Name _____________________________________________________________________
Test Date _Tues, 1/17____
UNIT VII – GENE EXPRESSION
I. DNA – A REVIEW (pp 287-294)
 DNA stands for _deoxyribonucleic acid__________.
 DNA contains the _genetic code_______ and the _working instructions____ for a cell.
 DNA is a _nucleic acid______ made up of monomers known as _nucleotides_______
o Each nucleotide contains:
 __5_-carbon sugar - _deoxyribose____
 _phosphate_____ group
 _nitrogen ___ base
 _adenine____________ & guanine (purines)
 _thymine____________ & cytosine (pyrimidines)

The shape of DNA is a _double helix______.
o _Covalent_____ bonds hold each nucleotide together
o _Covalent_____ bonds hold one nucleotide to another to form the backbone of DNA
o _Hydrogen____ bonds form between the nitrogen bases to form the double helix
 __A - T____
 __G - C____
II. RNA - _Ribonucleic Acid_________ (pp. 300-301)
DNA contains specific sequences of nucleotides known as _genes____. The instructions they contain are carried out through the
making of _proteins______. Protein synthesis occurs at the _ribosomes____ which are located in the _cytosol___,
but DNA is found in the _nucleus___. DNA cannot leave the nucleus because it is _double helix_, so another molecule is
required to take the genetic code from the _nucleus___ to the _cytoplasm____ so protein synthesis can take place. This
molecule is _RNA___.
A. RNA Structure – RNA differs from DNA in three ways.
Characteristic
DNA
RNA
Sugar
Deoxyribose
Ribose
Base
Thymine
Uracil
Structure
Double helix
Single helix
RNA is able to leave the nucleus through the _nuclear pores___ because it is a _single helix ____; 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. Provides a _disposable______ copy of the instructions in DNA so they can be taken
from the _nucleus__________ to the _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______. In addition to rRNA,
ribosomes are composed of _proteins____ and they are synthesized in the _nucleolus__________.
III. PROTEIN SYNTHESIS
(pp. 301-306)
Protein synthesis requires two steps:
A. Transcription
 Re-writing of the instructions contained in _DNA___ to _mRNA___
 Occurs in the _nucleus of eukaryotic cells_____
B. Translation
 Construction of the specified _protein
 Occurs at the _ribosome______
IV. TRANSCRIPTION
 First, the enzyme _RNA polymerase___________, unzips a specific portion of the DNA molecule by breaking the
_hydrogen_____ bonds between the _nitrogen bases________.
 Specific sequences of DNA nucleotides known as the _promoter________ indicate the beginning of a _gene____ while the end
of a gene is marked by a _termination____ sequence.
 RNA nucleotides are moved in _5’__ to _3’____, according to _base pairing rules____ and _mRNA___ is synthesized.
 There are several important ways that transcription differs from replication:
 Only _one side______ of the DNA molecule is copied in transcription while in replication, _both____ sides are copied.
 In RNA, the nucleotide that pairs with adenine is _uracil__. The nitrogen base, _thymine___, is not found in RNA.
 RNA polymerase does not require a _primer_____.
 In transcription, only _a portion of DNA representing one gene_____ is “unzipped” and copied; in replication, _the
entire DNA molecule____ is unzipped & copied. In addition, in _replication__, all DNA molecules in a cell are
unzipped & copied; this is not true in _transcription____.
 In a _eukaryotic____ cell, replication occurs during _S___ of the cell cycle while transcription primarily occurs during
_G1_______.
 All cells carry out _transcription____; only cells that are _dividing____ carry out _replication____


In prokaryotic cells, as mRNA is transcribed, _translation___ begins virtually simultaneously.
In eukaryotic cells,
 When the mRNA is transcribed, there are long sequences of _nucleotides______ that are not used for the synthesis of
the protein. These non-coding nucleotides are known as _introns_____. The DNA sequences that actually code for the
protein are known as _exons_________. Before mRNA exits the nucleus, the _introns_____ are edited (_cut__) out
and the remaining _exons_____ are spliced together to form the final mRNA.
 mRNA leaves the nucleus via the _nuclear pores____ and travels to the _ribosome_____, the site of protein synthesis.
V. TRANSLATION
Translation takes place at the _ribosome____. The message in _mRNA_________ is read by the ribosome,
_tRNA_____ brings the corresponding _amino acids_____ to make the final product – a _polypeptide____
chain, that eventually folds into a specific _conformation______ to form the _protein_____.
A. Ribosomes
 Synthesized in the _nucleolus______ of eukaryotic cells
 Composed of _rRNA_____ and _protein___________________
 Prokaryotic ribosomes are _smaller___ than eukaryotic ribosomes
 Composed of two __subunits_____ that come together when translation begins

Two locations for eukaryotic ribosomes
o Bound
 Attached to _rough endoplasmic reticulum__________________
 Synthesize proteins _destined for export out of the cell_______
o Free
 Found suspended in the _cytosol_______________
 Synthesize proteins _that will remain in the cell_________
B. Codons - A Mechanism to “Read” mRNA
The monomers of proteins are _amino acids______. There are _20____ amino acids used in building the proteins
essential for life yet there are only _4_____ different nucleotides DNA and RNA. In order to have a different code for
each of the 20 amino acids, nucleotides are read in groups of _three__ known as _codons_____. The codons
collectively make up the _genetic code____.
 The genetic code is _universal_______; in other words, a _codon___ codes for the same amino acid in all
_organisms______.
 The genetic code is _redundant_______. Since there are __4__ possibilities for each nucleotide, and __3____
nucleotides represent a codon, this allows for _4__ x _4___ x _4__ = _64___ variations. There are only _20____ amino
acids, so many different codons represent the same amino acid.
 There is one codon that designates “start” - _AUG____ which codes for _methionine (met)______.
 There are three “stop” codons:
o _UAG______
o _UAA______
o _UGA______
For the following codons, identify the corresponding amino acid:
a. UAC - ______________________________
b. AGA - ______________________________
c. GCA - ______________________________
d. CCU - ______________________________
For the following amino acids, give all possible codons:
a. arginine - ______________________________________________
b. glycine - _______________________________________________
C. tRNA
The function of tRNA is to transfer the _amino acid____ specified by the _mRNA codon_________ to the
_ribosome_____ for protein synthesis. The _cytosol________ of every cell is stocked with all _20___ amino
acids required for protein synthesis. tRNA forms _hydrogen__ bonds with itself, resulting in a folded molecule with
an _amino acid___ at one end that is specific for that tRNA. At the other end, there is a group of three nucleotides known
as the _anticodon________.
The anticodon binds to the _mRNA codon____ according to _base pairing rules____. This insures that the
proper amino acid is brought to the ribosome.
D. The Steps of Translation
 mRNA joins with a _small___ subunit of a ribosome and the start codon, _AUG__, is read.
 The large subunit attaches to the complex forming a complete _ribosome_____.
 A tRNA carrying a _methionine__ arrives at the _A__ site of the ribosome.
 Base pairing occurs between the _tRNA anticodon__ and _mRNA codon__, insuring the proper amino acid has been
delivered.
 The mRNA & ribosome slide against each other, moving the tRNA and its amino acid from the A site to the _P___ site.
 The next codon is read, and the tRNA with the corresponding _amino acid___ arrives at the _A__ site of the ribosome
carrying its amino acid.
 The methionine that had been shifted to the P site forms a _covalent___ bond known as a _peptide___ bond with the next
amino acid.
 Everything shifts _3__ places. The “empty” tRNA departs the ribosome, and the next codon is read. This process known as
_elongation_____, results in a growing chain of amino acids known as a _polypeptide_______ chain.
 Elongation continues until a _stop__ codon is reached.
 At this point, the ribosome releases the _polypeptide____ chain, which coils and folds to form a _protein___.
o The “empty” tRNAs will be re-charged by binding another amino acid.
o The mRNA will be _degraded___ or translated again.
o The protein may be modified, and used in the cell; for example, _enzymes_____, or packaged in a _vesicle___, sent to
the _Golgi apparatus_____, and transported out of the cell via _exocytosis______.
PROTEIN SYNTHESIS – A SUMMARY
VI. MUTATIONS
A mutation is a _change in the DNA sequence__________. Most mutations are harmful but they may also be _beneficial__ or
_silent_. Mutations are classified according to the scope of the change:
A. Chromosomal Mutations
 Involve the number or structure of the entire chromosome.
 Generally occur during _cell division____.
 Very serious consequences; usually results in death of cell.
 Types of chromosomal mutations:
B. Gene Mutations
 Affect one gene on a chromosome
 Usually due to a mistake in _replication_____
 Consequences are variable
 Known as point mutations; 2 main categories
o Substitution
 One nucleotide is exchanged for another
 May or may not affect resulting amino acid sequence
o Frameshift
 Caused by addition or deletion of nucleotide(s)
 Results in a different number of nucleotides, shifts the reading of the remainder of mRNA codons
following mutation
 Typically more serious