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A Level OCR Biology > Unit 2 - Nucleic acids & DNA replication > Flashcards

Unit 2 - Nucleic acids & DNA replication Flashcards

1
Q

What does DNA stand for?

A

Deoxyribonucleic acid

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2
Q

What does RNA stand for?

A

ribonucleic acid

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3
Q

What is the monomeric unit for nucleic acids?

A

Nucleotides

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4
Q

What are the 3 components of a nucleotide?

A
  • phosphate group
  • pentose (5 carbon sugar) either deoxyribose or ribose
  • nitrogenous base
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5
Q

Name the 5 nitrogenous bases

A
  • thymine
  • guanine
  • adenine
  • cytosine
  • uracil
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6
Q

Name the 4 bases present in DNA

A
  • thymine
  • adenine
  • guanine
  • cytosine
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7
Q

Name the 4 bases present in RNA

A
  • uracil
  • adenine
  • guanine
  • cytosine
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8
Q

Which base is never found in DNA?

A

Uracil

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9
Q

Which base is never found in RNA?

A

Thymine

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10
Q

How are polynucleotides formed?

A
  • condensation reactions occur
  • between the phosphate on carbon 5 of one nucleotide and the hydroxyl group on carbon 3 of another nucleotide
  • water is released
  • phosphodiester bond formed
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11
Q

What sort of bond is formed between nucleotides?

A

Phosphodiester

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12
Q

How are sequences of RNA/DNA given?

A

From the 5’ end to 3’ end

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13
Q

What are the two types of bases?

A

Purines and pyrimidines

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14
Q

What is the structure of purines?

A

Double ringed

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15
Q

What is the structure of pyrimidines?

A

Single ringed

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16
Q

Which bases are purines?

A

Adenine and guanine

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17
Q

Which bases are pyrimidines?

A

Cytosine, thymine, and uracil

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18
Q

What does complementary base pairing allow?

A

-DNA to be copied and transcribed

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19
Q

What type of bonds hold bases together?

A

Hydrogen bonds

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20
Q

How many hydrogen bonds form between Adenine and Uracil/Thymine?

A

2

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21
Q

How many hydrogen bonds form between Guanine and Cytosine?

A

3

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22
Q

In order to allow for the bases to form hydrogen bonds what must happen to one of the polynucleotide chains?

A

It must be rotated through 180 degrees

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23
Q

How does the sugar phosphate backbone run on one of the polynucleotide chains? How does the other polynucleotide chain run? What is this called?

A

From 5’ to 3’ and the other from 3’ to 5’

The antiparallel nature of DNA

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24
Q

Describe the steps involved in extracting DNA

A
  • sample ground up using pestle and mortar
  • sample mixed with detergent
  • salt is added
  • protease is added
  • ice cold ethanol is added down the side of the test tube
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25
Q

Why is the sample ground in the extraction of DNA?

A

it breaks down the cell walls

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26
Q

Why is detergent added to the sample in the extraction of DNA?

A

breaks down the cell membrane, releasing the cell contents into solution

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27
Q

Why is salt added to the sample in the extraction of DNA?

A

neutralises charges on phosphates in sugar phosphate backbone
breaks the hydrogen bonds between DNA and water
makes DNA less soluble

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28
Q

Why is protease added to the sample in the extraction of DNA?

A

breaks down the proteins associated with DNA

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29
Q

Why is ethanol added to the sample in the extraction of DNA?

A

Causes the DNA to form a white precipitate between the layer of the sample and the ethanol

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30
Q

Why is a low temperature maintained throughout the process of DNA extraction?

A

Reduces rate of enzyme controlled reactions that break down DNA

31
Q

Why should the ethanol be ice cold?

A

Helps to make DNA more insoluble

32
Q

At what point in the cell cycle does DNA replication occur?

33
Q

Why is it important that DNA replication results in 2 genetically identical molecules of DNA?

A
  • ensures the continuation of species

- ensures cells and structures in a species are maintained

34
Q

Why is DNA replication described as being semi-conservative?

A
  • Each one of the two daughter molecules contains one original strand of DNA and one newly synthesised strand
  • the original strand acts as a template for a new strand
35
Q

What is the relationship between the two daughter molecules?

A

They are genetically identical

36
Q

What is the relationship between the daughter molecules and the original DNA molecule?

A

they are identical

37
Q

Which two enzymes are used in DNA replication?

A

DNA helicase

DNA polymerase

38
Q

What is the role of DNA helicase?

A
  • unwinds DNA from histones

- unzips DNA by breaking Hydrogen bonds betwem complementary base pairs

39
Q

What is the role of DNA polymerase?

A

-catalyses the reaction to form phosphodiester bonds between the free nucleotides to form a sugar phosphate backbones

40
Q

Outline how DNA is replicated

A
  • DNA helicase unwinds DNA from histones and unzips DNA by breaking Hydrogen bonds between complementary base pairs
  • free nucleotides are attracted to the exposed nucleotides on the strand of DNA by base complementarity
  • DNA polymerase forms phosphodiester bonds between nucleotides
  • two genetically identical molecules of DNA are formed
41
Q

What does the unzipping mean for DNA nucleotides?

A

They are exposed

42
Q

Due to the nucleotides being exposed, what happens?

A

Free nucleotides are attracted to the exposed nucleotides by base complementarity and form hydrogen bonds

43
Q

Where do the free nucleotides come from?

A

Exogenous come from our diet.

We can also synthesis nucleotides.

44
Q

How do free nucleotides enter the nucleus?

A

From the cytoplasm through the nuclear pores

45
Q

What is a primer?

A

A short piece of RNA or DNA with a complementary sequence to part of the DNA template

46
Q

What direction does DNA synthesis occur?

A

5’ to 3’

47
Q

What is the difference between the leading strand and lagging strand?

A

The leading strand is built continuously whereas the lagging strand is built discontinuously

48
Q

Why is there a leading strand and a lagging strand?

A

The antiparallel nature of DNA

49
Q

Why are primers needed in DNA replication?

A

DNA polymerase requires them in order to bond to the 3’ OH group

50
Q

What does the discontinuous building of the lagging strand lead to the production of?

A

Okazaki fragments

51
Q

Which experiment was used to prove that DNA replication is semi-conservative?

A

The Meselsohn and Stahl experiment

52
Q

Which two techniques did the Meselsohn and Stahl experiment use?

A

Mass labelling and density centrifugation

53
Q

Which isotope was used for mass labelling in the Meselsohn and Stahl experiment?

54
Q

Why was N-15 used in the Meselsohn and Stahl experiment?

A

It could be incorporated into the nitrogenous bases and therefore track the movement of them

55
Q

Which part of the nucleotide was labelled in mass labelling?

A

The nitrogenous bases

56
Q

How did density centrifugation work in the Mehselson and Stahl experiment?

A
  • A CsCl gradient is produced by centrifugation
  • more dense molecules move to the bottom and lighter molecules to the top
  • When DNA is placed into this solution it migrates to the position in the gradient with the same buoyant density
57
Q

DNA was placed in a solution of what in the Meselsohn and Stahl experiment?

A

CsCl (caesium chloride)

58
Q

Outline the two control experiments in the Meselsohn and Stahl experiment

A
  • E.coli placed in growth factor containing N-15 (heavy nitrogen) which the E.coli incorporated into their DNA, DNA extracted and centrifuged with CsCl
  • E.coli placed in growth factor containing N-14 (normal nitrogen) which the E.coli incorporated into their DNA, DNA extracted and centrifuged with CsCl
59
Q

What was seen in the two control experiments in the Meselsohn and Stahl experiment?

A

N-15, the DNA settled further down the density gradient whilst the N-14 DNA was higher up the density gradient

60
Q

Why was E.coli used in the Meselsohn and Stahl experiment?

A
  • the environment in which it grows can be carefully controlled
  • it would incorporate the different isotopes of Nitrogen into its DNA
  • it divides every 20 minutes
61
Q

Why were the control experiments important to the Meselsohn and Stahl experiment?

A
  • They allowed the position of where each N isotope sits after centrifugation depending on its density to be identified
  • if there was a mixing of the two isotopes a band would be seen inbetween
62
Q

What was the key experiment in the Meselsohn and Stahl experiments?

A
  • E.coli was grown in a growth media containing N-15 for several divisions
  • it was then transferred to a growth media containing N-14 and only allowed to divide once
  • the DNA was then extracted and centrifuged with CsCl
63
Q

What was observed in the key Meselsohn and Stahl experiment?

A

A band of DNA was formed inbetween the N-15 and N-14 bands

64
Q

How did the key Meselsohn and Stahl experiment prove DNA replication is semi-conservative?

A
  • the initial molecule of DNA contained only N-15 nucleotides
  • as DNA replication is semi-conservative the DNA which was newly synthesised in the N-14 growth media would contain one strand with N-15 nucleotides and one strand with N-14 nucleotides
  • so the band would be in the middle
65
Q

If DNA replication was conservative what would have been observed in the key Meselsohn and Stahl experiment?

A

A band of DNA at the N-14 and N-15 positions

66
Q

If the E.coli were allowed to divide twice in the key Meselsohn and Stahl experiment in N-14 what would be seen?

A

A band at N-14 and in the middle in the ratio 1:1

67
Q

If the E.coli were allowed to divide three times in the key Meselsohn and Stahl experiment in N-14 what would be seen?

A

A band at N-14 and in the middle in the ratio 3:1

68
Q

Explain why complementary base pairing is important in DNA replication

A
  • reduces occurrences of mutation
  • allows for formation of hydrogen bonds
  • DNA is replicated without error
69
Q

Suggest 3 precautions that Meselson and Stahl would have taken in order to ensure that the centrifugation part of their investigation produced valid results

A
  • the tubes were spun for the same amount of time
  • the tubes were spun at the same speed
  • the solution the DNA is put in is the same concentration
70
Q

What is a mutation?

A

A change in the sequence of DNA bases within a gene

71
Q

How do mutations occur?

A

A base changing

72
Q

What are 2 possible consequences of mutations?

A
  • evolution

- genetic disorders

73
Q

Give 3 differences between DNA and RNA?

A

-DNA has a double stranded structure whilst RNA has a
single stranded structure
-In DNA A bonds to T and G bonds to C, in RNA A bonds to U and G bonds to C
-DNA is too big to leave the nucleus, RNA is small enough to leave the nucleus

A Level OCR Biology (42 decks)

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