DNA, RNA and proteins Flashcards
DNA RNA protein synthesis (214 cards)
1
Q
why was it though that proteins were hereditary material
A
because of their heterogeneity (diversity) and specificity of function
2
Q
what did Griffith find in his pneumonia vaccine study
A
- a vaccine of a mixture of heat killed pathogenic cells and living non-pathogenic cells caused a mouse to die
- he concluded the non-pathogenic bacteria had been transformed into pathogenic bacteria by an unknown heritable substance from the dead pathogenic cells
3
Q
what are viruses that infect bacteria called
A
bacteriophages
4
Q
what is a virus
A
DNA or RNA enclosed by a protective protein coat
5
Q
what did Hershey and chase do
A
- they used radioactive sulfur and phosphorus to trace the fates of protein and DNA, respectively, of T2 phages that infected bacterial cells
- they found that the only the DNA entered the bacterium and when it did the bacteria released new phages containing the DNA of T2
6
Q
what did Erwin chargaff do
A
he reported that the base composition of DNA varies from one species to another
- he found that the number of adenine bases equalled the number of thymine bases and that the number of cytosine bases equalled the number of guanine bases
7
Q
how was the structure of DNA found
A
Watson saw an x-ray crystallography image by Rosalind franklin
Watson and Crick started building models of a double helix using the base pairing rules of Erwin Chargaff and the image by Rosalind Franklin
the model they made had two strands running antiparallel with 10 base pairs per turn of the helix
A paired to T and C paired to G
8
Q
which bases are purines (double ring)
A
adenine and guanine
9
Q
which bases are pyrimidines (single ring)
A
cytosine and thymine
10
Q
how many hydrogen bonds does A form with T
A
2
11
Q
how many hydrogen bonds does C form with G
A
3
12
Q
what did Watson and crick propose as the method of DNA replication
A
- the 2 DNA strands of the double helix separate
- each parental strand can now serve as a template to make a new complementary strand
- nucleotides complementary to the parental strands are added to make the sugar phosphate backbone of the new strands
13
Q
what is the conservative model
A
two parental strands re-associate after acting as templates for new strands, thus restoring the parental double helix (parent double helix is somehow conserved)
14
Q
what is the semiconservative model
A
two strands of the parental molecule separate and each function as a template for synthesis of a new complementary strand
15
Q
what is the dispersive model
A
each strand of both daughter molecules contains a mixture of old and newly synthesised DNA
16
Q
what is the model of DNA replication
A
semiconservative model
17
Q
what did Meselson and stahl do
A
they had 2 mediums of E.coli - one containing N15 (heavy) and the other N14 (light)
they transferred the N15 medium to the N14 medium
after the first replication the DNA was extracted and centrifuged to separate the densities. only one band was formed containing the parent strand and the N14 strand
the DNA was extracted after the second replication and after centrifugation 2 bands were formed. there was the band that appeared previously and a lighter band. the lighter band is from replication of the N14 strand using N14 nucleotides
18
Q
how many DNA molecules does each somatic cell nucleus of a human have
A
46
19
Q
where does the replication of chromosomal DNA begin
A
origin of replication
20
Q
what is the origin of replication
A
short stretches of DNA that have a specific sequence of nucleotides and is where DNA replication begins
21
Q
how does DNA replication begin from the origin of replication
A
proteins that initiate replication recognise the origin of replication and attaches to the DNA separating the strands and opening up the replication bubble
22
Q
eukaryotic cells have one/multiple origins of replication per chromosome
A
multiple - thousands whereas bacterium like E.coli only has one
23
Q
why is it useful for eukaryotes to have multiple origins of replication
A
speeds up the copying of very long DNA molecules
24
Q
what is the replication fork
A
Y shaped region where the parental strands of DNA are being unwound
25
what do helicases do
they are enzymes that untwist the double helix at the replication forks, separating the 2 parental strands and making them available as replication templates
26
what do single strand binding proteins doo
they bind to unpaired DNA strands after they have separated, keeping them from repairing
27
what does topoisomerase do
it is an enzyme that helps relive the strain at either end of the replication fork by breaking, swivelling and re-joining DNA strands
28
the enzymes that synthesis DNA cannot initiate the synthesis of a polynucleotide, they can only...……………………………………………..
add nucleotides to the end of an already existing chain that is base paired with the template strand
29
what is a primer
a short RNA nucleotide chain synthesised by the enzyme primase
30
what does primase do to make primers
it starts a complementary RNA chain with a single RNA nucleotide and adds RNA nucleotides one at a time using the parental DNA strand as a template
31
at which end of the RNA primer do the DNA nucleotides begin to add
the 3' end (DNA elongates from 5' to 3')
32
what do DNA polymerases do
catalyse the synthesis of new DNA by adding nucleotides to the 3' end of a pre-existing chain
33
what are the 2 major DNA polymerases in E.coli
DNA polymerase III and I
34
how many DNA polymerases are found in eukaryotes
11 have been found so far although the general principles of them all are the same
35
what does each nucleotide contain
a base
a sugar
3 phosphate groups
36
why are nucleotides chemically reactive
due to the 3 phosphate groups - an unstable cluster of negative charge
37
what kind of reaction occurs when DNA polymerase adds a nucleotide to a growing strand
dehydration - the 2 phosphate groups are lost when nucleotides join - an exergonic reaction that helps drive the polymerization
38
what does the two DNA strands being antiparallel mean
they are oriented in opposite directions to each other
39
what is the leading strand
the strand that can have DNA nucleotides continuously added to it - only one primer is required to synthesise the entire leading strand
40
what is the lagging strand
it is synthesised discontinuously in a series of segments
41
what are the synthesised segments of the lagging strand called
okazaki fragments
42
describe the steps in the replication of the leading strand
1. RNA primer is made by primase
2. DNA polymerase III starts to synthesise the strand by adding nucleotides to the 3' end of the primer
3. the strand is elongated continuously in the 5' to 3' direction
43
describe the steps in the replication of the leading strand
1. RNA primer is made by primase
2. DNA polymerase III starts to synthesise the strand by adding nucleotides to the 3' end of the primer
forming okazaki fragment 1
3. after reaching the next RNA primer DNA polymerase III detaches
4. fragment 2 is primed and DNA polymerase III adds nucleotides, detaching when it reaches the fragment 1 primer
5. DNA polymerase I replaces the RNA with DNA, adding nucleotides to the end of each fragment
6. DNA ligase forms bonds between the newest DNA (that just replaced RNA) and the DNA of the fragments
44
why is it wrong to represent DNA pol as locomotives moving along a track
1. many proteins that participate in replication actually form a single large complex
2. The DNA replication protein complex may not move along DNA but DNA moves along it (unanswered)
45
in what direction does DNA pol I replace RNA with DNA
fragment to fragment in a 5' to 3' direction
46
what decreases the error frequency of DNA replication except from the specificity of base pairing
- many DNA polymerases proofread each nucleotide against the template as soon as it is covalently bonded to the growing strand
47
what is mismatch repair
when enzymes remove and replace incorrectly paired nucleotides that have evaded the checks of DNA polymerase
48
can incorrectly paired or altered nucleotides arise after replication
yes - changes due to harmful chemical and physical agents or spontaneous changes can cause discrepancies in the DNA
49
are discrepancies usually corrected in the DNA before they become permanent mutations
yes
50
what does a nuclease enzyme do
it can excise a damaged or incorrect DNA segment and the resulting gap is filled with correct nucleotides (using DNA pol and DNA ligase) using the undamaged strand as a template
51
describe nucleotide excision repair
1. enzymes detect and repair damaged DNA
2. a nuclease cuts the damaged DNA at 2 points and the damaged section is removed
3. DNA pol fills in the missing nucleotides using the undamaged strand as a template
4. DNA ligase seals the free end of the new DNA to the old DNA making the strand complete
52
what causes XP
- covalent linking of thymine bases can be caused by UV light
- these thymine dimers buckle the DNA and interfere with replication
- XP results when there is a defect in the nucleotide excision repair enzyme
53
once a mismatch nucleotide pair is replicated, the change is permanent/reversible
permanent - a mutation
54
mutations are the original source of what
variation
55
which end of the linear daughter DNA strands can never be completed and why
the 5' end
when the primers are removed, they cannot be replaced with DNA nucleotides because they can only be added to the 3' end of a strand
so repeated rounds of replication produce shorter and shorter strands
56
why does the shortening of prokaryotic DNA not occur
because their DNA is circular
57
what protects the genes of linear eukaryotic chromosomes from being eroded away during successive rounds of replication
eukaryotic chromosomal DNA molecules have special nucleotide sequences called telomeres at their ends
telomeres don't contain genes; instead the DNA contains multiple repetitions of one short nucleotide sequence
58
what are the 2 protective functions of telomeres
1. proteins associated with telomeric DNA prevent staggered ends of the daughter molecule from activating the cells system for monitoring DNA damage (because staggered ends often induce cell death)
2. telomeric DNA acts as a buffer zone that provides protection against gene shortening - they postpone the erosion of genes near the end of chromosomes - telomeres become shorter in every round of replication
59
what enzyme catalyses the lengthening of telomeres in germ cells
telomerases
60
cancerous cells usually have long/short telomeres
short - they have undergone many replications
61
telomerase activity is high/low in cancer cells
high - this allows cancer cells to persist
62
what is associated with more proteins: linear eukaryotic DNA or circular bacterial DNA
linear eukaryotic DNA
63
what is a nucleoid
region within the cell of a prokaryote that contains all or most of the genetic material (not membrane bound)
64
each eukaryotic chromosome contains a single ………………..
DNA double helix
65
what is the complex of DNA and protein called
chromatin
66
what is a nucleosome
a structural unit (bead like) of a eukaryotic chromosome, consisting of a length of DNA coiled around a core of histones.
67
what do histones do
they are proteins that are responsible for the first level of packaging in chromatin
the positive charged histone is attracted to the negative phosphate on DNA and the DNA wraps around it
68
as a cell prepares for mitosis what happens to the chromatin
it coils and folds up forming the characteristic short thick metaphase chromosome that can be seen with a light microscope
69
interphase chromatin is more/less condensed than chromatin of mitotic chromosomes
less
70
histone tails of nucleosomes interact to form what
30 nm fibre
71
30 -nm fibres form loops called what
looped domains
72
looped domains coil into what
metaphase chromosome
73
interphase …………… and ……………. of chromosomes as well as other chromosomal regions exist in a highly condensed state similar to metaphase chromatin
centromeres
| telomeres
74
what are the irregular clumps of interphase chromatin called
heterochromatin
75
what are the more dispersed regions of interphase chromatin called
euchromatin
76
what is more accessible for transcription: heterochromatin or euchromatin
euchromatin because the machinery can access it more easily as it is more loosely packed
77
how many base pairs are there per turn of the double helix
10
78
are DNA base hydrophilic or hydrophobic
hydrophobic - why they fold into the inside of the double helix
79
are the phosphate groups on DNA hydrophobic or hydrophilic
hydrophilic - why they are on the outside of the double helix
80
what does DNA polymerase use as its source of nucleotides
triphosphates - e.g. ATP and GTP
81
give 3 examples of things that can damage DNA (mutagenic agents)
x rays
chemicals
UV light
82
what does helicase do
unwinds parental double helix at replication forks
83
what does the single stranded binding protein do
binds to and stabilises single stranded DNA so it can be used as a template (stops the strands from joining back together)
84
what does topoisomerase do
breaks, swivels and re-joins DNA strands - this releases tension and corrects over ending at the replication forks
85
what foes primase do
leading strand - synthesises RNA primer at the 5' end of the strand
lagging strand - synthesises RNA primer at 5' end of the okazaki fragments
86
what does DNA polymerase III do
leading - adds nucleotides continuously onto the primer
| lagging - elongates each okazaki fragment by adding nucleotides to the primer
87
what does DNA polymerase I do
leading - replaces the primer with DNA nucleotides
| lagging - removes primer from each fragment and replaces it with DNA nucleotides
88
what does DNA ligase do
leading - joins the 3' end of the DNA that replaces the primer to the rest of the leading strand
lagging - joins the okazaki fragments together
89
where in the mitochondria is mitochondrial DNA found
near the membrane and it is double stranded and circular
90
which parent do we get our mtDNA from
mother
91
what are the 3 types of RNA
mRNA
tRNA
rRNA
92
what did Garrod discover
that genes dictate phenotype through proteins
| he postulated that the symptoms of an inherited disease reflect an inability to make a particular enzyme
93
what did beadle and tatum discover
the one gene one enzyme hypothesis
they placed bread mould on a medium
the cells were subject to x-rays to induce mutations
each surviving cell formed a colony of genetically identical cells
each colony were places in a vial containing medium
those colonies that did not grow were identified as nutritional mutants
these cells were then grown in the presence of other nutrients
the cells grew in the presence of arginine indicating that the cell was missing the enzyme for synthesis of arginine
94
what does a minimal medium contain
inorganic salts
biotin
glucose
95
what was the one gene one enzyme hypothesis restated as
one gene one polypeptide
96
what is transcription
the synthesis of mRNA using DNA
97
what is translation
the synthesis of a polypeptide using the information on mRNA
98
what is the site of translation
ribosomes
99
what is the main difference in protein synthesis between bacteria and eukaryotes
eukaryotes have nuclei so transcription and translation are separated whereas bacteria lack a nucleus so translation can begin when transcription is still in process
100
what needs to be done do convert pre-mRNA to mRNA
further processing
101
what is the primary transcript
it includes both sections that will be incorporated into the protein and sections that won't
102
for each gene one/both of the DNA strands are transcribed
only one - it acts as a template - the strand that is used depends on the orientation of the enzyme that transcribes it
103
is the RNA molecule synthesised antiparallel to the DNA
yes
104
what are mRNA nucleotide triplets called
codons
105
what is the non template strand of DNA often called
the coding strand because it has the same code as the RNA
106
in what direction are the codons read by translation machinery
5' to 3'
107
how many of the 64 triplets code for amino acids
61
108
which 3 codons do not designate amino acids
ones that are termination codons, marking the end of translation
109
what is the dual function of the codon AUG
it codes for the amino acid methionine and also functions as an initiation codon
110
what is meant by the genetic code being redundant
that more than one codon specifies the same amino acid
111
what does RNA polymerase do in transcription
separates the 2 DNA strands and adds RNA nucleotides to the 3' end of the growing chain that are complementary to the DNA strand template
112
how do DNA pol and RNA pol differ
RNA pol don't need to add their first nucleotide to a pre-existing primer
113
what is the DNA sequence where RNA pol attaches and initiates transcription called
the promotor
114
what is the DNA sequence where RNA pol detaches and terminates transcription called
the terminator
115
the promotor sequence is upstream/downstream from the terminator
upstream
116
what is the transcription unit
the stretch of DNA downstream from the promotor that is transcribed to RNA
117
which RNA pol is used to synthesise pre-mRNA in eukaryotes
RNA pol II
118
what are the 3 steps of transcription
initiation
elongation
termination
119
what happens in the initiation process of transcription
RNA pol binds to the promoter causing DNA strands unwind and initiation of RNA synthesis at the start point on the template strand
120
what happens in the elongation process of transcription
RNA pol moves downstream unwinding DNA and elongating the RNA transcript (5' - 3')
121
what happens in the termination process of transcription
RNA transcript is released and polymerase detaches from DNA
122
which RNA pol is used to synthesise pre-mRNA in eukaryotes
RNA pol II
123
what are the 3 steps of transcription
initiation
elongation
termination
124
what happens in the initiation process of transcription
RNA pol binds to the promoter causing DNA strands unwind and initiation of RNA synthesis at the start point on the template strand
125
what happens in the elongation process of transcription
RNA pol moves downstream unwinding DNA and elongating the RNA transcript (5' - 3')
after transcription DNA strands reform the double helix
126
what is the complex of transcription factors and RNA pol II bound to the promoter called
transcription initiation complex
127
what does a eukaryotic promoter often include
a TATA box about 25 nucleotides upstream from the start point
128
how does the RNA know where to bind on DNA in eukaryotes
transcription factors recognise the TATA box and bind to it and this allow RNA to bind at the correct position and orientation
129
what does the promoter contain
the TATA box and the start point of transcription
130
in bacteria how does the RNA pol know where to bind
part of the RNA pol itself recognises and binds to the promotor
131
what is the complex of transcription factors and RNA pol II bound to the promoter called
transcription initiation complex
132
when RNA polymerase unwinds and separates the DNA strands how many DNA nucleotides does it expose at a time
10-20
133
how do eukaryotes terminate their transcription
RNA pol transcribes a sequence on the DNA called the polyadenylation signal sequence which specifies a polyadenylation signal in pre-mRNA
once it is transcribed it is immediately bound to proteins in the nucleus
then further downstream these proteins cut the transcript free from polymerase releasing pre-mRNA which then undergoes processing
134
how do bacteria terminate their transcription
the terminator is on the DNA and when it is transcribed onto RNA transcription stops and pol detaches and the transcript is released and requires no further modification before translation
135
how is pre-mRNA modified to produce mRNA
enzymes modify the to ends of the pre-mRNA
the 5' end receives a 5' cap
the 3' end receives a poly-A tail
136
what is altered during RNA processing
both ends of the primary transcript
137
what functions do the 5' cap and the poly-A tail share
1. they facilitate the export of mature mRNA from the nucleus
2. they help protect mRNA from degradation by hydrolytic enzymes
3. they help ribosomes attach to the 5' end of mRNA once the mRNA reaches the cytoplasm
138
what are UTRs
parts of the mRNA that will not be translated into protein but they have other functions such as binding to ribosomes
139
what is RNA splicing
the introns are cut out of the pre-mRNA and the exons are joined together forming an mRNA molecule with a continuous coding sequence
140
what are the non coding segments of nucleic acid that lie between the coding segment called in pre-mRNA
intervening sequences or introns
141
what are the coding segments of pre-mRNA called
exons
142
do UTRs lie in introns or exons
exons - but the UTRs are not translated into protein
143
what is pre-mRNA splicing carried out by
the removal of introns is accomplished by a large complex made of proteins and small RNAs called a spliceosome
144
how does the spliceosome work in pre-mRNA splicing
it binds to short nucleotide sequences along an intron
the intron is then released and rapidly degraded
the spliceosome then joins the two exons together
145
what are ribozymes
RNA molecules that function as enzymes
146
how is RNA splicing accomplished in some organisms without using spliceosomes
the intron's RNA can act as a ribozyme catalysing its own excision
147
what 3 properties of RNA allow them to function as enzymes
1. RNA is single stranded so can base pair with a complementary region elsewhere in the molecule to give a specific 3D structure
2. some of the RNA bases contain functional groups that can participate in catalysis
3. RNA can H bond to other nucleic acids adding specificity to its catalytic activity
148
what is a common function of introns
they can be transcription factors regulating gene expression
149
what is alternative RNA splicing
different polypeptides can be expressed from the same gene depending on which sections of the mRNA are treated as exons
150
proteins often have regions called domains, what are these
a structural and functional region of a protein - commonly different exons will code for different domains
151
Exon shuffling:
introns increase the likelihood of ………………. between the exons of alleles of a gene
exons can also be mixed and matched between completely different genes (…………………)
crossing over
| nonallelic
152
what is the function of tRNA in translation
it transfers an amino acid from the cytoplasmic pool of amino acids to a growing polypeptide in a ribosome
153
each tRNA molecule enables translation of a given mRNA codon into a certain …………..
amino acid
154
describe the structure of tRNA
it is a single strand of RNA that can fold back on itself making H bonds with other parts of the chain giving it a 3D structure
the 5' and 3' ends are located close to each other due to folding
the 3' end acts as an attachment for an amino acid
the loop extending form the other end of the tRNA includes the anticodon
155
anticodons are conventionally written in what direction
3' --> 5' so that they match up with the 5' --> 3' direction on the mRNA
156
how is tRNA made
it is transcribed from DNA in the same way that mRNA is made
it then travels to the cytoplasm to be used in translation
157
are tRNA molecules used more than once
yes - once they have delivered the amino acid it exits the ribosome into the cytoplasm where it picks up another amino acid and repeats the process
158
how is the matching up of tRNA and the amino acid carried out
it is carried out by a family of enzymes called aminoacyl-tRNA synthetases (the active site of each one fits only a specific combination of tRNA and amino acid)
the enzyme catalyses the covalent attachment of tRNA and amino acid (powered by ATP hydrolysis)
159
how many different aminoacyl-tRNA synthetases are there
20 - one for each amino acid
160
there are 61 mRNA codons but only 45 tRNA molecules, how does this work
some tRNAs are able to bind to more than one codon
this can only happen because bas pairing of the third nucleotide of a codon and tRNA anticodon is more relaxed
e.g. U at the 5' end of tRNA can bind with A or G in the third position of the mRNA codon (the 3' end)
161
what is wobble
the flexible base pairing in the third position (3' end) of an mRNA codon to the third position (5' end) of the tRNA
162
how many subunits is a ribosome made of
2 - one large unit and one small unit each made up of proteins and one or more rRNAs
163
where are ribosome subunits made in eukaryotes
in the nucleolus
- rRNA genes are transcribed
- the RNA is then processed and assembled with proteins imported from the cytoplasm
- ribosomal subunits are then exported to the cytoplasm via nuclear pores
164
a large and a small subunit join to form a functional ribosome only when .............. is present
mRNA
165
how are bacterial and eukaryotic ribosomes different and how is this significant in medicine
eukaryotic ribosomes are larger and their molecular composition differs
medicine - certain antibiotics can inactivate bacterial ribosomes without affecting eukaryotic ribosomes
166
how many binding sites does a ribosome have for tRNA and what are they called
3
- P site
- A site
- E site
167
what does the P site of the ribosome do
holds the tRNA carrying the growing polypeptide chain
168
what does the A site of the ribosome do
holds the tRNA carrying the next amino acid to be added to the chain
169
what does the E site of the ribosome do
it is where discharged tRNAs leave the ribosome from
170
to which end of the polypeptide chain are new amino acids added to in the ribosome
the carboxyl end
171
what happens to the polypeptide in the ribosome as it becomes longer
it passes through an exit tunnel in the ribosomes large subunit
172
rRNAs/proteins are primarily responsible for the structure and function of the ribosome
rRNAs
173
what provides the energy needed for translation
GTP
174
what is the start codon of translation
AUG
175
what is the first step of translation
a small ribosomal subunit binds to both the mRNA and a specific initiator tRNA which carries methionine
176
where does the ribosome bind to mRNA in bacteria
it is bound to a specific RNA sequence just upstream from the AUG start codon
177
how does the ribosome bind mRNA and tRNA in eukaryotes
- the initiator tRNA and the small ribosomal subunit are bound
- this binds to the 5' cap of the mRNA then moves down stream until it reaches the start codon
- the initiator tRNA then H bonds to the AUG start codon
178
what is then attached to the small ribosome subunit carrying the initiator tRNA and mRNA
the large ribosomal subunit
179
what does the translation initiation complex consist of
the large ribosomal subunit, the small ribosomal subunit, the initiator tRNA and the mRNA
180
what is required to bring all the components of the transition initiation complex together
initiation factors and GTP
181
in what direction is the polypeptide synthesised in the ribosome
from N terminus to C terminus and always begins with methionine
182
in what direction does the mRNA flow through the ribosome
5' --> 3' (the 5' end is translated first)
183
each addition of an amino acid requires what kind of factors
elongation factors
184
describe the steps in the elongation step of translation
1. codon recognition - the anticodon of a tRNA base-pairs with the complementary mRNA codon in the A cite (GTP hydrolysis increases the efficiency of this step
2. peptide bond formation - an rRNA molecule of the large ribosomal subunit catalyses the formation of a peptide bond between the new amino acid and the previous one. This removes the growing polypeptide from the tRNA in the P site and attaches it to the amino acid on the tRNA in site A
3. translocation (requires GTP) - the ribosome translocates the tRNA in site A to the P site. At the same time the empty tRNA in the P site is transferred to the E site where it is released. Another tRNA can be brought to the A site
185
describe the initiation steps of translation
1. a small ribosomal subunit binds to mRNA
2. the initiator tRNA (in the P site with methionine attached) binds to the start codon in bacteria or binds to the 5' cap and flows down stream until it reaches the start codon in eukaryotes (the start codon is AUG and the anticodon is UAC)
3. the arrival of the large ribosomal subunit completes the initiation complex with initiation factors being required to bring all the components together. GTP hydrolysis makes the process more efficient
186
describe the steps in the termination of translation
1. when the ribosome reaches the stop codon on mRNA, the A site of the ribosome accepts a release factor (a protein shaped like tRNA) instead of an aminoacyl tRNA
2. the release factor promotes hydrolysis of the bond between the tRNA in the P site and the polypeptide chain, releasing the polypeptide
3. the ribosomal units and the other components of the assembly dissociate with the hydrolysis of 2GTP
187
what a post translational modifications
changes made to the polypeptide formed in translation to make it into a functional protein
188
polypeptides that are destined for the endomembrane system or for secretion are marked with what which targets the protein to the ER
signal peptide
189
polypeptides with signal peptides are recognised by what complex
SRP - signal recognition particle
190
in the ER other ………………… can be attached to polypeptides if they are to be sent to other organelles not part of the endomembrane system
signal peptides
191
describe the steps in the signal mechanism for targeting proteins to the ER
1. polypeptide synthesis on free ribosome in cytosol
2. An SRP binds to the signal peptide halting synthesis momentarily
3. SRP binds to receptor protein in ER membrane
4. SRP leaves and polypeptide synthesis resumes with simultaneous translocation across the membrane through the pore
5. signal peptide is cleaved by an enzyme in the receptor protein complex
6. the polypeptide detaches from the ribosome and folds into its final conformation in the ER
192
give examples of attachments that can modify amino acids
addition of sugars, lipids, phosphate groups etc
193
what is cleavage
when polypeptides are split into two or more pieces
194
can several ribosomes translate an mRNA molecule at the same time
yes - they are just separated by a few codons
195
what is a polyribosome
when multiple ribosomes are translating the same mRNA
196
what are the 3 stop codons on mRNA
UAG UAA UGA
197
summarise transcription and translation in a eukaryotic cell
1. RNA is transcribed from a DNA template
2. pre-mRNA is spliced and modified to produce mRNA which moves from the nucleus to the cytoplasm
3. mRNA leaves the nucleus and attaches to the ribosome
4. each amino acid attaches to its tRNA with the help of a specific enzyme and ATP
5. tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome one codon at a time
6. the polypeptide is released from the ribosome
198
what are mutations
changes to the genetic material of the cell that are the root of variation
199
what are point mutations
changes in a single nucleotide pair of a gene
200
how may a point mutation be passed to offspring
it the mutation is in a gamete or a cell that gives rise to a gamete
201
name 2 diseases caused by point mutations
sickle cell anaemia
| familial cardiomyopathy
202
what are the 2 categories of small scale mutations
1. single nucleotide pair substitutions
| 2. nucleotide pair insertions or deletions
203
what is a nucleotide pair substitution
the replacement of one nucleotide and its partner with another pair of nucleotides
204
what is a silent mutation
when the mutation confers no change in the phenotype e.g. a different codon can code for the same amino acid due too redundancy
205
what is a missense mutation
the change of one amino acid to another - these can causes changes in the protein or no change depending on the location of the mutation
206
substitution mutations are usually ………………. mutations
missense
207
what is a nonsense mutation
changes a codon for an amino acid into a stop codon, terminating translation prematurely
this usually leads to a non-functional protein
208
what is an insertion mutation
addition of a nucleotide pair in a gene
209
what is a deletion mutation
the loss of a nucleotide pair in a gene
210
which mutations have more disadvantageous effects
insertions and deletions
211
what is a frameshift mutation
when an insertion or deletion alters the reading frame of the genetic message
this results in excessive missense mutations usually at some point resulting in a nonsense mutation
proteins tend to be rendered be rendered non-functional unless the mutation occurs near the end of the gene so the reading frame isn't affected the whole way through
212
what are spontaneous mutations
those that occur as error in DNA replication and evade proofreading and repair systems
213
what are mutagens
physical and chemical agents that increase mutation rate
214
give examples of mutagenic agents
x rays
UV light
nucleotide analogs
