Nucleic acids 2-DNA Replication, the cell cycle and mitosis Flashcards
What type of replication occurs in DNA?
Semi conservative- Each daughter cell inherits one old and one new strand.
Explain how cells duplicate DNA (3)
The DNA helix is unwound by DNA helicase, an enzyme that uses ATP as source of energy to break hydrogen bonds between base pairs.
New DNA is synthesised by enzymes called DNA polymerases. DNA polymerases require a template strand, an olinucleotide primer , and a supply of of deoxynucleotide triphosphates
(dNTPs)
DNA polymerases add dNTPs to the 3’ end of a DNA molecule.
Describe the replication fork (3)
The site of DNA synthesis is called a replication fork: the fork moves along during the process.
The templates for the two new daughter strands have opposite orientations: 3’ to 5’ and 5’ to 3’
The replication fork is asymmetric. Both strands are synthesised in a 5’-3’ direction. The leading strand is
synthesised continuously, whereas the lagging strand is synthesised in short pieces termed Okazaki fragments
(bottom strand)
What are RNA primers (3)
RNA primes the synthesis of new DNA
A specialised RNA polymerase called DNA primase synthesises a short RNA fragment (~ 5 nucleotides). The RNA primer is only transient and removed at a later stage of replication.
For the synthesis of the leading strand, an RNA primer is needed only to start replication at a replication origin.
What is the synthesis of the lagging strand (5)
DNA primase synthesises multiple short RNA fragment primers
DNA polymerase adds to RNA primer, starting the Okazaki fragment, and finishes the particular section of
the DNA fragment
A special ribonuclease removes RNA primer using a 5’ to 3’ exonuclease activity.
A repair DNA polymerase then replaces RNA with DNA.
DNA ligase joins the two fragments together using ATP. This makes the DNA strand continuous.
What does the sliding clamp and single stranded DNA binding proteins make sure?
The sliding clamp makes sure the DNA polymerase stays attached during DNA synthesis
Single stranded DNA-binding proteins make sure the DNA remains extended
How accurate is DNA replication (3)
DNA replication has an error frequency of about 1 change per 109 base pairs.
Any incorrect bases are removed by 3’ to 5’ exonuclease activity of DNA polymerase. A new, correct nucleotide is then added.
Inaccurate RNA primers are replaced by accurate DNA
How does Replication of the E.coli chromosome occur
In E.coli, replication starts at a unique origin, OriC.
Two replication forks proceed simultaneously in opposite directions (creates a loop shapes of new DNA
forming).
The two forks meet at the other side of the circular chromosome. This is known as BI-DIRECTIONAL
REPLICATION
Describe the cell division cycle
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
G1, G0, S, and G2 are Interphase
Describe chromosome activity and visibility in mitosis
1) Interphase G2: Chromosomes not visible
2) Late prophase: Condensed chromosomes, each contains two sister chromatids
3) Metaphase: Condensed chromosomes, aligned on central plane of spindle
4) Anaphase: Sister Chromatids move to opposite poles of spindle. Centromere splits, ensuring each new cell
gets one half of new chromosomes
5) Telophase: Two daughter cells formed
6) Interphase G1: Condensation process reversed
How can nucleosides be used as drugs?
they act as chain terminators e.g. Acyclovir- herpes, and dideoxycytosine (ddC), Chain terminators do not have a free 3’ hydroxyl group, therefore polymerase cannot add anymore free dNTPs
What are the points where dna replication begin known as
Replication begins at discrete points on the DNA molecule called origin of replication.
What does the looping of the template for the lagging strand makes sure?
Two DNA polymerase can work together closely, and the orientation of the two DNA synthesis strands are the same.
