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39
DNA and Translation
Biology B
Pages 235-241
Name: ________________________ Date: __________ Period: ________
As we mentioned before, the ultimate goal of a gene on a
chromosome is gene expression. Gene expression is ultimately the
production of a protein product. That is for us to see what a gene does – it
MUST produce a protein molecule! We have discovered that to create a
protein product a small portion of the DNA must be transcribed (or copied).
The copy of the DNA is called a messenger RNA and is abbreviated,
mRNA. Transcription takes place in the nucleus. The mRNA that is
produced in the nucleus moves to the cytoplasm. (See picture below:)
At this point, the process is now called translation. That is, we are
now translating the message (the mRNA is the message from the DNA) into
a protein molecule. In the cytoplasm, the mRNA must bind to a ribosome.
If you remember,
ribosomes are cell
Cytoplasm
organelles that help
protein production.
Another kind of RNA
then comes into play.
Transfer RNA (or
tRNA see * in picture
to right) has a
*
specific amino acid
attached to it
(represented by a
small black dot).
When the mRNA
binds to the
ribosome, special
tRNA’s move to the
mRNA/ribosomematch their “anticodon” to the codon of the mRNA and begin forming protein molecules (see
pages 240-241) and/or above picture.
To help explain how the sequences of the DNA help direct this
process, we have to explain some basics. The nucleotide sequences in DNA
code for specific amino acids.
Page 2 (Cont. #39 Bio. B)
The code is broken down into three nucleotide sequences in a row
and are called codons. Specific codons (that is specific three nucleotide
sequences) code for a certain amino acid. Of the following DNA sequence,
the codons are labeled.
Double stranded DNA
T
Single
stranded
DNA
template
T
A
C
T
T
G
G
C
A
T
A
G
Double stranded DNA
A-T
G-C
C-G
T-A
T-A
A
U
G
A
A
C
C
G
U
A
U
C
C
A
codon
codon
codon
codon
G
G-C
G-C
T-A
A-T
C-G
A
T
G
A
A
C
C
G
T
A
T
C
Page 3 (Cont. #39 Bio. B)
Why are there three nucleotides to a codon?
Well, one possible answer is simple math. There are twenty different
amino acids that the body uses to make proteins. If we needed a unique way
to determine which amino acid we wanted in a protein, we would use one,
two, three, or more nucleotides in a row to “code” for a specific amino acid.
1.) If we only used one nucleotide to code for a specific amino acid, we
could only code for a maximum of FOUR unique amino acids.
2.) If we used two nucleotides to code for a specific amino acid, we could
only code for 16 different amino acids. (Not enough combinations to
code for 20 different amino acids.)
3.) If we used three nucleotides in a row to code for a specific amino
acid, we could code for 64 different amino acids! This would satisfy
the need to have a specific code for 20 different amino acids AND
code for “STOP” sequences.
What scientists discovered was that three nucleotides in a row was the
precise number needed to “code” for a specific amino acid. Scientists
also discovered that the three nucleotide “code” is universal. What
this means is that the 3 nucleotide sequence in human cells that codes for
a particular amino acid called Glycine, is the SAME nucleotide sequence
in bacteria cells that codes for Glycine.
Each tRNA holds only one amino acid. The amino acid that tRNA carries
depends on the anti-codon found
on one part of the tRNA
molecule. (see picture on page
240 and to the right:)
As you can see to the right,
the tRNA has an amino acid
(a.a.) attached to it and the anticodon matches up with the codon
on the mRNA and this continues
until the mRNA has a STOP codon. This sequence stops protein synthesis.
SO- the mRNA sequence controls which amino acids are going to be
put together and in what order. Remember, the mRNA sequence was
originally determined by the original sequence in the DNA.
Answer the following questions here and on your scan-tron.
_____ 1.) (T,F) All living cells make proteins.
Page 4 (Cont. Handout #39 Bio. B)
_____ 2.) Where does transcription take place?
a.) in the nucleus b.) in the cytoplasm c.) outside the cell
_____ 3.) When the DNA is “transcribed” during protein synthesis, what is
really happening is…
a.) DNA is making a copy of itself in preparation for mitosis
b.) DNA is making a copy of itself by making an mRNA molecule
c.) DNA is being broken down
d.) DNA is making sugar molecules directly
_____ 4.) When the mRNA leaves the nucleus, the proteins synthesis
process is now called?
a.) transcription b.) transceiver c.) translation d.) transportation
_____ 5.) In the cytoplasm, the mRNA binds to:
a.) the newly formed protein only
b.) the ribosome
c.) the mitochondria
d.) the Golgi body
_____ 6.) Where are the proteins assembled in the cytoplasm (see picturepg. 1 of handout)
a.) in the nucleus b.) in the mitochondria c.) on the ribosome
_____ 7.) What is a codon?
a.) a two nucleotide sequence that codes for a specific amino acid
b.) a three nucleotide sequence that codes for a specific amino acid
c.) a four nucleotide sequence that codes for a specific amino acid
d.) none of the above are correct
_____ 8.) What molecule moves (transfers) the amino acid to the ribosome?
a.) mRNA b.) DNA c.) tRNA d.) ribosome (rRNA)
_____ 9.) What part of the tRNA binds to the mRNA?
a.) anti-codon b.) codon c.) amino acid d.) DNA
_____ 10.) How does the mRNA and tRNA know when to stop making
protein?
a.) when cell runs out of amino acids b.) when the tRNA hits a stop codon
c.) when cell runs out of DNA
d.) when cell runs out of protein
Page 5 (Cont. Handout #39 Bio. B)
Answer to the following question on this handout ONLY! The first
codon (the first three nucleotides) is done for you…
11.) Given the following DNA sequence, show what the mRNA sequence
SHOULD look like: Circle EACH codon on the mRNA!
T-A-C-T-T-G-A-A-A-G-T-T-C-A-C-C-G-G-A-G-G
A-U-G-
Date: ____________________
Lesson Plan for Handout #39
Biology B
Objective: TLWD ability to explain what happens to the mRNA once it
leaves the nucleus and what the process is now called. Moreover the student
will be able to explain what is located on the tRNA and how and where
proteins are formed when given handout #39.
Content: Translation, protein production
NJCCCS: 5.3.12.E.1.
Method: Power point, white board, discussion
Homework: Handout #39
Comments: Try explaining transcription as the same as asking them to look
something up on the Internet and copying a portion to help you create a
portfolio? (Something like that- see if you can get the point across.)