Section 11.1 Summary – pages 281 - 287
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Transcript of Section 11.1 Summary – pages 281 - 287
DNA REPLICATION REVIEW
1. When does DNA divide?
2. Why does it happen at this time?
• What is the first step in DNA replication?
• Next, what is the job of the enzyme Helicase?
What is the replication fork?
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?
LEADING STRAND VS. LAGGING STRAND
Why does the DNA always have to build from the original DNA’s 3’?
Why does the DNA always have to build from the original DNA’s 3’?
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…
TranscriptionThe process of copying
a segment of DNAinto RNA.
This is the first step in protein synthesis.
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?
• 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!
• 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.
• The sugar in RNA is ribose;
• REMEMBER:DNA’s sugar is deoxyribose.
# 2
• 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?
Steps of Transcription
1. DNA is uncoiled by enzymes and unzips using Helicase, just as it does in DNA
replication.
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.
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.
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.
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.
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.
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)
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
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
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|
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
mRNA
From gene to protein
DNAtranscription
nucleus cytoplasm
aa
aa
aa
aa
aa
aa
aa
aa
aa
aa
aa
ribosome
proteintranslation