DNA replication and Telomeres Flashcards
DNA polymerase
catalyzes the addition of nucleotides to the 3’ end of a growing strand of DNA using a parent DNA strand as a template
also proofreads nucleotides added and removes those that are paired incorrectly
DNA helicase
uses the energy of ATP hydrolysis to unwind and separate the DNA double helix ahead of the replication fork
single-strand DNA binding protein
binds to single-stranded DNA exposed by DNA helicase and prevents base pairs from re-forming before the lagging strand can be replicated (aka keeps DNA double helix from reforming during replication)
DNA topoisomerase
produces transient breaks (aka nicks) in one strand of DNA double helix to relieve the tension built up by the unwinding of DNA ahead of the DNA helicase
reseals break after DNA has relaxed
sliding clamp (protein clamp)
keeps DNA polymerase attached to the template, allows the DNA polymerase to move along without falling off
primase
synthesizes RNA primers so that DNA polymerase can initiate synthesis
DNA ligase
uses the energy of ATP hydrolysis to join Okazaki fragments made on the lagging strand template
what were the three proposed models of DNA replication?
a. Conservative: the original/parental DNA molecule remains intact and an entirely new DNA molecule is synthesized
b. Semiconservative: each of the 2 strands of the parental DNA molecule serves as a template for the synthesis of a new strand
c. Dispersive: the original DNA molecule is fragmented and the daughter molecules consist of a mix of old and new DNA scattered in pieces throughout both strands
Name and describe the experiment that proved that DNA replication is semi-conservative
Meselson-Stahl Experiment
1. labeled DNA with nitrogen isotopes (15N-heavy and 14N-light) and grew DNA in the two mediums
2. used density gradient centrifugation (centrifuged at high speed for 48 hours to form cesium chloride density gradient): 15N-heavy will form a band near the bottom of the centrifuge tube and 14N-light will form a band near the top of the tube
3. after 1 generation, both daughter DNA molecules were of INTERMEDIATE weight
–> demonstrated that DNA replication is semi-conservative
what are replication origins and how do they differ between eukaryotes and prokaryotes?
replication origins: where DNA synthesis begins (typically in TATA regions)
Prokaryotes: only one origin of replication
Eukaryotes: multiple origins of replication; can simultaneously replicate regions of DNA
If DNA polymerase only works in the 5’ to 3’ direction, how do you duplicate both strands simultaneously?
asymmetry of the replication fork (leading - continuous and lagging - fragmented strands)
DNA made on lagging strand is made of short 5’ → 3’ sequences
how far could DNA replication go if the following protein was inactivated and how would the leading strand vs lagging strand be affected?
- DNA polymerase
- DNA ligase
- sliding clamp for DNA polymerase
- nuclease that removes RNA primers
- DNA helicase
- primase
DNA polymerase: RNA primers would be laid down at origin
DNA ligase: Lagging strands would be complete but unconnected fragments
Sliding clamp for DNA polymerase: Slows down the rate of DNA synthesis since DNA polymerase would fall off frequently
Nuclease that removes RNA primers: Lagging strand will have unconnected fragments of RNA and DNA
DNA helicase: Replication could not start since the DNA strands would be stuck together
Primase: RNA primers cannot begin on the leading or lagging strand → no DNA
when, where, and why does DNA replication occur?
when: occurs during interphase (before mitosis or meiosis) before cell divides
where: in nucleus for eukaryotic cells
why: before a cell undergoes division, it needs to get more DNA into the new daughter cells
describe the steps for leading strand replication
- DNA replication starts at the replication origin where helicase comes in to unwind the 2 strands: SSB proteins (single stranded binding) bind to the unzipped DNA strands to make sure they don’t come back together and topoisomerase keeps the DNA strands from getting supercoiled
- Primase makes RNA primers for both strands
- DNA polymerase comes in and begins adding onto the RNA primer, starting replication from 5’ to 3’ direction with no issues (same direction as the unzipping)
describe the steps of lagging strand replication
same as leading strand until DNA polymerase comes in on step 3.
1. primase continues to synthesize new RNA primer and DNA polymerase needs to keep backtracking to build the new DNA strand → new primers need to be placed constantly → creates DNA fragments called okazaki fragments
3. the new okazaki fragments are extended to the previous adjoined fragments
4. nuclease removes previous RNA primers
5. repair polymerase replaces RNA primer with DNA sequences
5. DNA ligase: seals the gaps between the okazaki fragments and glue them together
what is the problem with replicating the lagging strand of linear chromosomes?
lagging strand replication cannot be completed to the end of the chromosome
RNase H removes the RNA primers and then DNA polymerase comes in and adds the appropriate DNA nitrogen bases at the 3’ end → however, the last primer at the end of the lagging strand ends with a 5’ end, so DNA polymerase cannot add DNA there → this results in a shortening of the lagging strand each time replication/cell division occurs since some of the genome is not included in the replication → can lose important genes → this problem is solved by telomerase replication
How is the issue of telomere shortening solved?
the telomerase enzyme has its own primer (AAUCCC) and it elongates the parent strand adding repeating sequences of TTAGGG → the last two bases of DNA strand pair up with the last two bases and then telomerase enzyme drives DNA synthesis to elongate the original/parents DNA strand more (cannot synthesize the newly made strand since it is 5’ end) → the RNA primer lays down another template at the very end of the newly added part of DNA strand → DNA polymerase starts to add DNA to the 3’ end of the primer → back to the same situation where RNase H removes the primer but now FEN1 can remove the overhand flap without losing any actual parent DNA/genome
what is the relation between telomere length and aging?
Telomere length shortening plays a role in aging since there is an innate limit (hayflick limit) to the number of times a cell can divide in its lifetime before it undergoes senescence. This shortening is associated with aging as well as aging-related diseases (there are more senescent cells in older humans than young humans and cells senesce earlier in older humans as well since their telomere lengths are shorter)
telomeres
The protective caps at both ends of each chromosome
How does telomerase contribute to cancer?
Telomerase is the enzyme that extends the telomere sequence at the ends of the chromosomes to prevent the loss of genetic info during replication. It plays a role in cancer since it keeps extending the telomeres and thus chromosome stability increases and the cell never undergoes the normal cycle of senescence, which eventually leads to biological immortality of the cell and unchecked proliferation, which leads to cancer.
What are the major findings of Hahn et al 1999 Nature 400:464?
The first example of transforming normal human cells into tumorigenic cells by adding large amounts of hTERT, a RAS allele, and Large-T. It was the creation of human tumor cells with defined genetic elements. The experiment showed that an increase in hTERT activity leads to an increase in telomere length and thus chromosome stability, which gives way to biological immortalization of Large-T expressing HEK and BJ cells
by co-expressing telomerase (hTERT), simian virus 40 (SV40) large-T oncoprotein (SV40-T Ag), and an oncogenic allele of H-RAS (RAS), normal human cells could become tumorigenic
what is the full name of TERT and its importance of TERT expression and activity in cancer?
Telomerase Reverse Transcriptase
TERT is the catalytic subunit of telomerase. In cancer, TERT expression and activity is increased, which results in cells avoiding the usual senescence and leads to unchecked proliferation and ultimately cancer (most normal somatic cells, except for germ cells and stem cells, do not have TERT expression activated)
how is TERT and telomerase regulated in human cells?
Chromosomes in majority of human somatic cells have a set length of telomeres at their ends → each time cell divides, the telomeres’ length decrease until it runs out → cell undergoes senescence
Majority of somatic cells do NOT have telomerase/TERT activated
most normal somatic cells, except for germ cells and stem cells, do not have TERT expression activated (telomerase is not active in most human cells)
what are stem cells and germ cells?
stem cells: unique cells in the body with the ability to divide and develop (differentiate) into specialized cells with specific functions (can self-renewal for long periods of time –> why they have telomerase activity activated)
germ cells: sperm and egg cells
