DNA Replication

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DNA Replication

-Watson and Crick's model made sense
-DNA REPLICATION= process by which DNA is copied in a cell (before the cell divides by mitosis, meiosis, or binary fission)

Simple but complicated

-The helical DNA molecule must untwist as it replicates
- DNA must copy its two strands roughly all the time
~Nucleotides are added at a rate of about 50/second in mammals and 500/second in bacteria

Semi-Conservative Replication

-In each new DNA double helix, one strand is from the original and one molecule is new: this type or replication is called semi-conservative replication
-It's called semi-conservative because each of the new DNA molecules has "conserved" (kept) one of the two original DNA strands ("semi" because only 1 was kept, the other was new)

How DNA Replication Happens

-Starts at specific sites on the double helix also called the origins of replications
-Proteins, such as enzymes called helicases, start the process by attaching to the DNA and separating the strands
- Replication proceeds in both directions, creating replication "bubbles" (the Y-shaped region that results when two strands separate is called REPLICATION FORK

DNA Polymerases

-Each strand has a 3' end and a 5' end, which refer to carbon atoms of the nucleotide sugars (at one end the sugar's 3' carbon is attached to an -OH group; at the other the sugar's 5' carbon has a phosphate group
-the sugar phosphate backbone runs in opposite directions
- DNA polymerases~ add nucleotides that are complementary to each of the original DNA strands
- Important because DNA polymerases, the enzymes that link DNA nucleotides to a growing daughter strand, add them only to the 3' end, never to 5' (DNA strands only grow in the 5' to 3' direction

Action at the Replication Fork

-As the replication fork moves along the original DNA, synthesis of one strand follows the movement of the replication fork while synthesis on the other strand moves in the opposite direction (leaving gaps)
-Gaps left by the opposite moving strand are later joined together by the enzyme DNA ligase (because it ligates, or links, the pieces together into a single strand)

Okazaki Fragment

SInce DNA can be replicated in only one direction (nucleotides can be added only at the 3' end) only one of the two strands of a double helix can replicated continuously. The other strand is replicated in small segments (Okazaki fragments) that are joined together by DNA ligase

Last Step of Replication

At the end of replication, DNA polymerases are released. Two new DNA molecules identical to the original DNA molecule result

Prokaryotic Replication

In prokaryotic cells (have one circular chromosome), replication begins at one place along the chromosome but has 2 replication forks formed that proceed in opposite directions (replication continues until they meet and the entire molecule is copied)

Eukaryotic Replication

Nucleotides are added at a rate 50/second in mammals (would take about 53 days to replicate the largest human chromosome); however, the replication begins at many origins along DNA and 2 replication forks move in opposite directions

DNA Errors in Replication

-Only about 1 in a billion errors occur as paired nucleotides are added (DNA polymerases have repair functions to "proofread" DNA)
-When the base sequence is not identical to the original DNA's base sequence, a MUTATION occurs~ a change in the nucleotide sequence of a DNA molecule

Mutations

-Some errors escape repair
-Mutations can have serious effects in the function of an important gene and can disrupt important cell function
-Chemicals, ultraviolet, radiation from the sun, can damage DNA even to the point of cancer
-An effective mechanism for the repair of damaged DNA is crucial for survival

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