ch14: DNA structure and function (eukaryotes)

Question 1

eukaryotic replication is more complicated because (2)

Answer 1

• 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)

Question 2

the basic enzymology is similar because

Answer 2

it just requires new enzymatic activity for dealing with the ends

Question 3

eukaryotic organisms must have —— origin of replication

Answer 3

more than one
- multiple chromosomes
- more DNA to copy
- multiple origins of replication for each chromosome

Question 4

eukaryotic origins of replication are (4)

Answer 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

Question 5

initiation requires more factors (2)

Answer 5

• to assemble helicase and primase complexes onto template
• to load polymerase with sliding clamp unit

Question 6

primase is more complex (2)

Answer 6

• complex of an RNA polymerase and a DNA polymerase
• first makes short RNA primers, then extends with DNA

Question 7

elongation

Answer 7

main replication polymerase is a complex of 2 DNA polymerases

Question 8

similarities of both prokaryotic and eukaryotic DNA replication: 1) DNA is constrained and must be unwound to be replicated

Answer 8

prokaryotic: because it’s circular
eukaryotic: because it’s wound around histones

Question 9

similarities of both prokaryotic and eukaryotic DNA replication: 2) each origin of replication opens up

Answer 9

only once per DNA cycle

Question 10

similarities of both prokaryotic and eukaryotic DNA replication: 3) DNA replication proceeds in

Answer 10

both directions around the chromosome, always elongating in the 5’ to 3’ direction

Question 11

similarities of both prokaryotic and eukaryotic DNA replication: 4) a DNA primase (RNA polymerase) is

Answer 11

necessary to start DNA replication

Question 12

similarities of both prokaryotic and eukaryotic DNA replication: 5) a DNA polymerase must

Answer 12

remove the RNA primer and replace with DNA bases

Question 13

differences between prokaryotic and eukaryotic DNA replication (3)

Answer 13

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

Question 14

why is eukaryotic DNA linear

Answer 14

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

Question 15

gradual shortening of chromosomes (3)

Answer 15

• 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

Question 16

telomeres (3)

Answer 16

• 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

Question 17

telomerase (3)

Answer 17

• enzyme makes telomere using an internal template (not the DNA itself)
• slides over, makes repeated copies of telomere
• same sequence over and over again

Question 18

what does it mean when it is said that “telomerase is developmentally regulated”

Answer 18

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

Question 19

normal somatic cells dont generally express telomerase because

Answer 19

they only divide a finite amount of times

Question 20

when telomeres are gone: (3)

Answer 20

• 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)

Question 21

not necessarily a good thing to have extra long telomerase because

Answer 21

around 90% of all cancers have unregulated telomerase; allows them to divide uncontrollably

Question 22

mutagen

Answer 22

any agent that increases the number of mutations above background level

Question 23

DNA repair systems: two categories

Answer 23

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

Question 24

specific DNA repair: photorepair (2)

Answer 24

• only repairs damage caused by UV light
• photolyase is an enzyme that used energy from visible light to cleave thymine dimers

Question 25

nonspecific repair: excision repair

Answer 25

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

Question 26

error-prone repair

Answer 26

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)