1 Section A: DNA as the Genetic Material CHAPTER 16 THE MOLECULE BASIS OF INHERITANCE

2 Section B: DNA Replication نـَسْـــــخ الـ دنا

3 1- During DNA replication, DNA strands خيوط serve as templates قالبfor new complimentary مُكمِّلstrands When a cell copies a DNA molecule, each strand serves as a template for ordering nucleotides into a new complimentary strand.When a cell copies a DNA molecule, each strand serves as a template نموزج for ordering nucleotides into a new complimentary strand الجانب المُكمِّل. –Nucleotides line up along the template النموذج strand according طبقاً لـ to the base-pairing rules قوانين الازدواج. –Nucleotides line up تـَتـَراص along the template النموذج strand according طبقاً لـ to the base-pairing rules قوانين الازدواج. Fig. 16.7, Page 293

4 Semiconservative replicationSemiconservative replication New DNA will have one old strand and one newly made strand.New DNA will have one old strand and one newly made strand. The other two models are the conservative and the dispersive modelsThe other two models are the conservative and the dispersive models Fig. 16.8, Page 294 Types of DNA replication

5 Semiconservative DNA Replication نـَسْـــــخ Several enzymes carry out تنفذ DNA replication تضاعف :Several enzymes carry out تنفذ DNA replication تضاعف : Helicase, Helicase, Primase, Primase, Polymerase, إنزيم نسخ الـ د ن أ Polymerase, إنزيم نسخ الـ د ن أ Ligase الرابط. Ligase الرابط. * In bacteria it takes less than an hour to copy نسخ each of the 5 million base pairs and divide تنقسم to form two daughter cells. * A human cell can copy its 6 billion base pairs and divide into daughter cells in only a few hours. * This process is accurate دقيق, with only one error خطأ per billion nucleotides.

6 –Each DNA strand has a 3’ end with a free OH group attached to deoxyribose and a 5’ end with a free phosphate group attached to deoxyribose. –The 5’ -> 3’ direction of one strand runs counter to مُعاكس لـ the 3’ -> 5’ direction of the other strand. Fig , Page 296 DNA polymerases إنزيم نسخ الـ د ن أ catalyze the elongation امتداد of new DNA.DNA polymerases إنزيم نسخ الـ د ن أ catalyze the elongation امتداد of new DNA.

7 Fig [Replication Mechanism] In eukaryotes, there may be hundreds or thousands of bubbles فقاعات () per chromosome لكل كروموسوم.In eukaryotes, there may be hundreds or thousands of bubbles فقاعات ( each has origin sites for replication ) per chromosome لكل كروموسوم. –At the origin sites, the DNA strands separate forming a replication “bubble فقاعة النسخ ” with replication forks at each end. –At the origin sites, the DNA strands separate forming a replication “bubble فقاعة النسخ ” with replication forks شوكة النسخ at each end. –The replication bubbles elongate as the DNA is replicated and eventually fuse. –The replication bubbles elongate تستطيل as the DNA is replicated and eventually fuse تندمج مع بعضها.

8 Primer: ( قطعة صغيرة من ) is required to start a new chain مطلوبة لبدء سلسلة جديدة.Primer: البادئة (a short segment of RNA, قطعة صغيرة من 10 nucleotides long) is required to start a new chain مطلوبة لبدء سلسلة جديدة. Primase: () links ribonucleotides that are complementary مكملة to the DNA template into the primer.Primase: (an Enzyme انزيم) links ribonucleotides that are complementary مكملة to the DNA template into the primer. DNA polymerases: After formation of the primer, DNA polymerases can add deoxyribonucleotides to the 3’ end of the ribonucleotide chain.DNA polymerases: After formation of the primer, DNA polymerases can add deoxyribonucleotides to the 3’ end of the ribonucleotide chain. Another DNA polymerase replaces يستبدل primers with DNA.Another DNA polymerase replaces يستبدل primers with DNA. Fig , Page 297 U3, Ch 15 U3, Ch 15

9 The other parental strand ( ), the lagging strand, is copied away from the fork in short segments (Okazaki fragments ).The other parental strand (5’->3’ into the fork), the lagging strand, is copied away from the fork in short segments (Okazaki fragments قـِطـَع صغيرة). Okazaki fragments ( ) are joined by DNA ligaseOkazaki fragments (each about nucleotides) are joined by DNA ligase الإنزيم الرابط DNA polymerases can only add nucleotides to the free 3’ end of a growing DNA strand.DNA polymerases can only add nucleotides to the free 3’ end of a growing DNA strand. A new DNA strand can only elongate in the 5’->3’ direction.A new DNA strand can only elongate in the 5’->3’ direction. At the replication fork, one parental strand (), the leading strand, can be used by polymerases as a template for a continuous complimentary strand.At the replication fork, one parental strand (3’-> 5’ into the fork), the leading strand, can be used by polymerases as a template for a continuous complimentary strand. Fig , Page 297

10 Summary of DNA Replication Mechanism The two DNA-strands separate forming replication bubbles. Each strand functions as a template قالبfor synthesizing new complementary & lagging strands via primers, polymerase and ligase. G CT A A T G G TATAC C G TATAC C G CT A A T G Templates Polymerase Complementary (leading) strand Lagging strand (complementary) Primer 53 Okazaki fragments Ligase

11 Fig , Page البـَـــدْء الإستطالة

12 Helicase: untwists the double helix to separate the DNA strands by forming replication bubles.Helicase: untwists the double helix to separate the DNA strands by forming replication bubles. Replication enzymes: separates DNA strands, forming a replication “bubble”.Replication enzymes: separates DNA strands, forming a replication “bubble”. Replication bubble: formed at the origin sites of replication as DNA strands separate, and hence, replication forks formed at each end.Replication bubble: formed at the origin sites of replication as DNA strands separate, and hence, replication forks formed at each end. Replication site: it also called origin of replication which is a single specific sequence of nucleotides that is recognized by the replication enzymes and at which replication starts.Replication site: it also called origin of replication which is a single specific sequence of nucleotides that is recognized by the replication enzymes and at which replication starts. Primer: is a short piece of RNA (10 nucleotide long) which is synthesised by primase and used to initiate the leading strands of the new DNA.Primer: is a short piece of RNA (10 nucleotide long) which is synthesised by primase and used to initiate the leading strands of the new DNA. DNA-polymerase: builds up the new DNA strand by adding nucleotides to the primer (from 5’ to 3’ end).DNA-polymerase: builds up the new DNA strand by adding nucleotides to the primer (from 5’ to 3’ end). Leading strand: the elongation strand (3’-> 5’ into the fork) that initiate the new DNA after recognizing the sequence of the primer by special proteins.Leading strand: the elongation strand (3’-> 5’ into the fork) that initiate the new DNA after recognizing the sequence of the primer by special proteins. Lagging strand: Is the other parental strand (5’->3’ into the fork), is copied away from the fork in short segments (Okazaki fragments).Lagging strand: Is the other parental strand (5’->3’ into the fork), is copied away from the fork in short segments (Okazaki fragments). Okazaki fragments: the newly formed segments (5’->3’, away from the fork) then, form the lagging strand when connected by ligase towards the fork.Okazaki fragments: the newly formed segments (5’->3’, away from the fork) then, form the lagging strand when connected by ligase towards the fork. DNA-ligase: joins the Okazaki fragments of the newly formed bases to form the new lagging DNA strand.DNA-ligase: joins the Okazaki fragments of the newly formed bases to form the new lagging DNA strand.Definitions