Transmission and Expression of the Genome Flashcards
what is semi- conservative replication?
double stranded DNA separates into 2 strands- one of which is the template strand and the other forms a strand complimentary to the template (i.e one new and one old)
what does DNA polymerase require for a new strand of DNA to be formed?
- template strand
- requires dNTPs
- nucleotides are added at the 3’ end
- RNA primer
what are dNTPs?
deoxynucleoside triphosphates (dNTPs)
NB: The N is substituted for the base e.g. dATP or dCTP etc.
Recruitment of dNTPs and consequent binding via a phosphodiester bond to form sugar-phosphate chain by DNA polymerase causes loss of 2 phosphates
why do we need an RNA primer to replicate DNA?
DNA polymerase can only extend from a double stranded nucleotide. (can’t start from nothing)
So the first enzyme that comes in is Primase to produce a primer which is recognised by DNA polymerase and adds bases in a 5’ to 3’ direction.
Primer is then removed.
what is DNA polymerases role in DNA replication?
Adds 1000 bases per sec to the chain
has proof reading activity- its adds a base, moves back to check it (proof reading), removes it if wrong (by detecting size of double stranded base pair), and then moves on forward e.g C=C bond swapped for C=G bond
explain the mechanism of replication
1) DNA helix is unwound by helicases (energy is required to break H bonds)- leaves two exposed strands
2) DNA is produced in a 5’ to 3’ direction
Because of this, both strands cannot replicate fully. The strand that is replicating away from the fork develops through Okazaki fragments.
3) The leading strand- synthesised continuously as one piece by DNA polymerase towards replication fork- from 5’ to 3’
4) The lagging strand- is synthesised discontinuously in the form of Okazaki fragments which are later joined together by DNA ligase enzyme- from 3’ to 5’
where does DNA replication occur (on the chromosome)?
DNA is too large to have replication occurring at one site
There are multiple origins of replication present along length of chromosome – these are called REPLICATION BUBBLES
Replication bubbles form at multiple origins
Spread out in both directions until united with one another- efficient & faster replication
how are Okazaki fragments made and joined together?
multiple short fragments are being made in the lagging strand therefore multiple RNA primers need to be used
1) the Okazaki fragment has been synthesised with an old RNA primer on it
2) a new RNA primer anneals to the template strand
3) DNA polymerase adds to new RNA primer to start a new Okazaki fragment
4) once it reaches the old RNA primer from the previous Okazaki fragment, it gets removed and replaced by DNA
5) ligase joins the two fragments together.
what methods are used to repair mutations in DNA?
DNA polymerase has proof reading ability
stability of base pairing gives high fidelity
Mismatch repair systems- recognises and repairs insertion, deletion, and mis-incorporation of bases (corrects 99% of errors)
how do mismatch repair mechanisms work?
idk if I need to know this
what chemical reactions can damage DNA?
depurination
deamination
what is depurination?
when a nucleotide loses a purine base (could be pyramiding but less common)
what is deamination?
where cytosine can become uracil
problem arises when pairing because C-G and U-A
how does UV affect DNA?
UV can cause thymidine dimers:
Ultraviolet light is absorbed by a double bond in thymine (and cytosine) bases in DNA. This added energy opens up the bond and allows it to react with a neighbouring thymine and form a dimer
what is mitosis?
cell division of a diploid cell to produce two diploid daughter cells
repair
what is the purpose of mitosis?
cell division/ organismal growth
repair
what are kinetichores?
where spindle microtubules attach to
there are two kinetochores (one for each sister chromatid) at the centromere
what are the different spindle microtubules?
aster microtubules- extend from the spindle pole to the cell membrane
kinetochore microtubules- attach the chromosomes to the spindle pole
interpolar microtubules- extend from the spindle pole to the opposite spindle pole
what are the phases of mitosis?
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
what happens in prophase?
chromosomes condense
organelles called centrosomes (positioned and orientated by astral microtubules) produce kinetochore microtubules and interpolar microtubules (spindle)
they also move apart
what happens in prometaphase?
nuclear envelope breaks down
kinetochore microtubules bind to the kinetochore of the chromosome
chromosomes begin to migrate to the metaphase plate. Interpolar microtubules from different centrosomes, bind together.
what happens in metaphase?
chromosomes fully condensed & located at metaphase plate
(M-phase checkpoint – checks all centromeres are connected to microtubules).
what happens in anaphase?
each centromere splits
sister chromatids of each chromosome pulled to opposite poles due to the kinetochore microtubule (attached to the kinetochore) shortening
The interpolar microtubules extend further aiding in the movement of chromosomes to the poles.
happens fast
what happens in telophase?
chromosomes reach poles & decondense and start to become transcriptionally active again
nuclear membrane reforms
cytoplasm starts to divide.
