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MCB 3023 Exam 3 Questions with Detailed
Verified Answers
Question: How many bases per turn in the different types of DNA?
Ans✅ ✅: B-DNA- 10.5 bases per turn
A-DNA- 11 bases per turn
Z-DNA- 12 bases per turn and left handed
Question: What relieves supercoiling in DNA?
Ans✅ ✅: -Positive supercoiling- increases the base pairs per turn
-Negative supercoiling- decreases the base pairs per turn and makes the strands easier
to separate
-DNA gyrase (topoisomerase) relieves supercoiling in DNAS replication
Question: RNA
Ans✅ ✅: -Uracil instead of thymine
-Ribose instead of deoxyribose
-Single stranded but can form secondary structures that mimic double strands e.g.
hairpins
Question: Circular DNA replication in bacteria
, Page | 2
Ans✅ ✅: -Bidirectional from a single origin
-Replication forks move outwards from the origin
-Replication fork is the place the DNA helix is unwound
-Replicon- portion of the genome that contains the origin, replicated as a unit (entire
genome in bacteria)
-Genome forms a theta structure before separating at the termination point
Question: How many proteins make up the E.coli replisome?
Ans✅ ✅: At least 30
Question: What is the alpha phosphate?
Ans✅ ✅: The phosphate incorporated into the sugar phosphate backbone of DNA in
replication- PPi is released but this phosphate is attached to the 3' hydroxyl of one
dNTP and the 5' carbon of another dNTP
Question: DNA Polymerase III
Ans✅ ✅: -Holoenzyme complex of 10 proteins
-2 core enzymes that bind to each DNA strand for synthesis and proofreading
-Each enzyme has a beta clamp to tether it to the DNA
-Clamp loader links the core enzyme and loads the Beta clamp onto the DNA
-Tau protein dimer holds the holoenzyme together
-Encoded by DNAe gene in e.coli- highly conserved
-proofreads for mismatched bases with 3'->5' exonuclease activity
Question: Events at replication fork
, Page | 3
Ans✅ ✅: -DnaA proteins bind to OriC to initiate replication causing bending and
separation of strands
-Helicase DnaB unwinds strands with help of topoisomerase
-SSB attach and keep strands apart
-Single DNA polymerase III catalyses leading and lagging strand synthesis
-DNA polymerase removes primers by 5'->3' exonuclease activity
-DNA ligase seals gaps between okasaki fragments of lagging strand
Question: Termination of replication
Ans✅ ✅: -DNA replication stops when replisome reaches termination site
-Tus protein binds to ter site to halt replication forks
-Catenanes are interlocked chromosomes that do not separate and they are broken by
topoisomerases by DSB
-Dimerized chromosomes formed by recombination are separated by recombinase
enzymes that cross over at the ter site
-In linear chromosomes Res-T telomerase resolvase enzyme cuts DNA at repeats in
dimerized chromosome to form hairpin ends and 2 daughter chromosomes
Question: Cistron
Ans✅ ✅: -segment of DNA that codes for a single polypeptide
-Prokaryotes are often polycistronic and contain 1 or more coding regions for one or
more polypeptides
-Eukaryotes are always monocistronic
Question: What are the sequences leading up to the promotor on DNA?
, Page | 4
Ans✅ ✅: Promoter is the binding site for RNA polymerase
-35 and -10 pribnow box are the RNA polymerase recognitions and binding sites
-Transcription start site +1 starts and leader which may not code for AAs
-Shine Dalagarno sequence initiates translation
Question: Coding region of DNA
Ans✅ ✅: Starts with AUG start codon on coding strand- 3'TAC5' on template strand
First amino acid is N-formylmethionine
Ends with stop codon followed by trailer sequence (terminator sequence to stop
transcription)
Coding region is continuous and has no introns in prokaryotes
Question: tRNA and rRNA genes
Ans✅ ✅: -Code for more than one type of tRNA to ensure high conc
-tRNAs and rRNAs transcribed together under a single promoter
-More than one tRNA gene transcribed and separated by spacer sequences which are
then removed by ribonucleases
-rRNA genes also transcribed as one large precursor
Question: Operon
Ans✅ ✅: -genes encoding proteins in related processes that are close to each are
transcribed a single promoter
Question: RNA polymerase
MCB 3023 Exam 3 Questions with Detailed
Verified Answers
Question: How many bases per turn in the different types of DNA?
Ans✅ ✅: B-DNA- 10.5 bases per turn
A-DNA- 11 bases per turn
Z-DNA- 12 bases per turn and left handed
Question: What relieves supercoiling in DNA?
Ans✅ ✅: -Positive supercoiling- increases the base pairs per turn
-Negative supercoiling- decreases the base pairs per turn and makes the strands easier
to separate
-DNA gyrase (topoisomerase) relieves supercoiling in DNAS replication
Question: RNA
Ans✅ ✅: -Uracil instead of thymine
-Ribose instead of deoxyribose
-Single stranded but can form secondary structures that mimic double strands e.g.
hairpins
Question: Circular DNA replication in bacteria
, Page | 2
Ans✅ ✅: -Bidirectional from a single origin
-Replication forks move outwards from the origin
-Replication fork is the place the DNA helix is unwound
-Replicon- portion of the genome that contains the origin, replicated as a unit (entire
genome in bacteria)
-Genome forms a theta structure before separating at the termination point
Question: How many proteins make up the E.coli replisome?
Ans✅ ✅: At least 30
Question: What is the alpha phosphate?
Ans✅ ✅: The phosphate incorporated into the sugar phosphate backbone of DNA in
replication- PPi is released but this phosphate is attached to the 3' hydroxyl of one
dNTP and the 5' carbon of another dNTP
Question: DNA Polymerase III
Ans✅ ✅: -Holoenzyme complex of 10 proteins
-2 core enzymes that bind to each DNA strand for synthesis and proofreading
-Each enzyme has a beta clamp to tether it to the DNA
-Clamp loader links the core enzyme and loads the Beta clamp onto the DNA
-Tau protein dimer holds the holoenzyme together
-Encoded by DNAe gene in e.coli- highly conserved
-proofreads for mismatched bases with 3'->5' exonuclease activity
Question: Events at replication fork
, Page | 3
Ans✅ ✅: -DnaA proteins bind to OriC to initiate replication causing bending and
separation of strands
-Helicase DnaB unwinds strands with help of topoisomerase
-SSB attach and keep strands apart
-Single DNA polymerase III catalyses leading and lagging strand synthesis
-DNA polymerase removes primers by 5'->3' exonuclease activity
-DNA ligase seals gaps between okasaki fragments of lagging strand
Question: Termination of replication
Ans✅ ✅: -DNA replication stops when replisome reaches termination site
-Tus protein binds to ter site to halt replication forks
-Catenanes are interlocked chromosomes that do not separate and they are broken by
topoisomerases by DSB
-Dimerized chromosomes formed by recombination are separated by recombinase
enzymes that cross over at the ter site
-In linear chromosomes Res-T telomerase resolvase enzyme cuts DNA at repeats in
dimerized chromosome to form hairpin ends and 2 daughter chromosomes
Question: Cistron
Ans✅ ✅: -segment of DNA that codes for a single polypeptide
-Prokaryotes are often polycistronic and contain 1 or more coding regions for one or
more polypeptides
-Eukaryotes are always monocistronic
Question: What are the sequences leading up to the promotor on DNA?
, Page | 4
Ans✅ ✅: Promoter is the binding site for RNA polymerase
-35 and -10 pribnow box are the RNA polymerase recognitions and binding sites
-Transcription start site +1 starts and leader which may not code for AAs
-Shine Dalagarno sequence initiates translation
Question: Coding region of DNA
Ans✅ ✅: Starts with AUG start codon on coding strand- 3'TAC5' on template strand
First amino acid is N-formylmethionine
Ends with stop codon followed by trailer sequence (terminator sequence to stop
transcription)
Coding region is continuous and has no introns in prokaryotes
Question: tRNA and rRNA genes
Ans✅ ✅: -Code for more than one type of tRNA to ensure high conc
-tRNAs and rRNAs transcribed together under a single promoter
-More than one tRNA gene transcribed and separated by spacer sequences which are
then removed by ribonucleases
-rRNA genes also transcribed as one large precursor
Question: Operon
Ans✅ ✅: -genes encoding proteins in related processes that are close to each are
transcribed a single promoter
Question: RNA polymerase
