DNA structure + function

Question 1

Frederick Griffith (1928)-

Answer 1

Studied two strains of bacteria:

o S strain (smooth)
o R strain (rough)

Question 2

S strain

Answer 2

virulent, capsuled, causes disease

Question 3

R strain

Answer 3

avirulent- didn’t cause disease. rough bc lacked capsule

Question 4

Griffith found that when injected mice with S strain-

Answer 4

mouse died

Question 5

When injected mice with living R strain-

Answer 5

mouse lived

Question 6

When injected mice with S strain that was heat-killed first-

Answer 6

mouse lived

Question 7

when injected mice with a mixture of heat killed S and

living R-why?

Answer 7

mice died, and there were living S cells inside the mice–Heat killed S did not become spontaneously alive- rather-
although the heat killed the cells, the hereditary material
(including the part that coded for infection) wasn’t destroyed and was transferred from S stream to R stream. permanent transformation.

Question 8

transformation

Answer 8

bacteria can absorb DNA and make it part of them

Question 9

original thought on strains

Answer 9

Scientists originally thought that this hereditary material

was the proteins bc they are the most structurally complex

Question 10

Oswald Avery (with MacLeod and McCarty)

Answer 10

further studied these
bacteria and found that adding a DNA digesting enzyme prevented the transformation This points to the fact that DNA is the hereditary material

Question 11

Alfred Hershey and Martha Chase (1950s) -

Answer 11

studied the replication
of viruses called bacteriophages (viruses that enter bacteria). Knew that a bacteriophage had to enter the bacteria cell in order
to reproduce- but which part of the virus entered? nucleic acid or protein coat?
o Labeled a protein coat of a bacteriophage with a radioactive
substance- S35 – sulfur
o Labeled the DNA of bacteriophage with radioactive substance-P32–phosphorus
Allowed bacteriophage to attach to cell and infect them. Found S35 outside the bacteria cell and P32 inside the cell. Concluded that bacteriophages inject their DNA into the cells,
leaving protein on the outside- therefore the DNA is significant in reproduction and codes for traits

Question 12

Bacteriophage-

Answer 12

composed of nucleic acid surrounded by a protein coat

Question 13

Nucleotide-

Answer 13

building block of DNA. Consists of: Deoxyribose, phosphate group, 1/ 4 possible nitrogenous base

Question 14

Deoxyribose

Answer 14

pentose (=5carbon) sugar

Question 15

4 nitrogenous base

Answer 15

Adenine and Guanine- purines (2 ringed)

Thymine and Cytosine- pyrimidines (1 ringed)

Question 16

phosphodiester bond

Answer 16

The phosphate and sugar form the backbone and are linked by
covalent bond called phosphodiester bond. 5’ end starts with
phosphate group
o 3’ end starts with
OH group of the sugar

Question 17

Erwin Chargoff

Answer 17

in DNA, the number of adenines equals the number
of thymines, the number of guanines = the number of cytosines. Chargoff’s rules–Adenine bonds w Thymine,
Guanine w Cytosine

Question 18

Rosalind Franklin-

Answer 18

Took pictures of DNA using X-ray diffraction- directed beam of x-rays at DNA, which would be scattered by the atoms in the DNA in specific patterns.

Question 19

James Watson and Francis Crick, 1953-

Answer 19

got Nobel Prize for structure of DNA; Studied the pictures and determined it to be a helical shape with
repeating patterns. Built models and determined that it was a double helix shape.
o Sugar-phosphate backbone formed the
outside of the chains
Bases, in pairs, formed the
rungs of the chain
o 10 base pairs present in a full turn of the helix= 3.4 nm

o Two chains are antiparellel-
o Each rung had the same length- 2.0 nm= one purine (two rings) bound to one pyrimidine (one ring)
o Hydrogen bonds hold the bases together-

o 3’—- AGCTAC —–5’
5’—–TCGATG——3’

Question 20

antiparallel

Answer 20

run in opposite directions,
meaning that one starts with 5’ end (phosphate group) while the
other starts with
the 3’ end (the OH group of the sugar)

Question 21

how many H bonds between nitrogenous bases

Answer 21

two between adenine and thymine, and three between cytosine and guanine-
complementary to each other

Question 22

DNA replication

Answer 22

• method by which DNA is copied
  o Because nucleotides form complementary pairs, each strand can
  separate from the other and act as a

template for synthesizing the opposite strand

Question 23

semi-conservative replication-

Answer 23

correct. each new DNA

double helix is made from one original strand and one newly synthesized complimentary strand

Question 24

Conservative replication-

Answer 24

wrong. parent strands remained together, new strands would form a double helix

Question 25

Dispersive replication-

Answer 25

parent strands and new strands
would become
randomly mixed during replication

Question 26

Telomeres-

Answer 26

caps on each end of chromosomes that contain short noncoding DNA sequences that repeat many times. Shortening of telomeres contribute to cell aging and apoptosis-
programmed cell death.

Question 27

Purpose of telomeres

Answer 27

After a DNA strand finishes replication, there is a small
segment on the lagging strand that
does not get replicated
Therefore, a cell can therefore divide many times before it starts
to lose important genetic info

Question 28

Telomerase

Answer 28

special DNA replication enzyme that can lengthen
telomeric DNA. This is usually present in cells that divide an unlimited number of times, like
protists, and also cancer cells

Question 29

Matthew Meselson and Franklin Stahl

Answer 29

Used density gradient centrifugation to prove that DNA
replicated in a semi conservative fashion. Labeled the bases of DNA with 15N, a heavy isotope of
nitrogen
Put it in an area containing just 14N and allowed it to
replicate
After one generation, new DNA had an intermediate
density, between 14N and 15N, because it had
half of each type
After another round of cell division, two types of DNA
appeared in density gradient- one with intermediate
density, one just 14N density proved
replicated in a semi conservative fashion

Question 30

Semiconservative replication explains

Answer 30

how mutations, or genetic changes, could arise in genes and then be transmitted to succeeding generations

Question 31

First step of replication

Answer 31

DNA helicase unwinds and unzips the two strands of DNA at various points within the DNA molecule called origins of replication

Question 32

What happens in replication after helicase?

Answer 32

The two strands separate from one another, forming a

replication fork

Question 33

What happens in replication after replication fork is formed?

Answer 33

Single-strand binding proteins (SSBs) bind to the individual strands and stabilize them, preventing them from
reforming the double helix

Question 34

What happens in replication after individual strands stabilize

Answer 34

Topoisomerases produce breaks in the DNA molecule and then rejoin the strands, preventing
excessive coiling in the rest of the strand

Question 35

How is DNA replication initiated if DNA polymerase only adds

nucleotides to an existing nucleotide?

Answer 35

An RNA primer (short
piece of RNA) is synthesized first at the region where
replication begins, by an enzyme called DNA primase. DNA polymerase then replaces the primase and adds
nucleotides to the 3’ end. Enzymes later
break down the primer and fill the space in w DNA

Question 36

DNA assembly direction

Answer 36

DNA is read from 3’ to 5’, synthesized from 5’ to 3’

Question 37

Leading strand –

Answer 37

strand where one DNA polymerase is
constantly adding nucleotides to one strand from the 5’ end to
the 3’ end of new (3-5), moving smoothly and continuously towards the replication fork

Question 38

Lagging strand-

Answer 38

strand where another polymerase molecule
adds nucleotides to the 3’ end of the other new strand. Since this moves away from the replication fork, only short pieces, called Okazaki fragments, can be synthesized at a time.

Question 39

Okazaki fragment initiation

Answer 39

Each Okazaki fragment is initiated by its own primer.
The fragments are joined together by DNA ligase- forms
a phosphodiester linkage between nucleotides

Question 40

DNA proofreading mechanisms-

Answer 40

DNA polymerase can proofread new bases pairings
can correct mismatches and reverse additions of base pairs
When they cannot, replication stops
Repair enzymes can repair some changes-can sniff out damaged sites or mismatches

Question 41

Prokaryotic replication

Answer 41

Bacteria, which has DNA in a circle, have one origin of replication, so
the replication forks eventually meet each other

Question 42

Eukaryotic replication

Answer 42

Eukaryotic chromosomes are linear, so many origins of replication exist at one time until newly synthesized strand is formed

Question 43

What happens in replication after topoisomerases

Answer 43

DNA polymerases catalyze the formation of two brand-new strands of DNA from free nucleotides