DNA Replication

DNA Replication

The cell completely replicates the DNA within the nucleus

When does replication occur?

The S-Phase (synthesis) of Interphase

3 steps of replication

1.) initiation 2.) elongation 3.) termination

Initiation

Replication begins at “origins of replication” found multiple times on a eukaryotic chromosome and once on a prokaryotic chromosome. Helicase unwinds the DNA into two strands at the replication fork. Primase creates a primer that tells the new strand where to start.

Helicase

Enzyme that unwinds the double helix into two strands by breaking the hydrogen bonds which hold the bases together.

Replication fork

The part of the DNA which is unwound. Where the new strand is formed

Single-stranded binding proteins

Hold the replication fork open

Topoisomerase

Enzyme that prevents the unwound strands from supercooling.

Primase

Enzyme which places an RNA primer on the starting point of the new strand. The primer is necessary for the DNA to build on

Elongation

DNA Polymerase builds the new DNA strand using the original as the template. DNA is only synthesized in the 5’→3’ direction, creating a leading and lagging strand.

DNA Ploymerase

Adds nucleotides to the new DNA strand that starts at the RNA primer

Leading strand

Since DNA is built 5’→3’, one strand is built continuously: the leading strand

Lagging strand

The other strand being built cannot be built continuously because it has to wait for enough of the template strand to be unwound. The strand is built in fragments, which are later connected.

Okazaki fragments

The individual fragments of the lagging strand

Ligase

Enzyme which fills in the gaps left by the Okazaki fragments.

Semi conservative

Each new DNA strand contains one original strand and one new strand

Proofreading

DNA polymerase can “check” the bases to make sure they’re correct

Mismatch repair

Replacement of incorrect bases after the completion of a DNA strand

Telomeres

Repetitive bases at the end of a DNA strand. They do not code for anything. They protect the DNA from deteriorating during repeated replication. The full strand cannot be completed due to lagging strands so DNA replication is limited.

Apoptosis (replication)

Since strand get shorter after every replication, they eventually get too short, so apoptosis occurs. Apoptosis is programmed cell death

Telomerase

Lengthens telomeres during fetal development. Deactivates at a certain point, but reactivates in some mutated cells and can contribute to cancer

Central Dogma of Genetics

The flow of genetic information is conserved. DNA is the template for RNA, which is translated to make protein. All living things go through replication, transcription, and translation similarly.

Retroviruses

Don’t follow central dogma. Use RNA to make DNA to make RNA to make proteins. Some primitive viruses can’t even make DNA. Prions contain proteins that replicate