1.5 nucleic acids and their functions Flashcards
1
Q
what are some examples of nucleic acid?
A
- DNA
- RNA
- ATP
2
Q
what is a single monomer of nucleic acid called?
A
a (mono)nucleotide
3
Q
what are the 3 components of a nucleotide?
A
- pentose sugar (deoxyribose in DNA, ribose in RNA)
- phosphate group
- organic/nitrogenous base (always contain nitrogen)
4
Q
what bond is formed between the phosphate and the sugar (pentose) in a nucleotide?
A
an ester bond
5
Q
what bond is formed between the base and the sugar (pentose) in a nucleotide?
A
a glycosidic bond
6
Q
what does the base in a nucleotide always contain?
A
nitrogen
7
Q
what reaction creates the bonds between the phosphate, sugar and base in a nucleotide?
A
a condensation reaction
8
Q
what does DNA stand for?
A
deoxyribose nucleic acid
9
Q
what does RNA stand for?
A
ribose nucleic acid
10
Q
what is the name for the bases with a double ringed structure?
A
purines
11
Q
what are some examples of purines?
A
(double ringed structure)
adenine (A) and guanine (g)
12
Q
what is the name for the bases with a single ringed structure?
A
pyrimidines
13
Q
what are some examples of pyramidines?
A
(single ringed structure)
thymine, uracil and cytosine
14
Q
what do purines always pair with?
A
pyrimidines
15
Q
what bases join to each other?
A
A-T
C-G
16
Q
how many hydrogen bonds are found between A-T?
A
2 hydrogen bonds
17
Q
how many hydrogen bonds are found between C-G?
A
3 hydrogen bonds
18
Q
energy definition
A
- the ability to do ‘work’
- measured in joules (J)
19
Q
what does ATP stand for?
A
adenosine triphosphate
20
Q
what is ATP made up of?
A
- adenine (a base)
- ribose (a pentose sugar)
- 3 phosphate groups
21
Q
how does ATP release energy?
A
- the 3 phosphate groups are joined together by 2 high energy bonds
- ATP can be hydrolysed to break a bond which releases a large amount of energy (exergonic)
- hydrolysis of ATP to ADP is catalysed by the enzyme ATPase
22
Q
what are the 3 phosphate groups in ATP joined together by?
A
2 high energy bonds
23
Q
what does ADP stand for?
A
adenosine diphosphate
24
Q
what enzyme is the hydrolysis of ATP to ADP catalysed by?
A
ATPase
25
is the reaction from ATP —> ADP + Pi ∆H=-30.6KJmol^-1 reversible?
yes
26
how much energy is released from the hydrolysis of ATP to ADP?
30.6kJmol^-1
every mole of ATP hydrolysed releases 30.6 kJ
27
what does Pi stand for?
inorganic phosphate
28
what is the energy currency of the cell?
ATP
29
what are the 3 key roles of ATP?
- movement (e.g muscle contraction)
- active transport
- biosynthesis
30
which bond out of the 3 phosphate groups in ATP is broken to form ADP?
the bond furthest from the ribose group
31
the conversion of ATP to ADP + Pi (dephosphorylation) is an (endothermic/exothermic) reaction?
exothermic
(and is the main source of energy within the cell)
32
the reaction to convert ADP + Pi back into ATP is (endothermic/exothermic)?
endothermic
- so an energy source for this is needed
- in the majority of living cells the source of this energy comes from the oxidation of glucose: oxidative phosphorylation
33
what are the advantages of using ATP (compared to using glucose directly)?
- instant source of energy in the cell (ATP->ADP = a single reaction while breakdown of glucose involves many and takes longer)
- releases energy in small amounts as needed
- it is mobile and transports chemical energy to where it is needed IN the cell
- universal energy currency and can be used in many different chemical reactions (increasing efficiency and control by the cell)
- only 1 enzyme needed to release energy from ATP, but many are needed to release energy from glucose
34
can ATP be stored?
no
so it has to be continuously made within mitochondria of the cell that need it
35
what are some examples of processes in cells that require energy?
- muscle contraction
- protein synthesis
- active transport
- cell division
36
what is the relationship between adenine and thymine, and between cytosine and guanine?
they are complementary
37
what are the weak bonds formed between complementary bases?
hydrogen bonds
38
what is the shape of DNA molecules?
double helix
39
what is the arrangement of complementary polynucleotides in the double helix?
antiparallel
(the nucleotides in one strand are arranged in the opposite direction from those in the complementary strand)
(ie parallel but facing in opposite directions)
40
what is the pentose sugar found in DNA?
deoxyribose
41
what is the general name for a five carbon sugar?
pentose
42
what is a chain of nucleotides called?
polynucleotide
43
what is the name of two complementary bases held together by hydrogen bonds?
base pairs
44
what are nucleic acids?
polymers
45
when is ATP synthesised? when is it broken down?
- it is synthesised when energy is made available, such as in the mitochondria
- it is broken down when energy is needed, such as in muscle contraction
46
what is an endergonic reaction?
a reaction that requires an energy input
(e.g ATP synthesis)
47
what is an exergonic reaction?
a reaction that releases energy
e.g ATP hydrolysis
48
what is the addition of phosphate to ADP called?
phosphorylation
49
what are some examples of cellular activity that need ATP for energy?
- metabolic processes - to build large, complex molecules from smaller, simpler molecules e.g DNA synthesis from nucleotides
- active transport - to change the shape of carrier proteins in membranes and allow molecules or ions to be moved against a conc gradients
- movement - for muscle contraction
- nerve transmission - sodium-potassium pumps actively transport sodium and potassium ions across the axon membrane
- secretion - the packaging and transport of secretory products into vesicles in cells
50
when a phosphate group is transferred to another molecule, does it make the recipient molecule more reactive?
yes
it lowers the activation energy of a reaction involving that molecule
51
what is DNA composed of?
2 polynucleotide strands wound around each other in a double helix
52
what form the ‘backbone’ of DNA?
the deoxyribose sugar and phosphate groups on the outside of the DNA molecule
53
how is DNA suited to its functions?
- it is a very stable molecule and its information content passes essentially unchanged from generation to generation
- it is a very large molecule and carries a large amount of genetic information
- the two strands are able to separate, as they are held together by hydrogen bonds
- as the base pairs are on the inside of the double helix, within the deoxyribose-phosphate backbone, the genetic information is protected
54
what are the complementary base pairs in RNA?
adenine - uracil
guanine - cytosine
55
what are the components of RNA?
- phosphate group
- pentose sugar - ribose
- base
56
what elements do nucleotides contain?
carbon, hydrogen, oxygen, nitrogen, phosphorus
57
nucleotides polymerise by forming what bond? (in DNA)
phosphodiester bonds
(between the phosphate of one nucleotide and the sugar of another nucleotide)
58
does the double helix structure of DNA make it a stable molecule?
yes
so it is not easily damaged
59
is DNA a long or short molecule?
long
so it stores a great deal of information
60
is RNA a polynucleotide?
yes
but a single stranded one
61
what pentose sugar does RNA contain?
ribose
62
what purines does RNA contain?
adenine and guanine
63
what pyramidine bases does RNA contain?
cytosine and uracil (not thymine)
64
what are the 3 types of RNA involved in the process of protein synthesis?
- messenger RNA (mRNA)
- ribosomal RNA (rRNA)
- transfer RNA (tRNA)
65
what is messenger RNA (mRNA)?
- a long single-stranded molecule
- it is synthesised in the nucleus and carries the genetic code from the DNA to the ribosomes in the cytoplasm
- different mRNA molecules have different lengths, relating to the genes from which they are synthesised
66
where is mRNA synthesised?
in the nucleus
67
where does mRNA carry the genetic code from and to?
from the DNA in the nucleus to the ribosomes in the cytoplasm
68
what are the lengths of different mRNA molecules related to?
the genes from which they are synthesised
69
ribosomal RNA (rRNA) info
- made in the nucleolus
- found in the cytoplasm
- comprises large, complex molecules
- a component of ribosomes (along with protein)
- they are the site of translation of the genetic code into protein
- single polynucleotide strand
- highly folded to make a globular structure
70
where is rRNA made and found?
made in the nucleolus
found in the cytoplasm
71
what does rRNA comprise?
large, complex molecules
72
what are ribosomes made of?
ribosomal RNA and protein
(they are the site of translation of the genetic code into protein)
73
transfer RNA (tRNA) info
- small single-stranded molecule
- folds so that in places, there are base sequences forming complementary pairs
- its shape is described as a cloverleaf
- the 3’ end of the molecule has the base sequence cytosine-cytosine-adenine, where the specific amino acid the molecule carries is attached
- it also carries a sequence of three bases on the ‘middle leaf’ of the clover shape called the anticodon
- it carries amino acids to the ribosomes
- the type of amino acid carried is determined by the anticodon
74
what does mRNA do (simple)?
transfer genetic information from DNA to ribosomes
75
what does rRNA do (simple)?
it’s the RNA that makes up ribosomes along with protein
76
what does t(RNA) do (simple)?
it’s the RNA involved in protein synthesis
77
what is the difference in pentose between DNA and RNA?
DNA - deoxyribose
RNA - ribose
78
what is the difference in strands between DNA and RNA?
DNA - two in double helix
RNA - single-stranded
79
what is the difference in length between DNA and RNA?
DNA - long
RNA - tRNA and rRNA are short; mRNA varies but shorter than DNA
80
where is DNA found in cells? (eukaryotes? prokaryotes?)
- enclosed in the nuclei of eukaryotic cells
- is loose in the cytoplasm of prokaryotes
81
what are the 2 main roles of DNA?
1. a template for replication in dividing cells
2. protein synthesis
82
how does DNA’s role of replication work?
- DNA comprises two complementary strands, the base sequence of one strand determining the base sequence of the other
- if two strands of a double helix are separated, two identical double helices can be formed, as each parent strand acts as a template for the synthesis of a new complementary strand
(when cells divide, a complete copy of the DNA in the cell needs to be made. both DNA strands separate, and each strand acts as a template to synthesise a complementary strand)
83
how does DNA’s role of protein synthesis work?
- the sequence of bases represents the information carried in DNA (template strand) and determines the sequence of amino acids in proteins
84
what is the copying of DNA called?
replication
85
where does replication take place? when?
it takes place in the nucleus during interphase
86
why must replication happen?
chromosomes must make copies of themselves so that when cells divide, each daughter cell receives an exact copy of the genetic information
87
originally, what were the 3 possibilities imagined for the mechanism of DNA replication?
1. conservative replication
2. semi-conservative replication
3. dispersive replication
88
what is conservative replication?
the DNA molecule would be copied from the original, leaving the original DNA molecule as it was and having a new copy
where the parental doublr helix remains intact i.e is conserved, and a whole new double helix is made
89
what is semi-conservative replication?
the parental double helix separates into two strands, each of which acts as a template for synthesis of a new strand
leading to each new molecule having one original chain and one new one
(the type of DNA replication where each strand of DNA is used as a template to make a new strand. there are now 2 DNA molecules, each one with an original and a new replicated strand
90
what is dispersive replication?
sections of the DNA molecule would be copied and spliced together, making each new DNA molecule a mix of original and new DNA
the two new double helics contain fragments from both strands of the parental double helix
91
which DNA replication hypothesis was correct?
semi-conservative replication
(experiments were carried out)
92
what is the name of the experiment that proves semi-conservative replication?
the Meselson-Stahl experiment
93
what happens in the Meselson-Stahl experiment?
1. cultured the bacterium Escherichia coli for several generations in a medium containing amino acids made with the heavy isotope of nitrogen, N-15 (instead of normal light isotope N-14)
2. the bacteria incorporated the N-15 into their nucleotides and then into their DNA so that eventually, the DNA contained only N-15
3. they extracted the bacterial DNA and centrifuged it - the DNA settled at a low point in the tube bc the N-15 made it heavy
4. the N-15 bacteria were washed, then transferred to a medium containing the lighter isotope of nitrogen N-14, and were allowed to divide once more (the washing prevented contaminating the N-14 medium with N-15)
5. DNA from this first generation culture was centrifuged, and had a mid-point density (ruled out conservative replication)
6. DNA from the second generation grown in N-14 settled at the mid point and high point in the tube, in equal amounts. the sample at thr mid point had intermediate density and the sample at the high point was light, containing N-14 (ruled out dispersive replication)
7. one parental strand is conserved, therefore proving semi-conservative replication
94
which nitrogen isotope (N-14 and N-15) is heavy?
N-15
95
why was the N-15 bacteria washed before being transferred to a medium containing the lighter isotope of nitrogen N-14 in the Meselson-Stahl experiment?
the washing prevented contaminating the N-14 medium with N-15, so that N-15 was not incorporated into any new DNA strands
96
DNA from the first generation culture was centrifuged and had a mid-point density. why did this rule out conservative replication?
- bc that would produce a band showing the parental molecule that was entirely heavy
- the intermediate position could imply one strand of the new DNA moleculr was an original strand of N-15 DNA and the other half was newly made, with N-14, as in semi-conservative replication or it could imply that all strands contained a mixture of light and heavy, as in dispersive replication
97
the sample of DNA from second generations settled at the mid point and high point in the tube, in equal amount. why did this rule out dispersive replication?
- the sample at the midpoint had intermediate density and the sample at the high point was light, containing N-14
- bc if it was dispersive replication, there would alwats be a mixture of light and heavy in every strand and only onr band would form
98
where is position of the band for bacteria grown in N-14 medium in the Meselson-Stahl experiment?
at a high point
99
where is position of the band for bacteria grown in N-15 medium in the Meselson-Stahl experiment?
at a low point
100
where is the position of the band for bacteria grown in N-15 medium then washed and transferred to N-14 medium - after one generation, in the Meselson-Stahl experiment?
at an intermediate position
101
where is the position of the band for bacteria grown in N-15 medium then washed and transferred to N-14 medium - after two generation, in the Meselson-Stahl experiment?
at the midpoint and high point, in equal amounts
102
where is the position of the band for bacteria grown in N-15 medium then washed and transferred to N-14 medium - after three generation, in the Meselson-Stahl experiment?
at the midpoint and high point, but with more at the high point
103
which part of the DNA molecule contains the N-15?
nitrogenous base
104
what are the stages in semi-conservative replication?
- replication starts at a specific sequence on the DNA molecule
- DNA helicase unwinds and unzips DNA, breaking the hydrogen bonds that join the base pairs, and forming 2 separate strands
- the new DNA is built up from the 4 nucleotides (A,T,C,G) that are abundant in the nucleoplasm
- these nucleotides attach themselves to the bases on the old strands by complementary base pairing
- DNA polymerase joins the new nucleotides to each other by strong covalend bonds, forming the phosphate-sugar backbone
- a winding enzyme winds the new strands up to form double helices
- the two new molecules are identical to the old molecule
105
to replicate DNA, what does the enzyme DNA polymerase need?
- single-stranded DNA, as a template
- the 4 nucleotides, each containing deoxyribose and the base A, T, C or G
- ATP, to provide energy for synthesis
106
ATP is formed in an (endergonic/exergonic) reaction?
endergonic
(a reaction which uses energy)
107
what is the enzyme used to catalyse the reaction from ADP + Pi -> ATP + H2O (condensation reaction)
ATP synthetase
108
what is the enzyme used to catalyse the reaction from ATP + H2O -> ADP + Pi (hydrolysis reaction)
ATPase
109
what does it mean that ATP is the ‘universal energy current’
it means it providrs energy to all reactions in all cells in all species
110
is ATP soluble?
yes
(it can easily be transported in cells to where it is required)
it is also small
111
what happens to ATP when energy is available?
ATP is synthesised when energy is availablr
this has the advantage of converting the energy into a single useable form
112
what are the 2 functions of DNA in cells?
- as the base sequence codes for amino acid sequences in protein synthesis
- replicating prior to cell division so that each daughter cell gets equal DNA
113
what is a DNA molecule composed of?
two complementary polypeptide strabds
114
the sugar-phosphate molecules in DNA are joined by condensation reactions, making what?
a phosphodiester linkage
115
what type of bond join complementary bases together?
hydrogen
116
how is messenger RNA manufactured?
- using complementary base pajrs of RNA nucleotides using a strand of DNA as a template
- every 3 bases code for one amino acid
117
what are the 3 bases called on mRNA
codons
118
how is the shape of tRNA held in place?
by hydogen bonds between complementary base pairs
119
what is the function of tRNA?
to carry amino acids to the ribosomes
120
what is the anticodon on tRNA? what do they do
- a sequence of three bases on the ‘middle leaf’ of the clover shape
- they are complementary to the codons
- they ensure that the amino acids are lined up in the correct order for the primary structure of the protein
121
what are the main steps in extracting DNA from cells?
1. crush or blend the source cells in ice cold washing up liquid, salt and water to release the DNA
2. filter the cell debris and collect the filtrate
3. pour ice-cold alcohol down the side of the tube containing the extract
4. the DNA precipitates at the junction of the extract and alcohol
5. DNA can be stained red using acetic orcein
122
what bacteria did Meselson and Stahl use as their source of DNA in their experiment?
E.coli
bc E.coli are easily grown in a flask of culture medium and replicate their cells (and DNA) every 20 mins under optimal conditions
123
how were the ‘new’ and ‘original’ nucleotides distinguished in Meselson and Stahl’s experiment?
- by the isotope of nitrogen in the nitrogenous bases
N-15 is heavier in mass than N-14, so DNA molecules containing bases with N-15 and DNA molecules containing N-14 can be separated by mass in centrifugation
124
in the conservative replication of DNA experiment, where would the bands after one replication be expected to be in the centrifuge tube?
top (new) and bottom (original)
125
which models of DNA replication would give a band halfway up the tube after one generation, indicating that half the nucleotides in each new molecule were N-15 and half N-14?
dispersive and semi-conservative
126
by what does DNA replication happen?
a semi-conservative mechanism
127
what enzyme breaks the hydrogen bonds holding the two polynucleotide chains together in DNA replication?
DNA helicase
(the area where the helicase works is a ‘replication fork’)
128
what enzyme joins new nucleotides to their complementary bases in DNA replication?
how?
DNA polymerase
by catalysing the formation of phosphodiester bonds between the deoxyribose and phosphate groups working from the 5’ to 3’ direction. the original polynucleotide chains act as a template for thr aligning of new nucleotides
129
where is DNA confined to?
the nucleus
130
where are proteins assembled?
on ribosomes in the cytoplasm
131
what is a gene?
a section of DNA that codes for a polypeptide
132
in eukaryotes, what do genes contain?
- introns
- exons
133
what are exons?
the coding parts of a gene
134
what are introns?
the non-coding parts of genes
135
do prokaryotes have introns?
no - their genes are continuous
136
transcription steps to make pre-mRNA : (1st part of protein synthesis)
- the gene is unwound and unzipped by DNA helicase
- this means that the hydrogen bonds between the two polynucleotide chains are broken
- the bases are exposed
- once chain acts as a templatr for the formation on mRNA
- RNA nucleotides align opposite their complementary base pairs (e.g G on DNA pairs to C RNA nucleotide)
- RNA polymerase joins the nucleotides together, condensing the ribose phosphate backbone
- a pre-mRNA molecule is formed
- the pre-mRNA leaves the DNA when a stop sequence is reached
137
what does post-transcriptional modification involve?
- the removal of introns (these remain inside thr nucleus)
- the exons are spliced together
138
once the exons are spliced (introns removed), what is thr mRNA called?
functional mRNA
- the exons joined into functional mRNA exit the nucleus through a nuclear pore and attach to ribosomes
139
does pre-mRNA have the same base sequence as the coding strand of DNA?
no it is complementary to the coding strand of DNA
140
where does functional mRNA leave the nucleus through?
a nuclear pore
141
during splicing, can exons be joined in different orders?
yes
142
what is every three bases on mRNA called?
a codon
143
where does mRNA join to a ribosome at?
at the ‘start’ codon
144
how many codons can the ribosome accommodate?
2 codons
145
after the mRNA joins to a ribosome at the ‘start’ condon. tRNA in the cytoplasm is activated.
- the correct amino acid is attached to the amino acid binding site
- which amino acid is attached is determined by the anticodon; this is a sequence of three bases
- the process uses energy from ATP
146
when two tRNA molecules occupy both ribosome sites, the amino acids are brought close enough to form what bond?
a peptide bond
147
what happens once the first tRNA is released from the amino acid and ribosome in protein synthesis translation?
it returns to the cytoplasm to be reactivated
148
until when does the process of amino acids attaching, and the ribosome moving onto the next codon happen until? (in translation)
until a ‘stop’ codon is reached
149
what happens at the stop codon?
the polypeptide chain leaves the ribosome to move to the Golgi body for modification
150
what happens to the polypeptide chain at the golgi body? (in protein synthesis)
the polypeptide can be folded to make a protein
151
what type of code is the genetic code?
a triplet code
152
what does it mean that the genetic code is a triplet code?
three bases on DNA code for one amino acid
153
what is the name of three bases on DNA?
what is the name of three bases on mRNA?
what is the name of three bases on tRNA?
- triplet code for DNA
- codon for mRNA
- anticodons for tRNA
154
what determines which specific amino acid is carried by the tRNA?
the anticodon
155
what does it mean if a code is degenerate?
multiple codons can code for the same amino acid
156
is the genetic code univeral?
yes - it is the same in all living things, so each specific codon codes for the same amino acid in all species
(it is a linear code with no overlaps, so it can be ‘read’ without any ambiguity)
157
what are the 2 stages of protein synthesis?
1. transcription
2. translation
158
what is the process of translation?
- a ribosome attached to the mRNA at an initiation codon (AUG). the ribosome encloses 2 codons
- met-tRNA (UAC) diffuses to the ribosome and attaches to the mRNA initiation codon by complementary base pairings
- the next amino acid -tRNA attaches to the adjacent mRNA codon. the anticodon on the tRNA binds to the codon on the mRNA
- the bond between the amino acid and the tRNA is cut and a peptide bond is formed between the 2 amino acids. this operation is catalysed by an rRNA-protein complex called a ribozyne
- the ribosome moves along one codon so that a new amino acid-tRNA can attach. the free tRNA molecule leaves to collect another amino acid. cycle repeats
- the polypeptide chain elongates 1 am ac. at a time, and peels away from thr ribosome, folding up into a protein as it goes. this continuous until a stop codon is reachrd, when the ribosome falls apart, releasing the finished protein
159
what is the start codon?
met (AUG)
160
what happens to the mRNA once it has left the nucleus in transcription ?
it attaches onto a ribosome
161
what enzyme cuts the introns out of the pre-mRNA?
endonucleases
162
what enzyme joins the remaining exons together/splices them?
ligases
(then the mRNA is ready to leave the nucleus)
163
how does DNA determine the characteristic of an organism?
- DNA is coded by genes
- the base sequence directs which amino acids join together
- so, determines which proteins are made
- and because enzymes are proteins, it determines which reactions can take place in an organism
164
how many combinations can 4 bases with 3 bases coding for 1 amino acid make?
4^3 = 64
165
what does it mean that the genetic code is punctuated?
- there are 3 triplet codes that do not code for amino acids
- in mRNA they are called ‘stop’ codons and mark the end of a portion to be translated
166
in eukaryotes, why is the initial RNA version of the code much longer than the final mRNA?
(thr RNA is sometimes called pre-mRNA)
bc it contains sequences of bases that have to be removed (introns)
167
transcription definition?
one strand of the DNA acts as a template for the production of mRNA, a complementary section of part of the DNA sequence
(which carries information needed for protein synthesis from the nucleus to the cytoplasm)
(occurs on ribosomes in the cytoplasm)
168
translation definition?
the mRNA acts as a template to which complementary tRNA molecules attach, and the amino acids they carry are linked to form a polypeptide
(this occurs on ribosomes in the cytoplasm)
169
does dna leave the nucleus in protein synthesis?
no
170
transcription sequence of events:
- the enzyme DNA helicase breaks the hydrogen bonds between the bases in a specific region of the DNA molecule. this causes the two strands to separate anr unwind, exposing nucleotide bases
- the enzyme RNA polymerase binds to the template strand of DNA at the beginning of the sequence to be copied
- free RNA nucleotides align opposite the template strand, based on the complementary relationship between the bases in DNA and the free nucleotides
- RNA polymerase moves along the DNA forming bonds that add RNA nucleotides, 1 at a time, to the growing RNA strand. this results in the synthesis of a molecule of mRNA alongside the unwound portion of DNA. behind the RNA polymerase, the DNA strands rewind to form the double helix
- the RNA polymerase separates from the template strand when it reaches a ‘stop’ signal
- the production of the transcript is complete and newly formed RNA detaches from the DNA
171
what is each ribosome made of?
2 subunits
- the larger subunit has 2 sites for the attachment of tRNA molecules, so 2 tRNA molecules are associated with a ribosome at any one time
- the smaller subunit binds to the mRNA
172
translation sequence of events:
- initiation: a ribosome attaches to a ‘start’ codon at one end of the mRNA molecule
- the 1st tRNA, with an anticodon complementary to the first codon on the mRNA, attaches to the ribosome. the 3 bases of the codon on the mRNA bond to thr 3 complementary bases of the anticodon on the tRNA, with hydrogen bonds
- a 2nd tRNA, with an anticodon complementary to the second codon on the mRNA, attachrs to the other attachment site and the codon anf anticodon bond with hydrogen bonds
- elongation: the 2 amino acids are sufficiently close for a ribosomal enzyme to catalyse the formation of a peptide bond between them
- the 1st tRNA leaves the ribosome, leaving its attachment site vacant. it returns to the cytoplasm to bind to another copy of its specific amino acid
- the ribosome moves 1 codon along the mRNA strand
- the nect tRNA binds
- termination: the sequence repeats until a ‘stop’ codon is reached
- the ribosome - the mRNA - polypeptide complex separates
173
the 3 bases of the codon on the mRNA bond to thr 3 complementary bases of the anticodon on the tRNA, with what bonds?
hydrogen bonds
174
does the ribosome move along the mRNA or does the mRNA move through thr ribosome in translation?
- the ribosome moves along the mRNA
175
what is amino acid activation?
- once the tRNA is released from the ribosome, it is free to collect another amino acid from the amino acid pool in thr cytoplasm
- energy from ATP is needed to attach the amino acid to the tRNA
- this process of attachment is called amino acid activation
176
what happens once the ribosome makes the polypeptide chain in protein sybthesis?
- transported through the cytoplasm to the golgi body
- sometimes the primary structure of a polypeptide is functional but usually the polypeptide is folded into secondary, tertiary or quaternary structures and may be chemically modified as well (by combination wjth non-proteins e.g carbohydrates, lipids, phosphates)
- polypeptides produced from translation may be modified chemically and modified by folding
177
why is detergent used in extracting DNA?
it dissolves the lipids in the phospholipid membranes, allowing DNA to be released
178
why is a cold temperature used in extracting DNA?
it protects the DNA from cellular DNases
179
does DNA polymerase form hydrogen bonds in semi-conservative DNA replication?
no
it binds the complementary nucleotides (forming the phosphodiester bond)
180
where does transcription occur?
in the nucleus
181
where does translation occur?
at the ribosomes
182
where does post-translation modification occur?
in the golgi apparatus prior to packaging of the protein into vesicles
(post-translation modification is thr further modification needed to produce a protein with a secondary, tertiary or quaternary structure etc)
183
what is the ‘one gene one polypeptide hypothesis’?
- it was thought that genes coded only for enzymes which catalysed the production of other proteins. ‘1 gene 1 enzyme hypothesis’ (but then it was demonstrated that genes also code for other proteins leading to a modification of the hypothesis
- the ‘1 gene 1 polypeptide hypothesis’ is that each gene is transcribed and then translated into single polypeptide
(but this is now thought to be too simple as exons can be spliced in different orders forming different types of mRNA from one pre-mRNA. as the code is different, different proteins will be made, which refutes the one gene one polypeptide hypothesis)
why is one gene one enzyme and one gene one protein not used anymore?
(some proteins are made of more than one polypeptide/some genes code for more than one polypeptide + not all proteins are enzymes)
184
name the organelle that would contain thr greatest % of RNA. explain your answer
- ribosomes + rRNA is ribosomal RNA/constituent of ribosomes)
OR
- nucleolus + synthesis of RNA
185
explain why there would be large numbers of different mRNA molecules in a rabbit cell but only a maximum of 64 different tRNA molecules [4]
- different mRNA for every protein/polypeptide. / many different genes/one gene one polypeptide
- reference to base triplet code hypothesis givibg 4^3 triplets
- tRNA molecules have anticodons which have three bases (complementary to codons) / tRNA are specific to one amino acid
186
what is the role of DNA polymerase in DNA replication?
- to join the (free) nucleotides together/ catalyse the addition of nucleotides to form a (new strand/backbone/phosphodiester bonds)
187
what is the difference in bases between DNA and RNA?
DNA - adenine, thymine, cytosine, guanine
RNA - adenine, uracil, cytosine, guanine
188
what would the results be (of the position in the centrifuge) be with conservative replication?
- always some heavy DNA present
- increasing mass of light DNA
- no intermediate DNA
- because original heavy DNA is not split
189
what would the results be (of the position in the centrifuge) be with dispersive replication?
- apart from when only in N-15, all DNA would be intermediate in molecular mass
- getting lighter
- because original heavy DNA split between all new molecules
(but same width)
190
adenine 28.6% thymine 28.4%
why are they not exactly the same?
they are not identical becausenof experimental error
191
why is it necessary that the genetic code is a triplet code and why the code is said to be degenerate?
- a triplet code can producr 64 different combinations, which is enough to code for the 20 different amino acids (2 bases would only give 16 combinations which is not enough for 20 amino acids)
- as tbere are more than 20 codes, some amino acide are coded for by more than one mRNA codon
192
how can different proteins be produced from a single gene?
- splicing : cutting the pre-mRNA to remive introns and join exons
- more than 1 arrangement of exons can be made
- each protein has a unique sentence of amino acids
193
why are the strands of a DNA double helix described as ‘antiparallel’?
the complementary strands run parallel but in opposite directions
194
what is the genetic code?
- the rules by which triplets in a DNA base sequence code for the sequence of amino acids in a polypeptide chain
- the genetic code is degenerate, universal and non-overlapping
195
why is the genetic code described as degenerate?
more than one triplet can code for a particular amino acid
196
why is the genetic code described as universal?
the same codons code for all the same amino acids in almost all organisms
197
what is meant by non-overlapping?
each base in a sequence is read once and is only part of one triplet
198
what are the variables that need to be controlled in the centrifugation process?
- spin at same speed
- same time
- same density
- same temperature
199
in 1958, Meselson and Stahl conducted a series of experiments that demonstrated that DNA replication is semi-conservative.
other theories of DNA replication included: conservative replication, in which the original DNA is retained as a double stranded molecule; and dispersive replication, where the original DNA is split into many fragments which are then dispersed through the replicated molecules.
using your own knowledge of Meselson and Stahl’s experiments, explain how their experiment and the results supported the theory of semi-conservative replication. predictive how the results would have differed if the other theories had been correct [9 QER)
- description of experiment including details of use of heavy and light isotopes of N and use of ultracentrifugation
- explanation of how bands seen in centrifuge tube are formed giving details of heavy and light strands for each generation in terms of the heavy and light isotopes of N
- the relative amount of DNA in each band are explained
conservative replication:
- always some heavy DNA present
- increasing mass of light DNA
- no intermediate DNA
- because original heavy DNA is not split
dispersive replication:
- apart from G0, all DNA would be intermediate in molecular mass
- getting lighter between G0 and G3
- because original heavy DNA split between all new molecules
