ch14: DNA structure and function (eukaryotes) Flashcards
eukaryotic replication is more complicated because (2)
- larger amount of DNA in multiple chromosomes (a lot more to copy)
- linear structure compared to circular (need to deal with ends instead of circles)
the basic enzymology is similar because
it just requires new enzymatic activity for dealing with the ends
eukaryotic organisms must have —— origin of replication
more than one
- multiple chromosomes
- more DNA to copy
- multiple origins of replication for each chromosome
eukaryotic origins of replication are (4)
- not sequence specific, can be adjusted
- depends on chromatin structure
- number of origins can be adjusted
- each origin opens up only once per DNA replication cycle
initiation requires more factors (2)
- to assemble helicase and primase complexes onto template
- to load polymerase with sliding clamp unit
primase is more complex (2)
- complex of an RNA polymerase and a DNA polymerase
- first makes short RNA primers, then extends with DNA
elongation
main replication polymerase is a complex of 2 DNA polymerases
similarities of both prokaryotic and eukaryotic DNA replication: 1) DNA is constrained and must be unwound to be replicated
prokaryotic: because it’s circular
eukaryotic: because it’s wound around histones
similarities of both prokaryotic and eukaryotic DNA replication: 2) each origin of replication opens up
only once per DNA cycle
similarities of both prokaryotic and eukaryotic DNA replication: 3) DNA replication proceeds in
both directions around the chromosome, always elongating in the 5’ to 3’ direction
similarities of both prokaryotic and eukaryotic DNA replication: 4) a DNA primase (RNA polymerase) is
necessary to start DNA replication
similarities of both prokaryotic and eukaryotic DNA replication: 5) a DNA polymerase must
remove the RNA primer and replace with DNA bases
differences between prokaryotic and eukaryotic DNA replication (3)
pro: single circular molecule of DNA
euk: multiple linear molecules of DNA
pro: have one single origin of replication
euk: have more tha one origin of replication
pro: fewer enzymes driving replication
euk: larger protein complexes and more enzymes driving replication
why is eukaryotic DNA linear
we have nucleases in our cells that recognize and chop up free DNA which protects us against viruses; we went to protect the chromosome ends
gradual shortening of chromosomes (3)
- unable to replicate last section of lagging strand
- if gene sequences are lost, then the DNA is defective
- so DNA has to be added to the ends of chromosomes to form a buffer
telomeres (3)
- composed of short repeated sequences of DNA
-specialized structures found on the ends of eukaryotic chromosomes - protect ends of chromosomes from nucleases and maintain the integrity of linear chromosomes
telomerase (3)
- enzyme makes telomere using an internal template (not the DNA itself)
- slides over, makes repeated copies of telomere
- same sequence over and over again
what does it mean when it is said that “telomerase is developmentally regulated”
telomerase activity is high during embryonic and childhood development in humans, low in most adult somatic cells, except stem cells and other cells that continuously divide
normal somatic cells dont generally express telomerase because
they only divide a finite amount of times
when telomeres are gone: (3)
- important genes may be lost from the ends of chromosomes
- cell might think the end is a DNA double-strand break and try to repair it (might perform crossover with incorrect DNA; might try to connect two chromosome ends)
- result in aging/cell death (apoptosis)
not necessarily a good thing to have extra long telomerase because
around 90% of all cancers have unregulated telomerase; allows them to divide uncontrollably
mutagen
any agent that increases the number of mutations above background level
DNA repair systems: two categories
1) specific DNA repair: targets a single kind of lesion in DNA and repairs only that damage
2) nonspecific DNA repair: use a single mechanism to repair multiple kinds of lesions in DNA
specific DNA repair: photorepair (2)
- only repairs damage caused by UV light
- photolyase is an enzyme that used energy from visible light to cleave thymine dimers
nonspecific repair: excision repair
damaged region is removed and replaces though DNA synthesis
1. recognition of damage
2. removal of the damaged region
3. resynthesis using the information on the undamaged strand as a template
error-prone repair
when all other repair systems fail, enzyme just sticks in any base and hopes the cell survives (in e. coli this is called the SOS response)
