DNA Duplication

Question 1

T. H. Morgan

Answer 1

genes are located on chromosomes
DNA and protein were candidates for genetic material

Question 2

Frederick Griffith experiment

Answer 2

1928
worked with 2 strains of bacteria: Rough (harmless) and Smooth (pathogenic)
Transformation: gave combination of heat killed S cells and living R cells, result was mouse died - assimilation of genetic material

Question 3

bacteriophages

Answer 3

viruses that infect bacteria specifically

Question 4

Hershey-Chase experiment

Answer 4

1952
showed DNA is genetic material of T2 bacteriophage
phages grown with radioactive sulfur (found in AA) and phosphorus (found in DNA)
centrifuge to form pellet demonstrated which molecules entered the bacterial cell infected

Question 5

Erwin Chargaff

Answer 5

1950
DNA varies from one species to the next
base pairing A-T, G-C and quantities matching

Question 6

Maurice Wilkins and Rosalind Franklin

Answer 6

used X-ray crystallography to deduce DNA was helical and double stranded
also the width of DNA, and spacing of nitrogenous bases

Question 7

James Watson and Francis Crick

Answer 7

1953
double helix model with antiparallel sugar-phosphate backbones
model explains Chargaff’s rules and fits with crystallography data

Question 8

semiconservative model

Answer 8

each daughter cell has 1 parent strand and 1 newly synthesized strand
after 2 replications: 1:1 ratio

Question 9

conservative model

Answer 9

parent strands rejoin every time
after 2 replications: 1:3

Question 10

dispersive model

Answer 10

each strand is a mix of old and new

Question 11

Meselson-Stahl experiment

Answer 11

old nucleotides labeled with heavy N, new nucleotides labeled with light N
replication 1: hybrid band (conservative not possible)
replication 2: hybrid and light (dispersive not possible)

Question 12

replication begins at

Answer 12

origin of replication with a replication bubble
there may be 100s or 1000s of origins in eukaryotes

Question 13

what stabilizes single stranded DNA being replicated?

Answer 13

single strand binding proteins

Question 14

DNA polymerases requires

Answer 14

an RNA primer synthesized by primase, from which DNA polymerase can synthesize a new strand
dNTPs - deoxyribose nucleotide triphosphate

Question 15

what provides energy for DNA polymerase?

Answer 15

loss of pyrophosphates from dNTPs as they are cleaved

Question 16

rate of elongation

Answer 16

500 nucleotides per second in bacteria
50 nucleotides per second in humans

Question 17

lagging strand

Answer 17

synthesized in direction away from replication fork (following leading strand)
in Okazaki fragments joined by DNA ligase

Question 18

DNA pol III vs. DNA pol I

Answer 18

DNA Pol III - DNA synthesis 5’ –> 3’
DNA Pol I - replaces RNA primers with DNA

Question 19

Movement of DNA replication machine

Answer 19

new evidence says DNA pol reels in parental DNA and extrudes daughter DNA

Question 20

proofreading of DNA polymerase

Answer 20

intrinsic proofreading of DNA pol III
mismatched bases repaired by enzymes
nucleotide excision repair

very low error rates after proofreading

Question 21

nucleotide excision repair steps

Answer 21

nuclease cuts out and replaces damaged DNA segments due to harmful chemicals and radiation (ex. thymine dimerization)
1. nuclease cuts in 2 places and removes segment
2. DNA polymerase resynthesizes
3. DNA ligase seals free ends

Question 22

thymine dimer

Answer 22

covalent bond formed between adjacent thymines due to UV radiation causes DNA distortion

Question 23

why does DNA get shorter with replication rounds?

Answer 23

DNA pol can only add nucleotides to 3’ end, last primer cannot be replaced with DNA (making ends shorter every time)
not an issue in prokaryotes with circular DNA

Question 24

telomeres

Answer 24

special nucleotide sequences at the ends of DNA to be corroded away first with DNA replication sparing genes
shortening of telomeres associated with aging, loss of cell viability and development of age-related disease
degradation of telomeres acts as a time stop on cell division (after enough replication, cell signals for apoptosis)

Question 25

telomerase

Answer 25

enzyme found in germ cells that lengthens telomeres to overcome end of DNA replication issues
not found in most somatic cells in humans, only cells that replicate a lot
expressed in some stem cells
expressed by 80-90% of cancer cells

Question 26

DNA location/structure in bacteria

Answer 26

nucleoid: DNA is supercoiled, circular and associated with a small amount of protein in chromosomes
Also contain plasmids outside of chromosomes

Question 27

plasmids

Answer 27

extrachromasomal circular fragments of DNA duplicated independently from chromosome
can spread from one bacterium to another (leads to bacterial resistance)

Question 28

DNA location/structure in eukaryotes

Answer 28

located in nucleus, mitochondria and chloroplasts (all replicated separately)
chromosomes associated with larges amount of protein (histones) which act as spools to condense and protect DNA

Question 29

nucleosome

Answer 29

unit of chromatin - stretch of DNA wrapped around 8 histone proteins

Question 30

euchromatin vs heterochromatin

Answer 30

euchromatin is loosely packed - interphase
some regions during interphase are heterochromatin (condensed, restricted access to DNA)
histone modification leads to changes in gene expression

Question 31

chromosomes during interphase

Answer 31

each chromosomes occupies a different space in nucleus
2 homologues not located together
prior to mitosis chromatin condenses into chromosomes