DNA is a functional genetic material as it:

Transcription

1 DNA DNA is a functional genetic material as it: varies between species and individuals can store information remains constant within a species Replicates undergoes mutations 1

2 `It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material Watson & Crick Nature (1953) Original drawing by Francis Crick e9e/default.asp#

3 DNA replication occurs with great fidelity Somatic cell DNA stability and reproductive-cell cell DNA stability are essential. Why? Identity Pan troglodytes 98.77% sequence identity Genetic diseases

4 DNA Replication Process of duplication of the entire genome prior to cell division Biological significance extreme accuracy of DNA replication is necessary in order to preserve the integrity of the genome in successive generations In eukaryotes, replication only occurs during the S phase of the cell cycle.

5 Synthesis Phase (S phase) S phase during interphase of the cell cycle Nucleus of eukaryotes DNA replication takes place in the S phase. S phase G 1 G interphase 2 Mitosis -prophase -metaphase -anaphase -telophase 5

6 DNA is suitable for replication DNA duplication (or replication) is semiconservative as each new filament contains an old strand and a new strand. 6

7 DNA Replication Semiconservative Daughter DNA is a double helix with 1 parent strand and 1 new strand Found that 1 strand serves as the template for new strand

8 DNA is suitable for replication DNA duplication involves three steps: 1.Unrolling and unwinding; 2.Complementary base paring; 3.Joining. DNA polymerase is an enzyme that assists steps 2 and 3. See cecie star 8

9 DNA Replication Let s meet the team Large team of enzymes coordinates replication e/default.asp# Both movies

10 DNA polymerase DNA polymerase adds nucleotides to the 3' end of the DNA and needs a primer to start the replication of a strand. Telomeres are special noncodifying repetitive nucleotide sequences 10

11 DNA Replication Enzyme Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds Single-Strand Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted 11

12 DNA Replication Enzyme Topoisomerase prevents torsion by DNA breaks It attaches to the 2 forks of the bubble to relieve stress on the DNA molecule as it separates Enzyme Enzyme DNA 12

13 DNA Replication Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides Primase is the enzyme that synthesizes the RNA Primer DNA polymerase can then add the new nucleotides 13

14 DNA Replication DNA polymerase can only add nucleotides to the 3 end of the DNA This causes the NEW strand to be built in a 5 to 3 3 direction 5 3 Nucleotide DNA Polymerase RNA Primer 5 Direction of Replication 14

15 Synthesis of the New DNA Strands The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork 5 3 Nucleotides DNA Polymerase RNA Primer 5 15

16 Synthesis of the New DNA Strands The Lagging Strand is synthesized discontinuously against overall direction of replication This strand is made in MANY short segments It is replicated from the replication fork toward the origin DNA Polymerase Lagging Strand RNA Primer Leading Strand

17 Lagging Strand Segments Okazaki Fragments - series of short segments on the lagging strand Must be joined together by an enzyme 5 Okazaki Fragment RNA Primer DNA Polymerase 3 3 Lagging Strand 17 5

18 Joining of Okazaki Fragments The enzyme Ligase joins the Okazaki fragments together to make one strand DNA ligase 5 Okazaki Fragment 1 Okazaki Fragment Lagging Strand

19 Replication of Strands Replication Fork Point of Origin See animation in your book at pag

20 Replication: 1st step Unwind DNA unwinds part of DNA helix I d love to be helicase & unzip your genes stabilized by helicase single-stranded binding proteins replication fork

21 Replication: 2nd step Build daughter DNA strand add new complementary bases DNA Polymerase III

22 Replication Adding bases can only add nucleotides to 3 end of the growing DNA strand need a primer nucleotide to bond to 5 energy DNA Polymerase III 3 3 5

23 DNA replication The second strand of DNA is duplicated in opposite direction of the replication fork. Replication is continuous for the leading strand but discontinuous for the lagging strand. hill.com/sites/ /student_view0/chapter2/animation_quiz_- _dna_replication_fork.html quiz hill.com/sites/ /student_view0/chapter2/animation_quiz_- _dna_replication_fork quiz_3_.html 23

24 Chromosome erosion we have a problem! DNA polymerases can only add to 3 end of an existing DNA strand DNA polymerase I growing replication fork 3 DNA polymerase III 5 Loss of bases at 5 ends in every replication chromosomes get shorter with each replication limit to number of cell divisions? 3

25 Telomeres Repeating, non-coding sequences at the end of chromosomes = protective cap limit to ~50 cell divisions growing replication fork 3 telomerase enzyme extends telomeres can add DNA bases at 5 end different level of activity in different cells high in stem cells & cancers -- Why? TTAAGGG TTAAGGG TTAAGGG 5 3

26 Telomeres are repetitive sequences found at the ends of many eukaryotic chromosomes. Corresponding to the telomeres, after the primer is removed, a fragment of single-strand DNA remains that cannot be replicated, and is lost. In some cells, the enzyme telomerase catalyzes the addition of any lost telomeric sequence 2m 2m 26

27 Replication fork direction of replication 5m

28 Editing & proofreading DNA 1000 bases/second = lots of typos! DNA polymerase proofreads & corrects typos repairs mismatched bases removes abnormal bases repairs damage throughout life reduces error rate from 1 in 10,000 to 1 in 100 million bases 4m

29 Fast & accurate! It takes E. coli <1 hour to copy 5 million base pairs in its single chromosome divide to form 2 identical daughter cells Human cell copies its 6 billion bases & divide into daughter cells in only few hours remarkably accurate only ~1 error per 100 million bases ~30 errors per cell cycle 2m (Go in hallmarks of cancer Becoming immortal and promoting mutation)

30 What does it really look like?

31 DNA explains some of its important functions: genetic information is stored in the sequence of the variable part of the molecule, the nitrogenous bases; replication is simple and effective because of the complementarity of the bases; genetic material is subject to mutations; genetic information is copied in the RNA molecule and expressed inthe phenotype. 31

32 Any Questions??

33 Question: What would be the complementary DNA strand for the following DNA sequence? DNA 5 -CGTATG5 CGTATG-3 33

34 Answer: DNA 5 -CGTATG5 CGTATG-3 DNA 3 -GCATAC3 GCATAC-5 34

35 What kind of enzyme synthesizes the new DNA strand? RNA polymerase DNA Polymerase Primase Helicase Topoisomerase

36 Eukaryotic chromosomes have multiple origins of replication True False

37 In what direction is the newly synthesized DNA produced? 5'-3' 3'-5' In the direction of the major groove Both 5'-3' and 3'-5' depending on which strand is being replicated

38 What is the sequence (1 to 6) in which these proteins function during DNA replication 3 RNA primase 6 DNA ligase 4 DNA polymerase 1 Topoisomerase 2 DNA helicase 5 tethering proteins

39 Why is an RNA primer necessary for DNA replication? The RNA primer is necessary for the activity of DNA ligase. The RNA primer creates the 5 and 3 ends of the strand. DNA polymerase can only add nucleotides to RNA molecules. DNA polymerase can only add nucleotides to an existing strand

40 Why has DNA evolved as the genetic material but not RNA? Because DNA is more stable

41 replication-flash-cards/ 41