Download - Section 11.1 Summary – pages 281 - 287
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DNA REPLICATION REVIEW
1. When does DNA divide?
2. Why does it happen at this time?
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• What is the first step in DNA replication?
• Next, what is the job of the enzyme Helicase?
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What is the replication fork?
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1. What happens next?
2. What enzyme is responsible for this action?
3. What is the significance of the three phosphates on the free nucleotides?
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LEADING STRAND VS. LAGGING STRAND
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Why does the DNA always have to build from the original DNA’s 3’?
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Why does the DNA always have to build from the original DNA’s 3’?
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The Flow of Genetic Information.How you get from a gene to a protein.
transcriptiontranslation
replication
proteinRNADNADNA gets
all the glory, but proteins do
all the work!
We’ve talked about replication, now we will focus on TRANSCRIPTION…
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TranscriptionThe process of copying
a segment of DNAinto RNA.
This is the first step in protein synthesis.
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DNA holds the instructions to make proteins, but can NOT leave the nucleus.
Proteins are made outside of the nucleus on the tiny organelles called ribosomes.
The DNA needs a messenger to take a copy of the instructions to the
ribosomes so the proteins can be made.
What’s the problem?
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• Other than DNA, what is another type of nucleic acid?
• There are 3 types of RNA, but today we will only mostly focus on one- mRNA.
Messenger RNA (mRNA) delivers a copy of DNA’s instructions from the
nucleus to the ribosomes.
SIDE-NOTE!
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• RNA’s structure differs from DNA’s structure in three ways.
# 1
RNA is single stranded—it looks like one-half of a zipper—whereas DNA is double stranded.
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• The sugar in RNA is ribose;
• REMEMBER:DNA’s sugar is deoxyribose.
# 2
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• Finally, both DNA and RNA can contain four nitrogenous bases, BUT RNA does not have
Thymine.
• Thymine is replaced by a similar base called uracil (U).
Uracil forms a base pair with adenine in
RNA.
# 3
What do you notice
instead?
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Steps of Transcription
1. DNA is uncoiled by enzymes and unzips using Helicase, just as it does in DNA
replication.
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Steps of Transcription
2. An enzyme called RNA polymerase recognizes and binds to a DNA at a site
called a “promoter”.
A promoter is a base sequence in DNA that signals the start of a gene.
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Steps of Transcription
3. The RNA Polymerase then moves along the DNA strand’s 3’ 5’prime direction, joining one free RNA nucleotide after another until
it reaches a termination sequence.
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Steps of Transcription
4. Once the termination sequence is reached, the entire ‘coding gene’ has been
transcribed into an mRNA strand, and the mRNA is cut away from the DNA.
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Transcription differs from DNA replication in three key respects…
1. Instead of copy the whole DNA strand, only a selected gene within the DNA is copied.
2. Instead of DNA polymerase attaching free nucleotides, it is RNA polymerase.
3. At the end of transcription there is a single, free strand of RNA nucleotides, not a double
helix.
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Transcription is complete at this point. The mRNA at this point is considered “pre-mRNA”.
HOWEVER, the mRNA is not yet ready to leave the nucleus and deliver DNA’s message.
It first needs some “finishing touches”…… called Post-Transcriptional mRNA processing.
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A A AA
A3' poly-A tail
mRNA
5'5' cap
3'
G PPP
50-250 A’s
Post-transcriptional processing• First a cap and tail are added to the mRNA strand.
– Why? • The cap helps the mRNA bind to the ribosome
5 GTP cap (modified Guanine nucleotide)
• The tail keeps enzymes in the cytoplasm from “attacking” the mRNA strand.
poly-A tail (string of Adenine nucleotides)
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Remember - Eukaryotic genes have “junk”!
• Eukaryotic genes are not continuous– exons = the real gene
• expressed / coding DNA
– introns = the junk, stuff you don’t need in order to build a protein.• inbetween sequence
eukaryotic DNA
exon = protein coding (expressed) sequence
intron = non-coding (in-between) sequence
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mRNA splicing – Taking out the “Trash”
eukaryotic DNA
exon = coding (expressed) sequence
intron = noncoding (inbetween) sequence
primary mRNAtranscript
mature mRNAtranscript
pre-mRNA
spliced mRNA
• Edit out the introns
• A “mature” mRNA strand has now been formed– It is much shorter than the original– Exons are Exported, Introns stay IN the nucleus
~10,000 bases
~1,000 bases
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Splicing must be accurate• No room for mistakes!
– a single base added or lost affects the entire protein
AUG|CGG|UCC|GAU|AAG|GGC|CAU
AUGCGGCTATGGGUCCGAUAAGGGCCAUAUGCGGUCCGAUAAGGGCCAU
AUG|CGG|GUC|CGA|UAA|GGG|CCA|U
AUGCGGCTATGGGUCCGAUAAGGGCCAUAUGCGGGUCCGAUAAGGGCCAU
Met|Arg|Ser|Asp|Lys|Gly|His
Met|Arg|Val|Arg|STOP|
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RNA splicing enzymes
snRNPs
exonexon intron
snRNA
5' 3'
spliceosome
exonexcisedintron
5'
5'
3'
3'
3'
lariat
exonmature mRNA
5'
• Spliceosome– Made of small nuclear
ribosomes
– recognize splice site
sequence• cut & paste gene
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mRNA
From gene to protein
DNAtranscription
nucleus cytoplasm
aa
aa
aa
aa
aa
aa
aa
aa
aa
aa
aa
ribosome
proteintranslation