DNA stands for deoxyribose nucleic acid

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2 DNA DNA stands for deoxyribose nucleic acid This chemical substance is present in the nucleus of all cells in all living organisms DNA controls all the chemical changes which take place in cells

3 DNA Structure Watson & Crick 1953 Proposed the double helix shape of DNA and received a Nobel Prize Maurice Wilkins and Rosalind Franklin Produced images of DNA using x-ray diffraction Franklin passed before Nobel Prize was awarded

4 Four requirements for DNA to be genetic material Must carry information Cracking the genetic code Must have the ability to be reproduced DNA replication Must allow for information to change Mutation Must govern the expression of the phenotype Gene function

5 DNA molecule DNA is a very large molecule made up of a long chain of sub-units The sub-units are called nucleotides Each nucleotide is made up of a sugar called deoxyribose a phosphate group - and an organic base

6 Ribose & deoxyribose Ribose is a sugar, like glucose, but with only five carbon atoms in its molecule Deoxyribose is almost the same but lacks one oxygen atom Both molecules may be represented by the symbol

7 The bases The most common organic bases are Adenine (A) Thymine (T) Cytosine (C) Guanine (G)

8 Nucleotides The deoxyribose, the phosphate and one of the bases Combine to form a nucleotide adenine deoxyribose

9 Joined nucleotides A molecule of DNA is formed by millions of nucleotides joined together in a long chain sugar-phosphate backbone + bases

10 2-stranded DNA

11 Bonding 1 The bases always pair up in the same way Adenine forms a bond with Thymine Adenine Thymine and Cytosine bonds with Guanine Cytosine Guanine

12 Bonding 2 adenine thymine cytosine guanine

13 Pairing up A

14 The paired strands are coiled into a spiral called A DOUBLE HELIX

15 THE DOUBLE HELIX bases sugar-phosphate chain

16 Replication overview Before a cell divides, the DNA strands unwind and separate Each strand makes a new partner by adding the appropriate nucleotides The result is that there are now two doublestranded DNA molecules in the nucleus So that when the cell divides, each nucleus contains identical DNA This process is called replication

17 The strands separate

18 Each strand builds up its partner by adding the appropriate nucleotides

19 Overview of Processes DNA replication 5 ->3 DNA proof reading Lagging strand, backstitching, Okazaki fragment Proteins involved: 1. DNA polymerase, primase 2. DNA helicase and single-strand DNAbinding protein (SSB) 3. DNA ligase, and enzyme to degrade RNA 4. DNA topoisomerases

20 5 and 3

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22 ATP datp

23 Semi-Conservative DNA Replication Step 1: Untwisting and Unzipping Helicase separates the strands by un-pairing the bases How can you remember helicase?

24 Replication Fork- Y-shaped region where the parental strands of DNA are being unwound Leading Strand- will be replicated continuously Lagging Strand- will be replicated in sections Template Strands-?

25 Step 2 Stabilization Single-Strand Binding Proteins- will bind to unpaired DNA strands to keep them stable

26 Step 3 Maintenance Topoisomerase- enzyme that breaks, swivels and rejoins parental DNA strands

27 Step 4 Initiation The enzymes that synthesize DNA cannot start without a guide Primase- an enzyme that adds a strip of RNA nucleotides to the parental strands This strip is approximately 5-10 nucleotides long and is referred to as the Primer

28 RNA Primers- point of initiation of DNA replication Base pairs or nucleotides are similar with the exception of Uracil. Uracil replaces Thymine and so, pairs with Adenine Uracil

29 Step 5 DNA Synthesis DNA Polymerase III- starts at the primer, adding bases in only one direction DNA Polymerase moves from the 5 end toward the 3 end

30 Caveats of Elongation DNA has a defined 5 and 3 end which gives it directionality DNA exists as antiparallel strands which means the new strands must also be antiparallel What s the issue?

31 Leading Strand This template strand is called as such because it is replicated continuously The DNA Polymerase binds to replication fork at the primer and simply adds free nucleotides to the leading strand until it has completely synthesized a complimentary strand.

32 Lagging Strand For the second template strand it is a bit more complicated The DNA Polymerase must synthesize the complimentary strand working away from the replication fork **this must also be synthesized 5 to 3 It is synthesized discontinuously Okazaki Fragments- the segment of the lagging strand ( bp in eukaryotes)

33 Lagging Strand Thus, several primers are needed on the lagging strand DNA Polymerase I- enzyme that will replace the RNA primers Adds to the 3 end of each Okazaki Fragment Exonuclease- detaches the RNA primers DNA Ligase- joins the sugar-phosphate backbones of the strands together to make a continuous strand

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38 DNA Repair Errors occur during replication There is approximately 1 error in every 10 billion nucleotides after replication However, errors due occur during the process and more frequently 1 in 100,000 bps

39 Proofreading While the base pairs are being added, DNA Polymerase check to make sure they are added correctly It is not a flawless proofread and correction of these nucleotides is referred to as mismatch repair

40 Proofreading Nucleotides can be incorrectly paired or even altered after replication DNA has to frequently be repaired Chemical or physical agents can lead to alterations of bps and warrant repair Reactive chemicals, radioactive emissions, X-rays, UV rays and other molecules can lead to nucleotides changes Often, these changes are repaired before mutations can occur

41 Nuclease An enzyme called a nuclease will excise the altered nucleotides Then other enzymes will replace those nucleotides with the correct bps, and then make the strand whole again This is done by using the unaffected strand of DNA as a template This is an example of nucleotide excision repair

42 Application UV light can lead to skin cancer UV light can catalyze the production of thymine dimers This causes a covalent bond to form between thymine nucleotides

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44 Xeroderma Pigmentosum Xeroderma pigmentosum (XP) is a rare autosomal recessive genetic disorder of DNA repair in which the ability to repair damage caused by ultraviolet (UV) light is deficient

45 Problems with Replication Over the course of several replications, the DNA faces some changes The Strands of DNA actually become shorter

46 Telomeres

47 Prokaryote VS Eukaryote DNA Replication Eukaryotes X-shaped Chromosome Semi-Conservative Replicates in Nucleus Many points of Origin Two templates Leading and Lagging Strands At least 11 major Polymerases 50 nucleotides/sec Prokaryotes Ring-shaped Chromosome Semi-Conservative Replicates in Cytoplasm One point of origin One template Bidirectional and Continuous 2 major polymerases 500 nucleotides/sec

48 Prokaryote DNA Replication

49 Prokaryote VS Eukaryote DNA Replication