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DNA, RNA and proteins Flashcards

DNA RNA protein synthesis

1
Q

why was it though that proteins were hereditary material

A

because of their heterogeneity (diversity) and specificity of function

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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
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3
Q

what are viruses that infect bacteria called

A

bacteriophages

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4
Q

what is a virus

A

DNA or RNA enclosed by a protective protein coat

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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
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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

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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

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8
Q

which bases are purines (double ring)

A

adenine and guanine

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9
Q

which bases are pyrimidines (single ring)

A

cytosine and thymine

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10
Q

how many hydrogen bonds does A form with T

A

2

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11
Q

how many hydrogen bonds does C form with G

A

3

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12
Q

what did Watson and crick propose as the method of DNA replication

A
  1. the 2 DNA strands of the double helix separate
  2. each parental strand can now serve as a template to make a new complementary strand
  3. nucleotides complementary to the parental strands are added to make the sugar phosphate backbone of the new strands
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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)

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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

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15
Q

what is the dispersive model

A

each strand of both daughter molecules contains a mixture of old and newly synthesised DNA

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16
Q

what is the model of DNA replication

A

semiconservative model

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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

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18
Q

how many DNA molecules does each somatic cell nucleus of a human have

A

46

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19
Q

where does the replication of chromosomal DNA begin

A

origin of replication

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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

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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

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22
Q

eukaryotic cells have one/multiple origins of replication per chromosome

A

multiple - thousands whereas bacterium like E.coli only has one

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23
Q

why is it useful for eukaryotes to have multiple origins of replication

A

speeds up the copying of very long DNA molecules

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24
Q

what is the replication fork

A

Y shaped region where the parental strands of DNA are being unwound

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25
Q

what do helicases do

A

they are enzymes that untwist the double helix at the replication forks, separating the 2 parental strands and making them available as replication templates

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26
Q

what do single strand binding proteins doo

A

they bind to unpaired DNA strands after they have separated, keeping them from repairing

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27
Q

what does topoisomerase do

A

it is an enzyme that helps relive the strain at either end of the replication fork by breaking, swivelling and re-joining DNA strands

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28
Q

the enzymes that synthesis DNA cannot initiate the synthesis of a polynucleotide, they can only………………………………………………..

A

add nucleotides to the end of an already existing chain that is base paired with the template strand

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29
Q

what is a primer

A

a short RNA nucleotide chain synthesised by the enzyme primase

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30
Q

what does primase do to make primers

A

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

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31
Q

at which end of the RNA primer do the DNA nucleotides begin to add

A

the 3’ end (DNA elongates from 5’ to 3’)

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32
Q

what do DNA polymerases do

A

catalyse the synthesis of new DNA by adding nucleotides to the 3’ end of a pre-existing chain

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33
Q

what are the 2 major DNA polymerases in E.coli

A

DNA polymerase III and I

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34
Q

how many DNA polymerases are found in eukaryotes

A

11 have been found so far although the general principles of them all are the same

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35
Q

what does each nucleotide contain

A

a base
a sugar
3 phosphate groups

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36
Q

why are nucleotides chemically reactive

A

due to the 3 phosphate groups - an unstable cluster of negative charge

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37
Q

what kind of reaction occurs when DNA polymerase adds a nucleotide to a growing strand

A

dehydration - the 2 phosphate groups are lost when nucleotides join - an exergonic reaction that helps drive the polymerization

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38
Q

what does the two DNA strands being antiparallel mean

A

they are oriented in opposite directions to each other

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39
Q

what is the leading strand

A

the strand that can have DNA nucleotides continuously added to it - only one primer is required to synthesise the entire leading strand

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40
Q

what is the lagging strand

A

it is synthesised discontinuously in a series of segments

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41
Q

what are the synthesised segments of the lagging strand called

A

okazaki fragments

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42
Q

describe the steps in the replication of the leading strand

A
  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
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43
Q

describe the steps in the replication of the leading strand

A
  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
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44
Q

why is it wrong to represent DNA pol as locomotives moving along a track

A
  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)
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45
Q

in what direction does DNA pol I replace RNA with DNA

A

fragment to fragment in a 5’ to 3’ direction

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46
Q

what decreases the error frequency of DNA replication except from the specificity of base pairing

A
  • many DNA polymerases proofread each nucleotide against the template as soon as it is covalently bonded to the growing strand
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47
Q

what is mismatch repair

A

when enzymes remove and replace incorrectly paired nucleotides that have evaded the checks of DNA polymerase

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48
Q

can incorrectly paired or altered nucleotides arise after replication

A

yes - changes due to harmful chemical and physical agents or spontaneous changes can cause discrepancies in the DNA

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49
Q

are discrepancies usually corrected in the DNA before they become permanent mutations

A

yes

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50
Q

what does a nuclease enzyme do

A

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

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51
Q

describe nucleotide excision repair

A
  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
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52
Q

what causes XP

A
  • 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
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53
Q

once a mismatch nucleotide pair is replicated, the change is permanent/reversible

A

permanent - a mutation

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54
Q

mutations are the original source of what

A

variation

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55
Q

which end of the linear daughter DNA strands can never be completed and why

A

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

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56
Q

why does the shortening of prokaryotic DNA not occur

A

because their DNA is circular

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57
Q

what protects the genes of linear eukaryotic chromosomes from being eroded away during successive rounds of replication

A

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

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58
Q

what are the 2 protective functions of telomeres

A
  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
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59
Q

what enzyme catalyses the lengthening of telomeres in germ cells

A

telomerases

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60
Q

cancerous cells usually have long/short telomeres

A

short - they have undergone many replications

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61
Q

telomerase activity is high/low in cancer cells

A

high - this allows cancer cells to persist

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62
Q

what is associated with more proteins: linear eukaryotic DNA or circular bacterial DNA

A

linear eukaryotic DNA

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63
Q

what is a nucleoid

A

region within the cell of a prokaryote that contains all or most of the genetic material (not membrane bound)

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64
Q

each eukaryotic chromosome contains a single ………………..

A

DNA double helix

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65
Q

what is the complex of DNA and protein called

A

chromatin

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66
Q

what is a nucleosome

A

a structural unit (bead like) of a eukaryotic chromosome, consisting of a length of DNA coiled around a core of histones.

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67
Q

what do histones do

A

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

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68
Q

as a cell prepares for mitosis what happens to the chromatin

A

it coils and folds up forming the characteristic short thick metaphase chromosome that can be seen with a light microscope

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69
Q

interphase chromatin is more/less condensed than chromatin of mitotic chromosomes

A

less

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70
Q

histone tails of nucleosomes interact to form what

A

30 nm fibre

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71
Q

30 -nm fibres form loops called what

A

looped domains

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72
Q

looped domains coil into what

A

metaphase chromosome

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73
Q

interphase …………… and ……………. of chromosomes as well as other chromosomal regions exist in a highly condensed state similar to metaphase chromatin

A

centromeres

telomeres

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74
Q

what are the irregular clumps of interphase chromatin called

A

heterochromatin

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75
Q

what are the more dispersed regions of interphase chromatin called

A

euchromatin

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76
Q

what is more accessible for transcription: heterochromatin or euchromatin

A

euchromatin because the machinery can access it more easily as it is more loosely packed

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77
Q

how many base pairs are there per turn of the double helix

A

10

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78
Q

are DNA base hydrophilic or hydrophobic

A

hydrophobic - why they fold into the inside of the double helix

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79
Q

are the phosphate groups on DNA hydrophobic or hydrophilic

A

hydrophilic - why they are on the outside of the double helix

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80
Q

what does DNA polymerase use as its source of nucleotides

A

triphosphates - e.g. ATP and GTP

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81
Q

give 3 examples of things that can damage DNA (mutagenic agents)

A

x rays
chemicals
UV light

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82
Q

what does helicase do

A

unwinds parental double helix at replication forks

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83
Q

what does the single stranded binding protein do

A

binds to and stabilises single stranded DNA so it can be used as a template (stops the strands from joining back together)

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84
Q

what does topoisomerase do

A

breaks, swivels and re-joins DNA strands - this releases tension and corrects over ending at the replication forks

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85
Q

what foes primase do

A

leading strand - synthesises RNA primer at the 5’ end of the strand
lagging strand - synthesises RNA primer at 5’ end of the okazaki fragments

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86
Q

what does DNA polymerase III do

A

leading - adds nucleotides continuously onto the primer

lagging - elongates each okazaki fragment by adding nucleotides to the primer

87
Q

what does DNA polymerase I do

A

leading - replaces the primer with DNA nucleotides

lagging - removes primer from each fragment and replaces it with DNA nucleotides

88
Q

what does DNA ligase do

A

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
Q

where in the mitochondria is mitochondrial DNA found

A

near the membrane and it is double stranded and circular

90
Q

which parent do we get our mtDNA from

A

mother

91
Q

what are the 3 types of RNA

A

mRNA
tRNA
rRNA

92
Q

what did Garrod discover

A

that genes dictate phenotype through proteins

he postulated that the symptoms of an inherited disease reflect an inability to make a particular enzyme

93
Q

what did beadle and tatum discover

A

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
Q

what does a minimal medium contain

A

inorganic salts
biotin
glucose

95
Q

what was the one gene one enzyme hypothesis restated as

A

one gene one polypeptide

96
Q

what is transcription

A

the synthesis of mRNA using DNA

97
Q

what is translation

A

the synthesis of a polypeptide using the information on mRNA

98
Q

what is the site of translation

A

ribosomes

99
Q

what is the main difference in protein synthesis between bacteria and eukaryotes

A

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
Q

what needs to be done do convert pre-mRNA to mRNA

A

further processing

101
Q

what is the primary transcript

A

it includes both sections that will be incorporated into the protein and sections that won’t

102
Q

for each gene one/both of the DNA strands are transcribed

A

only one - it acts as a template - the strand that is used depends on the orientation of the enzyme that transcribes it

103
Q

is the RNA molecule synthesised antiparallel to the DNA

A

yes

104
Q

what are mRNA nucleotide triplets called

A

codons

105
Q

what is the non template strand of DNA often called

A

the coding strand because it has the same code as the RNA

106
Q

in what direction are the codons read by translation machinery

A

5’ to 3’

107
Q

how many of the 64 triplets code for amino acids

A

61

108
Q

which 3 codons do not designate amino acids

A

ones that are termination codons, marking the end of translation

109
Q

what is the dual function of the codon AUG

A

it codes for the amino acid methionine and also functions as an initiation codon

110
Q

what is meant by the genetic code being redundant

A

that more than one codon specifies the same amino acid

111
Q

what does RNA polymerase do in transcription

A

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
Q

how do DNA pol and RNA pol differ

A

RNA pol don’t need to add their first nucleotide to a pre-existing primer

113
Q

what is the DNA sequence where RNA pol attaches and initiates transcription called

A

the promotor

114
Q

what is the DNA sequence where RNA pol detaches and terminates transcription called

A

the terminator

115
Q

the promotor sequence is upstream/downstream from the terminator

A

upstream

116
Q

what is the transcription unit

A

the stretch of DNA downstream from the promotor that is transcribed to RNA

117
Q

which RNA pol is used to synthesise pre-mRNA in eukaryotes

A

RNA pol II

118
Q

what are the 3 steps of transcription

A

initiation
elongation
termination

119
Q

what happens in the initiation process of transcription

A

RNA pol binds to the promoter causing DNA strands unwind and initiation of RNA synthesis at the start point on the template strand

120
Q

what happens in the elongation process of transcription

A

RNA pol moves downstream unwinding DNA and elongating the RNA transcript (5’ - 3’)

121
Q

what happens in the termination process of transcription

A

RNA transcript is released and polymerase detaches from DNA

122
Q

which RNA pol is used to synthesise pre-mRNA in eukaryotes

A

RNA pol II

123
Q

what are the 3 steps of transcription

A

initiation
elongation
termination

124
Q

what happens in the initiation process of transcription

A

RNA pol binds to the promoter causing DNA strands unwind and initiation of RNA synthesis at the start point on the template strand

125
Q

what happens in the elongation process of transcription

A

RNA pol moves downstream unwinding DNA and elongating the RNA transcript (5’ - 3’)
after transcription DNA strands reform the double helix

126
Q

what is the complex of transcription factors and RNA pol II bound to the promoter called

A

transcription initiation complex

127
Q

what does a eukaryotic promoter often include

A

a TATA box about 25 nucleotides upstream from the start point

128
Q

how does the RNA know where to bind on DNA in eukaryotes

A

transcription factors recognise the TATA box and bind to it and this allow RNA to bind at the correct position and orientation

129
Q

what does the promoter contain

A

the TATA box and the start point of transcription

130
Q

in bacteria how does the RNA pol know where to bind

A

part of the RNA pol itself recognises and binds to the promotor

131
Q

what is the complex of transcription factors and RNA pol II bound to the promoter called

A

transcription initiation complex

132
Q

when RNA polymerase unwinds and separates the DNA strands how many DNA nucleotides does it expose at a time

A

10-20

133
Q

how do eukaryotes terminate their transcription

A

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
Q

how do bacteria terminate their transcription

A

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
Q

how is pre-mRNA modified to produce mRNA

A

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
Q

what is altered during RNA processing

A

both ends of the primary transcript

137
Q

what functions do the 5’ cap and the poly-A tail share

A
  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
Q

what are UTRs

A

parts of the mRNA that will not be translated into protein but they have other functions such as binding to ribosomes

139
Q

what is RNA splicing

A

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
Q

what are the non coding segments of nucleic acid that lie between the coding segment called in pre-mRNA

A

intervening sequences or introns

141
Q

what are the coding segments of pre-mRNA called

A

exons

142
Q

do UTRs lie in introns or exons

A

exons - but the UTRs are not translated into protein

143
Q

what is pre-mRNA splicing carried out by

A

the removal of introns is accomplished by a large complex made of proteins and small RNAs called a spliceosome

144
Q

how does the spliceosome work in pre-mRNA splicing

A

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
Q

what are ribozymes

A

RNA molecules that function as enzymes

146
Q

how is RNA splicing accomplished in some organisms without using spliceosomes

A

the intron’s RNA can act as a ribozyme catalysing its own excision

147
Q

what 3 properties of RNA allow them to function as enzymes

A
  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
Q

what is a common function of introns

A

they can be transcription factors regulating gene expression

149
Q

what is alternative RNA splicing

A

different polypeptides can be expressed from the same gene depending on which sections of the mRNA are treated as exons

150
Q

proteins often have regions called domains, what are these

A

a structural and functional region of a protein - commonly different exons will code for different domains

151
Q

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 (…………………)

A

crossing over

nonallelic

152
Q

what is the function of tRNA in translation

A

it transfers an amino acid from the cytoplasmic pool of amino acids to a growing polypeptide in a ribosome

153
Q

each tRNA molecule enables translation of a given mRNA codon into a certain …………..

A

amino acid

154
Q

describe the structure of tRNA

A

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
Q

anticodons are conventionally written in what direction

A

3’ –> 5’ so that they match up with the 5’ –> 3’ direction on the mRNA

156
Q

how is tRNA made

A

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
Q

are tRNA molecules used more than once

A

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
Q

how is the matching up of tRNA and the amino acid carried out

A

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
Q

how many different aminoacyl-tRNA synthetases are there

A

20 - one for each amino acid

160
Q

there are 61 mRNA codons but only 45 tRNA molecules, how does this work

A

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
Q

what is wobble

A

the flexible base pairing in the third position (3’ end) of an mRNA codon to the third position (5’ end) of the tRNA

162
Q

how many subunits is a ribosome made of

A

2 - one large unit and one small unit each made up of proteins and one or more rRNAs

163
Q

where are ribosome subunits made in eukaryotes

A

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
Q

a large and a small subunit join to form a functional ribosome only when ………….. is present

A

mRNA

165
Q

how are bacterial and eukaryotic ribosomes different and how is this significant in medicine

A

eukaryotic ribosomes are larger and their molecular composition differs
medicine - certain antibiotics can inactivate bacterial ribosomes without affecting eukaryotic ribosomes

166
Q

how many binding sites does a ribosome have for tRNA and what are they called

A

3

  • P site
  • A site
  • E site
167
Q

what does the P site of the ribosome do

A

holds the tRNA carrying the growing polypeptide chain

168
Q

what does the A site of the ribosome do

A

holds the tRNA carrying the next amino acid to be added to the chain

169
Q

what does the E site of the ribosome do

A

it is where discharged tRNAs leave the ribosome from

170
Q

to which end of the polypeptide chain are new amino acids added to in the ribosome

A

the carboxyl end

171
Q

what happens to the polypeptide in the ribosome as it becomes longer

A

it passes through an exit tunnel in the ribosomes large subunit

172
Q

rRNAs/proteins are primarily responsible for the structure and function of the ribosome

A

rRNAs

173
Q

what provides the energy needed for translation

A

GTP

174
Q

what is the start codon of translation

A

AUG

175
Q

what is the first step of translation

A

a small ribosomal subunit binds to both the mRNA and a specific initiator tRNA which carries methionine

176
Q

where does the ribosome bind to mRNA in bacteria

A

it is bound to a specific RNA sequence just upstream from the AUG start codon

177
Q

how does the ribosome bind mRNA and tRNA in eukaryotes

A
  • 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
Q

what is then attached to the small ribosome subunit carrying the initiator tRNA and mRNA

A

the large ribosomal subunit

179
Q

what does the translation initiation complex consist of

A

the large ribosomal subunit, the small ribosomal subunit, the initiator tRNA and the mRNA

180
Q

what is required to bring all the components of the transition initiation complex together

A

initiation factors and GTP

181
Q

in what direction is the polypeptide synthesised in the ribosome

A

from N terminus to C terminus and always begins with methionine

182
Q

in what direction does the mRNA flow through the ribosome

A

5’ –> 3’ (the 5’ end is translated first)

183
Q

each addition of an amino acid requires what kind of factors

A

elongation factors

184
Q

describe the steps in the elongation step of translation

A
  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
Q

describe the initiation steps of translation

A
  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
Q

describe the steps in the termination of translation

A
  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
Q

what a post translational modifications

A

changes made to the polypeptide formed in translation to make it into a functional protein

188
Q

polypeptides that are destined for the endomembrane system or for secretion are marked with what which targets the protein to the ER

A

signal peptide

189
Q

polypeptides with signal peptides are recognised by what complex

A

SRP - signal recognition particle

190
Q

in the ER other ………………… can be attached to polypeptides if they are to be sent to other organelles not part of the endomembrane system

A

signal peptides

191
Q

describe the steps in the signal mechanism for targeting proteins to the ER

A
  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
Q

give examples of attachments that can modify amino acids

A

addition of sugars, lipids, phosphate groups etc

193
Q

what is cleavage

A

when polypeptides are split into two or more pieces

194
Q

can several ribosomes translate an mRNA molecule at the same time

A

yes - they are just separated by a few codons

195
Q

what is a polyribosome

A

when multiple ribosomes are translating the same mRNA

196
Q

what are the 3 stop codons on mRNA

A

UAG UAA UGA

197
Q

summarise transcription and translation in a eukaryotic cell

A
  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
Q

what are mutations

A

changes to the genetic material of the cell that are the root of variation

199
Q

what are point mutations

A

changes in a single nucleotide pair of a gene

200
Q

how may a point mutation be passed to offspring

A

it the mutation is in a gamete or a cell that gives rise to a gamete

201
Q

name 2 diseases caused by point mutations

A

sickle cell anaemia

familial cardiomyopathy

202
Q

what are the 2 categories of small scale mutations

A
  1. single nucleotide pair substitutions

2. nucleotide pair insertions or deletions

203
Q

what is a nucleotide pair substitution

A

the replacement of one nucleotide and its partner with another pair of nucleotides

204
Q

what is a silent mutation

A

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
Q

what is a missense mutation

A

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
Q

substitution mutations are usually ………………. mutations

A

missense

207
Q

what is a nonsense mutation

A

changes a codon for an amino acid into a stop codon, terminating translation prematurely
this usually leads to a non-functional protein

208
Q

what is an insertion mutation

A

addition of a nucleotide pair in a gene

209
Q

what is a deletion mutation

A

the loss of a nucleotide pair in a gene

210
Q

which mutations have more disadvantageous effects

A

insertions and deletions

211
Q

what is a frameshift mutation

A

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
Q

what are spontaneous mutations

A

those that occur as error in DNA replication and evade proofreading and repair systems

213
Q

what are mutagens

A

physical and chemical agents that increase mutation rate

214
Q

give examples of mutagenic agents

A

x rays
UV light
nucleotide analogs

B (19 decks)

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