backbones are antiparallel ; they run in opposite directions


• The sugar-phosphate backbones cause the two strands of DNA to twist in a right hand direction


It is this unique structure that allows DNA to:

• Carry information – In the form of the sequence of nitrogenous

bases• Self-replicate

– Because each strand is complementary to the other strand

– Why is DNA replication important ?


DNA replication

• Theory also described by Watson and Crick• semiconservative model of replication


DNA replication begins at special sequences called origins of replication

• This allows for the process to be speedy

• Creates replication ‘bubbles’

• Replication forks– Sites of DNA

elongation– DNA

polymerase enzymes


• DNA replication as a process• Be sure to know the steps• Be sure to know the enzymes and their

respective actions


1. Helicase enzyme – breaks hydrogen bonds between base-pairs; unwinds double helix, creates replication fork. Topoisomeraseenzyme prevents breaks due to un-twisting


2. single-strand binding protein stabilize replication fork and keeps it open so bases are exposed


3. DNA polymerase III adds correct complementary nucleotides. New DNA must grow from 5’ 3’. This means primase enzyme must add an RNA primer as a starting block so there is an open 3’ end.


4. At the replication fork there are two newly synthesized strands; leading strand which is continuous (and growing from 5’ 3’) and the lagging strand which is being synthesized in short segments called Okazaki fragments


5. DNA polymerase I replaces the RNA nucleotides with DNA nucleotides and DNA ligase joins the Okazaki fragments together


• This continues at each replication fork until special sequences called telomeres are reached. These telomeres signal the end of the DNA


Organization of DNA

• Double helix• DNA wrapped around

histone proteins to form nucleosomes

• Strings of nucleosomes coil to form chromatin

• During prophase, chromatin condenses by looping onto a protein scaffoldfolding to begin to form chromosomes

• Metaphase, replicated chromosome

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