Chapter 16. The Molecular Basis of Inheritance. Biology Kevin Dees

Transcription

1 Chapter 16 The Molecular Basis of Inheritance

2 DNA Life s instructions!!!! Deoxyribonucleic Acid Nucleic acid polymer from nucleotide monomers Unique in that it can: Self replicate Carry information

3 History of DNA James Watson (American) and Francis Crick (English) have been granted the majority of the credit with the discovery of DNA in however, they merely put the puzzle pieces together.

4 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

5 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

6 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

7 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

8 The double-helix Notice that the sugar-phosphate backbone also gives direction to the strands of DNA monomers 5 end 3 end

9 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

10 The sugarphosphate backbones cause the two strands of DNA to twist in a right hand direction

11 It is this unique structure that Carry information allows DNA to: 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?

12 DNA replication Theory also described by Watson and Crick semiconservative model of replication

13 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

14 DNA replication as a process Be sure to know the steps Be sure to know the enzymes and their respective actions

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

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

17 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.

18 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

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

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

21 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 scaffold folding to begin to form chromosomes Metaphase, replicated chromosome Organization of DNA