Unit 9: The Central Dogma Honors Biology

 The process of DNA replication is fundamentally similar for prokaryotes and eukaryotes.  DNA replication is a semi-conservative process – this means that when the double helix is unwound, each original strand serves as a template or pattern for the newly synthesized strand.

 The replisome is a complex molecular machine that carries out replication of DNA.  The replisome contains all the necessary enzymes for replication.  Major Components ◦ DNA Helicase ◦ DNA Primase ◦ DNA Polymerases ◦ DNA ligases ◦ DNA Binding Proteins

 The process of replication can be thought of as having a beginning where the process starts; a middle where the complementary nucleotides are added to the exposed parental strands; and an end where the process is terminated or stopped.  Write these steps down:  INITIATION: formation of replication fork  ELONGATION: a number of enzymes work together to accomplish the task of assembling a new strand  TERMINATION: An end point to the replication

 DNA helicases unwind and unzip the DNA molecule forming a replication fork.  A replication fork is a Y-shaped region where the new strands of DNA are elongating.

 The elongation process is semidiscontinuous.  The leading strand of the DNA molecule (exposed 3’ end) is synthesized continuously in the direction of the opening replication fork.  The lagging strand of the DNA molecule (exposed 5’ end) is synthesized discontinuously in the direction opposite to the opening of the replication fork.

 Remember - The two strands of the DNA double helix are antiparallel. This means that their sugar – phosphate backbones run in opposite directions!

 DNA polymerases add nucleotides only to the FREE 3’ end of a growing DNA strand, NEVER to the 5’ end.  DNA polymerases synthesize ONLY in the direction 5’ to 3’.

 DNA Primase synthesizes a short complementary segment to the exposed 3’ end of the DNA strand.  DNA polymerases extend the complementary segment toward the replication fork as it opens.  The ability of the DNA polymerase to remain attached to the template (leading strand) is known as PROCESSIVITY.

 The synthesis of the lagging strand is described as discontinuous since it is synthesized in pieces that get glued together.  DNA primase synthesizes a short segment of complementary DNA to the exposed 5’ end.  DNA polymerases add complementary nucleotides in the direction away from the opening replication fork.  DNA ligases glue the Okazaki Fragments together forming the new strand.

Leading StrandLagging Strand  Exposed 3’ end of the original DNA molecule  Synthesized continuously toward the opening replication fork.  The ability of a DNA polymerase to remain attached to the leading strand is known as processivity.  Exposed 5’ end of the original DNA molecule  Synthesized discontinuously away from the opening replication fork.  The Okazaki fragments are glued together by DNA Ligases.

 Prokaryotes have circular DNA so replication ends when the process comes around to the origin again.  In eukaryotes, end points for each chromosome are indicated by telomeres.  Telomeres are specialized structures on the ends of eukaryotic chromosomes that are composed of specific repeated DNA sequences.

 Telomeres are special nucleotide sequences on the ends of the linear eukaryotic chromosomes.  The typical repeating unit in human telomeres is TTAGGG. The number of repetitions in a telomere varies between 100 and  The telometric DNA protects an organism’s genes from being eroded through successive rounds of DNA replications.

 Remember that DNA polymerases can only add nucleotides in the 5’ to 3’ direction.  The exposed 5’ end represents the lagging strand.  The replisome does not contain the machinery to complete the 5’ ends of the daughter strands of the DNA. As a result, repeated replication produces shorter and shorter DNA molecules.

 Briefly describe the functions of the following enzymes during DNA replication: DNA helicases, DNA polymerases, and DNA ligases.  Briefly provide an overview of DNA replication using the terms: initiation, elongation, termination.  Distinguish between the leading strand and lagging strand during DNA replication?