BIOL2146 DNA Replication and Gene Expression
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Transcript of BIOL2146 DNA Replication and Gene Expression
8/3/2019 BIOL2146 DNA Replication and Gene Expression
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DNA replication
• Model for DNAreplication
• DNA strands areantiparallel
• DNA synthesis
– Leading strand
– Lagging strand
• Reference
– Campbell: Chap. 16
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Model for DNA replication:
> semiconservative
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Each DNA strand acts as a template
for a new strand• The 2 strands are complimentary
– Each stores the information necessary to
reconstruct the other strand – Due to base pairing: G – C and A – T
• Replicated (copied) DNA has: – One original strand
– One new strand
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DNA Replication:
> the basic concept
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Replication is bidirectional:
> DNA synthesis proceeds in both direction
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“DNA polymerase” adds nucleotides
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The two strands of DNAare antiparallel
DNA synthesis occurs inthe 5’ 3’ direction
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DNA Replication:
> the problem of antiparallel DNA strands• DNA synthesis is in 5’ 3’ direction
• DNA synthesis of one strand is continuous
• Synthesis of other strand is discontinuous
– Enzyme must work away from replication fork
– Synthesized in short DNA fragments
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The synthesis of one strand is
discontinuous
> lagging strand
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Lagging strand
• short segments of newly synthesisedDNA
• Okazaki fragments
– Synthesis occurs in a backward direction
– 100-200 nucleotides long
• DNA ligase joins the fragments
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Enzymes involved in DNA replication
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Summary of DNA replication
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Transcription:
> information readout• Role of transcription
• Structure of gene
• RNA polymerase – Transcription
• RNA processing
• Reference:
– Campbell; Chap 17
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Role of transcription: copy of a gene
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One gene – One protein
• Nucleotide sequence of a gene
– Provides the code for the amino acid sequence of a specificprotein
• Information from gene must be transferred toribosomes
– Where protein synthesis occurs
• Gene sequence is copied to mRNA
– Transcription
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Transcription of genes
• Only the DNA of genes are transcribedinto mRNA
– Human genome: 3 x 109 bases
– 35,000 genes
– Only 5 % of DNA is composed of genes
• 5 % of DNA is transcribed into mRNA
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mRNA (messenger RNA)
• Single stranded molecule
• Copy of one DNA strand from the gene• Sugar backbone is ribose
• Bases are A, G, C, or U – Compared to DNA: A, G, C, or T
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The level of gene transcription is
regulated
• Multiple copies of mRNA can be made
• The amount of mRNA synthesizeddetermines the amount of protein produced
Gene 1 Gene 2
mRNA
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General structure of a gene
• Promoter
– DNA sequence
– Binds RNApolymerase
– Determines start-siteof transcription
• Terminator
– DNA sequence
– Termination signal
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Initiation of
transcription• Eukaryotic promoter
– TATA box
– Binds transcriptionfactor
– RNA polymerasethen binds
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RNA polymerase
• What does it do ?
– unwinds the two strands of DNA
– Elongates (synthesizes) in 5’ to 3’ direction
– adds nucleotides to the 3’ OH end
– Copies from the DNA template strand
– mRNA produced is complimentary ratherthan identical to the template strand
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Elongation
• RNA polymerase moves along the DNAand unwinds the double helix
– 10-20 bases at a time
• mRNA synthesis: 60 nucleotides / sec
• DNA double helix then reforms
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Termination
• transcription continues until RNA polymerasereaches the termination signal
• in prokaryotes: stops immediately
• in eukaryotes: RNA polymerase continues
– 10-35 nucleotides past the termination signal
– addition of a poly A tail
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Split genes and RNA splicing
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RNA processing:
> eukaryotic cells• Modification of
mRNA transcript
– RNA splicing (cutand pasteprocedure)
– 5’ cap
– Poly(A) tail
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Protein synthesis: Translation
• 3 types of RNA
– mRNA
– rRNA
– tRNA
• Triplet genetic code
• Translation
– overview
• Reference:
– Campbell; Chap. 17
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3 types of RNA are involved in
Translation• mRNA (messenger RNA)
– Carries information from gene
• rRNA (ribosomal RNA) – Major component of ribosomes
– Location of protein synthesis (translation)
• tRNA (transfer RNA) – Carries amino acids to ribosome
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Polyribosomes
• mRNA is translated by several ribosomes
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Why a tripletgenetic code ?
4 nucleotides
20 amino acids
4 x 4 = 16
4 x 4 x 4 = 64
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64 codons
61 code for Amino acids
3 codonsStop signal
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The genetic code has
redundancy• An amino acid can be
specified by more than onecodon
• Nearly universal
• Non-overlapping
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The reading frame:
> 3 possibilities; only one is correct
• Translation of mRNA – Must begin at the correct nucleotide
– All proteins start with Methionine
– Start codon is AUG
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Mutations in DNA affect protein
structure• Mutation in DNA
code
• Change aminoacid sequence
– Change protein
activity
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Structure of aribosome
• Two subunits
– Large / small
• Both composed ofproteins & rRNA
• Has 3 sites
– A: Aminoacyl-tRNA
– P: Peptidyl-tRNA
– E: Exit site
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Structure & function of tRNA
• tRNA has a specific 3-D shape
• Each is linked to a specific amino acid – At one end of tRNA molecule
• Anticodon sequence – Located at other end
– Binds to the codon on mRNA strand
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Translation:> the basic concept
• mRNA slides through theribosome
• Codons are bound byspecific tRNA-amino acid
• tRNA adds it’s amino acid
to growing polypeptidechain