RNA and Transcription
Chapter 12
Introduction
• We learned how to replicate DNA, but we
haven’t learned how genes really work. To
help us learn that, we need to first learn
about RNA.
RNA
• Is like DNA, but has
ribose sugar instead
of deoxyribose
• Contains uracil (U)
instead of thymine (T)
• Generally singlestranded
• RNA is a disposable copy
of a segment of DNA
• Can have many jobs, but
main job is to produce
proteins from DNA
3 Types of RNA
• mRNA
• tRNA
• rRNA
mRNA
• Messenger RNA:
serve as messengers
from the DNA to the
rest of the cell
tRNA
• Transfer RNA: used
to transfer amino
acids to the ribosome
in translation (coming
up!)
rRNA
• Ribosomal RNA:
help make up
ribosomes
Transcription
DNA → mRNA
Where does Transcription Occur?
• Eukaryotes: In the
nucleus
• Prokaryotes: In the
cytoplasm
How Transcription Happens
1. An enzyme RNA
Polymerase (like
DNA polymerase,
but makes RNA)
binds to an area on
DNA called the
promoter.
How Transcription Happens
2. RNA polymerase
unzips the DNA
strands and begins
using ONE as a
template
– Uses the leading
strand!
How Transcription Happens
3. RNA polymerase
adds complimentary
RNA nucleotides
to the DNA strand
How Transcription Happens
4. RNA polymerase
stops at a
termination
sequence and
releases the new
mRNA
Translation
Chapter 12
What is Translation
mRNA → polypeptide (protein)
First, let’s learn about the
genetic code
• mRNA has instructions written
for translation in it’s sequence
of nucleotides
• Those nucleotides need to be
read groups of 3 (called
codons)
• If the entire mRNA nucleotide
sequence is considered a
sentence, the groups of 3
nucleotides can be considered
the words
• The 3 nucleotide “words”
specify a single amino acid that
will be used to make the
polypeptide chain
The genetic code
• If the mRNA
sequence reads:
CAGUCCGAAGUC
Glutamine- Serine-
Glutamic
- Valine
Acid
• What would the
amino acid sequence
be?
Steps of Translation
1. mRNA (transcribed
from DNA
remember??) leaves
the nucleus and
enters the
cytoplasm
Steps of Translation
2. mRNA attaches to a
ribosome
Steps of Translation
3. tRNA that is
complementary to the
mRNA codon (called the
anticodon) comes to
the ribosome
(remember that each
tRNA holds a specific
amino acid)
–
Always starts with the
AUG codon (codes for
Methionine)
Steps of Translation
4. Like an assembly
line, the tRNAs keep
coming to the
ribosome and the
amino acids bond
together forming a
continuous chain
Steps of Translation
5. mRNA continues to be
translated until a stop
codon is reached
(UAA, UAG, UGA)
– The tRNA that
corresponds to the stop
codons DOES NOT code
for an amino acid, and
instead helps to release
the finished polypeptide
chain
Now that we know about translation,
let’s review some stuff!
– DNA is the genetic
plan
– RNA is like the
blueprints to the plan
– Proteins are the end
product
But why are proteins
important?
• Many proteins are
enzymes which help
start and regulate
chemical reactions in the
cell
• Genes on chromosomes
code for proteins
– Can produce pigments to
control color
– Create molecules that
determine blood type
– Regulate growth, and lots
more!
• Proteins are important
for everything that living
cells do!