Chapter 13: Replication

Question 1

how does DNA replication occur in eukaryotes?

Answer 1

the DNA strand is split into two single strands, each strand is then used a template for the synthesis of a new complementary strand, the new strands are called daughter strands and the original strands are called parental strands

Question 2

how does DNA replication occur in bacteria?

Answer 2

replication begins at the one origin of replication and runs bidirectionally as two replication forks move in opposite directions away from the origin

Question 3

what happens during the initiation step of replication in bacterial DNA replication?

Answer 3

DnaA proteins bind to sequences in the origin of replication next to the AT-rich region, other proteins cause the region to bend around the DnaA proteins which causes the two strands on the AT-rich region to separate, DNA helicase (DnaB) and DnaC then use ATP to unwind the DNA in both directions away from the origin creating two replication forks

Question 4

what makes up the replisome complex and where is it located in bacterial DNA replication?

Answer 4

DNA polymerase holoenzyme and the primosome make up the replisome region at the replication fork

Question 5

what are the 7 issues that must be resolved in DNA replication of bacteria?

Answer 5

1. unwinding of the helix
2. reducing coiling generated during unwinding
3. synthesis of an RNA primer for initiation
4. discontinuous synthesis of the second strand
5. removal of the RNA primers
6. joining the gap-filling DNA to the adjacent strand
7. proofreading

Question 6

what proteins are responsible for unwinding the DNA double helix in bacterial DNA replication?

Answer 6

DNA helicase breaks the H bonds between the strands, topoisomerase II (DNA gyrase) relaxes supercoiliing in front of the DNA helicase, and single-stranded binding proteins keep the two strands separated until after they are copied

Question 7

what proteins are responsible for synthesizing DNA in bacterial DNA replication?

Answer 7

primase creates one RNA primer for the leading strand and multiple RNA primers for the lagging strand, DNA polymerase III synthesizes the DNA daughter strand using the RNA primers on the strands as a template to add nucleotides on at a time, then DNA polymerase I removes the primers and fills in the gaps, and DNA ligase links the two strands together

Question 8

what makes up the primosome complex and where is it located in bacterial DNA replication?

Answer 8

DNA helicase and primase are bound together to form the primosome which is directly attached to the replisome where it leads the way at the replication fork

Question 9

what occurs at the replisome complex in bacterial DNA replication?

Answer 9

two DNA polymerase III proteins replicate both the leading (continuous) and lagging (discontinuous) strands together, together they form a dimeric DNA polymerase that moves as a unit towards the replication fork

Question 10

how does proofreading work in bacterial DNA replication?

Answer 10

as the strand passes through DNA polymerase any mismatched base pair will cause strand to enter the exonuclease site where it is digested until the incorrect nucleotide is removed

Question 11

how does DNA replication occur in eukaryotes?

Answer 11

DNA replication proceeds bidirectionally from many different origins of replication and runs until the replication forks meet

Question 12

how does initiation of DNA replication occur in eukaryotes?

Answer 12

a pre-replication complex (preRC) is assembled when the origin recognition complex (ORC) that acts as the initiator or replication binds to MCM helicase which complete DNA replication licensing and allows replication to proceed when 22 more proteins activate the MCM helicase

Question 13

what proteins are responsible for synthesizing DNA in eukaryotic DNA replication?

Answer 13

alpha, delta, and epsilon polymerase synthesize the nuclear DNA while gamma polymerase synthesizes the mitochondrial DNA, epsilon synthesizes the leading strand and delta synthesizes the lagging strand while alpha, delta, and epsilon also act as lesion-replicating polymerases that can cross regions of damage

Question 14

what makes and removes RNA primers in eukaryotic DNA replication?

Answer 14

DNA polymerase delta elongates the okazaki fragment until it hits another primer, creating a flap, flap endonuclease then follows behind and removes the flap until the entire primer was been lifted and removed, then DNA ligase seals the two fragments together

Question 15

what causes the DNA fragments to shorten each time in eukaryotic DNA replication?

Answer 15

DNA polymerases synthesize DNA only in the 5’ to 3’ direction and cannot initiate DNA synthesis without a primer meaning they cannot synthesize the end of the lagging strands because there is no room for a primer at the very end of the strand

Question 16

how do telomeres solve the issue of shortening DNA fragments observed in eukaryotic DNA replication?

Answer 16

telomeres are special DNA repeat sequences at the end of the DNA, the length of these telomeres is maintained by telomerase which contains protein and RNA that is complimentary to the DNA sequence of the telomeric repeat which allows telomerase to bind the the overhang of the telomere

Question 17

what is homologous recombination?

Answer 17

the idea that similar DNA segments break and rejoin to form new combinations during crossing-over in meiosis resulting in gene conversion

Question 18

what is gene conversion?

Answer 18

the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical, creating two different identical alleles

Question 19

what is the DNA mismatch repair theory of gene conversion?

Answer 19

branch migration during homologous recombination travels down the sequence, if it passes a region that has a minor difference in sequence such as base pairs that don’t match the DNA mismatch repair will yield 4 possible options to repair the error, if the same option is used for both then the two alleles become the same

Question 20

what is the gap repair synthesis theory of gene conversion?

Answer 20

if a double-stranded break occurs in a chromosome and erases the recessive gene encoded there then the dominant allele in an adjacent chromosome could create a D loop that causes the gap repair synthesis to use strands from the dominant allele to fill in the gap in the recessive allele, making both chromosomes now carry the dominant allele