Name _______________________________________________________ Test Date _______________
UNIT 9 – PROTEIN SYNTHESIS
I. THE IMPORTANCE OF RNA (pp. 300-301)
The instructions in _DNA_____ are carried out through the making of _proteins___. Protein synthesis
occurs at the _ribosomes___ in the _cytoplasm_______ of the cell, but DNA is found in the
_nucleus____. DNA cannot leave the nucleus because _it is too “fat”____, so another molecule is
required to take the genetic code from the nucleus to the _ribosomes___ so protein synthesis can take
place. This molecule is _ribonucleic acid___ or _RNA____.
A. RNA Structure – RNA differs from DNA in three ways:
Characteristic
Sugar
Base
Structure
DNA
RNA
RNA is able to leave the nucleus through the _nuclear pores__________ because it is a
_single helix (skinnier )__; thus, the genetic code can be carried out.
B. Types of RNA – There are three principal forms of RNA involved in carrying out the genetic instructions
of DNA:
1. mRNA - _messenger___ RNA. Carries instructions (_recipe___) from DNA in _nucleus____ to
_ribosome_____. “_Blueprint” for genetic code.
2. tRNA - _transfer___ RNA. Carries (transfers) _amino acids___ to ribosome according to instructions
encoded in _mRNA_____.
3. rRNA - _ribosomal__ RNA. Structural component of _ribosomes_____; also produces enzymes____
to bond _amino acids___ together to form _proteins____.
II. PROTEIN SYNTHESIS
(pp. 301-306)
Protein synthesis takes place in two steps:
A. Transcription
The word “transcribe” means to _re-write__. Transcription creates a _disposable____ copy of DNA’s
instructions that can be carried out to the _ribosome__. Transcription takes place in the _nucleus__.
B. Translation
Translates or _interprets___ the _nucleotide_____ sequence of _m_RNA to an _amino acid__
sequence to make a protein. Translation takes place at the _ribosome____.
III. A CLOSER LOOK AT TRANSCRIPTION, THE FIRST STEP IN PROTEIN SYNTHESIS
A. First, the enzyme _RNA polymerase____, unzips part of the DNA molecule.
B. RNA nucleotides are moved in according to base pairing rules and _mRNA__ is synthesized. There are
2 important ways that transcription differs from replication:
1. Only _one side___ of the DNA molecule is copied in transcription.
2. In RNA, the nucleotide that pairs with adenine is _uracil___. The nitrogen base, _thymine____, is
not found in RNA.
C. When the mRNA is first transcribed, there are long sequences of _nucleotides____ that are not
required for the synthesis of the protein called _introns____. The DNA sequences that code for the
protein are known as _exons___. Introns are edited out (cut out) of the mRNA before it leaves the
nucleus and the remaining _exons___ are spliced together to form the final mRNA.
D. mRNA leaves the _nucleus___ through the _nuclear pores_____ and travels to the _ribosome___,
the site of protein synthesis.
For the following “gene”, give the corresponding mRNA sequence:
TACCCGGAATTCGAG
___________________________________________________________________
IV. A CLOSER LOOK AT TRANSLATION, THE SECOND STEP IN PROTEIN SYNTHESIS
A. Codons – A Way to “Read” mRNA
The _monomers__ (building blocks) of proteins are _amino acids__. There are _20___ amino acids
used to build the proteins essential for life. The mRNA message is read by the ribosome in groups of
three _nucleotides__ called _codons___. Each codon codes for a specific amino acid. A codon can
be made up of any combination of the 4 nucleotides; in other words, there are 4 X 4 X 4, or _64__
possibilities for codons. Since there are only 20 amino acids, many different codons represent the same
amino acid. There are codons that are start (_AUG__) and stop signals for the amino acid chain. The
genetic code is _universal__; that is, the same _codon__ codes for the same _amino acid_ in all
organisms.
CODON CHART
For the following codons, give the name of the corresponding amino acid:
a. UAC - _____________________
b. GCA - _____________________
c. AGA - ______________________
d. CCU - ______________________
For the following amino acids, give all the possible codons:
a. arginine - _______________________________________________
b. glycine - ________________________________________________
B. tRNA
The function of tRNA is to transfer the _amino acids_____ specified by the _mRNA______
to the _ribosome_____ for protein synthesis. The _cytosol___ of the cell is stocked with
all 20 amino acids required for protein synthesis. The tRNA molecule carries an _amino acid___ at
one end and at the other end, it has a group of _3__ nucleotides known as an _anticodon__. The
anticodon binds to the mRNA codon cording to base pairing rules and insures that the proper amino acid
is brought to the ribosome.
C. Building a Protein
The _mRNA___ slides through the ribosome a codon___ at a time. As the codon is read, the
tRNA with the corresponding _anticodon_____ comes to the ribosome carrying its amino acid. The next
mRNA codon is read, and another _tRNA____ arrives with its amino acid, according to
its anticodon and base-pairing rules. As another tRNA arrives at the ribosome, the first tRNA is
bumped off, but it leaves its amino acid behind. This process continues and a chain of amino acids
forms until a stop codon is reached. Then the ribosome releases the amino acid chain, which coils &
folds to form a _protein_____.
V. GENETIC MUTATIONS
(pp. 307-308)
A mutation is a _change in the DNA sequence_______. Although many mutations are harmful, some
mutations are _silent__, and others may be very _beneficial____ to an organism. There are two categories
of mutations:
A. Chromosomal Mutations
A chromosomal mutation involves a change in the _structure_____ of the entire chromosome or a
change in the total _number_____ of chromosomes. Does not alter individual _genes____. These errors
generally occur during _meiosis___ or _mitosis_____.
B. Types of Chromosomal Mutations
C. Gene Mutations
A gene mutation is a change in one gene on an individual chromosome. This may result in a change in
only one _nucleotide______ or many nucleotides making up that gene might be altered. The incidence
of gene mutations is relatively low due to the action of _enzymes___ that _proofread______ the DNA
sequence after _replication___.There are two types of gene mutations:
1. Point Mutations – This is a change in one or just a few _nucleotides__, but the total number of
nucleotides in the gene is not changed. This might have no effect, or change one amino acid.
Therefore, the resulting _protein___ may or may not be altered.
2. Frameshift Mutations – This involves the _addition___ or _deletion_____ of a _nucleotide___.
When a nucleotide is inserted or deleted, this shifts the reading of the remainder of the _codons____;
therefore, the _translation___ of the remainder of the mRNA is altered. This will usually result in
tremendous changes in the _amino acid__ chain and completed protein.
Suggested Study Questions
p. 315 (3-10, 12-16, 18-21, 23)