2 DNA Replication, The Cell Cycle And Mitosis Flashcards
Q: Why is it referred to as semi conservative replication?
A: each daughter cell inherits one old and one newly synthesised strand (old one was the template)
Q: What are the 2 steps in semiconservative replication?
A: opening of DNA helix
DNA synthesis
Q: How is the DNA helix opened? (2)
A: E- topoisomerase unravels the DNA
DNA Helicase separates the two DNA strands - uses ATP to break hydrogen bonds between base pairs
Q: How is the DNA synthesised? (2) Direction?
A: The new strand is made by DNA Polymerase
DNA Polymerases add dNTPs to the 3’ end of DNA. DNA SYNTHESIS OCCURS IN A 5’ TO 3’ DIRECTION
Q: What does DNA polymerase require? (3)
A: template strand
oligonucleotide primer
supply of deoxynucleotide triphosphates dNTPs
Q: What is an oligonucleotide primer? What is it made of and by?
A: short sequence of nucleotides- made of RNA by specialised RNA polymerase called DNA primase
Q: What is a deoxynucleotide triphosphate dNTP?
A: nucleotide with three phosphates attached
Q: What provides the energy for DNA synthesis?
A: Hydrolysis of triphosphate
Q: How can DNA synthesis be blocked? (3) Give an example.
A: incorporation of modified nucleotides (chain terminators)
they don’t have a free OH group on the 3’ end
once incorporated -> no more can be added
acyclovir is an example
Q: What is the replication fork? Explain the asymmetry of the replication fork.
A: The site of DNA synthesis is called the replication fork - the fork moves along during the process
The two daughter strands have opposite orientations. One runs from 5’ to 3’ and the other runs from 3’ to 5’
Q: Where does replication begin on a DNA molecule?
A: specific point on DNA molecule called the ORIGIN OF REPLICATION- helicase does not work randomly
Q: Why does DNA synthesis differ with the leading and lagging DNA strands? Which DNA strand is the leading one? Summarise what DNA synthesis is like with the leading strand.
A: DNA POLYMERASE CAN ONLY ATTACH NUCLEOTIDES AT THE 3’ END
The LEADING STRAND is the one which has it’s 3’ end closest to the replication fork
DNA synthesis of the leading strand is continuous because it’s 3’ end is in the same direction as the movement of the replication fork
Q: Which DNA strand is the lagging one? Summarise what DNA synthesis is like with the lagging strand.
A: The LAGGING STRAND has it’s 3’ end in the opposite end, away from the replication fork
DNA synthesis of the lagging strand is discontinuous - DNA is synthesised in short pieces called OKAZAKI FRAGMENTS
Q: How does RNA prime the DNA for synthesis? (2) How do the leading and lagging strands differ? Both strands?
A: A type of RNA polymerase called DNA Primase synthesises a short RNA fragment. The fragment is removed at a later stage of replication.
DNA polymerase adds nucleotides to the 3’ end of the RNA primer and continues
For the leading strand, only one RNA primer is needed to start replication at the replication origin
The lagging strand requires multiple
With both strands, DNA is synthesised in the 5’ to 3’ direction
Q: Describe synthesis of the lagging strand. (5)
A: New RNA primer made by primase
DNA polymerase adds nucleotides to the 3’ end of the RNA primer, starting a new Okazaki fragment
Continues to synthesise the until it reaches the end of the previous Okazaki fragment
RNA primer of previous Okazaki fragment is removed and replaced by DNA.
DNA ligase joins the two adjacent Okazaki fragments to growing chain
Q: How are okazaki fragments joined? (3E)
A: Ribonuclease removes RNA primer using 5’ to 3’ exonuclease activity (exonuclease activity - ability of an enzyme to remove one nucleotide at a time from the end of a polynucleotide chain)
Repair DNA Polymerase - replaces RNA with DNA
DNA Ligase - joins two Okazaki fragments together
Q: What are the 2 replication fork proteins? What do they do?
A: Single strand DNA binding protein - prevents the single stranded DNA from locally folding
Sliding Clamp - makes sure DNA polymerase is in the right place
Q: Why is a proofreading mechanism needed? How does it operate? (2)
A: Ensures no mistakes are made because mutations can be dangerous
Before adding a new nucleotide, DNA polymerase checks to make sure previous nucleotide is correct
any incorrect base is removed by 3’ to 5’ exonuclease activity of DNA polymerase and a new correct nucleotide is then added
Q: Explain the replication of the E.coli chromosome. (2) Diagram.
A: replication begins at a unique origin, OriC
2 replication forks proceed simultaneously in opposite directions (each replication fork is synthesising a leading and lagging strand)
the 2 forks meet at the other side of the circular chromosome
Q: Summarise the replication of mammalian chromosomes. (2)
A: Eukaryotic chromosomes are linear and v long -> have multiple replication origins distributed at different intervals
each replication origin gives bidirectional replication forks
replication ends when the forks have met
Q: Define the different phases of the cell cycle. (5)
A: G1, G0, S and G2 are interphase
M phase- mitosis- cell division
G1 phase- Gap phase 1- prior to DNA synthesis
S phase- period of DNA synthesis (replication)
G2 phase- gap phase 2 - between DNA synthesis and mitosis
G0- cells which have stopped dividing
Q: What are chromosomes like in G1?
A: DNA of each chromosome present as a single linear double helix of DNA
Q: What are chromosomes like in S?
A: by the end, the DNA has replicated
Q: What are chromosomes like in G2?
A: each chromosome has 2 identical sister chromatids
Q: What are chromosomes like in M?
A: the 2 chromatids separate to the daughter cells
Q: Summarise how the chromosomes organise at metaphase and then anaphase. Diagram.
A: condensed chromosomes are aligned on the central plane of spindle
sister chromatids are separated and taken to opposite poles by microtubule spindles
