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OCR Biology Paper 1 > 2.3 - Nucleotides & nucleic acids > Flashcards

2.3 - Nucleotides & nucleic acids Flashcards

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1
Q

What are the building blocks of DNA and RNA?

A

Nucleic acids (nucleotides)

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2
Q

Describe the structure of a nucleic acid

A
  • Inorganic phosphate
  • Ribose sugar
  • Deoxyribose in DNA
  • Ribose in RNA
  • Nitrogenous base
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3
Q

What type of bond forms between nucleic acids?

A

Phosphodiester bond

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4
Q

List the nitrogenous bases

A
  • A= adenine
  • T = thymine (DNA only)
  • C = cytosine
  • G = guanine
  • U = uracil (RNA only)
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5
Q

Describe the structure of ATP

A
  • 3 inorganic phosphate groups
  • Ribose sugar
  • Adenine base
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6
Q

Why do cells require energy?

A
  • Synthesis of large molecules
  • Transport
  • Movement
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7
Q

Why is ATP a universal energy currency?

A

Used for energy transfer in all organisms

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8
Q

Explain why the properties of ATP make it a good energy source

A

Small
- Moves into and out of cells easily

Water soluble
- Metabolic process occur in aqueous environments (e.g. cytoplasm)

Intermediate bond strength
- Little energy wasted as heat

Releases small quantities of energy
- Quantities suitable for cellular needs, so little energy wasted as heat

Easily regenerated
- Can be recharged with energy

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9
Q

What is produced when one phosphate bond of ATP is hydrolysed?

A

ADP + Pi

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10
Q

What type of bond forms between adjacent nucleotides?

A

Phosphodiester bond

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11
Q

What type of reaction causes phosphodiester bonds to form?

A

Condensation reaction

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12
Q

What type of reaction causes phosphodiester bonds to break?

A

Hydrolysis reaction

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13
Q

Describe the structure of DNA

A
  • Double helix
  • Long
  • Two antiparallel strands of nucleotides
  • Nucleotides consist of a phosphate, deoxyribose (pentose sugar), base (A, T, C or G)
  • Hydrogen bonding between complementary base pairs (A-T, C-G)
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14
Q

Describe the structure of RNA

A
  • Single stranded
  • Short
  • Nucleotides consist of a phosphate, ribose (pentose sugar), base (A, U, C or G)
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15
Q

Describe the complementary base pairing rules for DNA

A
  • A-T (2 hydrogen bonds)
  • C-G (3 hydrogen bonds)
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16
Q

Describe the complementary base pairing rules for RNA

A
  • A-U (2 hydrogen bonds)
  • C-G (3 hydrogen bonds)
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17
Q

Outline the bonding between DNA nucleotides

A
  • Hydrogen bonds between complementary bases on opposite strands
  • Covalent (phosphodiester) bonds between deoxyribose sugar and phosphate
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18
Q

Describe how the structure of DNA makes it suitable for its role

A

Polymer
- contains a lot of information

Hydrogen bonds
- easy to break and separate strands

Double stranded
- each strand acts as a template for replication

Antiparallel strands
- Allow double helix to twist and provide compact shape

Complementary base pairing
- allows DNA to be replicated without error, reduces frequency of mutations

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19
Q

Describe how to extract a pure sample of DNA from plant cells

A

1) Grind up cells to break cell walls
2) Mix sample with detergent
- Breaks down cell membrane
3) Add salt
- Breaks hydrogen bonds between DNA and water molecules
4) Add protease enzyme
- Breaks down histones
5) Add alcohol
- Causes DNA to form precipitate
- DNA will form white precipitate between sample and alcohol

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20
Q

What is semi-conservative replication?

A
  • Mechanism by which DNA is copied
  • Each molecule formed has one new strand and one from parent molecule
21
Q

How is semi-conservative DNA replication carried out?

A
  • DNA helicase unwinds DNA double helix
  • Separates strands by breaking hydrogen bonds between bases
  • Free DNA nucleotides in nucleus are assembled on each of the parent strands
  • Nucleotides complementary base pair with exposed DNA strands
  • Hydrogen bonds formed between complementary bases (A-T and C-G)
  • Both strands act as template
  • DNA polymerase forms phosphodiester bonds between adjacent nucleotides
  • New DNA molecules rewind into double helices
  • Two new DNA strands are identical to template strands due to complementary base pairing
  • Each new DNA molecule has 1 old strand and 1 new strand
22
Q

Outline the role of helicase in DNA replication

A

Helicase unwinds double strand and separates strands by breaking hydrogen bonds

23
Q

Outline the role of DNA polymerase in DNA replication

A

DNA polymerase forms covalent bonds between nucleotides to form new strand of DNA

24
Q

Describe the Meselson-Stahl experiment for DNA replication

A
  • E. coli grown in a 15N medium until all nitrogen contained in DNA was 15N (heavy)
  • Sample of DNA isolated by centrifuge
  • Bacteria moved to 14N medium (light)
  • DNA isolated by centrifuge after 1, 2 and 3 replication cycles
  • After one replication cycle, the DNA was all of intermediate density
  • After two replication cycles, two bands of DNA were seen, one of intermediate
    density and one of light density
  • After three replication cycles, the lighter density band contained more DNA
25
Q

Explain why the results from the Meselson-Stahl experiment proved that DNA was replicated in a semi-conservative manner

A
  • After one replication cycle, the DNA was all of intermediate density
  • Consistent with semi-conservative replication model, which predicts all DNA
    molecules will consist of one 15N-labeled DNA strand and one 14N-labeled DNA
    strand
  • Rules out conservative replication model, which predicts both heavy DNA and light
    DNA will be present, but no intermediate density will be present
  • After two replication cycles, two bands of DNA were seen, one of intermediate density and one of light density
  • Consistent with the semi-conservative model: half should be intermediate density
    DNA and half should be light density DNA
  • Rules out dispersive replication model - predicts after one replication cycle, the
    density of all DNA molecules will gradually become lower, so no intermediate
    density DNA should remain after second replication cycle
26
Q

What is the role of DNA?

A
  • Stores all the organism’s genetic information
  • Codes for sequences of amino acids
  • Called the genetic code
27
Q

Define gene

A
  • A section of DNA which codes for a particular sequence of amino acids
  • Heritable factor
  • Occupies a specific position on a chromosome (locus)
28
Q

What are ‘coding sequences’?

A
  • Sections of DNA that code for proteins
  • i.e. genes
29
Q

Describe what is meant by the universality of the genetic code

A
  • Same genetic code is found in all organisms
  • Each codon in the mRNA is translated to the same amino acid
30
Q

Define mutation

A

Random and spontaneous change in the base sequence of a gene

31
Q

Define codon

A

3 bases on mRNA that correspond to one amino acid on the polypeptide

32
Q

Define anticodon

A

3 bases on tRNA which are complementary to the mRNA codon

33
Q

If there are 4 RNA bases (A, U, C and G), and 3 bases per codon, how many different codons is it possible to make?

A

4*3 = 64

34
Q

Why are there only 20 amino acids coded for by the 64 possible codons?

A

Genetic code is degenerate
- Multiple codons code for same amino acid (e.g. AAA and AAG both code for lysine)

Some codons are ‘stop’ codons - do not code for an amino acid

35
Q

What is the benefit of a degenerate genetic code?

A
  • Helps protect against negative effects from mutations by having multiple codons code for
    same amino acid
36
Q

What is the benefit of the DNA code being non-overlapping?

A
  • DNA can be read from base 1
  • Ensures correct RNA codons match with correct bases on DNA
37
Q

Define transcription

A
  • Synthesis of mRNA copied from the DNA base sequence
  • Carried out by RNA polymerase
  • Occurs in nucleus
38
Q

Describe the process of transcription

A
  • Occurs in nucleus - produces single stranded mRNA molecule
  • DNA helicase unwinds and unzips double helix, breaking hydrogen bonds
  • One strand of exposed DNA acts as template (antisense strand)
  • RNA nucleotides align with exposed DNA bases by complementary base pairing
  • Adenine (DNA) - Uracil (RNA)
  • Cytosine - Guanine
  • Thymine (DNA) - Adenine (RNA)
  • RNA polymerase links RNA nucleotides together
  • Phosphodiester bonds form between nucleotides
  • RNA strand separates and leaves nucleus through nuclear pores as mRNA
39
Q

Distinguish between the sense and antisense strands of DNA during transcription

A
  • Only the antisense strand is transcribed to mRNA
  • Sense strand is not transcribed
  • Has the same base sequence as mRNA (with thymine instead of uracil)
40
Q

At which end of the RNA molecule is the next RNA nucleotide added?

A

3’ (strand grows 5’ to 3’)

41
Q

Define translation

A
  • Synthesis of polypeptides
  • Occurs on ribosomes
42
Q

Describe the process of translation

A
  • Occurs on ribosomes and is synthesis of polypeptides
  • mRNA binds to ribosome
  • tRNA binds to mRNA at site where its anticodon corresponds to complementary codon on
    mRNA
  • Each tRNA molecule brings an amino acid that is specific to tRNA’s anticodon
  • Peptide bonds form between amino acids
  • Ribosome moves along mRNA to elongate polypeptide chain
  • Once tRNA has delivered its amino acid, it detaches from ribosome and next tRNA attaches
  • Polypeptide chain is released when ‘stop’ codon is reached
43
Q

What do DNA replication, transcription and translation have in common?

A

Complementary base pairing

44
Q

Outline how translation depends on complementary base pairing

A
  • Translation converts a sequence of mRNA codons to a sequence of amino acids
  • Triplet of bases on tRNAs pair with complementary triplet of bases on mRNA
  • Base pairing occurs when A pairs with U and G pairs with C
  • Specific amino acids are attached to specific tRNA
  • mRNA has codons and tRNA has anticodons
45
Q

What is the difference in function between free ribosomes and bound ribosomes?

A
  • Free ribosomes make proteins for use within the cell
  • Bound ribosomes make proteins for secretion or use in lysosomes
46
Q

Where are ribosomes found?

A
  • Bound to RER (rough endoplasmic reticulum)
  • Free in cytoplasm
47
Q

Describe differences in ribosomes in eukaryotes and prokaryotes

A
  • 80S type in eukaryotes
  • 70S type in prokaryotes
48
Q

What are ribosomes made out of?

A
  • Protein - to stabilise
  • rRNA (ribosomal RNA) - enzyme activity

OCR Biology Paper 1 (20 decks)

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