DNA Replication

How DNA Replication Occurs DNA replication is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission One copy moves to each new cell Since the two strands are complementary, each strand serves as a template to make a new strand

Steps of DNA Replication Helicases separate the DNA Strands These move along the DNA molecule breaking the hydrogen bonds between the complementary bases This allows the two strands to separate The Y shaped region where the strands are separating is called the replication fork (not spoon)

Steps of DNA Replication DNA polymerases add complementary nucleotides to each of the original strands Nucleotides float freely in the nucleus As nucleotides are added, covalent bonds form between the sugar of one nucleotide and the phosphate group of the next Hydrogen bonds form between the bases

Steps of DNA Replication DNA polymerases are finished and fall off - Result is two molecules of DNA identical to the original - Molecules are ready to move into new cells during division - Called semi-conservative replication because each of the new DNA molecules has kept (or conserved) one of the two (or semi) original DNA strands.

Steps of DNA Replication Click picture for video

Action at the Replication Fork DNA synthesis occurs in different directions on each strand As the replication fork moves along the original DNA, synthesis follows the movement of the replication fork Synthesis on the other strand moves in the opposite direction, away from the replication fork This leaves gaps in the newly synthesized strand, which are later joined by an enzyme DNA ligase

Prokaryotic and Eukaryotic Replication Circular chromosome Replication begins at one place along the chromosomes Two replication forks are formed and proceed in opposite directions Replication continues along each fork until they meet an the whole molecule is copied

Prokaryotic and Eukaryotic Replication Each chromosome is long, but not circular At the rate a polymerase adds nucleotides (50/sec), it would take 53 days to replicate the largest human chromosome Instead, replication begins at many points or origins along the DNA, and two forks move in opposite directions In a fruit fly, replication begins simultaneously at about 3500 sites in a DNA molecule

Errors in DNA Replication DNA replication usually occurs with great accuracy (only one error for every billion nucleotides added) How is this done??? Polymerases have repair or “proofreading” functions in the same way the computer checks for spelling errors For example, if A pairs with C, the polymerase removes the C and replaces it with T Click picture for video

Errors in DNA Replication When mistakes do occur, the base sequence of the newly formed DNA differs from the original A change in the nucleotide sequence of a DNA molecule is called a mutation These can have serious effects on the function of a gene – therefore disrupting cell function

DNA Replication and Cancer Mutations can arise and lead to altered cells and organisms The changes allow individuals to survive and reproduce better Sometimes non-repaired mutations can cause cancer Tumors are caused by mutations that affect cell division

How is the exact replication of DNA ensured? By complementary base pairing and proofreading by DNA polymerases

What are replication forks? Areas of DNA where the double helix separates prior to replication