DNA Structure And Replication

Question 1

Describe what is meant by the central dogma.

Answer 1

• flow of genetic information from DNA —> RNA —> protein

Question 2

Describe Chargaff’s rules

Answer 2

• amount of each dNTP varies between organisms, but dA=dT and dC=dG in ALL organisms.

Question 3

How was the function of DNA discovered?

Answer 3

• by 1940s, hereditary material known to reside on one or more chromosomes.
• so seemed logical that protein is genetic material because of diversity needed, but found to be DNA

Question 4

How was the structure of DNA discovered?

Answer 4

Rosalind Franklin and Maurice Wilkins used X-ray diffraction suggested helix of two strands with a uniform width that stacks ashes with sugar-phosphate on the outside.

Question 5

If DNA contains more than one chain of nucleotides, what forces hold them together?

Answer 5

• hydrogen bons forming between a purine on one strand, and pyrimidine on the other.

Question 6

Characterisitics of the double strand of DNA?

Answer 6

• anti-parallel
• complimentary

Question 7

What does 5’ and 3’ mean

Answer 7

• 5’: free phosphate group on 5’ carbon
• 3’: free hydroxyl group on 3’ carbon

Question 8

Formula for number of possibilités of nucleotides

Answer 8

If a chain has n nucleotides, 4^n

Humans have 4^250000000 possibilities.

Question 9

Describe Watson and Crick’s model of DNA double helix

Answer 9

• played great deal of importance on complimentary bases
• suggested a copying mechanism

Question 10

Models of DNA replication

Answer 10

Dispersive
Conservative
Semiconservative

Question 11

Where does DNA replication start?

Answer 11

At the ori (specific sequence of bases in the DNA. (Also called the origination of replication)

Question 12

How does DNA replication start?

Answer 12

• strands separate t the original, and synthesis of new DNA will occur from both parent strands in BOTH directions away from the ori.

Question 13

Describe the DNA replication process

Answer 13

• helicase unwinds double helix, and strands are kept separated by Single-stranded DNA Binding Protein.
• incoming dNPT hybridized to parental template, forming phosphodiester bond with 3’ end of chain (Bi directional) by DNA polymerase.

Question 14

What does it mean if DNA replication is bidirectional?

Answer 14

• new DNA needs to be synthesized on both strands on both sides of the ori
• synthesis only occurs in the 5’ to 3’ direction, new strands have to be antiparallel to the template.

Question 15

Role of topoisomerase

Answer 15

• prevent DNA from getting tangled and relieves pressure in supercooled DNA during DNA replication.

Question 16

What is the name of the strand where DNA synthesis begins and proceeds normally?

Answer 16

• leading strand

Question 17

What is the name of the strand where DNA synthesis starts on the other side of the ori strats a short distance away from the ori and works back toward the ori? What does this form?

Answer 17

• lagging strand
• forms Okazaki fragments.

Question 18

Why is DNA replication called semi-discontinuous?

Answer 18

-because one strand is lagging (formed in Okazaki fragments)
- and the other strand is formed continuously.

Question 19

What do primers do?

Answer 19

• they provide the 3’ OH group needed for polymerase to build on the existing nucleotides.

Question 20

Why aren’t primers made of just 1 base pair?

Answer 20

• because its too unstable, so primers are short strands of RNA opposite an as DNA template.

Question 21

How do primases know when to stop?

Answer 21

• at the end of each Okazaki fragments in a lagging strand.

Question 22

What is a nick?
Which enzyme fixes this?

Answer 22

• there are no covalent linkages between nucleotides.
• DNA polymerase 1

Question 23

Which enzyme is the ‘after’ enzyme to fix nicks?

Answer 23

• Ligase

Question 24

What does X and Y mean it terms of X dépendant and Y synthesizing enzyme

Answer 24

• X: what its using as a template
• Y: What it is making

Question 25

What is the name of he family of enzymes that hydrolyse a phosphodiester bond? What is it called if it acts within or at the end of the molecule?

Answer 25

• nuclease
• exonuclease (End of the molecule)
• endonuclease (within the molecule)

Question 26

Role of telomerase

Answer 26

Extends unreplicated end by binding to the 3’ end of the overhanging strand of parental DNA and extends the strand using its own RNA template.

Question 27

How are millions or billions of base pairs of DNA in such a small space in prokaryotes

Answer 27

DNA is supercoiled
- done by topoisomerase which nick DNA, wind, unwind then reseal DNA.

Question 28

How do we get 2 meters of DNA into a nucleus that is 5–8
μm in diameter? In eukaryotes?

Answer 28

Chromatin, and wrapping around histone proteins

Question 29

Histones

Answer 29

• small, basic proteins (basic amino acids)
• sequence is highly conserved among species
• bacteria dont have histones, but they have histones like proteins.
• 5 major types of histones: H1, H2A, H2B, H3, H4

Question 30

What does the core nucleosome consist of?

Answer 30

• 2 of each histones: H2A, H2B, H3, H4 (8 proteins)
• 146/147 base pairs of DNA
• packing of DNA and histones into nucleosomes yields chromatin fiber of approx 10 nm in diameter.

Question 31

What types of chromatin exist in interphase?

Answer 31

• hetrochromatin: tightly condensed
• Euchromatin: loosely condensed