Transcription and
Translation
The Central Dogma of Molecular Biology:
DNA --> RNA --> Protein
Protein synthesis
requires two steps:
transcription and
translation.
Before we get into transcription and
translation, let’s review some vocabulary
gene - coded DNA instructions that control the production of
proteins in cells
DNA - deoxyribonucleic acid, the genetic material that tells your
cells how to make proteins
What are the four nucleotide bases in the DNA code?
RNA - ribonucleic acid. It carries out the instructions coded in
DNA. What are the four nucleotide bases in RNA?
mRNA - messenger RNA
RNA polymerase - an enzyme that attaches to DNA at specific
sequences to start transcription
ribosomes - small particles made of protein that are found on the
rough endoplasmic reticulum. They bind mRNA and tRNA during
translation.
codon - a three base sequence in DNA or RNA that codes for one
amino acid
DNA Contains Codes
Three bases in DNA code for one amino acid. The DNA code is copied
to produce mRNA. The order of amino acids in the polypeptide
(the protein) is determined by the sequence of 3-letter codes (codons)
in mRNA.
Transcription
To transcribe something is to copy it. In transcription,
DNA is copied into mRNA. RNA molecules are
produced by copying part of the DNA sequence into a
complementary RNA sequence.
Transcription
To transcribe DNA to mRNA, we need an enzyme called
RNA polymerase. It is similar to DNA polymerase.
During transcription, RNA polymerase binds to DNA and
separates the DNA strands. RNA polymerase then uses one
strand of DNA as a template from which nucleotides are
assembled into a strand of mRNA.
Where does
transcription
take place?
How does RNA polymerase know what
part of the DNA to bind to? How does
it know when to start and stop?
There are regions of DNA called
promoters, which have specific sequences.
Promoters act as signals in the DNA to tell
RNA polymerase where to bind. There is
also a similar sequence in the DNA that
signals RNA polymerase to stop
transcription.
RNA Editing
Like a rough draft of a paper, many RNA molecules
need to be edited before they are ready for
translation. The DNA of many eukaryotes contains
sequences called introns that do not code for any
proteins and must be cut out of the RNA sequence.
The remaining sequences that do code for proteins
are called exons.
To help you remember, think of it this way – INtrons
go IN the trash
RNA Editing
What is the point of this? Nobody knows for sure.
RNA editing may play a role in evolution.
Mutations in introns have no effect on protein
synthesis, but mutations in exons will probably have
an effect and change the protein that will be
produced.
Once the RNA has been
edited, it is released to the
cytoplasm and attaches to
a ribosome on the rough
endoplasmic reticulum
so that translation can begin
Translation
To translate something is to express it in
another language. mRNA is translated
into amino acids, which form chains
called polypeptides, which get folded
and shaped into proteins.
More Vocabulary!
tRNA - transfer RNA, it transfers an amino acid to the
growing polypeptide chain. Each tRNA molecule
carries one particular amino acid
codon - a sequence of three nucleotides that together
form a unit of genetic code in a DNA or RNA
molecule. Each codon codes for one amino acid
amino acids - the subunits that make up polypeptides
and proteins
polypeptide - a chain of amino acids
protein - a finished polypeptide that has been folded
into shape
Translation
Translation begins when an mRNA molecule
in the cytoplasm attaches to the ribosome. As each
codon of the mRNA molecule moves through the
ribosome, the proper amino acid is brought to the
brought to the ribosome by tRNA. In the ribosome,
the amino acid
is transferred to
the growing
polypeptide
chain
Video!
During the video, write down at least one
question you have about transcription and/or
translation.
QuickTime™ and a
decompressor
are needed to see this picture.
How do we find out what the
fox says?
In this activity, you will examine the DNA sequence of a fictitious
fox. These foxes say different things that are determined by their
genes. Our fake foxes have one gene that determines what
phrases they say. Your job is to analyze the genes of its DNA and
determine what the fox says.
The gene sequences we will be using are much
smaller than -real-gene sequences found
in living organisms. Each gene has two
versions that result in a different trait
(the phrase that the fox says)
being expressed in the fox.
How do we find out what the
fox says?
Each of the DNA samples on your worksheets was taken
from volunteer foxes. The DNA was then transcribed to its
complementary mRNA strand. Your job is to analyze the
mRNA sample and determine what the fox says based on the
sequence. You will choose one sample (the sequences from
one fox) to analyze.
No foxes were harmed in the making of this presentation.
You will also determine four
other traits that the fox has
The first gene determines what the fox
says. The rest of the genes determine
the fox’s fur color (brown or red), eye
color (brown or black), tail length (short or
long), and fur length (short or long).
The Codon Chart
Use this codon chart to determine which
amino acids are in your fox’s sequence.
Example
The mRNA of gene 1 in the Ylvis Fox is
GUC AGC AAA
Looking at the codon chart,
GUC = val, AGC = ser, and AAA = lys
Looking at the table, the amino acid sequence
(the polypeptide or protein) val-ser-lys results in
the Ylvis Fox’s first phrase being
“ring-ding-dingeringeding”
Continue this process until you have decoded
all five genes, and you will know what the fox
says and what it looks like!
Example
Once you have
determined all five of
your fox’s traits, draw
a picture of your fox
and give it a text
bubble showing what
it says
Just in case you were
curious…
QuickTime™ and a
decompressor
are needed to see this picture.