book 3 (DNA replication) Flashcards
1
Q
describe the process of DNA replication
A
- replication begins at origins of replication
- DNA helicase recognises & binds to the origins of replication, causing the DNA molecule to unwind and unzip by breaking the hydrogen bonds between the bases
- DNA strands are separated to form a replication bubble whereby both DNA strands serve as templates for DNA replication - single-stranded binding proteins binds to separated strands of parental DNA to stabilise and prevent the 2 complementary strands from renaturing
- DNA primase attaches to DNA template strand & synthesises an RNA primer that is complementary to the template
- DNA polymerase III adds free deoxyribonucleotides to the free 3’-OH ends of existing polynucleotides complementary to the template strand
- DNA polymerase III catalyses the formation of a phosphodiester bond between adjacent nucleotides
- daughter strands are elongated in the 5’ to 3’ direction as DNA polymerase can only add free deoxyribonucleotides to free 3’-OH group of existing polynucleotide
- one of the daughter strands, called leading strand, is synthesised continuously & the other strand is the lagging strand, synthesised discontinuously in the form of Okazaki fragments - DNA polymerase I hydrolyses the RNA primer and fills the gaps with complementary deoxyribonucleotides
- DNA ligase catalyses the formation of phosphodiester bonds between 2 Okazaki fragments
- at the end, both parental strand and daughter strands rewind into a double helix, whereby each resultant double helix consists of one template where each resultant double helix consists of one template parental strand and one newly-synthesized daughter strand.
2
Q
explain the end replication problem
A
- for linear DNA, DNA polymerase is unable to fill in the gaps when the last RNA primer on the lagging strand is being removed as there is no free 3’-OH groups available for DNA polymerase to add deoxyribonucleotides to complete the 5’ ends of the daughter DNA strand
- as a result the 3’ end of template DNA is not copied to the 5’ end of the daughter DNA strand, resulting in staggered ends, 3’ overhang
- repeated rounds of replication produces shorter and shorter DNA molecule, hence resulting in progressive telomere shortening each time a cell divides.
3
Q
explain the structure of telomeres
A
- non-coding repetitive DNA sequences found at the ends of linear DNA molecules of eukaryotic chromosomes
- consists of short repeated sequences
- eg 5’-TTAGGG-3’ sequence
4
Q
explain the functions of telomeres
A
it serves as a disposable buffer to protect the coding DNA from erosion during DNA replication as DNA shortens with each round of replication
5
Q
explain the functions of protective nucleoprotein cap
A
- protects the end of the chromosomes from degradation by nucleases
- prevents the end-joining of chromosome ends which may lead to chromosomal mutations
- prevents staggered chromosomal ends from activating the cell’s DNA damage monitoring system and triggering a cell signalling pathway that leads to unintentional cell death.
6
Q
explain what is meant by semi-conservative replication
A
- whereby both DNA strands unzip, unwind & is separated via the breaking of hydrogen bonds between complementary base pairs
- each DNA strand acts as a template for the synthesis of a complementary DNA strand
- Overall resulting in the formation of 2 newly synthesised DNA molecules, each comprising of one original parental strand and one daughter strand
7
Q
Explain why replication only occurs in a 5’ to 3’ direction
A
- DNA polymerases can only add free deoxyribonucleotides to the 3’-OH end of an existing polynucleotide chain as the free 3’OH group is needed for condensation reaction during the formation of
phosphodiester bonds - active sites of DNA polymerase has a specific 3D conformation that is complementary to that of the conformation of the substrates ( 3’OH groups & 5’ phosphate groups)
