1. 10-1 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Chapter 10: Genes, chromosomes and DNA

2. 10-2 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Chromosome structure DNA is organised into chromosomes Each chromosome is a single DNA molecule In eukaryotic cells, chromosomes are located in the nucleus and are characteristically linear Each species has a unique chromosome complement—shape, size and number Centromere is located in the middle of the chromosome and is essential for segregation during cell division The ends of the chromosomes are called telomeres

3. 10-3 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.1: Stained human chromosomes

4. 10-4 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Chromosome structure (cont.) Eukaryotic chromosomes are made up primarily of protein and DNA Histones assemble in groups of eight and form a core upon which the DNA is bound—called a nucleosome core particle This, together with the linking DNA, is referred to as a nucleosome

5. 10-5 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.3: Model of a nucleosome particle

6. 10-6 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.4: A condensed chromosome in metaphase

7. 10-7 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Genetic information is stored in DNA and not protein 1944: Oswald Avery, Colin MacLeod and Maclyn McCarty demonstrated that it is the DNA component, not the protein, that carries the genetic information Observation about two different forms of the same Streptococcus pneumoniae bacterial strain—one was virulent (S, for smooth strain) and the other was not (R, for rough strain) –mice injected with live non-virulent bacteria and heat-killed virulent bacterial material died –neither preparation on its own killed the mice –non-virulent strain was ‘transformed’ by the virulent material –the virulence acquired from the heat-killed strain was passed on to progeny of the transformed bacteria

8. 10-8 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.5: Transforming principle in Streptococcus pneumoniae

9. 10-9 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Genetic information is stored in DNA and not protein (cont.) Hershey & Chase’s (1952) experiment Bacteriophage DNA or protein was specifically radioactively labelled Bacteriophage-infected bacteria—new bacteriophage produced by infected organisms The presence of radiolabel inside infected bacteria was only detected when the DNA was radiolabelled No radiolabelled protein was found inside the bacteria

10. 10-10 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.6: Radioactive labelling of DNA with 32 P or protein with 35 S

11. 10-11 Copyright  2009 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University The structure of DNA explains its capacity to store biological information Model of DNA was proposed by James Watson and Francis Crick in 1953 based on crystallographic analysis by Rosalind Franklin They found that DNA is a double-stranded molecule twisted into a helix Each strand, comprising a sugar-phosphate backbone and attached bases, is connected to a complementary strand by non-covalent hydrogen bonding between paired bases The bases are adenine (A), thymine (T), cytosine (C) and guanine (G) Each base or nucleotide has three parts: a phosphate group, a pentose sugar and an organic base Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University

12. 10-12 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.7: Schematic outline of the molecular structure of DNA

13. 10-13 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University DNA structure (cont.) Bases are purines (A and G) and pyrimidines (C and T) Purines have a pair of fused rings; pyrimidines only have one A and T are connected by two hydrogen bonds; G and C are connected by three hydrogen bonds The number of bonds is the basis of specific pairing between the bases

14. 10-14 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University DNA structure (cont.) Bases are linked together by phosphodiester bonds Nucleic acids have distinct ends –the 3’ end has a free hydroxyl group on the 3’ carbon of a sugar –the 5’ end has a free phosphate group at the 5’ carbon of the sugar The two strands of the helix are antiparallel: the 5’ end of one strand is directly apposed to the 3’ end of the other strand

15. Question 1 If Cytosine makes up 24% of the bases in the DNA from an organism, approximately what proportion of the bases will be thymine? a) 12 b) 24 c) 26 d) 38 e) cannot be determined from the above information 10-15 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University

16. 10-16 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University DNA replication How does DNA build a copy of itself? DNA is replicated semi-conservatively—meaning that each separate strand provides the template for building the new strand by the base-pairing rules Semi-conservative replication allows synthesis of new strands with high fidelity New DNA molecules consist of one ‘old’ strand from the original molecule and one newly synthesised strand

17. 10-17 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.8a: Semiconservative replication

18. 10-18 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.8b: Sequence-based representation of replicating DNA

19. 10-19 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University DNA replication in prokaryotes Bacteria have a single circular chromosome Replication begins at a single origin of replication where unwinding begins A replication fork is formed on each side of the origin as small lengths of DNA separate for synthesis of new strands Each replication fork contains a leading strand growing toward the fork and a lagging strand growing away from the fork The two replication forks eventually meet at the terminus

20. 10-20 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.10: DNA synthesis in circular chromosomes

21. 10-21 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Enzymes needed for replication in eukaryotes Gyrases, required to unwind the supercoiled helices, and helicases to separate the strands New-strand synthesis is performed by a family of enzymes called DNA polymerases –Polymerisation requires a template to copy –DNA polymerases require priming to initiate strand extension –An enzyme called primase builds an RNA primer (~5–10 nucleotides long) at the origin of replication –A short RNA primer with a 3’ OH group is added to the template strand by primase –DNA polymerase I checks the added base and also removes RNA primers after replication –DNA polymerase III attaches bases only in the 5’  3’ direction from the primer sequence

22. 10-22 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.13: Initiation of DNA synthesis

23. Enzymes needed for replication (cont.) Synthesis always proceeds 5’  3’ on the strand being produced, therefore: –one strand is synthesised continuously (leading strand) But how will the other (lagging strand) be synthesised? DNA polymerase must work discontinuously in the direction away from the replication fork and is synthesised as a series of small segments –primases must attach a series of primers along the template strand –DNA polymerase extends the primers away from the replication fork –small segments of DNA called Okazaki fragments will be joined together by DNA ligase –DNA polymerase eventually replaces the RNA nucleotides with DNA nucleotides 10-23 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University

24. 10-24 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.11: Replication fork of Escherichia coli

25. 10-25 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Replication in eukaryotes Because eukaryotic cells contain much more DNA than prokaryotes, there must be multiple origins of replication on each chromosomes in order for replication to take place in a timely fashion Two replication forks are formed at each origin Synthesis proceeds 5’ to 3’ at each unit of replication (replicon), with leading and lagging strands

26. 10-26 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.14: DNA synthesis in a chromosome of a eukaryote

27. Question 2 If you were to look at the size of DNA fragments during replication, one group would show up as very large molecules (of perhaps millions of bases long) and the other group would be very short molecules (only 200 bases). These two types of DNA molecules would most likely represent: a)leading strands and Okazaki fragments b)lagging strands and Okazaki fragments c)Okazaki fragments and RNA primers d)leading strands and RNA primers e)RNA primers and mitochondrial DNA 10-27 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University

28. 10-28 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Telomeres during replication Remember: DNA polymerases only replicate DNA in a 5’ to 3’ direction and need a primer to get things started When the primer is removed from the 5’ end of the new strand, a gap is left from which DNA polymerase cannot extend At each round of cell division chromosomes would therefore become shorter This leads to a progressive shortening of DNA strands (between 20 and 50 nucleotides are lost with each round of replication).

29. 10-29 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Telomeres during replication (cont.) To overcome this problem –During development of the foetus, cells add nucleotides to the 5’ end of every chromosome, up to 10–15 kb long –These nucleotides are called telomeres (repeat DNA sequences) –In humans the repeated sequence is AGGGTT –Nucleotides are added to chromosome ends by an enzyme called telomerase

30. 10-30 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Fig. 10.15: Replication at ends (telomeres) of eukaryotic chromosomes

31. 10-31 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Telomeres during replication (cont.) Mammalian somatic cells do not continue telomerase activity after birth, so chromosomes become shorter with age This limits the number of divisions each cell can undergo As telomeres grow shorter, eventually cells reach the limit of their replicative capacity and progress into senescence (cell aging) and the cell eventually dies

32. 10-32 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University Telomerases and cellular ageing Restoration of telomerase activity allows cells to proliferate indefinitely and has important implications for ageing: could it be considered as the ‘fountain of youth’? Telomerase becomes reactivated in cancer cells, giving them immortality Cancer researchers have become very interested in designing drugs that target and inactivate telomerase

33. Summary of important replication enzymes Primase: synthesises RNA primers Gyrase: unwinds supercoiled DNA Helicase: separates the DNA strands DNA polymerase III: adds nucleotides to growing daughter strands DNA polymerase I: replaces primer RNA nucleotides with DNA nucleotides DNA ligase: joins Okazaki fragments 10-33 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University