CH13 – RNA and Protein Synthesis
Section 13.1 – RNA
SC.912.L.16.5
Key Questions
• How does RNA differ from DNA?
• How does the cell make RNA?
Vocabulary
• RNA
• Messenger RNA
• Ribosomal RNA
• Transfer FNA
• Transcription
• RNA polymerase
• Promoter
• Intron
• exon
Think About It
• Now we know that DNA is the genetic
material, and the sequence of the nucleotide
bases in its strands must carry some sort of
code. For the code to work, the cell has to be
able to understand it…so…what exactly do the
bases code for and where is the cell’s
decoding system?
The Role of RNA
How does RNA differ from DNA?
When Watson and Crick solved the double-helix
structure of DNA they knew how DNA could be
copied.
All the cell had to do was separate the 2 strands and
then use the base-pairing to make a new
complementary strand for each.
But…
…the structure by itself did not explain how a
gene actually works.
It took a lot more research to discover that
another nucleic acid – ribonucleic acid (RNA),
was instrumental in putting the genetic code
into action
RNA – like DNA – consists of a long chain of
nucleotides.
Genes contain coded DNA instructions that tell cells
how to build proteins.
The first step in decoding these genetic instructions is
to copy part of the base sequence from DNA into
RNA, which then uses these instructions to direct the
production of protein which help to establish an
organism’s characteristics.
Comparing RNA and DNA
Review:
Each DNA nucleotide is made up of :
5 carbon sugar
phosphate group
nitrogenous base
There are 3 main differences between DNA and RNA:
1. Sugar in RNA is ribose, instead of deoxyribose.
2. RNA is generally a single-strand, not a doublestrand
3. RNA contains uracil, instead of thymine.
These chemical differences make it easy for enzymes
in the cell to tell DNA and RNA apart.
RNA Strand
Master Plans vs. Blueprints
• A master plan has all the information needed to
construct a building – DNA.
• Blueprints are inexpensive, disposable copies of the
master plan – RNA.
• The DNA molecule stays safely in the cell’s nucleus
while the RNA molecules go to the protein-building
sites in the cytoplasm – the ribosomes.
Functions of RNA
There are 3 types of RNA:
• Messenger RNA (mRNA) – carries instructions for
assembling amino acids into proteins from the DNA
and into other parts of the cell.
• Ribosomal RNA (rRNA) – proteins are assembled
on ribosomes, small organelles composed of two
subunits which are made up of several dozen
proteins and rRNA.
Functions of RNA
• Transfer RNA (tRNA) - during the
construction of proteins, tRNA
transfers each amino acid to the
ribosome as it is specified by coded
messages in mRNA.
RNA Synthesis
• How does the cell make RNA?
Cells invest large amounts of raw material and
energy into making RNA molecules.
Transcription
• Most of the work of making RNA takes place
during transcription.
• In transcription, segments of DNA serve as
templates to produce complementary RNA
molecules.
• Bases of the transcribed RNA complement
the base sequence of the template DNA.
• In prokaryotes, RNA synthesis and protein
synthesis take place in the cytoplasm.
• In eukaryotes, RNA is produced in the cell’s
nucleus and then moves to the cytoplasm to
play a role in the production of protein.
• The ability to copy a single DNA sequence into
RNA makes it possible for a single gene to
produce hundreds or even thousands of RNA
molecules.
The enzyme in RNA transcription is RNA polymerase.
RNA polymerase binds to DNA during transcription
and separates the DNA strands.
It then uses one strand of the DNA as a template from
which to assemble nucleotides into a
complementary strand of RNA
Promoters
• How does RNA polymerase know where to
start and stop making a strand of RNA?
The answer is that RNA polymerase doesn’t bind
to DNA just anywhere. The enzyme binds only
to promoters, regions of DNA that have
specific base sequences.
RNA Editing
• Just like a writer’s first draft, RNA molecules
sometimes need a bit of editing before they
are ready to be read.
• These pre-mRNA molecules have bits and
pieces cut out of them before they can go into
action.
• The cut out, discarded portions are called
introns.
• In eukaryotes, introns are taken out of the
pre-mRNA molecules while they are still in the
nucleus.
• The remaining pieces, exons, are then spliced
back together to form the final mRNA.
Introns and Exons
• Promoters are signals in the DNA molecule
that show RNA polymerase exactly where to
begin making RNA.
• There are similar signals in DNA that cause
transcription to stop when a new RNA
molecule is completed
• Why do cells use energy to make a large RNA
molecule and then throw away parts?
• Biologists aren’t complete sure; however,
introns and exons may play a role in evolution,
making it possible for very small changes in
DNA sequences to have dramatic effects on
how genes affect cellular function.
13.1 Assessment (page 365)
1a. Review Describe three main differences
between RNA and DNA.
RNA – ribose
RNA – single strand
RNA – uracil
DNA – deoxyribose
DNA – double strand
DNA - thymine
1b. Explain List the three main types of RNA, and
explain what they do.
mRNA – carries instructions for polypeptide synthesis
from DNA in the nucleus to ribosomes in the
cytoplasm
rRNA – forms an important part of both subunits of a
ribosome, where proteins are assembled.
1b. Continued.
tRNA carries amino acids to a ribosome
and matches them to the coded mRNA
message.
1c. Infer Why is it important for a single gene to be
able to produce hundreds or thousands of the same
RNA molecules?
Proteins must be continuously synthesized in the cell,
so the instructions coded in genes must be used over
and over again. A single gene must be able to
produce hundreds or thousands of the same RNA
molecules for protein synthesis.
2a. Review Describe what happens during
transcription.
During transcription, the enzyme RNA
polymerase binds to DNA and separates the
DNA strands. It then uses one strand of DNA
as a template to assemble nucleotides into a
complementary strand of RNA.
2b. Predict What do you think would happen
if introns were not removed from pre-mRNA?
If introns were not removed, the instructions
carried by mRNA for assembling amino acids
into a protein might be incorrect, and the
resulting protein might no function properly.