Lecture 16 Flashcards
What bases pair with what?
Adenine with thymine (uracil in RNA), Cytosine with guanine.
Why do our cells need to replicate DNA?
To make more cells, this may be required for growth or repair or meiosis.
What model of replication does DNA undergo?
Semiconservative (old with new).
How does prokaryotic DNA replication work?
Prokaryotes contain a single circular chromosome, this is split at a single origin point which is a region rich in adenine and thymine (due to only having two hydrogen bonds). This origin point then expands by splitting along the DNA strands in both directions to form replication forks, as this occurs new DNA nucleotides will attach to the two seperated strands via enzymes. Eventually the replication forks will meet and this will lead to the termination of replication.
What direction does the parental strand run in? Why?
3’-5’, as the daughter strand must grow 5’-3’.
What steps are necessary to copy DNA? which enzymes facillitate these steps?
Addition of new nucleotides (needs nucleotides, done by DNA polymerase III), a starting point (done by the formation of an RNA primer by primase), unwinding of the helical double strand (helicase), release of the tension generated by unwinding (topoisomerase (DNA gyrase) cutting and rejoining DNA strands), prevention of unwound DNA strands from rejoining (single strand binding proteins), the replacement of the RNA primer (DNA polymerase I), joining of newly synthesised fragments together (DNA ligase).
Why is replication semi discontinuous? What are the seperated regions in the lagging strand called?
Replication is semi discontinuous due to having a continuous strand (leading) and a discontinuous strand (lagging strand), the lagging strand is travelling in the 3’ to 5’ direction and as such must be synthesised backwards using fragments known as okazaki fragments.
What bonds are created by DNA ligase?
Phosphodiester bonds join the DNA nucleotides together.
How does DNA polymerase I achieve its two functions?
The RNase activity (removal of the RNA primers) makes use of the endonuclease enzyme to degrade the RNA part, The DNA polymerase activity is done by adding nucleotides (complementary to the parental DNA template of the lagging strand).
When are the X shaped chromosomes visible? Why is this the only time?
Only after DNA replication, this is because the DNA becomes very compacted at this point.
How does eukaryotic replication of DNA differ from prokaryotes?
Has multiple linear chromosomes rather than a single circular one, has multiple origins of replication rather than just one, both prokaryotic and eukaryotic replication of DNA are bidirectional.
How does DNA polymerase III proof read?
Newly inserted bases are checked against the template by 3’ to 5’ exonuclease found on the DNA polymerase III. If an incorrect base is found it is removed by the 3’ to 5’ exonuclease and DNA polymerase III is moved back to repeat the process.
What kind of things can cause errors in the DNA after DNA replication? How can these be corrected generally?
Incorrectly inserted bases by DNA polymerase III not corrected, radiation damage( e.g UV causing formation of pyrimidine dimers), chemical modifications of bases (natural and chemical causes).
These must be repaired by endonucleases and the general process is the incorrect base is spotted, the endonuclease removes it and flanking DNA regions, DNA polymerase will then redo this removed segment and DNA ligase will join it up to the existing DNA.
What is a consequence of uncorrected DNA errors?
The error becomes a permanent part of the DNA template (permanent DNA change, a mutation).
What is PCR?
Polymerase chain reaction (PCR) is an in vitro(outside of an organism) process done in labs which acts to amplify targeted DNA regions. It utilises cycles of heating and cooling and leads to rapid exponential increase in DNA molecules.
