DNA Replication, the cell cycle and mitosis Flashcards
DNA replication is semi-conservative
Each daughter cell inherits 1 old and 1 new strand
Topoisomerase
Unwinds DNA
DNA polymerase
Synthesises the new strand
Adds nucleotides to the 3’ end
DNA polymerases require
Template strand
Oligonucleotide primer
Supply of deoxynucleotide triphosphates (dNTPs)
DNA (and RNA) synthesis occurs in a … direction
5’ to 3’
Replication begins at specific points on the DNA molecule called …
Origin of replication
What is the site of DNA synthesis is called?
Replication fork
What do the templates for the 2 new daughter strands have?
opposite orientations: 3’ to 5’ and 5’ to 3’
How is the leading strand synthesised?
synthesised continuously
has its 3’ end closest to replication fork
How is the lagging strand synthesised?
synthesised in short pieces- Okazaki fragments
has its 3’ end away from the replication fork
RNA primes the synthesis of new DNA
DNA primase (an RNA polymerase) synthesises a short RNA fragment -only transient, removed at a later stage of replication
DNA polymerase adds to 3’ end of RNA primer and continues to synthesise the Okazaki fragment…
Until it reaches the end of the previous Okazaki fragment
Old RNA primer erased and replaced by DNA
DNA ligase joins new Okazaki fragment to growing chain
Ribonuclease
removes RNA primer using a 5’ to 3’ exonuclease activity
Repair DNA polymerase
replaces RNA with DNA
DNA Ligase
joins 2 Okazaki fragments together
Single strand DNA binding protein
Prevents the single stranded DNA from locally folding
Sliding clamp
Ensures DNA polymerase is in the right place
Lagging strand forms a loop
So both daughter strands can be synthesised in a coordinated manner
What does the proofreading mechanism do?
Ensures no mistakes are made because mutations can be dangerous
Proofreading mechanism
Before a new nucleotide is added, DNA polymerase checks the previous nucleotide for correct base-pairing
Incorrect bases are removed by 3’ to 5’ exonuclease activity
In E.coli, replication starts at a unique origin, OriC
2 replication forks proceed simultaneously in opposite directions.
2 forks meet at the other side of the circular chromosome
Eukaryotes have large, linear DNA so there are multiple origins of replication
Each replication origin gives bi-directional replication forks.
Replication is finished when all the forks have met
M phase: Mitosis
2 chromatids separate to the daughter cells
G1 phase: Gap 1
DNA of each chromosome present as a single linear double helix of DNA
S phase: Synthesis
DNA is replicated
G2 phase: Gap 2
each chromosome has 2 identical sister chromatids
G0 phase
cells have stopped dividing
Late prophase
Condensed chromosomes,
each contains 2 sister chromatids
Metaphase
Condensed chromosomes,
aligned on central plane of spindle
Anaphase
Sister chromatids separate and are pulled to spindle poles
Telophase
Sister chromatids move to opposite poles of spindle
Cytokinesis
Division of cytoplasm
Interphase (G1)
Condensation process reversed
Flow of genetic information
DNA replication
Transcription
Translation
DNA helicase
separates DNA strands by using ATP as a source of energy to break hydrogen bonds
Deoxynucleotide triphosphates (dNTPs)
nucleotide with 3 phosphates attached
Oligonucleotide primer
made of RNA by specialised RNA polymerase: DNA primase
What drives the DNA synthesis? (Energy)
Hydrolysis of triphosphate releases energy
Exonuclease activity
ability of an enzyme to remove 1 nucleotide at a time from the end of a polynucleotide chain
