Transcript SBI 4U Genetics 3

Genetics 3:
Transcription:
Making RNA from DNA
Comparing DNA and RNA
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DNA nitrogenous bases: A, T, G, C
RNA nitrogenous bases: A, U, G, C
DNA: Deoxyribose vs RNA: ribose sugar
DNA: double stranded vs RNA: single
DNA: full set of chromosomes vs RNA: just
single genes
• DNA: stays in nucleus vs RNA: can leave nucleus
and go to endoplasmic reticulum and cytoplasm
Types of RNA
• All are made in the nucleus first
• mRNA: (messenger) template for translation
• tRNA: (transfer) brings amino acids to ribosome
in translation
• rRNA: (ribosomal) with proteins, it makes up the
ribosome
• snRNA: (small nuclear) modifies mRNA
• miRNA: (micro) regulates gene expression
• siRNA: (small interfering) also regulates gene
expression
Main Point of Transcription:
• To copy only the one section of DNA that
we need
• To make RNA that is allowed to leave the
nucleus
Transcription Phase 1: Initiation
• For each gene, only one strand of the DNA is
transcribed and is called the template strand.
 The other strand is called the coding strand because the
mRNA you make will actually match this strand (with T’s
being replaced by U’s of course…)
• Either side can work as the template. It depends on the
gene.
• Transcription begins when an RNA polymerase binds
tightly to the promoter region and unwinds and opens
up a section of the double helix.
• The promoter region consists of a sequence of
nucleotides in DNA that indicates where the RNA
polymerase complex needs to bind.
Transcription Phase 2: Elongation
• The RNA polymerase complex works its way along the DNA
molecule, making a strand of mRNA that is complementary to
the template strand of DNA.
• They work in the 5’ to 3’ direction (adding a new nucleotide to
the free -OH group)
• They only transcribe one strand, so no Okazaki fragments this
time.
• As soon as the RNA polymerase complex moves from the
promoter region, another complex can bind on.
• No proofreading, so this process is much faster than DNA
replication
• An error would only result in a messed up protein, so it’s not a
big deal
Transcription Phase 3: Termination
• Specific nucleotide sequences in the
template DNA serve as a stop signal to
transcription.
• When the RNA polymerase complex
reaches this signal, they detach from the
DNA strand and the mRNA is released.
• Double helix reforms.
Diagram
mRNA Modification in
Eukaryotes.
• In prokaryotes, the mRNA is used right away in
protein synthesis.
• In eukaryotes, what we have just made is known
as precursor mRNA (pre-mRNA).
• In order to get our mature mRNA we have to:
 Add a 5’cap: modified G nucleotide added to the 5’ end
which enables our protein synthesis machinery to
recognize it.
 Add a 3’ poly-A tail: series of A nucleotides added the
the 3’ end that keep the mRNA stable
 Remove introns: introns are non-coding regions that are
interspersed among the coding regions (exons) through
splicing
Splicing
• Particles composed of snRNA (small
nuclear RNA) and proteins called snRNPs
(pronounced “snurps”) recognize regions
where exons and introns meet and bind on.
• snRNPs interact with other proteins to form
a larger spliceosome complex and removes
the introns.
Diagrams:
Questions:
• Page 250: 8, 10, 11, 12, 14.
• Page 254: 7, 9, 12
• Page 256: 3, 5, 7