Ch 16 DNA Replication

Question 1

Properties of DNA

Answer 1

• double stranded helix
• uniform diameter
• right handed twist
• chargaff’s rule A=T, C=G
• Strands are anti-parallel

Question 2

semiconservative DNA replication

Answer 2

DNA strands separate and each one serves as a template to copy a second strand. End up with two helixes that both have one new strand and one old strand.

Question 3

four requirements of semiconservative DNA replication

Answer 3

a. Energy to break apart helix and fuel enzymes
b. Enzymes to do the work of copying the DNA
c. Nucleotides (A, T, C, G) prepared by cells in advance
d. Sequence/DNA template

Question 4

Describe how DNA acts as its own template.

Answer 4

Once the DNA splits apart, a copy is synthesized based on the nucleotides of the old strands (one is leading, one is lagging

Question 5

antiparallel

Answer 5

Side by side in opposite directions, 3’ to 5’, 5’ to 3’

Question 6

leading strand

Answer 6

runs 5’ to 3’ (DNA pol III can synthesize continuously)

Question 7

lagging strand

Answer 7

runs 3’ to 5’ (DNA pol III must synthesize in segments)

Question 8

Okazaki fragments

Answer 8

segments between primer on lagging strand

Question 9

nuclease

Answer 9

DNA cutting enzyme (“nucle”, “-ase”) for nucleotide excision repair of errors

Question 10

telomere

Answer 10

Eukaryotic cells have linear chromosomes shaped like Xes, so origin of replication is not also the end. There’s no room on the lagging strand to replace the 5’ end. Telomeres are “nonsense” DNA repeating itself over and over found at the end of chromosomes, so no information is lost when the end is not replicated.

Question 11

helicase

Answer 11

unzipping, breaking through hydrogen bonds that hold DNA nitrogenous bases together

Question 12

single stranded binding

Answer 12

binds to and stabilizes single stranded DNA until it can be used as a template so that they don’t reconnect

Question 13

Topoisomerase

Answer 13

“nicks” the helix so that it doesn’t snap

Question 14

Primase

Answer 14

makes the primer which instructs polymerase where to begin working (made of RNA)

Question 15

DNA polymerase I

Answer 15

removes RNA nucleotides of primer from 5’ end and replaces w/ DNA. proofreads.

Question 16

DNA polymerase III

Answer 16

the builder. adds nucleotides onto the helix one at a time

Question 17

Ligase

Answer 17

glue, takes care of gaps between Okazaki fragments (primase has to continue to tell polymerase where to build, which leaves gaps of RNA that need to be replaced with DNA)

Question 18

limitations of DNA pol III

Answer 18

Has to build DNA in the 5’ to 3’ direction

Cannot attach to DNA w/o a 3’ end

Question 19

DNA replication - leading strand

Answer 19

DNA alpha helix opens up, unzipped by helicase enzyme

2. Topoisomerase nicks the helix so that it doesn’t snap
3. Single stranded binding proteins lay on both sides of the helix so that it doesn’t reform (strands don’t bind together again)
4. Primase lays down a short segment of RNA (RNA primer is only a few nucleotides long to give the next enzyme somewhere to bond) on the leading strand
5. RNA primer primes the enzyme for DNA polymerase III to add nucleotides one at a time continuously, toward replication fork
6. DNA polymerase I removes the primer and replaces it with DNA

Question 20

DNA replication - lagging strand

Answer 20

1. DNA alpha helix opens up, unzipped by helicase enzyme
2. Topoisomerase nicks the helix so that it doesn’t snap
3. Single stranded binding proteins lay on both sides of the helix so that it doesn’t reform (strands don’t bind together again)
4. Instead of one primer, there are multiple primers laid down (new strand must replicate 5’ -> 3 prime, but lagging strand replicates in opposite direction, so primers go down repeatedly at the beginning of each fragment)
5. DNA pol III follows primer to start replicating segments of DNA, replicates in the direction away from replication fork
6. DNA pol III completes an Okazaki fragment. When it hits primer from previous fragment, it jumps backwards (toward the replication fork) to attach to strand again
7. New Okazaki fragment is created after jump
8. DNA pol I removes primers between fragments
9. Ligase seals up any gaps in the helix (from topoisomerase)

Question 21

4 differences between strands

Answer 21

a. Leading is fast, lagging is slow. Replication is the same rate because of delay between fragments
b. Leading starts with one primer, lagging starts with multiple primers at each segment
c. Leading is continuous replication, lagging is Okazaki fragments
d. Leading is copied in the 5’ to 3’ direction, lagging is 3’ to 5’

Question 22

telomerase

Answer 22

Enzyme which lengthens telomeres in germ cells. Found in large amounts in cancer cells.

Question 23

bacteriophage/phage

Answer 23

bacteria eating virus

Question 24

virus

Answer 24

DNA or RNA w/ protective coat (usually protein), takes over cell’s metabolic machinery. DNA enters the host cell, the protein does not