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 Strand Lagging 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 1000.

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?